Cell

Extract(1)
Unicellular organisms are capable of
(i) independent existence and
(ii) performing the essential functions of life. Anything less than a complete structure of a cell does not ensure independent living.
Hence, cell is the fundamental structural and functional unit of all living organisms.

Anton Von Leeuwenhoek first saw and described a live cell.
Robert Brown later discovered the nucleus.
The invention of the microscope and its improvement leading to the electron microscope revealed all the structural details of the cell. Extract(2)

Cell theory

In 1838, Matthias Schleiden, a German botanist, examined a large number of plants and observed that all plants are composed of different kinds of cells which form the tissues of the plant.

At about the same time, Theodore Schwann (1839), a British Zoologist,
studied different types of animal cells and reported that cells had a thin outer layer which is today known as the ‘plasma membrane’.
He also concluded, based on his studies on plant tissues, that the presence of cell wall is a unique character of the plant cells.

On the basis of this, Schwann proposed the hypothesis that the bodies of animals and plants are composed of cells and products of cells.

Schleiden and Schwann together formulated the cell theory.
This theory however, did not explain as to how new cells were formed.

Rudolf Virchow (1855) first explained that cells divided and new cells are formed from pre-existing cells (Omnis cellula-e cellula).
He modified the hypothesis of Schleiden and Schwann to give the cell theory a final shape.

Cell theory as understood today is:

(i) all living organisms are composed of cells and products of cells.
(ii) all cells arise from pre existing cells.

An overview of cell

You have earlier observed cells in an onion peel and/or human cheek cells under the microscope.
Let us recollect their structure.
The onion cell which is a typical plant cell,
has a distinct cell wall as its outer boundary and just within it is the cell membrane.
The cells of the human cheek have an outer membrane as the delimiting structure of the cell.
Inside each cell is a dense membrane bound structure called nucleus.
This nucleus contains the chromosomes which in turn contain the genetic material, DNA.
Cells that have membrane bound nuclei are called eukaryotic
whereas cells that lack a membrane bound nucleus are prokaryotic.
In both prokaryotic and eukaryotic cells,
a semi-fluid matrix called cytoplasm occupies the volume of the cell.
The cytoplasm is the main arena of cellular activities in both the plant and animal cells.
Various chemical reactions occur in it to keep the cell in the - living state

Besides the nucleus,
the eukaryotic cells have other membrane bound distinct structures called organelles like the endoplasmic reticulum (ER),
the golgi complex,
lysosomes, mitochondria, microbodies and vacuoles.
The prokaryotic cells lack such membrane bound organelles.

Ribosomes are non-membrane bound organelles found in all cells -- both eukaryotic as well as prokaryotic.
Within the cell, ribosomes are found not only in the cytoplasm but also within the two organelles
– chloroplasts (in plants) and mitochondria and on rough ER.

Animal cells contain another non-membrane bound organelle called centrosome which helps in cell division.
Cells differ greatly in size, shape and activities (Figure 8.1).
For example, Mycoplasmas, the smallest cells, are only 0.3 μm in length while bacteria could be 3 to 5 μm.
The largest isolated single cell is the egg of an ostrich.
Among multicellular organisms, human red blood cells are about 7.0 μm in diameter.
Nerve cells are some of the longest cells.
Cells also vary greatly in their shape.
They may be disc-like, polygonal, columnar, cuboid, thread like, or even irregular.
The shape of the cell may vary with the function they perform.

Extract(4) Prokaryotic cells -The prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma and PPLO (Pleuro Pneumonia Like Organisms).
They are generally smaller and multiply more rapidly than the eukaryotic cells (Figure 8.2).

They may vary greatly in shape and size. The four basic shapes of bacteria are bacillus (rod like),
coccus (spherical),
vibrio (comma shaped) and spirillum (spiral).

The organisation of the prokaryotic cell is fundamentally similar even though prokaryotes exhibit a wide variety of shapes and functions.
-All prokaryotes have a cell wall surrounding the cell membrane except in mycoplasma. The fluid matrix filling the cell is the cytoplasm. There is no well-defined nucleus.
-The genetic material is basically naked, not enveloped by a nuclear membrane.
-In addition to the genomic DNA (the single chromosome/circular DNA), many bacteria have small circular DNA outside the genomic DNA. These smaller DNA are called plasmids.
-The plasmid DNA confers certain unique phenotypic characters to such bacteria. One such character is resistance to antibiotics. In higher classes you will learn that this plasmid DNA is used to monitor bacterial transformation with foreign DNA.
-Nuclear membrane is found in eukaryotes.
-No organelles are found in prokaryotic cells except organisms for ribosomes.
-Prokaryotes have something unique in the form of inclusions. A specialised differentiated form of cell membrane called mesosome is the characteristic of prokaryotes. They are essentially infoldings of cell membrane.

Cell Envelope and its Modifications

- Most prokaryotic cells, particularly the bacterial cells, have a chemically complex cell envelope.
- The cell envelope consists of a tightly bound three layered structure i.e.,
the outermost glycocalyx
followed by the cell wall and
then the plasma membrane.
- Although each layer of the envelope performs distinct function, they act together as a single protective unit.

Bacteria can be classified into two groups on the basis of the differences in the cell envelopes and the manner in which they respond to the staining procedure developed by Gram
viz.,
- those that take up the gram stain are Gram positive and
the others that do not are called Gram negative bacteria.

Glycocalyx differs in composition and thickness among different bacteria.
-It could be a loose sheath called the slime layer in some,
-while in others it may be thick and tough,
called the capsule.

The cell wall determines the shape of the cell and provides a strong structural support to prevent the bacterium from bursting or collapsing.
The plasma membrane is selectively permeable in nature and interacts with the outside world.
This membrane is similar structurally to that of the eukaryotes.

A special membranous structure is the mesosome which is formed by the extensions of plasma membrane into the cell.
- These extensions are in the form of vesicles, tubules and lamellae.
-They help in cell wall formation, DNA replication and distribution to daughter cells.
- They also help in respiration, secretion processes, to increase the surface area of the plasma membrane and enzymatic content.
- In some prokaryotes like cyanobacteria, there are other membranous extensions into the cytoplasm called chromatophores which contain pigments.

Bacterial cells may be motile or non-motile.
- If motile, they have thin filamentous extensions from their cell wall called flagella. Bacteria show a range in the number and arrangement of flagella.
- Bacterial flagellum is composed of three parts –
filament, hook and basal body.
The filament is the longest portion and extends from the cell surface to the outside.
- Besides flagella,
Pili and Fimbriae are also surface structures of the bacteria but do not play a role in motility.
- The pili are elongated tubular structures made of a special protein.
- The fimbriae are small bristle like fibres sprouting out of the cell.
- In some bacteria, they are known to help attach the bacteria to rocks in streams and also to the host tissues.

Ribosomes and Inclusion Bodies

In prokaryotes,
- ribosomes are associated with the plasma membrane of the cell.
They are about 15 nm by 20 nm in size and are made of two subunits -
50S and 30S units which when present together form 70S prokaryotic ribosomes.
- Ribosomes are the site of protein synthesis.
- Several ribosomes may attach to a single mRNA and form a chain called polyribosomes or polysome.
- The ribosomes of a polysome translate the mRNA into proteins.

Inclusion bodies:
- Reserve material in prokaryotic cells are stored in the cytoplasm in the form of inclusion bodies.
These are not bound by any membrane system and lie free in the cytoplasm,
e.g., phosphate granules, cyanophycean granules and glycogen granules.
Gas vacuoles are found in blue green and purple and green photosynthetic bacteria.

Extract(5)

Eukaryotic Cell

-The eukaryotes include all the protists, plants, animals and fungi.
In eukaryotic cells there is an extensive compartmentalisation of cytoplasm through the presence of membrane bound organelles.
-Eukaryotic cells possess an organised nucleus with a nuclear envelope.
In addition,
-eukaryotic cells have a variety of complex locomotory and cytoskeletal structures.
-Their genetic material is organised into chromosomes.
- All eukaryotic cells are not identical.
Plant and animal cells are different as the former possess cell walls, plastids and a large central vacuole which are absent in animal cells.
- On the other hand,
animal cells have centrioles which are absent in almost all plant cells (Figure 8.3).

Cell Membrane

The detailed structure of the membrane was studied only after the advent of the electron microscope in the 1950s.
Meanwhile, chemical studies on the cell membrane, especially in human red blood cells (RBCs), enabled the scientists to deduce the possible structure of plasma membrane.
- These studies showed that the cell membrane is mainly composed of lipids and proteins.
The major lipids are phospholipids that are arranged in a bilayer.
-Also, the lipids are arranged within the membrane with the polar head towards the outer sides and the hydrophobic tails towards the inner part.This ensures that the nonpolar tail of saturated hydrocarbons is protected from the aqueous environment (Figure).
-In addition to phospholipids membrane also contains cholesterol.
-The lipid component of the membrane mainly consists of phosphoglycerides.
- Later, biochemical investigation clearly revealed that the cell membranes also possess protein and carbohydrate.
-The ratio of protein and lipid varies considerably in different cell types. In human beings, the membrane of the erythrocyte has approximately 52 per cent protein and 40 per cent lipids.

Depending on the ease of extraction, membrane proteins can be classified as integral and peripheral.
- Peripheral proteins lie on the surface of membrane while the integral proteins are partially or totally buried in the membrane. An improved model of the structure of cell membrane was proposed by Singer and Nicolson (1972) widely accepted as fluid mosaic model (Figure).
-According to this, the quasi-fluid nature of lipid enables lateral movement of proteins within the overall bilayer. This ability to move within the membrane is measured as its fluidity.
- The fluid nature of the membrane is also important from the point of view of functions like cell growth, formation of intercellular junctions, secretion, endocytosis, cell division etc.
-One of the most important functions of the plasma membrane is the transport of the molecules across it.
-The membrane is selectively permeable to some molecules present on either side of it.
- Many molecules can move briefly across the membrane without any requirement of energy and this is called the passive transport.
- Neutral solutes may move across the membrane by the process of simple diffusion along the concentration gradient, i.e., from higher concentration to the lower.
- Water may also move across this membrane from higher to lower concentration. Movement of water by diffusion is called osmosis.
- As the polar molecules cannot pass through the nonpolar lipid bilayer, they require a carrier protein of the membrane to facilitate their transport across the membrane.
- A few ions or molecules are transported across the membrane against their concentration gradient, i.e., from lower to the higher concentration.
Such a transport is an energy dependent process, in which ATP is utilised and is called active transport, e.g., Na+/K+ Pump.

Cell Wall

As you may recall, a non-living rigid structure called the cell wall forms an outer covering for the plasma membrane of fungi and plants.
- Cell wall not only gives shape to the cell and protects the cell from mechanical damage and infection, it also helps in cell-to-cell interaction and provides barrier to undesirable macromolecules.

-Algae have cell wall, made of cellulose, galactans, mannans and minerals like calcium carbonate, while in other plants it consists of cellulose, hemicellulose, pectins and proteins.
-The cell wall of a young plant cell, the primary wall is capable of growth, which gradually diminishes as the cell matures and the secondary wall is formed on the inner (towards membrane) side of the cell.
- The middle lamella is a layer mainly of calcium pectate which holds or glues the different neighbouring cells together. The cell wall and middle lamellae may be traversed by plasmodesmata which connect the cytoplasm of neighbouring cells.

Endomembrane System

While each of the membranous organelles is distinct in terms of its structure and function, many of these are considered together as an endomembrane system because their functions are coordinated.
-The endomembrane system include endoplasmic reticulum (ER), golgi complex, lysosomes and vacuoles. Since the functions of the mitochondria, chloroplast and peroxisomes are not coordinated with the above components, these are not considered as part of the endomembrane system.

The Endoplasmic Reticulum (ER)

-Electron microscopic studies of eukaryotic cells reveal the presence of a network or reticulum of tiny tubular structures scattered in the cytoplasm that is called the endoplasmic reticulum (ER) (Figure).
-Hence, ER divides the intracellular space into two distinct compartments, i.e., luminal (inside ER) and extra luminal (cytoplasm) compartments.
- The ER often shows ribosomes attached to their outer surface.
-The endoplasmic reticulun bearing ribosomes on their surface is called rough endoplasmic reticulum (RER).
-In the absence of ribosomes they appear smooth and are called smooth endoplasmic reticulum (SER).
RER is frequently observed in the cells actively involved in protein synthesis and secretion. They are extensive and continuous with the outer membrane of the nucleus.
- The smooth endoplasmic reticulum is the major site for synthesis of lipid. In animal cells lipid-like steroidal hormones are synthesised in SER.

Golgi apparatus

Camillo Golgi (1898) first observed densely stained reticular structures near the nucleus.
-These were later named Golgi bodies after him. They consist of many flat, disc-shaped sacs or cisternae of 0.5μm to 1.0μm diameter (Figure).
- These are stacked parallel to each other. Varied number of cisternae are present in a Golgi complex.
- The Golgi cisternae are concentrically arranged near the nucleus with distinct convex cis or the forming face and concave trans or the maturing face.
- The cis and the trans faces of the organelle are entirely different, but interconnected.
- The golgi apparatus principally performs the function of packaging materials, to be delivered either to the intra-cellular targets or secreted outside the cell.
- Materials to be packaged in the form of vesicles from the ER fuse with the cis face of the golgi apparatus and move towards the maturing face.
- This explains, why the golgi apparatus remains in close association with the endoplasmic reticulum. A number of proteins synthesised by ribosomes on the endoplasmic reticulum are modified in the cisternae of the golgi apparatus before they are released from its trans face. Golgi apparatus is the important site of formation of glycoproteins and glycolipids.

Lysosomes

These are membrane bound vesicular structures formed by the process of packaging in the golgi apparatus.
- The isolated lysosomal vesicles have been found to be very rich in almost all types of hydrolytic enzymes (hydrolases – lipases, proteases, carbohydrases) optimally active at the acidic pH.
- These enzymes are capable of digesting carbohydrates, proteins, lipids and nucleic acids.

Vacuoles

The vacuole is the membrane-bound space found in the cytoplasm. It contains water, sap, excretory product and other materials not useful for the cell.
- The vacuole is bound by a single membrane called tonoplast. In plant cells the vacuoles can occupy up to 90 per cent of the volume of the cell.
- In plants, the tonoplast facilitates the transport of a number of ions and other materials against concentration gradients into the vacuole, hence their concentration is significantly higher in the vacuole than in the cytoplasm.
- In Amoeba the contractile vacuole is important for excretion. In many cells, as in protists, food vacuoles are formed by engulfing the food particles.

Mitochondria

Mitochondria (sing.: mitochondrion), unless specifically stained, are not easily visible under the microscope.
-The number of mitochondria per cell is variable depending on the physiological activity of the cells. In terms of shape and size also, considerable degree of variability is observed.
-Typically it is sausage-shaped or cylindrical having a diameter of 0.2-1.0μm (average 0.5μm) and length 1.0-4.1μm.
- Each mitochondrion is a double membrane-bound structure with the outer membrane and the inner membrane dividing its lumen distinctly into two aqueous compartments,
i.e., the outer compartment and the inner compartment.
-The inner compartment is filled with a dense homogeneous substance called the matrix.
-The outer membrane forms the continuous limiting boundary of the organelle. The inner membrane forms a number of infoldings called the cristae (sing.: crista) towards the matrix (Figure).
-The cristae increase the surface area.
- The two membranes have their own specific enzymes associated with the mitochondrial function. Mitochondria are the sites of aerobic respiration.
-They produce cellular energy in the form of ATP, hence they are called ‘power houses’ of the cell.
-The matrix also possesses single circular DNA molecule, a few RNA molecules, ribosomes (70S) and the components required for the synthesis of proteins. The mitochondria divide by fission.

Plastids

Plastids are found in all plant cells and in euglenoides.
These are easily observed under the microscope as they are large. They bear some specific pigments, thus imparting specific colours to the plants. Based on the type of pigments plastids can be classified into chloroplasts, chromoplasts and leucoplasts.
- The chloroplasts contain chlorophyll and carotenoid pigments which are responsible for trapping light energy essential for photosynthesis.
-In the chromoplasts fat soluble carotenoid pigments like carotene, xanthophylls and others are present.
-This gives the part of the plant a yellow, orange or red colour.
-The leucoplasts are the colourless plastids of varied shapes and sizes with stored nutrients:
-Amyloplasts store carbohydrates (starch),
-e.g., potato; elaioplasts store oils and fats whereas the aleuroplasts store proteins. Majority of the chloroplasts of the green plants are found in the mesophyll cells of the leaves.
-These are lens-shaped, oval, spherical, discoid or even ribbon-like organelles having variable length (5-10μm) and width (2-4μm).
-Their number varies from 1 per cell of the Chlamydomonas, a green alga to 20-40 per cell in the mesophyll. Like mitochondria, the chloroplasts are also double membrane bound.
-Of the two, the inner chloroplast membrane is relatively less permeable.
-The space limited by the inner membrane of the chloroplast is called the stroma.
-A number of organised flattened membranous sacs called the thylakoids, are present in the stroma (Figure). Thylakoids are arranged in stacks like the piles of coins called grana (singular: granum) or the intergranal thylakoids.
- In addition, there are flat membranous tubules called the stroma lamellae connecting the thylakoids of the different grana. The membrane of the thylakoids enclose a space called a lumen. The stroma of the chloroplast contains enzymes required for the synthesis of carbohydrates and proteins.
- It also contains small, doublestranded circular DNA molecules and ribosomes. Chlorophyll pigments are present in the thylakoids. The ribosomes of the chloroplasts are smaller (70S) than the cytoplasmic ribosomes (80S).

Ribosomes

Ribosomes are the granular structures first observed under the electron microscope as dense particles by George Palade (1953).
- They are composed of ribonucleic acid (RNA) and proteins and are not surrounded by any membrane.
- The eukaryotic ribosomes are 80S while the prokaryotic ribosomes are 70S.
-Each ribosome has two subunits, larger and smaller subunits (Fig 8.9).
-The two subunits of 80S ribosomes are 60S and 40S while that of 70S ribosomes are 50S and 30S.
-Here ‘S’ (Svedberg’s Unit) stands for the sedimentation coefficient; it is indirectly a measure of density and size. Both 70S and 80S ribosomes are composed of two subunits.
-

Cytoskeleton

An elaborate network of filamentous proteinaceous structures present in the cytoplasm is collectively referred to as the cytoskeleton.
The cytoskeleton in a cell are involved in many functions such as mechanical support, motility, maintenance of the shape of the cell.

Cilia and Flagella

Cilia (sing.: cilium) and flagella (sing.: flagellum) are hair-like outgrowths of the cell membrane.
-Cilia are small structures which work like oars, causing the movement of either the cell or the surrounding fluid.
- Flagella are comparatively longer and responsible for cell movement.
-The prokaryotic bacteria also possess flagella but these are structurally different from that of the eukaryotic flagella.
- The electron microscopic study of a cilium or the flagellum show that they are covered with plasma membrane.
-Their core called the axoneme, possesses a number of microtubules running parallel to the long axis.
- The axoneme usually has nine pairs of doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules. Such an arrangement of axonemal microtubules is referred to as the 9+2 array (Figure).
-The central tubules are connected by bridges and is also enclosed by a central sheath, which is connected to one of the tubules of each peripheral doublets by a radial spoke. Thus, there are nine radial spokes.
-The peripheral doublets are also interconnected by linkers. Both the cilium and flagellum emerge from centriole-like structure called the basal bodies.

Centrosome and Centrioles

Centrosome is an organelle usually containing two cylindrical structures called centrioles.
-They are surrounded by amorphous pericentriolar materials.
-Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation like the cartwheel. They are made up of nine evenly spaced peripheral fibrils of tubulin protein.
-Each of the peripheral fibril is a triplet.
-The adjacent triplets are also linked. The central part of the proximal region of the centriole is also proteinaceous and called the hub, which is connected with tubules of the peripheral triplets by radial spokes made of protein.
- The centrioles form the basal body of cilia or flagella, and spindle fibres that give rise to spindle apparatus during cell division in animal cells.

Nucleus

Nucleus as a cell organelle was first described by Robert Brown as early as 1831.
- Later the material of the nucleus stained by the basic dyes was given the name chromatin by Flemming.
- The interphase nucleus (nucleus of a cell when it is not dividing) has highly extended and elaborate nucleoprotein fibres called chromatin, nuclear matrix and one or more spherical bodies called nucleoli (sing.: nucleolus) (Figure).
- Electron microscopy has revealed that the nuclear envelope, which consists of two parallel membranes with a space between (10 to 50 nm) called the perinuclear space, forms a barrier between the materials present inside the nucleus and that of the cytoplasm.
- The outer membrane usually remains continuous with the endoplasmic reticulum and also bears ribosomes on it.
-At a number of places the nuclear envelope is interrupted by minute pores, which are formed by the fusion of its two membranes.
-These nuclear pores are the passages through which movement of RNA and protein molecules takes place in both directions between the nucleus and the cytoplasm.
-Normally, there is only one nucleus per cell, variations in the number of nuclei are also frequently observed.
-Can you recollect names of organisms that have more than one nucleus per cell? Some mature cells even lack nucleus, e.g., erythrocytes of many mammals and sieve tube cells of vascular plants.
-Would you consider these cells as ‘living’?

The nuclear matrix or the nucleoplasm contains nucleolus and chromatin.
-The nucleoli are spherical structures present in the nucleoplasm. The content of nucleolus is continuous with the rest of the nucleoplasm as it is not a membrane bound structure. It is a site for active ribosomal RNA synthesis.
-Larger and more numerous nucleoli are present in cells actively carrying out protein synthesis.
- You may recall that the interphase nucleus has a loose and indistinct network of nucleoprotein fibres called chromatin.
But during different stages of cell division, cells show structured chromosomes in place of the nucleus. Chromatin contains DNA and some basic proteins called histones, some non-histone proteins and also RNA.
-A single human cell has approximately two metre long thread of DNA distributed among its forty six (twenty three pairs) chromosomes.

Every chromosome (visible only in dividing cells) essentially has a primary constriction or the centromere on the sides of which disc shaped structures called kinetochores are present (Figure 8.12).
- Centromere holds two chromatids of a chromosome. Based on the position of the centromere, the chromosomes can be classified into four types (Figure).

- The metacentric chromosome has middle centromere forming two equal arms of the chromosome.
-The sub-metacentric chromosome has centromere slightly away from the middle of the chromosome resulting into one shorter arm and one longer arm.
- In case of acrocentric chromosome the centromere is situated close to its end forming one extremely short and one very long arm, whereas the telocentric chromosome has a terminal centromere.

Sometimes a few chromosomes have non-staining secondary constrictions at a constant location.
- This gives the appearance of a small fragment called the satellite.

Microbodies

Many membrane bound minute vesicles called microbodies that contain various enzymes, are present in both plant and animal cells.

Spot Assignment

1. Who discovered the living cell?
2. Who discovered nucleus?
3. Who proposed the cell theory?
4. Write two characteristics of cell theory?
5. Who proposed “Omnis cellula-e cellula”?
6. Which are important cell organelles?
7. Name one non membrane organelle of a cell.
8. Give one function of centrosome.
9. What are mycoplasma?
10. What is full form of PPLO?
11. Which are four basic shapes of bacteria?
12. What are plasmids?
5.1 Cell Membrane- The detailed structure of the membrane was studied only after the advent of the electron microscope in the 1950s. Meanwhile, chemical studies on the cell membrane, especially in human red blood cells (RBCs), enabled the scientists to deduce the possible structure of plasma membrane. These studies showed that the cell membrane is mainly composed of lipids and proteins. The major lipids are phospholipids that are arranged in a bilayer. Also, the lipids are arranged within the membrane with the polar head towards the outer sides and the hydrophobic tails towards the inner part. This ensures that the nonpolar tail of saturated hydrocarbons is protected from the aqueous environment (Figure 8.4). In addition to phospholipids membrane also contains cholesterol. The lipid component of the membrane mainly consists of phosphoglycerides. Later, biochemical investigation clearly revealed that the cell membranes also possess protein and carbohydrate. The ratio of protein and lipid varies considerably in different cell types. In human beings, the membrane of the erythrocyte has approximately 52 per cent protein and 40 per cent lipids. Depending on the ease of extraction, membrane proteins can be classified as integral and peripheral. Peripheral proteins lie on the surface of membrane while the integral proteins are partially or totally buried in the membrane. An improved model of the structure of cell membrane was proposed by Singer and Nicolson (1972) widely accepted as fluid mosaic model (Figure 8.4). According to this, the quasi-fluid nature of lipid enables lateral movement of proteins within the overall bilayer. This ability to move within the membrane is measured as its fluidity. The fluid nature of the membrane is also important from the point of view of functions like cell growth, formation of intercellular junctions, secretion, endocytosis, cell division etc. One of the most important functions of the plasma membrane is the transport of the molecules across it. The membrane is selectively permeable to some molecules present on either side of it. Many molecules can move briefly across the membrane without any requirement of energy and this is called the passive transport. Neutral solutes may move across the membrane by the process of simple diffusion along the concentration gradient, i.e., from higher concentration to the lower. Water may also move across this membrane from higher to lower concentration. Movement of water by diffusion is called osmosis. As the polar molecules cannot pass through the nonpolar lipid bilayer, they require a carrier protein of the membrane to facilitate their transport across the membrane. A few ions or molecules are transported across the membrane against their concentration gradient, i.e., from lower to the higher concentration. Such a transport is an energy dependent process, in which ATP is utilised and is called active transport, e.g., Na+/K+ Pump. 5.2 Cell Wall- As you may recall, a non-living rigid structure called the cell wall forms an outer covering for the plasma membrane of fungi and plants. Cell wall not only gives shape to the cell and protects the cell from mechanical damage and infection, it also helps in cell-to-cell interaction and provides barrier to undesirable macromolecules. Algae have cell wall, made of cellulose, galactans, mannans and minerals like calcium carbonate, while in other plants it consists of cellulose, hemicellulose, pectins and proteins. The cell wall of a young plant cell, the primary wall is capable of growth, which gradually diminishes as the cell matures and the secondary wall is formed on the inner (towards membrane) side of the cell. The middle lamella is a layer mainly of calcium pectate which holds or glues the different neighbouring cells together. The cell wall and middle lamellae may be traversed by plasmodesmata which connect the cytoplasm of neighbouring cells. 5.3 Endomembrane System- While each of the membranous organelles is distinct in terms of its structure and function, many of these are considered together as an endomembrane system because their functions are coordinated. The endomembrane system include endoplasmic reticulum (ER), golgi complex, lysosomes and vacuoles. Since the functions of the mitochondria, chloroplast and peroxisomes are not coordinated with the above components, these are not considered as part of the endomembrane system. The Endoplasmic Reticulum (ER) Electron microscopic studies of eukaryotic cells reveal the presence of a network or reticulum of tiny tubular structures scattered in the cytoplasm that is called the endoplasmic reticulum (ER) (Figure 8.5). Hence, ER divides the intracellular space into two distinct compartments, i.e., luminal (inside ER) and extra luminal (cytoplasm) compartments. The ER often shows ribosomes attached to their outer surface. The endoplasmic reticulun bearing ribosomes on their surface is called rough endoplasmic reticulum (RER). In the absence of ribosomes they appear smooth and are called smooth endoplasmic reticulum (SER). RER is frequently observed in the cells actively involved in protein synthesis and secretion. They are extensive and continuous with the outer membrane of the nucleus. The smooth endoplasmic reticulum is the major site for synthesis of lipid. In animal cells lipid-like steroidal hormones are synthesised in SER. Golgi apparatus Camillo Golgi (1898) first observed densely stained reticular structures near the nucleus. These were later named Golgi bodies after him. They consist of many flat, disc-shaped sacs or cisternae of 0.5μm to 1.0μm diameter (Figure 8.6). These are stacked parallel to each other. Varied number of cisternae are present in a Golgi complex. The Golgi cisternae are concentrically arranged near the nucleus with distinct convex cis or the forming face and concave trans or the maturing face. The cis and the trans faces of the organelle are entirely different, but interconnected. The golgi apparatus principally performs the function of packaging materials, to be delivered either to the intra-cellular targets or secreted outside the cell. Materials to be packaged in the form of vesicles from the ER fuse with the cis face of the golgi apparatus and move towards the maturing face. This explains, why the golgi apparatus remains in close association with the endoplasmic reticulum. A number of proteins synthesised by ribosomes on the endoplasmic reticulum are modified in the cisternae of the golgi apparatus before they are released from its trans face. Golgi apparatus is the important site of formation of glycoproteins and glycolipids. Lysosomes - These are membrane bound vesicular structures formed by the process of packaging in the golgi apparatus. The isolated lysosomal vesicles have been found to be very rich in almost all types of hydrolytic enzymes (hydrolases – lipases, proteases, carbohydrases) optimally active at the acidic pH. These enzymes are capable of digesting carbohydrates, proteins, lipids and nucleic acids. Vacuoles -The vacuole is the membrane-bound space found in the cytoplasm. It contains water, sap, excretory product and other materials not useful for the cell. The vacuole is bound by a single membrane called tonoplast. In plant cells the vacuoles can occupy up to 90 per cent of the volume of the cell. In plants, the tonoplast facilitates the transport of a number of ions and other materials against concentration gradients into the vacuole, hence their concentration is significantly higher in the vacuole than in the cytoplasm. In Amoeba the contractile vacuole is important for excretion. In many cells, as in protists, food vacuoles are formed by engulfing the food particles. Mitochondria- Mitochondria (sing.: mitochondrion), unless specifically stained, are not easily visible under the microscope. The number of mitochondria per cell is variable depending on the physiological activity of the cells. In terms of shape and size also, considerable degree of variability is observed. Typically it is sausage-shaped or cylindrical having a diameter of 0.2-1.0μm (average 0.5μm) and length 1.0-4.1μm. Each mitochondrion is a double membrane-bound structure with the outer membrane and the inner membrane dividing its lumen distinctly into two aqueous compartments, i.e., the outer compartment and the inner compartment. The inner compartment is filled with a dense homogeneous substance called the matrix. The outer membrane forms the continuous limiting boundary of the organelle. The inner membrane forms a number of infoldings called the cristae (sing.: crista) towards the matrix (Figure 8.7). The cristae increase the surface area. The two membranes have their own specific enzymes associated with the mitochondrial function. Mitochondria are the sites of aerobic respiration. They produce cellular energy in the form of ATP, hence they are called ‘power houses’ of the cell. The matrix also possesses single circular DNA molecule, a few RNA molecules, ribosomes (70S) and the components required for the synthesis of proteins. The mitochondria divide by fission. Plastids- Plastids are found in all plant cells and in euglenoides. These are easily observed under the microscope as they are large. They bear some specific pigments, thus imparting specific colours to the plants. Based on the type of pigments plastids can be classified into chloroplasts, chromoplasts and leucoplasts. The chloroplasts contain chlorophyll and carotenoid pigments which are responsible for trapping light energy essential for photosynthesis. In the chromoplasts fat soluble carotenoid pigments like carotene, xanthophylls and others are present. This gives the part of the plant a yellow, orange or red colour. The leucoplasts are the colourless plastids of varied shapes and sizes with stored nutrients: Amyloplasts store carbohydrates (starch), e.g., potato; elaioplasts store oils and fats whereasthe aleuroplasts store proteins. Majority of the chloroplasts of the green plants are found in the mesophyll cells of the leaves. These are lens-shaped, oval, spherical, discoid or even ribbon-like organelles having variable length (5-10µm) and width (2-4µm). Their number varies from 1 per cell of the Chlamydomonas, a green alga to 20-40 per cell in the mesophyll. Like mitochondria, the chloroplasts are also double membrane bound. Of the two, the inner chloroplast membrane is relatively less permeable. The space limited by the inner membrane of the chloroplast is called the stroma. A number of organised flattened membranous sacs called the thylakoids, are present in the stroma (Figure 8.8). Thylakoids are arranged in stacks like the piles of coins called grana (singular: granum) or the intergranal thylakoids. In addition, there are flat membranous tubules called the stroma lamellae connecting the thylakoids of the different grana. The membrane of the thylakoids enclose a space called a lumen. The stroma of the chloroplast contains enzymes required for the synthesis of carbohydrates and proteins. It also contains small, double stranded circular DNA molecules and ribosomes. Chlorophyll pigments are present in the thylakoids. The ribosomes of the chloroplasts are smaller (70S) than the cytoplasmic ribosomes (80S). Ribosomes -Ribosomes are the granular structures first observed under the electron microscope as dense particles by George Palade (1953). They are composed of ribonucleic acid (RNA) and proteins and are not surrounded by any membrane. The eukaryotic ribosomes are 80S while the prokaryotic ribosomes are 70S. Each ribosome has two subunits, larger and smaller subunits (Fig 8.9). The two subunits of 80S ribosomes are 60S and 40S while that of 70S ribosomes are 50S and 30S. Here ‘S’ (Svedberg’s Unit) stands for the sedimentation coefficient; it is indirectly a measure of density and size. Both 70S and 80S ribosomes are composed of two subunits. Figure 8.8 Sectional view of chloroplast An elaborate network of filamentous proteinaceous structures present in the cytoplasm is collectively referred to as the cytoskeleton. The cytoskeleton in a cell are involved in many functions such as mechanical support, motility, maintenance of the shape of the cell. 8.5.8 Cilia and Flagella Cilia (sing.: cilium) and flagella (sing.: flagellum) are hair-like outgrowths of the cell membrane. Cilia are small structures which work like oars, causing the movement of either the cell or the surrounding fluid. Flagella are comparatively longer and responsible for cell movement. The prokaryotic bacteria also possess flagella but these are structurally different from that of the eukaryotic flagella. The electron microscopic study of a cilium or the flagellum show that they are covered with plasma membrane. Their core called the axoneme, possesses a number of microtubules running parallel to the long axis. The axoneme usually has nine pairs of doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules. Such an arrangement of axonemal microtubules is referred to as the 9+2 array (Figure 8.10). The central tubules are connected by bridges and is also enclosed by a central sheath, which is connected to one of the tubules of each peripheral doublets by a radial spoke. Thus, there are nine radial spokes. The peripheral doublets are also interconnected by linkers. Both the cilium and flagellum emerge from centriole-like structure called the basal bodies. Centrosome and Centrioles - Centrosome is an organelle usually containing two cylindrical structures called centrioles. They are surrounded by amorphous pericentriolar materials. Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation like the cartwheel. They aremade up of nine evenly spaced peripheral fibrils of tubulin protein. Each of the peripheral fibril is a triplet.The adjacent triplets are also linked. The central part of the proximal region of the centriole is also proteinaceous and called the hub, which is connected with tubules of the peripheral triplets by radial spokes made of protein. The centrioles form the basal body of cilia or flagella, and spindle fibres that give rise to spindle apparatus during cell division in animal cells. Nucleus - Nucleus as a cell organelle was first described by Robert Brown as early as 1831. Later the material of the nucleus stained by the basic dyes was given the name chromatin by Flemming. The interphase nucleus (nucleus of a cell when it is not dividing) has highly extended and elaborate nucleoprotein fibres called chromatin, nuclear matrix and one or more spherical bodies called nucleoli (sing.: nucleolus) (Figure 8.11). Electron microscopy has revealed that the nuclear envelope, which consists of two parallel membranes with a space between (10 to 50 nm) called the perinuclear space, forms a barrier between the materials present inside the nucleus and that of the cytoplasm. The outer membrane usually remains continuous with the endoplasmic reticulum and also bears ribosomes on it. At a number of places the nuclear envelope is interrupted by minute pores, which are formed by the fusion of its two membranes. These nuclear pores are the passages through which movement of RNA and protein molecules takes place in both directions between the nucleus and the cytoplasm. Normally, there is only one nucleus per cell, variations in the number of nuclei are also frequently observed. Can you recollect names of organisms that have more than one nucleus per cell? Some mature cells even lack nucleus, e.g., erythrocytes of many mammals and sieve tube cells of vascular plants. Would you consider these cells as ‘living’? The nuclear matrix or the nucleoplasm contains nucleolus and chromatin. The nucleoli are spherical structures present in the nucleoplasm. The content of nucleolus is continuous with the rest of the nucleoplasm as it is not a membrane bound structure. It is a site for active ribosomal RNA synthesis. Larger and more numerous nucleoli are present in cells actively carrying out protein synthesis. You may recall that the interphase nucleus has a loose and indistinct network of nucleoprotein fibres called chromatin. But during different stages of cell division, cells show structured chromosomes in place of the nucleus. Chromatin contains DNA and some basic proteins called histones, some non-histone proteins and also RNA. A single human cell has approximately two metre long thread of DNA distributed among its forty six (twenty three pairs) chromosomes. Every chromosome (visible only in dividing cells) essentially has a primary constriction or the centromere on the sides of which disc shaped structures called kinetochores are present (Figure 8.12). Centromere holds two chromatids of a chromosome. Based on the position of the centromere, the chromosomes can be classified into four types (Figure 8.13). The metacentric chromosome has middle centromere forming two equal arms of the chromosome. The sub-metacentric chromosome has centromere slightly away from the middle of the chromosome resulting into one shorter arm and one longer arm. In case of acrocentric chromosome the centromere is situated close to its end forming one extremely short and one very long arm, whereas the telocentric chromosome has a terminal centromere. Sometimes a few chromosomes have non-staining secondary constrictions at a constant location. This gives the appearance of a small fragment called the satellite. Microbodies Many membrane bound minute vesicles called microbodies that contain various enzymes, are present in both plant and animal cells. NCERT Ex.


1. Which of the following is not correct?
(a) Robert Brown discovered the cell.
(b) Schleiden and Schwann formulated the cell theory.
(c) Virchow explained that cells are formed from pre-existing cells.
(d) A unicellular organism carries out its life activities within a single cell.
2. New cells generate from
(a) bacterial fermentation
(b) regeneration of old cells
(c) pre-existing cells
(d) abiotic materials
3. Match the following Column I Column II (a) Cristae (i) Flat membranous sacs in stroma (b) Cisternae (ii) Infoldings in mitochondria (c) Thylakoids (iii) Disc-shaped sacs in Golgi apparatus
4. Which of the following is correct:
(a) Cells of all living organisms have a nucleus.
(b) Both animal and plant cells have a well defined cell wall.
(c) In prokaryotes, there are no membrane bound organelles.
(d) Cells are formed de novo from abiotic materials.
5. What is a mesosome in a prokaryotic cell? Mention the functions that it performs.

6. How do neutral solutes move across the plasma membrane? Can the polar molecules also move across it in the same way? If not, then how are these transported across the membrane?

7. Name two cell-organelles that are double membrane bound. What are the characteristics of these two organelles?
State their functions and draw labelled diagrams of both.

8. What are the characteristics of prokaryotic cells?

9. Multicellular organisms have division of labour. Explain.

10. Cell is the basic unit of life. Discuss in brief.

11. What are nuclear pores? State their function.

12. Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.

13. Describe the structure of the following with the help of labelled diagrams. (i) Nucleus (ii) Centrosome
14. What is a centromere?
How does the position of centromere form the basis of classification of chromosomes. Support your answer with a diagram showing the position of centromere on different types of chromosomes. NCERT QAs
Set 1

1. Which of the following is not correct?
(a) Robert Brown discovered the cell.
(b) Schleiden and Schwann formulated the cell theory.
(c) Virchow explained that cells are formed from pre-existing cells.
(d) A unicellular organism carries out its life activities within a single cell.
1. (a) Robert Brown did not discover the cell. The cell was discovered by Robert Hook.
2. New cells generate from
(a) bacterial fermentation (b) regeneration of old cells
(c) pre-existing cells (d) abiotic materials
2. (c) According to the biogenic theory, new cells can only arise from pre-existing cells. Only complete cells, in favourable conditions, can give rise to new cells.
3. Match the following
Column I Column II (a) Cristae (i) Flat membranous sacs in stroma (b) Cisternae (ii) Infoldings in mitochondria (c) Thylakoids (iii) Disc-shaped sacs in Golgi apparatus 3. Column I Column II (a) Cristae (ii) Infoldings in mitochondria (b) Cisternae (iii) Disc-shaped sacs in Golgi apparatus (c) Thylakoids (i) Flat membranous sacs in stroma
4. Which of the following is correct:
(a) Cells of all living organisms have a nucleus.
(b) Both animal and plant cells have a well defined cell wall.
(c) In prokaryotes, there are no membrane bound organelles.
(d) Cells are formed de novo from abiotic materials.
4. (c) Membrane-bound organelles are organelles surrounded by a double or a single membrane like
Nucleus, mitochondria, chloroplasts, Lysosomes, ER, Golgi bodies etc. are examples of such organelles. These cell organelles are absent in prokaryotes.
(a) Only eukaryotic cells have nuclei. They are absent in prokaryotes.
(b) Cell walls are only present in plant cells. They are absent in all animal cells.
(d) All cells arise from pre-existing cells.

5. What is a mesosome in a prokaryotic cell? Mention the functions that it performs.
5. Mesosome is a convoluted membranous structure formed in a prokaryotic cell by the invagination of the plasma membrane. Its functions are as follows:
(1) These extensions help in the synthesis of the cell wall and replication of DNA. They also help in the equal distribution of chromosomes into the daughter cells.
(2) It also increases the surface area of the plasma membrane to carry out various enzymatic activities.
(3) It helps in secretion processes as well as in bacterial respiration.
6. How do neutral solutes move across the plasma membrane? Can the polar molecules also move across it in the same way?
If not, then how are these transported across the membrane?
6. Plasma membrane is the outermost covering of the cell and regulates the movement of substances into the cell and out from it.
It allows the entry of only some substances and prevents the movement of other materials. Hence, the membrane is selectively-permeable.
Movement of neutral solutes across the cell membrane – Neutral molecules move across the plasma membrane by simple passive diffusion.
Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration.
Movement of polar molecules across the cell membrane – The cell membrane is made up of a phospholipid bilayer and proteins.
The movement of polar molecules across the non-polar lipid bilayer requires carrier-proteins. Which are integral protein particles having certain affinity for specific solutes.
As a result, they facilitate the transport of molecules across the membrane.
7. Name two cell-organelles that are double membrane bound. What are the characteristics of these two organelles?
State their functions and draw labelled diagrams of both.
7. Mitochondria and chloroplasts are the two organelles that are double-membrane-bound.
Characteristics of the mitochondria
Mitochondria are double- membrane-bound structures. The membrane of a mitochondrion is divided into the inner and outer membranes, distinctly divided into two aqueous compartments – outer and inner compartments. The outer membrane is very porous, while the inner membrane is deeply-folded.
These folds are known as cristae. They are the sites for ATP-generating chemical reactions. The membrane and matrix of a mitochondrion contains specific enzymes meant for aerobic respiration. They have their own DNA and ribosomes. Thus, they are able to make their own proteins. This is why they are considered as semi-autonomous organelles.
Characteristics of chloroplasts
Chloroplasts are double-membrane-bound structures.
They are divided into outer and inner membranes, further divided into two distinct regions:
(i) Grana are stacks of flattened discs containing chlorophyll molecules. The flattened membranous sacs are called thylakoids.
The thylakoids of adjacent grana are connected by membranous tubules called stroma lamellae.
(ii) Stroma is a homogenous mixture in which grana are embedded.
It contains several enzymes that are used for the synthesis of carbohydrates and proteins.
It also contains its own DNA and ribosomes and hence semi-autonomous organelle.
Functions of the mitochondria:
(i) They are the sites for cellular respiration.
(ii) They provide energy in the form of ATP for all vital activities of living cells.
(iii) They have their own DNA and ribosomes. Hence, they are regarded as semi-autonomous organelles.
(iv) They have several enzymes, intermediately required for the synthesis of various chemicals such as fatty acids, steroids, and amino acids.
Functions of chloroplasts:
(i) They trap solar energy and utilise it for manufacturing food for plants. Hence, they are involved in the process of photosynthesis.
(ii) They contain the enzymes required for the synthesis of carbohydrates and proteins.
8. What are the characteristics of prokaryotic cells?
8. The characteristics of prokaryotic cells are as follows:
i. They are generally small in size. The size of a prokaryotic cell varies from 0.5 – 5 µm.
ii. There is no well-defined nucleus. The genetic material is basically naked, not enveloped by a nuclear membrane.
iii. The genetic materials of prokaryotic cells are naked. They contain single, circular chromosomes. In addition to the genomic DNA, they have a small, circular plasmid DNA.
iv. The prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma and PPLO (Pleuro Pneumonia Like Organisms). They are generally smaller and multiply more rapidly than the eukaryotic cells.
v. They may vary greatly in shape and size. The four basic shapes of bacteria are bacillus (rod like), coccus (spherical), vibrio (comma shaped) and spirillum (spiral).
vi. All prokaryotes have a cell wall surrounding the cell membrane except in mycoplasma. The fluid matrix filling the cell is the cytoplasm.
vii. In addition to the genomic DNA (the single chromosome/circular DNA), many bacteria have small circular DNA outside the genomic DNA. These smaller DNA are called plasmids.
viii. The plasmid DNA confers certain unique phenotypic characters to such bacteria. One such character is resistance to antibiotics.
ix. They have specialised membranous structures called mesosomes. Mesosomes are formed by the invagination of the cell membrane.
These extensions help in the synthesis of the cell wall and replication of DNA. They also help in the equal distribution of chromosomes into the daughter cells.
x. Membrane-bound cell organelles such as mitochondria, plastids, and endoplasmic reticulum are absent from a prokaryotic cell.
xi. Most prokaryotic cells contain a three-layered structure – outermost glycocalyx, middle cell wall, and the innermost plasma membrane. This structure acts as a protective unit.
Examples of prokaryotic cells include blue green algae, bacteria, etc.
9. Multicellular organisms have division of labour. Explain.
9. Multicellular organisms are made up of millions and trillions of cells.
All these cells perform specific functions. All the cells specialised for performing similar functions are grouped together as tissues in the body.
Hence, a particular function is carried out by a group of cells at a definite place in the body. Similarly, different functions are carried out by different groups of cells in an organism and this is known as division of labour in multicellular organisms.
10. Cell is the basic unit of life. Discuss in brief.
10. Cells are the basic units of life capable of doing all the required biochemical processes that a normal cell has to do in order to live.
The basic needs for the survival of all living organisms are the same. All living organisms need to respire, digest food for obtaining energy, and get rid of metabolic wastes.
Cells are capable of performing all the metabolic functions of the body. Hence, cells are called the functional units of life.
11. What are nuclear pores? State their function.
11. Nuclear pores are tiny holes present in the nuclear membrane of the nucleus. They are formed by the fusion of two nuclear membranes.
Nuclear envelope has a large number (1000-10000) of pores with diameter of 200-800 A⁰ (100-1000 A⁰ ).
Nuclear pores may have diaphragms, septa, annuli, blebs or micropores with a central micropore surrounded by nine peripheral micropores.
The central micropore or channel has nucleoplasmin for movement of substances.
These holes allow specific substances to be transferred into a cell and out from it. They allow molecules such as RNA and proteins to move in both directions, between the nucleus and the cytoplasm.
12. Both lysosomes and vacuoles are endomembrane structures,
yet they differ in terms of their functions. Comment.
12. Lysosomes are membrane-bound vesicular structures holding a variety of enzymes such as lipases, proteases and amylases.
The purpose of lysosomes is to digest worn out cells.
They are involved in the intracellular digestion of foreign food particles and microbes.
Sometimes, they also act as suicidal bags. They are involved in the self digestion or autolysis of cells.
Autolysis is self-destruction of a cell, tissue or organ with the help of lysosomes. Lysosomes performing autolysis do not enclose the structures to be broken down.
Instead, they themselves burst to release the digestive enzymes. Autolysis occurs in ageing, dead and diseased cells.
Thus lysosomes are a kind of waste disposal systems of a cell.
On the other hand, vacuoles are non-cytoplasmic areas present inside the cytoplasm which are separated from the latter by specific membranes.
Depending upon the contents and functions, vacuoles are of four types- sap vacuoles, contractile vacuoles, food vacuoles and air vacuoles.
They might store the waste products of cells. In unicellular organisms, the food vacuole contains the consumed food particles.
It also plays a role in expelling excess water and some wastes from the cell.
13. Describe the structure of the following with the help of labelled diagrams. (i) Nucleus (ii) Centrosome
13. (i) Nucleus
Nucleus is a specialized double membrane bound protoplasmic body, which contains all the genetic information for controlling cellular metabolism and transmission to the posterity.
A nucleus in the non-dividing or metabolic phase is called interphase nucleus. Like other cellular structures, living unstained nucleus does not show much internal differentiation.
Nucleus controls all the cellular activities of the cell. It is spherical in shape.
1. Nuclear Envelope (=Karyotheca).
It bounds the nucleus on the outside.
The nuclear envelope separates the nucleus from the cytoplasm. It is made up of two lipoprotein and trilaminar membranes, each of which is 60-90Ao thick. The inner membrane is smooth.
The outer membrane may be smooth or its cytoplasmic surface may bear ribosomes like the rough endoplasmic reticulum. The two membranes of the nuclear envelope are separated by an electron transparent perinuclear space. The space is 100-700 A⁰ in width.
The outer membrane is often connected to endoplasmic reticulum.
The narrow space between the two membranes is called the perinuclear space. Nuclear membrane has tiny holes called nuclear pores.
These holes allow specific substances to be transferred into a nucleus and out from it.
Nucleoplasm/Nuclear matrix:
It is a homogenous granular fluid present inside the nucleus. It contains the nucleolus and chromatin.
Nucleolus is a spherical structure that is not bound by any membrane. It is rich in protein and RNA molecules, and is the site for ribosome formation.
Chromatin is an entangled mass of thread-like structures. It contains DNA and some basic proteins called histones.
(ii) Centrosome
Centrosome consists of two cylindrical structures called centrioles. Centrioles are minute submicroscopic subcylindrical structures of 0.3 - 0.5, μm length and 0.15 μm , diameter which usually occur in pair (diplosome) inside a specialised cytoplasm called centrosphere or kinoplasm.
The complex is called centrosome (Boveri, 1888) or central apparatus.
The two centrioles lie at right angles to each other. Each centriole has a cartwheel constitution. There is a whorl of nine peripheral triplet fibrils of tubulin (9 + 0) tilted at an angle of 40°.
The three subfibres of a triplet from outside to inside are C, B and A. Though each subfibre should have 13 protofilaments like a microtubule, both C and A subfibres share 2-3 protofilaments with B-subfibre. Matrix fills the centriole. Adjacent triplets are interconnected by proteinaceous C - A linkers.
A proteinaceous rod or hub occurs in the centre. Subfibre A of each triplet is connected to hub by means of a radial proteinaceous strand called spoke. Spokes are also connected to C - A linkers with two types of thickenings, X and Y.
These centrioles help in organising the spindle fibres and astral rays during cell division.
They form the basal body of cilia and flagella as well. 14. What is a centromere? How does the position of centromere form the basis of classification of chromosomes. Support your answer with a diagram showing the position of centromere on different types of chromosomes.
Answer
Centromere is a constriction present on the chromosomes where the chromatids are held together.
Chromosomes are divided into four types based on the position of the centromere.
(i) Metacentric chromosome
The chromosomes in which the centromere is present in the middle and divides the chromosome into two equal arms is known as a metacentric chromosome. During anaphase, they appear V-Shaped.
(ii) Sub-metacentric chromosome
The chromosome in which the centromere is slightly away from the middle region is known as a sub-metacentric chromosome. In this, one arm is slightly longer than the other.
During anaphase, they appear L-Shaped.
(iii) Acrocentric chromosome
The chromosome in which the centromere is located close to one of the terminal ends is known as an acrocentric chromosome.
In this, one arm is extremely long and the other is extremely short.
During anaphase, they appear J-Shaped. (iv) Telocentric chromosome
The chromosome in which the centromere is located at one of the terminal ends is known as a telocentric chromosome. During anaphase, they appear i-Shaped.
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Competition

503.1110208

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Notes

Cell

Extract(1)
Unicellular organisms are capable of
(i) independent existence and
(ii) performing the essential functions of life. Anything less than a complete structure of a cell does not ensure independent living.
Hence, cell is the fundamental structural and functional unit of all living organisms.

Anton Von Leeuwenhoek first saw and described a live cell.
Robert Brown later discovered the nucleus.
The invention of the microscope and its improvement leading to the electron microscope revealed all the structural details of the cell.
Extract(2)

Cell theory

In 1838, Matthias Schleiden, a German botanist, examined a large number of plants and observed that all plants are composed of different kinds of cells which form the tissues of the plant.

At about the same time, Theodore Schwann (1839), a British Zoologist,
studied different types of animal cells and reported that cells had a thin outer layer which is today known as the ‘plasma membrane’.
He also concluded, based on his studies on plant tissues, that the presence of cell wall is a unique character of the plant cells.

On the basis of this, Schwann proposed the hypothesis that the bodies of animals and plants are composed of cells and products of cells.

Schleiden and Schwann together formulated the cell theory.
This theory however, did not explain as to how new cells were formed.

Rudolf Virchow (1855) first explained that cells divided and new cells are formed from pre-existing cells (Omnis cellula-e cellula).
He modified the hypothesis of Schleiden and Schwann to give the cell theory a final shape.

Cell theory as understood today is:

(i) all living organisms are composed of cells and products of cells.
(ii) all cells arise from pre existing cells.

An overview of cell

You have earlier observed cells in an onion peel and/or human cheek cells under the microscope.
Let us recollect their structure.
The onion cell which is a typical plant cell,
has a distinct cell wall as its outer boundary and just within it is the cell membrane.
The cells of the human cheek have an outer membrane as the delimiting structure of the cell.
Inside each cell is a dense membrane bound structure called nucleus.
This nucleus contains the chromosomes which in turn contain the genetic material, DNA.

Cells that have membrane bound nuclei are called eukaryotic
whereas cells that lack a membrane bound nucleus are prokaryotic.
In both prokaryotic and eukaryotic cells,
a semi-fluid matrix called cytoplasm occupies the volume of the cell.
The cytoplasm is the main arena of cellular activities in both the plant and animal cells.
Various chemical reactions occur in it to keep the cell in the - living state

Besides the nucleus,
the eukaryotic cells have other membrane bound distinct structures called organelles like the endoplasmic reticulum (ER),
the golgi complex,
lysosomes, mitochondria, microbodies and vacuoles.
The prokaryotic cells lack such membrane bound organelles.

Ribosomes are non-membrane bound organelles found in all cells -- both eukaryotic as well as prokaryotic.
Within the cell, ribosomes are found not only in the cytoplasm but also within the two organelles
– chloroplasts (in plants) and mitochondria and on rough ER.

Animal cells contain another non-membrane bound organelle called centrosome which helps in cell division.
Cells differ greatly in size, shape and activities (Figure 8.1).
For example, Mycoplasmas, the smallest cells, are only 0.3 μm in length while bacteria could be 3 to 5 μm.
The largest isolated single cell is the egg of an ostrich.
Among multicellular organisms, human red blood cells are about 7.0 μm in diameter.
Nerve cells are some of the longest cells.
Cells also vary greatly in their shape.
They may be disc-like, polygonal, columnar, cuboid, thread like, or even irregular.
The shape of the cell may vary with the function they perform.

Extract(4) Prokaryotic cells -The prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma and PPLO (Pleuro Pneumonia Like Organisms).
They are generally smaller and multiply more rapidly than the eukaryotic cells (Figure 8.2).

They may vary greatly in shape and size. The four basic shapes of bacteria are bacillus (rod like),
coccus (spherical),
vibrio (comma shaped) and spirillum (spiral).

The organisation of the prokaryotic cell is fundamentally similar even though prokaryotes exhibit a wide variety of shapes and functions.
-All prokaryotes have a cell wall surrounding the cell membrane except in mycoplasma. The fluid matrix filling the cell is the cytoplasm.
There is no well-defined nucleus.
-The genetic material is basically naked, not enveloped by a nuclear membrane.
-In addition to the genomic DNA (the single chromosome/circular DNA), many bacteria have small circular DNA outside the genomic DNA. These smaller DNA are called plasmids.
- The plasmid DNA confers certain unique phenotypic characters to such bacteria. One such character is resistance to antibiotics. In higher classes you will learn that this plasmid DNA is used to monitor bacterial transformation with foreign DNA.
-Nuclear membrane is found in eukaryotes.
-No organelles are found in prokaryotic cells except organisms for ribosomes.
-Prokaryotes have something unique in the form of inclusions. A specialised differentiated form of cell membrane called mesosome is the characteristic of prokaryotes. They are essentially infoldings of cell membrane.

Cell Envelope and its Modifications

- Most prokaryotic cells, particularly the bacterial cells, have a chemically complex cell envelope.
- The cell envelope consists of a tightly bound three layered structure i.e.,
the outermost glycocalyx
followed by the cell wall and
then the plasma membrane.
- Although each layer of the envelope performs distinct function, they act together as a single protective unit.

Bacteria can be classified into two groups on the basis of the differences in the cell envelopes and the manner in which they respond to the staining procedure developed by Gram
viz.,

- those that take up the gram stain are Gram positive and
the others that do not are called Gram negative bacteria.

Glycocalyx differs in composition and thickness among different bacteria.
-It could be a loose sheath called the slime layer in some,
-while in others it may be thick and tough,
called the capsule.

The cell wall determines the shape of the cell and provides a strong structural support to prevent the bacterium from bursting or collapsing.
The plasma membrane is selectively permeable in nature and interacts with the outside world.
This membrane is similar structurally to that of the eukaryotes.

A special membranous structure is the mesosome which is formed by the extensions of plasma membrane into the cell.
- These extensions are in the form of vesicles, tubules and lamellae.
-They help in cell wall formation, DNA replication and distribution to daughter cells.
- They also help in respiration, secretion processes, to increase the surface area of the plasma membrane and enzymatic content.
- In some prokaryotes like cyanobacteria, there are other membranous extensions into the cytoplasm called chromatophores which contain pigments.

Bacterial cells may be motile or non-motile.
- If motile, they have thin filamentous extensions from their cell wall called flagella. Bacteria show a range in the number and arrangement of flagella.
- Bacterial flagellum is composed of three parts –
filament, hook and basal body.
The filament is the longest portion and extends from the cell surface to the outside.
- Besides flagella,
Pili and Fimbriae are also surface structures of the bacteria but do not play a role in motility.
- The pili are elongated tubular structures made of a special protein.
- The fimbriae are small bristle like fibres sprouting out of the cell.
- In some bacteria, they are known to help attach the bacteria to rocks in streams and also to the host tissues.

Ribosomes and Inclusion Bodies

In prokaryotes,
- ribosomes are associated with the plasma membrane of the cell.
They are about 15 nm by 20 nm in size and are made of two subunits -
50S and 30S units which when present together form 70S prokaryotic ribosomes.
- Ribosomes are the site of protein synthesis.
- Several ribosomes may attach to a single mRNA and form a chain called polyribosomes or polysome.
- The ribosomes of a polysome translate the mRNA into proteins.

Inclusion bodies:
- Reserve material in prokaryotic cells are stored in the cytoplasm in the form of inclusion bodies.
These are not bound by any membrane system and lie free in the cytoplasm,
e.g., phosphate granules, cyanophycean granules and glycogen granules.
Gas vacuoles are found in blue green and purple and green photosynthetic bacteria.

Extract(5)

Eukaryotic Cell

-The eukaryotes include all the protists, plants, animals and fungi.
In eukaryotic cells there is an extensive compartmentalisation of cytoplasm through the presence of membrane bound organelles.
-Eukaryotic cells possess an organised nucleus with a nuclear envelope.
In addition,
-eukaryotic cells have a variety of complex locomotory and cytoskeletal structures.
-Their genetic material is organised into chromosomes.
- All eukaryotic cells are not identical.
Plant and animal cells are different as the former possess cell walls, plastids and a large central vacuole which are absent in animal cells.
- On the other hand,
animal cells have centrioles which are absent in almost all plant cells (Figure 8.3).

Cell Membrane

The detailed structure of the membrane was studied only after the advent of the electron microscope in the 1950s.
Meanwhile, chemical studies on the cell membrane, especially in human red blood cells (RBCs), enabled the scientists to deduce the possible structure of plasma membrane.
- These studies showed that the cell membrane is mainly composed of lipids and proteins.
The major lipids are phospholipids that are arranged in a bilayer.
-Also, the lipids are arranged within the membrane with the polar head towards the outer sides and the hydrophobic tails towards the inner part.This ensures that the nonpolar tail of saturated hydrocarbons is protected from the aqueous environment (Figure).
-In addition to phospholipids membrane also contains cholesterol.
-The lipid component of the membrane mainly consists of phosphoglycerides.
- Later, biochemical investigation clearly revealed that the cell membranes also possess protein and carbohydrate.
-The ratio of protein and lipid varies considerably in different cell types. In human beings, the membrane of the erythrocyte has approximately 52 per cent protein and 40 per cent lipids.

Depending on the ease of extraction, membrane proteins can be classified as integral and peripheral.
- Peripheral proteins lie on the surface of membrane while the integral proteins are partially or totally buried in the membrane. An improved model of the structure of cell membrane was proposed by Singer and Nicolson (1972) widely accepted as fluid mosaic model (Figure).
-According to this, the quasi-fluid nature of lipid enables lateral movement of proteins within the overall bilayer. This ability to move within the membrane is measured as its fluidity.
- The fluid nature of the membrane is also important from the point of view of functions like cell growth, formation of intercellular junctions, secretion, endocytosis, cell division etc.
-One of the most important functions of the plasma membrane is the transport of the molecules across it.
-The membrane is selectively permeable to some molecules present on either side of it.
- Many molecules can move briefly across the membrane without any requirement of energy and this is called the passive transport.
- Neutral solutes may move across the membrane by the process of simple diffusion along the concentration gradient, i.e., from higher concentration to the lower.
- Water may also move across this membrane from higher to lower concentration. Movement of water by diffusion is called osmosis.
- As the polar molecules cannot pass through the nonpolar lipid bilayer, they require a carrier protein of the membrane to facilitate their transport across the membrane.
- A few ions or molecules are transported across the membrane against their concentration gradient, i.e., from lower to the higher concentration.
Such a transport is an energy dependent process, in which ATP is utilised and is called active transport, e.g., Na+/K+ Pump.

Cell Wall

As you may recall, a non-living rigid structure called the cell wall forms an outer covering for the plasma membrane of fungi and plants.
- Cell wall not only gives shape to the cell and protects the cell from mechanical damage and infection, it also helps in cell-to-cell interaction and provides barrier to undesirable macromolecules.

-Algae have cell wall, made of cellulose, galactans, mannans and minerals like calcium carbonate, while in other plants it consists of cellulose, hemicellulose, pectins and proteins.
-The cell wall of a young plant cell, the primary wall is capable of growth, which gradually diminishes as the cell matures and the secondary wall is formed on the inner (towards membrane) side of the cell.
- The middle lamella is a layer mainly of calcium pectate which holds or glues the different neighbouring cells together. The cell wall and middle lamellae may be traversed by plasmodesmata which connect the cytoplasm of neighbouring cells.

Endomembrane System

While each of the membranous organelles is distinct in terms of its structure and function, many of these are considered together as an endomembrane system because their functions are coordinated.
-The endomembrane system include endoplasmic reticulum (ER), golgi complex, lysosomes and vacuoles.
Since the functions of the mitochondria, chloroplast and peroxisomes are not coordinated with the above components, these are not considered as part of the endomembrane system.

The Endoplasmic Reticulum (ER)

-Electron microscopic studies of eukaryotic cells reveal the presence of a network or reticulum of tiny tubular structures scattered in the cytoplasm that is called the endoplasmic reticulum (ER) (Figure).
-Hence, ER divides the intracellular space into two distinct compartments, i.e., luminal (inside ER) and extra luminal (cytoplasm) compartments.
- The ER often shows ribosomes attached to their outer surface.
-The endoplasmic reticulun bearing ribosomes on their surface is called rough endoplasmic reticulum (RER).
-In the absence of ribosomes they appear smooth and are called smooth endoplasmic reticulum (SER).
RER is frequently observed in the cells actively involved in protein synthesis and secretion. They are extensive and continuous with the outer membrane of the nucleus.
- The smooth endoplasmic reticulum is the major site for synthesis of lipid. In animal cells lipid-like steroidal hormones are synthesised in SER.

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Endoplasmic Reticulum (Ergastoplasm of Garnier, 1897)

E.R. or EPR was discovered by Porter et al (1945) and Thompson (1945). It was given the present name by Porter (1953). Endoplasmic reticulum is a system of membrane lined channels found in all eucaryotic cells except mature erythrocytes. It constitutes more than 30 - 60% of total cell membranes.
In muscle cells, it is called sacroplasmic reticulum. Myeloid bodies (granules at base of retinal pigment cells) and Nissl granules are believed to be formed from E.R. E.R is little developed in meristematic cells.
Membranes of endoplasmic reticulum are thinner than most other cell membranes. E.R is often connected with outer membrane of nuclear envelope. It may also open at plasmalemma.
Enzymes occur both on the cytoplasmic surface (e.g., P-450, cyt b5, some reductases, nucleotidase) and luminar surface (e.g., glucose 6-phosphatase, peptidases, β -glucuronidase). It is made of a few tubules in adipose tissue, few vesicles in spermatocytes and reticulocytes but is best developed in metabolically active cells of liver, pancreas, plasma cells, interstitial cells and fibroblasts.
Endoplasmic reticulum has three parts-cisternae (parallel interconnected flattened sacs of 40-50 nm thickness), tubules (often branched, network, 50-100 nm in diameter) and vesicles (round or oval, 25-500 nm diameter). Membranes of E.R. may bear ribosomes when E.R. is called granular/rough endoplasmic reticulum or RE.R It is abundant in cells engaged in active secretion. Attachment between ribosome and endoplasmic reticulum is through glycoprotein called ribophorin.
The union is actually between 60 S ribosome subunit and E.R. by means of two types of glycoproteins-ribophorin I (mol. wt. 65000) and ribophorin II (mol. wt. 64000).

R.E.R often contains minute pores below ribosomes to pass synthesised polypeptides into its lumen for transport. E.R. without attached ribosomes is called agranular/smooth endoplasmic reticulum or S.E.R. It occurs in cells engaged in producing large quantity of lipids. SER also takes part in synthesis of vitamins and carbohydrates and detoxification.

Detoxification of pollutants, carcinogens and drugs is carried out by P-450 and P-448 present on S.E.R of liver cells and mitochondria. S.E.R. occurs in adipose cells, muscle. cells, glycogen metabolising liver cells, steroid synthesising cells, retinal cells, leucocytes, interstitial cells, intestinal epithelial cells. etc. In muscle cells, endoplasmic or sarcoplasmic reticulum is associated with storage and release of Ca²⁺ ions. S.E.R has more tubules and vesicles. It gives rise to spherosomes. R.E.R has more of cisternae. It is abundant in cells engaged in production and excretion of proteins, e.g., plasma cells, goblets cells, pancreatic acinus cells, certain liver cells. Broken pieces of endoplasmic reticulum appear as microsomes (Claude, 1941). Transitional E.R. is R.E.R without ribosomes. Annulate E.R. possesses pores (Mecullo, 1972). New E.R. may develop from existing one, nuclear envelope or ordinary biomembranes.

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Golgi apparatus

Camillo Golgi (1898) first observed densely stained reticular structures near the nucleus.
-These were later named Golgi bodies after him. They consist of many flat, disc-shaped sacs or cisternae of 0.5µm to 1.0µm diameter (Figure).
- These are stacked parallel to each other. Varied number of cisternae are present in a Golgi complex.
- The Golgi cisternae are concentrically arranged near the nucleus with distinct convex cis or the forming face and concave trans or the maturing face.
- The cis and the trans faces of the organelle are entirely different, but interconnected.
- The golgi apparatus principally performs the function of packaging materials, to be delivered either to the intra-cellular targets or secreted outside the cell.
- Materials to be packaged in the form of vesicles from the ER fuse with the cis face of the golgi apparatus and move towards the maturing face.
- This explains, why the golgi apparatus remains in close association with the endoplasmic reticulum. A number of proteins synthesised by ribosomes on the endoplasmic reticulum are modified in the cisternae of the golgi apparatus before they are released from its trans face. Golgi apparatus is the important site of formation of glycoproteins and glycolipids.

Lysosomes

These are membrane bound vesicular structures formed by the process of packaging in the golgi apparatus.
- The isolated lysosomal vesicles have been found to be very rich in almost all types of hydrolytic enzymes (hydrolases – lipases, proteases, carbohydrases) optimally active at the acidic pH.
- These enzymes are capable of digesting carbohydrates, proteins, lipids and nucleic acids.

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Golgi Apparatus (= Golgi Complex, named by Cajal 1914 after Golgi)

It is complex organelle made of membrane-lined stack of cisternae, network of tubules, vesicles and vacuoles which was first seen by George (1867) but studied by Camillo Golgi in 1898 in nerve cells of Barn Owl and Cat through metallic impregnation technique (osmium chloride + silver salts). Due to metallic impregnation artefacts, the apparatus was once called internal reticular apparatus/canalicular system/apparato reticolare. The concept changed after observation of the apparatus under electron microscope by Dalton and Felix (1954). Golgi apparatus is present in all eucaryotic cells except RBC and sieve tube elements. It is also absent in procaryotes and sperm cells of seedless embryophytes.
A unit of Golgi apparatus is called Golgisome. In plant cells, Golgi apparatus consists of a number of isolated units called dictyosomes while in animal cells it occurs as single compact or loose complex (localised or diffused).
The number of Golgi bodies/dictyosomes is generally 9-10 in a plant cell but is very high in cells engaged in secretory activity (e.g., root cap cells) and rapid division. Golgi apparatus is surrounded by a clear zone of exclusion (Mori-e, 1977) in which ribosomes, mitochondria, plastids, storage granules, etc. are absent.
A golgisome or dictyosome has a central stack of 3 -10 curved but parallel membrane-lined narrow sacs called cisternae. Cisternae are interconnected. Unicisternal dictyosomes occur in fungi and ciliated protozoans. Golgi apparatus has two faces, distal maturing and proximal forming (Mollenhauer and Whaley, 1963). The convex forming face (cis-face) receives materials from endoplasmic reticulum (GER = Golgi associated E.R.) and cytosol while concave maturing face (trans-face, normally towards plasma membrane) gives out large Golgian vacuoles and small vesicles having transformed materials.
Vesicles develop from the tubules as well. They are of two types, smooth and coated. Golgian vacuoles form lysosomes. Membranes of proximal cisterna are thin (50-60 A⁰) as those of E.R. They progressively become thick towards the distal side, reaching a thickness of 75 - 80 A⁰. A cisterna encloses a lumen of 100 -150 A⁰. Intercisternal space is 100 - 300 A⁰ having cytosol, granules, cementing material or parallel fibres. Golgi appratus receives materials from cytosol and endoplasmic reticulum in the form of transitional vesicles. The latter fuse with the cisterna of convex forming face.
Both membranes and biochemicals flow to the distal cisternae for elaboration. The movement may occur through vesicles that bud off from one cisternal edges to the next and so on. Golgi apparatus produces materials for secretion, takes part in transformation of membranes, formation of a number of products from glycoproteins (e.g., mucin from goblet cells), complex heteropolysaccharides (e.g., mucilage from root cap cells), hormones, melanin, matrix of connective tissue, middle lamella, acrosome and lysosomes.

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Vacuoles

The vacuole is the membrane-bound space found in the cytoplasm. It contains water, sap, excretory product and other materials not useful for the cell.
- The vacuole is bound by a single membrane called tonoplast. In plant cells the vacuoles can occupy up to 90 per cent of the volume of the cell.
- In plants, the tonoplast facilitates the transport of a number of ions and other materials against concentration gradients into the vacuole, hence their concentration is significantly higher in the vacuole than in the cytoplasm.
- In Amoeba the contractile vacuole is important for excretion. In many cells, as in protists, food vacuoles are formed by engulfing the food particles.

Mitochondria

Mitochondria (sing.: mitochondrion), unless specifically stained, are not easily visible under the microscope.
-The number of mitochondria per cell is variable depending on the physiological activity of the cells. In terms of shape and size also, considerable degree of variability is observed.
-Typically it is sausage-shaped or cylindrical having a diameter of 0.2-1.0µm (average 0.5µm) and length 1.0-4.1µm.
- Each mitochondrion is a double membrane-bound structure with the outer membrane and the inner membrane dividing its lumen distinctly into two aqueous compartments,
i.e., the outer compartment and the inner compartment.
-The inner compartment is filled with a dense homogeneous substance called the matrix.
-The outer membrane forms the continuous limiting boundary of the organelle. The inner membrane forms a number of infoldings called the cristae (sing.: crista) towards the matrix (Figure).
-The cristae increase the surface area.
- The two membranes have their own specific enzymes associated with the mitochondrial function. Mitochondria are the sites of aerobic respiration.
-They produce cellular energy in the form of ATP, hence they are called ‘power houses’ of the cell.
-The matrix also possesses single circular DNA molecule, a few RNA molecules, ribosomes (70S) and the components required for the synthesis of proteins. The mitochondria divide by fission.

Plastids

Plastids are found in all plant cells and in euglenoides.
These are easily observed under the microscope as they are large. They bear some specific pigments, thus imparting specific colours to the plants. Based on the type of pigments plastids can be classified into chloroplasts, chromoplasts and leucoplasts.
- The chloroplasts contain chlorophyll and carotenoid pigments which are responsible for trapping light energy essential for photosynthesis.
-In the chromoplasts fat soluble carotenoid pigments like carotene, xanthophylls and others are present.
-This gives the part of the plant a yellow, orange or red colour.
-The leucoplasts are the colourless plastids of varied shapes and sizes with stored nutrients:
-Amyloplasts store carbohydrates (starch), e.g., potato;
elaioplasts store oils and fats whereas the aleuroplasts store proteins. Majority of the chloroplasts of the green plants are found in the mesophyll cells of the leaves.
-These are lens-shaped, oval, spherical, discoid or even ribbon-like organelles having variable length (5-10 µm) and width (2-4 µm).
-Their number varies from 1 per cell of the Chlamydomonas, a green alga to 20-40 per cell in the mesophyll. Like mitochondria, the chloroplasts are also double membrane bound.
-Of the two, the inner chloroplast membrane is relatively less permeable.
-The space limited by the inner membrane of the chloroplast is called the stroma.
-A number of organised flattened membranous sacs called the thylakoids, are present in the stroma (Figure). Thylakoids are arranged in stacks like the piles of coins called grana (singular: granum) or the intergranal thylakoids.
- In addition, there are flat membranous tubules called the stroma lamellae connecting the thylakoids of the different grana. The membrane of the thylakoids enclose a space called a lumen. The stroma of the chloroplast contains enzymes required for the synthesis of carbohydrates and proteins.
- It also contains small, doublestranded circular DNA molecules and ribosomes. Chlorophyll pigments are present in the thylakoids. The ribosomes of the chloroplasts are smaller (70S) than the cytoplasmic ribosomes (80S).
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Characteristics of chloroplasts

Chloroplasts are double-membrane-bound structures.
They are divided into outer and inner membranes, further divided into two distinct regions:
(i) Grana are stacks of flattened discs containing chlorophyll molecules. The flattened membranous sacs are called thylakoids.
The thylakoids of adjacent grana are connected by membranous tubules called stroma lamellae.
(ii) Stroma is a homogenous mixture in which grana are embedded.
It contains several enzymes that are used for the synthesis of carbohydrates and proteins.
It also contains its own DNA and ribosomes and hence semi-autonomous organelle.
Plastids Plastids (Haeckel, 1866) are semiautonomous cell organelles which are surrounded by double membrane envelope, take part in storage and synthesis of organic compounds which occur in plants and some protistans. Plastidome includes the whole plastid complex of a cell. Plastids develop from colourless precursors called proplastids. Plastids undergo multiplication through fission-like division. Plastids are of three main types (Schimper, 1883)
-leucoplasts,
chromoplasts and
chloroplasts../ Chromoplasts can develop from proplastids, leucoplasts and chloroplasts; chloroplasts can arise from pre-existing chloroplasts, proplaslids and leucoplasts while leucoplasts are formed from proplastids and leucoplasts. All plastids possess naked DNA, 70 S ribosomes and lamellae. 1. Leucoplasts. They are colourless plastids of various shapes that generally occur in nongreen plant cells near the nucleus. Leucoplasts may get specialised into
(a) Amyloplasts. Storage of starch.
(b) Aleuroplasts or Proteinoplasts. Storage of proteins.
(c) Elaioplasts (Oleosomes). Storage of fats.
2. Chromoplasts. The plastids are coloured other than green due to presence of carotenoids but lacking of chlorophylls, change of fruit colour from green to yellow orange or red during ripening is due to conversion of chloroplasts to chromoplasts, e.g., red chillies, tomato. A chromoplast has lipid globules and carotenoids in both crystallised and dissolved states. Chromoplasts come to have various shapes due to crystallisation of carotenoids. Chromoplasts commonly attract animals to flowers and fruits for performing pollination and fruit dispersal. .
3. Chloroplasts (Schimper, 1883 ; Autoplasts of Meyer, 1883). They are green plastids which take part in photosynthesis and temporary or permanent storage of starch. In green algae, chloroplasts are of various shapes and sizes but chloroplasts of higher plants are disc-shaped with diam_ter of 4 - 6 µm and thickness of 2-4 µm. In embryophytes they are the second largest cell organelles (smaller only to nucleus). In many algae they are the largest, e.g., Chlamydomonas, Chlorella, Ulothrix, Spirogyra.
The number varies from one (e.g., Chlamydomonas, Chlorella, Ulothrix, Oedogonium), two (e.g., Zygnema) 20-40 (e.g., chlorenchyma cells of leaf) to numerous (e.g., internodal cell of Chara).
Each chloroplast is surrounded by a double membrane envelope. Outer membrane is smooth. It regulates passage of materials between the organelle and cytoplasm. Inner membrane runs parallel to outer membrane. Though not apparent in the mature state, it is folded extensively inward to produce lamellae.
Internally, the chloroplast contains matrix or stroma and thylakoids or photosynthetic lamellae. Matrix contains crystallo-colloidal complex having 2-6 copies of circular or rarely linear chloroplast DNA (cpDNA), 70S ribosomes, plastoglobuli (lipid globules with quinones, vitamin K, vitamin E), starch grains, granules of phytoferritin, enzymes of Calvin cycle, etc.

Rubisco (ribulose biphosphate carboxylase, RuBP or RuDP) is the most abundant enzyme (or protein) of the biological world. Thylakoids (Menke, 1961) or baggy trousers are structural elements of chloroplasts. They are membrane-lined flattened sacs. At places they form stacks called grana.
A chloroplast may have 40-60 grana with each granum having 10-100 thylakoids. Thus there is differentiation of granal and stromal (intergranal) thylakoids. Space present in a granal thylakoid is called loculus while that of stromal thylakoids is termed as fret channel. It contains mobile parts of electron transport chain embedded in a semifluid complex.
More than 50% of chloroplast proteins and various components connected with photosynthesis are present in thylakoids. Photosynthetic pigments are located in the membranes of thylakoids in specific areas (according to Park and Biggins, 1963) called quantasomes (20 x 10 nm).
Photosynthetic pigments of higher plants include chl a, chl b, carotenes and xanthophylls.
A quantasome is believed to have 230 chlorophyll (160 a + 70 b) and 50 carotenoid molecules. Photosynthetic pigments, especially chlorophyll a, are connected with trapping of solar energy and formation of ATP and NADPH. Photosynthetic pigments form two complexes, PS I and PS II.
Photosystem I is present on stroma thylakoids and unappressed parts of granal thylakoids. Its trap centre or photocentre is P700. Photosystem II occurs in appressed part of granal thylakoids. Its trap centre is P 680 Other structures present in thylakoid membranes are of cytochrome complex and CFo-CF1 (ATP-synthetase).
The matrix or stroma contains naked DNA - circular as well as linear, RNA, ribosomes (70 S), plastoglobuli (lipid/fat globuli) and enzymes of carbon assimilation. Grana are absent in chloroplasts of algae and bundle sheath chloroplasts of C4 plants.
They are agranal. Green algae have a special starch storing structure in their chloroplasts. The same is called pyrenoid. Motile algal forms possess a photosensitive structure called stigma or eye spot. In photoautrophic procaryotes, thylakoids occur freely in the cytoplasm. A chloroplast like structure is not organised.
Unlike isolated plant cells, isolated cell organelles like chloroplasts and mitochondria cannot be kept in distilled water as they would burst. They can be maintained only in solutions of specific concentration, e.g., 0.25% sucrose.
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Ribosomes

Ribosomes are the granular structures first observed under the electron microscope as dense particles by George Palade (1953).
- They are composed of ribonucleic acid (RNA) and proteins and are not surrounded by any membrane.
- The eukaryotic ribosomes are 80S while the prokaryotic ribosomes are 70S.
-Each ribosome has two subunits, larger and smaller subunits (Fig 8.9).
-The two subunits of 80S ribosomes are 60S and 40S while that of 70S ribosomes are 50S and 30S.
-Here ‘S’ (Svedberg’s Unit) stands for the sedimentation coefficient; it is indirectly a measure of density and size. Both 70S and 80S ribosomes are composed of two subunits.
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Cytoskeleton

An elaborate network of filamentous proteinaceous structures present in the cytoplasm is collectively referred to as the cytoskeleton.
The cytoskeleton in a cell are involved in many functions such as mechanical support, motility, maintenance of the shape of the cell.

Cilia and Flagella

Cilia (sing.: cilium) and flagella (sing.: flagellum) are hair-like outgrowths of the cell membrane.
-Cilia are small structures which work like oars, causing the movement of either the cell or the surrounding fluid.
- Flagella are comparatively longer and responsible for cell movement.
-The prokaryotic bacteria also possess flagella but these are structurally different from that of the eukaryotic flagella.
- The electron microscopic study of a cilium or the flagellum show that they are covered with plasma membrane.
-Their core called the axoneme, possesses a number of microtubules running parallel to the long axis.
- The axoneme usually has nine pairs of doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules. Such an arrangement of axonemal microtubules is referred to as the 9+2 array (Figure).
-The central tubules are connected by bridges and is also enclosed by a central sheath, which is connected to one of the tubules of each peripheral doublets by a radial spoke. Thus, there are nine radial spokes.
-The peripheral doublets are also interconnected by linkers.
Both the cilium and flagellum emerge from centriole-like structure called the basal bodies.

Centrosome and Centrioles

Centrosome is an organelle usually containing two cylindrical structures called centrioles.
-They are surrounded by amorphous pericentriolar materials.
-Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation like the cartwheel. They are made up of nine evenly spaced peripheral fibrils of tubulin protein.
-Each of the peripheral fibril is a triplet.
-The adjacent triplets are also linked. The central part of the proximal region of the centriole is also proteinaceous and called the hub, which is connected with tubules of the peripheral triplets by radial spokes made of protein.
- The centrioles form the basal body of cilia or flagella, and spindle fibres that give rise to spindle apparatus during cell division in animal cells.

Nucleus

Nucleus as a cell organelle was first described by Robert Brown as early as 1831.
- Later the material of the nucleus stained by the basic dyes was given the name chromatin by Flemming.
- The interphase nucleus (nucleus of a cell when it is not dividing) has highly extended and elaborate nucleoprotein fibres called chromatin, nuclear matrix and one or more spherical bodies called nucleoli (sing.: nucleolus) (Figure).
- Electron microscopy has revealed that the nuclear envelope, which consists of two parallel membranes with a space between (10 to 50 nm) called the perinuclear space, forms a barrier between the materials present inside the nucleus and that of the cytoplasm.
- The outer membrane usually remains continuous with the endoplasmic reticulum and also bears ribosomes on it.
-At a number of places the nuclear envelope is interrupted by minute pores, which are formed by the fusion of its two membranes.
-These nuclear pores are the passages through which movement of RNA and protein molecules takes place in both directions between the nucleus and the cytoplasm.
-Normally, there is only one nucleus per cell, variations in the number of nuclei are also frequently observed.
-Can you recollect names of organisms that have more than one nucleus per cell? Some mature cells even lack nucleus, e.g., erythrocytes of many mammals and sieve tube cells of vascular plants.
-Would you consider these cells as ‘living’?

The nuclear matrix or the nucleoplasm contains nucleolus and chromatin.
-The nucleoli are spherical structures present in the nucleoplasm. The content of nucleolus is continuous with the rest of the nucleoplasm as it is not a membrane bound structure. It is a site for active ribosomal RNA synthesis.
-Larger and more numerous nucleoli are present in cells actively carrying out protein synthesis.
- You may recall that the interphase nucleus has a loose and indistinct network of nucleoprotein fibres called chromatin.
But during different stages of cell division, cells show structured chromosomes in place of the nucleus. Chromatin contains DNA and some basic proteins called histones, some non-histone proteins and also RNA.
-A single human cell has approximately two metre long thread of DNA distributed among its forty six (twenty three pairs) chromosomes.

Every chromosome (visible only in dividing cells) essentially has a primary constriction or the centromere on the sides of which disc shaped structures called kinetochores are present (Figure 8.12).
- Centromere holds two chromatids of a chromosome. Based on the position of the centromere, the chromosomes can be classified into four types (Figure).

- The metacentric chromosome has middle centromere forming two equal arms of the chromosome.
-The sub-metacentric chromosome has centromere slightly away from the middle of the chromosome resulting into one shorter arm and one longer arm.
- In case of acrocentric chromosome the centromere is situated close to its end forming one extremely short and one very long arm, whereas the telocentric chromosome has a terminal centromere.

Sometimes a few chromosomes have non-staining secondary constrictions at a constant location.
- This gives the appearance of a small fragment called the satellite.
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Spherosomes (Pemer, 1953)
Spherosomes or sphaerosomes are single membrane covered small spherical organelles (0.5 -1.0 µn) which synthesise and store fats. They develop from E.R. In some plants, spherosomes have lysosomal activity, e.g., Maize root tip.

Microbodies (Rhodin, 1954)
They are single membrane covered small cell organelles which take part in oxidation reactions other than those of respiration. Microbodies often possess a crystalline core and granular matrix.
They are of two types - peroxisomes and glyoxisomes.
1. Peroxisomes (De Duve et al, 1965; De Duve, 1969). The microbodies have enzymes for peroxide biosynthesis.
They include (a) Peroxide producing oxidative enzymes like peroxidase, urate oxidase or uricase (hence uricosome), amino acid oxidase and
(b) Peroxide destroying enzyme catalase. Peroxisomes occur widely in animal cells, protistans, fungi, eucaryotic algae, bryophytes, pteridophytes and higher plants but quite common in photosynthetic cells. They take part in. oxidative breakdown of extra biochemicals like purines, amino acids, alcohol, toxins, drugs etc. In mesophyll cells, peroxisomes interact with chloroplasts and mitochondria to take part in photorespiration (Tolbert et al, 1969). For this they have glycolate oxidase for metabolism of glycolate. Their number can be 70 -100 per mesophyll cell.
2. Glyoxisomes/Glyoxysomes (Briedenbach, 1967 ; also Beevers, 1963). The microbodies occur only in fat rich plant cells where they take part in, β-oxidation of fats and perform glyoxylate cycle. Glyoxisomes possess catalase.
Cytoskeletal Structures
They are fibrous or fine tubular structures which constitute the supportive structures of the cell. The term cytoskeleton was coined by Koltzoff (1928). Cytoskeletal structures are of three types -
micro tubules,
microfilaments and
intermediate fibres.

1. Microtubules.

Microtubules were discovered by DeRobertis and Franchi (1953) in nerve axons and named as neurotubules. The term was coined by Slautterback (1963). They are unbranched hollow tubules of indefinite length, 25 nm of thickness with 15 nm core and boundary formed of 13 helically arranged protofilamellts of globular molecules of α and , β -tubulins.
Lateral projections occur for establishing cross-bridges. Microtubules grow from nucleating regions. Their tips can grow and shorten quickly.
GTP, Ca²⁺, Mg²⁺ and a calmodulin bound protein are required for assembly.
Colchicine prevents it.
Microtubutes are basic structures of spindle apparatus, centrioles, basal bodies, cilia and flagella. They are also present in other cellular structures like sensory hair, nerve processes, sperm tail, etc. Microtubules present in cytoplasm provide shape and polarity to cells. They are involved in cell movements (alongwith microfilaments), chromosome movements and intracellular transport. Microtubules are absent in procaryotes (except Anabaena), Amoeba and Slime Moulds.

2. Microfilaments (Paleviz et at, 1974).

They are double helical cylindrical rods or filaments of actin like protein that occur in both muscular and nonmuscular cells. Microfilaments have an indefinite length but the diameter is 5 - 8 nm.
They occur in sheets, hexagonal bundles and three dimensional micro trabecular lattice, take part in cytoplasmic streaming, movement of pigment granules, amoeboid movements and other sol-gel changes (through formation and breakage of cross-linkages), membrane undulations, microvilli and cleavage.

3. Intermediate Fibres.

They are solid, unbranched protein fibrils of 8 -10 nm thickness which are noncontractile. Intermediate fibres (IF) are of four types - keratin filaments, neurofibrils, glial filaments and heterogeneous filaments (viz., desmin filaments, vimentin filaments, synemin filaments). They occur in cell-cell junctions and in the form of basket around nucleus of animal cells.

Centrioles

They are minute submicroscopic subcylindrical structures of 0.3 - 0.5 ,μm length and 0.15 μm , diameter which usually occur in pair (diplosome) inside a specialised cytoplasm called centrosphere or kinoplasm. The complex is called centrosome (Boveri, 1888) or central apparatus. The two centrioles lie at right angles to each other. Each centriole has a cartwheel constitution. There is a whorl of nine peripheral triplet fibrils of tubulin (9 + 0) tilted at an angle of 40°. The three subfibres of a triplet from outside to inside are C, B and A. Though each subfibre should have 13 protofilaments like a microtubule, both C and A subfibres share 2-3 protofilaments with B-subfibre.

Matrix fills the centriole. Adjacent triplets are interconnected by proteinaceous C - A linkers.

A proteinaceous rod or hub occurs in the centre. Subfibre A of each triplet is connected to hub by means of a radial proteinaceous strand called spoke. Spokes are also connected to C - A linkers with two types of thickenings, X and Y. Centrioles are surrounded by massules or pericentriolar satellites for formation of new centrioles in G2 phase. (Typical replication is absent). ATP-ase activity is present. Centrioles are required to form basal bodies, cilia, flagella and astral spindle poles. They occur in most animal cells. Amongst protists, fungi and metaphytes they are found in flagellate forms or forms having flagellate stages (e.g., ciliates, primitive fungi, brown algae, yellow green algae, many green algae, bryophytes, pteriodophytes and cycads).

Basal Bodies
Basal bodies, basal granules, kinetosomes or blepharoplasts are microcylinders that lie below the plasmalemma at the base of flagella and cilia. The structure is similar to a centriole with a diameter of 0.15 – 0.2 μm, 9 + 0 constitution with a cartwheel structure in the proximal region. In the distal half, central hub and spokes disappear. Proximal region bears rootlets of microfilament bundles for support. Basal plate is dense plate-like band that lies between basal body and shaft of a cilium or flagellum. Here C-subfibres disappear converting triplets into doublets. The central singlet fibrils are also formed here.

Cilia and Flagella

They are vibratile hair-like narrow protoplasmic processes present on the free surface of the cell which are connected with motility. Each one is made of four parts - basal body, rootlets, basal plate and shaft. Shaft consists of an external membrane or sheath (extension of plasmalemma), a semifluid matrix and an axoneme. Axoneme has nine peripheral doublet fibrils, two central singlet fibrils, central sheath, linkers and spokes. The fibrils have 9 + 2 arrangement (Fawcett and Porter, 1954). Peripheral doublet fibrils are tilted at an angle of 10°. Each peripheral doublet fibril has a slightly broader outer subfibre B and slightly narrow inner subfibre A. They are made of microtubules both sharing 2 - 3 protofilaments. Subfibre A has about 15 nm long two bent arms, outer with a hook. They come in contact with subfibre B of adjacent fibril during contraction. It allows sliding of adjacent doublets. The singlet central fibrils are called C1 and C2. A double proteinaceous bridge occurs between them. A proteinaceous central sheath surrounds the two central fibrils. Adjacent doublet fibrils are connected by proteinaceous B - A linkers.
Subfibre A of each doublet is also connected to central sheath by a proteinaceous radial spoke. The spoke broadens towards the central sheath to form head. A narrow flat transition junction occurs between spoke head and central sheath. The central fibrils and side arms of subfibre A are made of dynein protein with A TP-ase activity. B - A linkers and radial spokes are made of protein nexin.
Cilia are shorter (5 -10 µm as compared to 150 µm for flagella), more numerous (number of flagella 1 to 4), have sweeping or pendular movement (flagella have undulatory motion) and beat in a coordinated rhythmic movement (flagella slow independent movements), either synchronous (=isochronous, simultaneous) or metachronous (one after the other). Cilia function like an oar. They have a power or effective stroke which pushes the fluid in one direction with a jerk while the cell is pulled in the opposite direction. It is followed by slow recovery or return stroke when the cilia regain their original position, ready for another power stroke.
Flagella show independent undulatory movements with undulations starting either from base and proceeding to top (the cell is pushed along) or from top to base (the cell is pulled along). Certain flagella possess lateral hair called Dimmer filaments. They are called tinsel nagella. Flagella devoid of flimmers are called smooth or whiplash nagella. In tinsel flagella, the tinsel filaments also show movements. If it is from base to top, the cell is pulled instead of being pushed.
Cilia and flagella beat at the rate of 10-40 per second: It gives very high speed (much faster than higher animals). The maximum speed is found in flagellate Monas stigmatica -40 lengths or 260 μm/sec. Paramaecium caudatum swims at the rate of 12 lengths or 1500 μm/sec.

Vacuoles (Spallanzani)

They are noncytoplasmic sacs which are separated from cytoplasm by a membrane. Vacuoles are of four types.

1. Sap Vacuoles. They enclose sap or water with dissolved inorganic and organic substances. A sap vacuole is surrounded by a membrane called tonoplast. Plant cells usually have a single large central vacuole. Animal cells have numerous small sap vacuoles. Sap vacuoles maintain osmotic pressure for turgidity and osmosis. They also store useful as well as waste substances.

2. Contractile Vacuoles. They occur in some simple fresh water forms (e.g., Amoeba, Paramecium, Chlamydomonas). Contractile vacuoles are surrounded by a few feeding canals. They pick up water from surrounding cytoplasm, expand (diastole) and collapse ( systole) to throw water to the outside. Contractile vacuoles perform osmoregulation and excretion.

3. Food Vacuoles. A food vacuole is a complex of lysosome and phagosome. Digestion occurs inside them.

4. Gas or Air Vacuoles (= Pseudovacuoles). Gas or air vacuoles occur in some procaryotes. Each gas vacuole is made of a large number of submicroscopic hexagonal gas vesicles. A gas vesicle is surrounded by a thin protein membrane. Gas vacuoles store metabolic gases and take part in buoyancy regulation.

Nucleus

Nucleus (Robert Brown, 1831) is a double membrane covered protoplasmic body that contains hereditary information. With the help of grafting experiments on Acetabularia, Hammerling (1953) proved that nucleus is a store-house of hereditary information. A cell usually contains a single nucleus (uninucleate, monokaryotic). However, binucleate (e.g., Paramecium caudatum), trinucleate (e.g., Paramecium aurelia) and multinucleate forms also occur. Multinucleate form is called plasmodium in slime moulds. It is syncytium in animal cells (also by fusion of cells) and coenocytic in plant cells. A nucleus.is absent in some mature cells like mammalian RBCs and sieve tube cells. They are called anucleate cells. The term enucleate is used more commonly for cells from which nucleus has been removed. Nucleus is commonly rounded but can be oval, disc-shaped, lobed or irregularly branched. A typical nucleus is 5 - 25 μm in diameter. It is the largest amongst cell components. It is called master organelle as it controls all cell activities. Chemically it contains DNA (9 -12%), basic proteins (15%), nonbasic proteins (acid proteins, neutral proteins, enzymes-65%), RNA (5%), lipids (3%), minerals (traces). Phosphorus occurs in DNA, RNA and some acid proteins. Nucleus has five parts-nuclear envelope, nucleoplasm, nuclear matrix, chromatin and nucleolus.
1. Nuclear Envelope (= Karyotheca). It is made up of two membranes separated by 10- 70 nm perinuclear space. The outer membrane may have ribosomes and interconnections withE.R. Nuclear envelope has a large number (1000-10000) of pores with diameter of 200-800 A0 (100-1000 A0 ). Nuclear pores may have diaphragms, septa, annuli, blebs or micropores with a central micropore surrounded by nine peripheral micropores. The central micropore or channel has nucleoplasmin for movement of substances. 2. Nucleoplasm (Strasburger, 1882 ; Nuclear Sap, Karyolymph, Karyoplasma). It is a colloidal complex that fills the nucleus. Nucleoplasm contains raw materials for synthesis of DNA and RNA. 3. Nuclear Matrix. It is a network of proteinaceous fibrils which is thickened towards the outside to form fibrous lamina (Harris and James, 1952) or nuclear lamina in contact with inner membrane of nuclear envelope. The proteins are acidic in nature. 4. Chromatin (Flemming, 1879). It is a fibrous hereditary material formed by DNA-histone complex. Chromatin fibres are of two types, narrow (30-80 A) elongated lightly stained transcriptionally active euchromatin and broader (250A0 or more) darkly stained transcriptionally inactive granular heterochromatin (Heitz, 1928). Replication occurs late in heterochromatin. Large granular heterochromatic parts are called karyosomes, false nucleoli or chromocentres. Constitutive heterochromatin occurs in all cells and in all stages including chromosomes. It consists of repetitive bases. Facultative heterochromatin is formed in certain cells to inactivate certain genes. One X-chromosome in human females becomes facultatively heterochromatic during early embryogenesis. It becomes normal in oocytes. Barr body develops due to it. Chromatin is actually folded nucleosome (DNA + histones) containing DNA chains. During nuclear division, chromatin fibres fold variously to produce chromosomes. 5. Nucleolus. It was originally discovered by Fontana (1781) and given the present name by Bowman (1840). It is naked (without covering membrane) roughly rounded darkly stained structure that is attached to chromatin at specific spot called nucleolar organiser region or NOR. Nucleolus contains four parts - amorphous, fibrillar, granular and chromatin. Both fibrillar and granular parts are formed of RNA and protein. Nucleolus is the site for elaboration of rRNA and synthesis of ribosomes. It is, therefore, called ribosomal factory. Ribosomal protein is synthesised in the cytoplasm. Nucleoid. In prokaryotes, DNA is circular and folded variously to form a nearly compact structure called nucleoid or genophore. There is no structure like nuclear envelope, nucleolus or nucleoplasm. The anionic charges present on internucleotide phosphate are balanced by polyamines. Histones are absent. DNA is therefore, naked. Cell Inclusions (Ergastic/Deutoplasmic/Paraplasmic Substances) They are nonliving substances of four categories - reserve, excretory, pigment and mineral. Due to their presence, a cell may become different from other surrounding cells. It is then called idioblast. 1. Reserve Food. It can be starch (plant cells), glycogen (animal cells, fungi and bacteria), fat droplets and aleurone grains. Aleurone grains are storage proteins developed inside aleuroplasts. They can be amorphous, with crystalloid alone (aleurone grains of Maize, Wheat), with globloid alone and with both crystalloid and globloid (e.g., Castor seed). Crystalloid is protein-carbohydrate while globloid has lipid and phytin. 2. Excretory/Secretory Products. They include mucus, gums, tannins, resins, alkaloids, latex, etc. (i) Latex. It is a crystallo-colloid fluid secreted by latex tubes or laticifers of two types, latex cells(non articulated laticifers, e.g., Banyan, Calotropis, Oleander) and latex vessels (articulated laticifers, e.g., Poppy, Rubber plant, Sonchus). Latex can be watery (e.g., Banana), milky (e.g., Banyan) or coloured (e.g., Poppy). Latex of Hevea brasiliensis yields rubber, that of Poppy forms opium while latex of Papaya contains protein digesting enzyme papain. (ii) Gums. They are degradation products of cell wall, e.g., gum arabic (Acacia senegal). (iii) Gum-Resin. It is mixture of gum and resin, e.g., root of Femla asafoetida (asafoetida). (iv) Resins. They are acidic oxidation products of essential oils which are insoluble in water but soluble in alcohol/turpentine. An example of hard resin is shellac. Pine resin and Canada Balsam are oleo-resins (resins with associated essential oils). (v) Tannins. They are astringent, acidic, phenolic compounds, related to glucosides, found in leaves (e.g., Tea), bark (e.g.,Acacia nilotica, Walnut or Juglans regia), fruit (e.g., Caesalpinia, Betel Nut). Dyes related to tannins are cutch (heart wood of Acacia catechu) and haematoxylin (heartwood of Haematoxylon). (vi) Alkaloids. They are bitter nitrogenous by-products, often poisonous and with medicinal properties, e.g., quinine (bark of Cinchona officinalis), atropine (leaves and tops of Atropa belladana), nicotine (leaves of Nicotiana tabacum) , morphine (latex of Papaver somniferum), reserpine (roots of Rauvolfia serpentina) , colchicine (corms of Colchicum autumnale) , thein (Tea leaves). (vii) Glucosides. They are aromatic compounds having glucose/carbohydrates, e.g., saponin, digitoxin, digitalin, amygdalin. Many of them are medicinal. (viii) Essential Oils. They- are volatile aromatic oils secreted by special glands, e.g., Lavender, Rosemary oil, Menthol, Eucalyptus oil. (ix) Nectar. It is sugary secretion of parts of flowers for attracting insects and other animals for pollination. Nectar contains glucose, fructose and sucrose. 3. Pigments. The major pigments dissolved in cell sap are anthocyanins (bluish or reddish depending upon pH) and anthoxanthins (yellowish). 4. Mineral Crystals. Calcium carbonate occurs as a mass of crystals around a cellulose core to form cystolith (leaf hypodermis of Banyan, group in epidermis of Momordica). Cystolith is like a bunch of grapes but is worm-like in Justicia. Calcium oxalate forms needle-shaped raphides (e.g., Lemna, Eichhomia), prismatic crystals (e.g., scales of Onion), star-shaped sphaeraphide or druse (e.g., Colocasia, Chenopodium) or powdery mass named crystal sand (e.g., Atropa).
Chromosomes
Chromosomes were discovered by Hofmeister (1848), studied by Strasburger (1875) and given the present name by Waldeyer (1888) after their staining by dyes like Janus green. Chromosomes are thread like DNA-protein entities which possess, replicate and transcribe coded information for controlling structure and function of biological systems. Late prophase and metaphase are the best time to observe chromosomes. Chromosomes are 0.5-30.0 µm in length and 0.2-3.0 µm in breadth. Haploid set of chromosomes alongwith all its genes is called genome. In somatic cells of animals and higher plants, there are two genomes, two sets or 2x or diploid number of chromosomes where there are two chromosomes (homologous chromosomes) of each type. Their gametes or gametophytic cells have one genome or 1x or haploid number of chromosomes.
The late prophase or metaphase chromosome has two similar threads or chromatids attached to each other by a narrow nonstainable area called centromere. The two parts of the chromatid/chromosome on either side of centromere are called arms. They may be isobrachial (= equal) or heterobrachial (unequal in length).
Depending upon position of the centromere, a chromosome can be
(i)Metacentric. Centromere in middle, anaphasic stage V-shaped.
(ii)Submetacentric: Centromere submedian, anaphasic stage L-shaped.
(iii) Acrocentric: Centromere subterminal, anaphasic stage J-shaped.
(iv)Telocentric.Centromere terminal, anaphasic stage I-shaped.
Depending upon number of centromeres, chromosomes are monocentric (common type), dicentric (two centromeres, often in Zea mays or Maize), polycentric (many centromeres, e.g., germ line chromosome of Parascaris equorum), acentric (without centromere) or holocentric (whole surface as centromere). Centromeric index is the ratio of lengths of the two arms of chromosomes. Chromomeres (Pfitzner, 1882) are swollen or dense areas arranged linearly along the chromosomes. They are believed to represent genes but gene activity has also been observed in interchromomeric regions.
Chemical Composition. DNA-4O%. Histone -50%. Other (Acid) Proteins-8.5%. RNA -1.5%. Traces of lipids, Ca, Mg and Fe. Histones are low molecular weight basic proteins which occur alongwith DNA in 1 : 1 ratio. Nonhistone chromosomal or NHC proteins are of three types - structural, enzymatic and regulatory. Structural NHC proteins form the core or axis of the chromosome. They are also called scaffold proteins. Enzymatic proteins form enzymes for chemical transformations, e.g., phosphatase, RNA polymerase, DNA polymerase. Regulatory proteins control gene expression. HMG (high mobility group) proteins get linked to histones for releasing DNA to express itself.

Ultra-Structure. A chromosome has seven parts.

1. Pellicle. It is outer thin sheath of nongenetic material.
2. Matrix. It is ground substance of chromosome which is made of nongenetic materials like RNA, acid proteins and lipids.
3. Chromonema (Vejdowsky, 1912). It is DNA-histone thread or chromatin strand which forms the bulk of chromosome. A chromosome with two chromatids has two chromonemata while an anaphasic chromosome has single chromonema (old estimates 32-64). It is coiled. Basically the chromonema is made of nucleosome chains. They are formed by complexing of DNA with histone proteins.
Nucleosomes are oblate structures having a histone octamer (nu body) of four types of histone molecules (2 each) - H2A, H2B, H3 and H4, with their charged parts towards outside and hydrophobic areas towards centre. The charged parts help DNA to coil over nu body (1.75 coils). DNA strand between two nucleosomes is called linker/interbead DNA. It bears H1 histone. There are two view points about the arrangement of nucleosome chains during formation of chromonema. (i) Solenoid Model (Finch and KIug, 1976). Nucleosome chain has a diameter of 100.-125 A0. It is rolled like a cylindrical coil of wire called solenoid. Solenoid is about 300 A0 in diameter. Its single coil has about six nucleosomes. Solenoid is further coiled into super-solenoid or chromonema of 2000-4000 A0 thickness. (ii) Radial Loop Model (Laemmli, 1977). A chromatid or chromosome has a core of scaffold proteins. It bears a number of lateral loops of 300 A0 thickness that are themselves formed of solenoid of nucleosome chains. The loops are spread in interphase nucleus. They get folded and compacted to form chromatid. Scaffold proteins also come closer during chromatid formation.
4. Primary Constriction/Centromere. It is narrow very lightly stained part of chromosome where the two chromatids are attached in prophasic and metaphasic chromosome. Internally the centromere part has little chromonemal coiling with a small amount of β-heterochromatin. The chromosome also possesses α-heterochromatin on either side of centromere. Within the centromere is present a fibrous or granular structure of about 400 nm diameter, one on either side. It is called kinetochore. Its molecular structure and even precise location has not been characterised (Lewin, 2000). Kinetochore has points for attachment of microtubules of chromosomal fibres. Centromere/primary constriction divides into two during early anaphase.
5. Secondary Constrictions. They are narrow areas of two types, joints and NOR. Joints are areas involved in breaking and fusion of chromosome segments. NOR or nucleolar organiser region is secondary constriction capable of forming nucleolus in interphase.
6. SateIlite/Trabant. Knob like part distal to NOR. The chromosome is called sat chromosome (due to poor stainability of NOR).
7. Telomeres. They are nonsticky terminal ends of the chromosome having moderately repetitive DNA.
Karyotype It is chromosome complement of a cell/organism providing description of various aspects of all the chromosomes like number, relative size, position of centromere, length of arms and centromeric ratio, secondary constrictions and satellites. Idiogram is a karyotype consisting of photograph or diagram of all the metaphasic chromosomes arranged in homologous pairs according to their decreasing length, thickness, position of centromere, length of arms, shape and other characteristics. Sex chromosomes are placed at the end but in case of Drosophila, they are kept at position I. For obtaining karyotype, cells cultured under aseptic conditions are administered colchicine (for arresting division at metaphase), killed, fixed, crushed and stained. Banding Technique (Casperson et ai, 1970). It is a technique in which chromosomes are first subjected to a variety of chemical treatments and then stained with special fluorescent dyes that produce bands of specific colours and strength as against other regions which remain unstained or lightly stained. The two common stains used are Giemsa and Quinacrine Mustard. Depending upon treatment animal karyotyping has four types of bands-Q, C, G, R. Plant karyotyping uses two types of banding techniques-C and N. Fluorescent in. Situ Hybridisation (FISH). It uses DNA probes, labelled with radioactive or nonradioactive molecules for locating specific DNA sequences over the chromosomes. In multicolour fluorescence in situ hybridisation (Mc FISH) different DNA probes labelled with different fluorochromes are used to locate different DNA sequences on the same chromosome. Flow Cytometry. Chromosomes are extracted from a large number of similar cells and stained with DNA binding fluorochrome. The stained chromosomes are passed through a flow cytometer having a light focussing device and a sensor for measuring the fluorescence and size of individual chromosomes. With the help of computer, the result is prepared in the form of histogram. It can detect differences of 1.5 - 4 MbP (megabase pairs) indicating deletibn, duplication, aneuploidy, etc. Uses. (i) Karyotype indicates primitive or advanced nature of the organism. Advanced nature depends upon asymmetry. A karyotype is asymmetric if it has fewer metacentric chromosomes and there is a large difference between the smallest and the largest chromosomes. Karyotype is symmetric when the number of metacentric chromosomes is large and there is a gradual change from smallest to largest chromosomes. (ii) Change in chromosome number is detected immediately. (iii) By comparing karyotypes of different species, similarities and evolutionary relationships can be known, e.g., humans and primates. (iv)With the help of flow cytometry as well as banding technique, deletion and duplication can be detected. Giant Chromosomes 1. Polytene Chromosomes (Salivary Chromosomes). They were discovered by Balbiani (1881) in the salivary glands of Chironomus tantans. Later they were found in salivary glands (and malpighian tubules) of many insect larvae including Drosophila. They also occur in antipodal cells, suspensor cells and endosperm cells of plants. The name polytene was given by Kollar. Significance of polytene chromosomes was found out by Painter et al (1933). Polytene chromosomes are formed by somatic pairing of homologus chromosomes followed by their repeated replication or endomitosis. They are in permanent prophase. The cells (salivary of larva) seldom divide and degenerate during the development of adult organism. A polytene chromosome may have 1000 (e.g., Drosphila) to 16000 (Chironomus) strands. Each chromosome may reach a length of 200-600 µm. All the polytene chromosomcs may remain attached to a common chromocentre by their pericentric heterochromatin which is slow to replicate. All the four polytene chromosomes of Drosophila melanogaster measure 2000 , µm. Stained with basic dyes, the chromosomes show dark bands and light interbands. Bands are formed by coming together of chromomeres of all the chromonemata. They, therefore, seem to represent genes. Bridges counted 5149 dark bands in the polytene chromosomes of Drosophila. Loss or gain of chromosome segments can be known. In different developmental stages different dark bands show swellings called puffs/Balbiani rings/bulbs. In the region of a puff or bulb the chromonemata develop lateral loops where DNA becomes active and produces copies of RNA After the activity, the puffs are withdrawn. Certain workers use the term of balbiani rings for larger puffs while some others refer them to lateral loops in the puffs. 2. Lampbrush Chromosomes (Ruckert, 1892). They are large sized diplotene chromosome bivalents with a length of 400 -1000 µm each and a total length of 5900 µm in Triturus (Salamander = Newt). A lampbrush chromosome is made of two homologous chromosomes held at several places by chiasmata. Each chromosome has an axis with alternate chromomeres and interchromomeric areas. Many of the chromomeres give out lateral loops of various sizes which are thin in the region of origin and thick in the area where they are wound back into chromomeres. Loops possess a number of copies of the same gene and are meant for rapid transcription and production of materials like yolk. Hence, lampbrush chromosomes occur in oocytes. The loops may also produce informosomes (mRNA + proteins) for future development of embryo. Besides oocytes, lampbrush chromosomes have been reported in spermatocytes and giant nucleus of Acetabularia.

Functions of Chromosomes

1. Chromosomes are a link between parents and offspring.
2. They contain genes and hence hereditary information.
3. Sex chromosomes determine sex.
4. Chromosomes control cell growth, cell division, cell differentiation and cell metabolism through directing synthesis of particular proteins and enzymes.
5. Haploid and diploid chromosome number determine gametophytic and sporophytic traits.
6. Chance separation, crossing over and random coming together of chromosomes bring about variations.
7. New species develop due to change in number, form and gene complements of chromosomes.

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Notes,
Prokaryotic Cell and its Organisation

A prokaryotic cell is the one in which the genetic material is not organised into nucleus (Gk. pro - before, karyon-nucleus) and all membrane bound organelles are absent so that there is single envelope system of organisation.
Prokaryotic cells occur in bacteria, archaebacteria, mycoplasma(PPLO), spirochaetes, rickettsiae, chlamydiae and cyanobacteria (or blue green algae).

Cell Size

The average size is 2.0-2.6μm in length and 1.1-1.5μm in width. The smallest prokaryotic cells are those of mycoplasma (0.1- 0.15 μm). The smallest bacterium is Dialister pneumosintes (0.15μm).
Cell size is quite large in a few bacteria and cyanobacteria reaching 500μm (e.g., Spirochaetes, Oscillatoria). Bacterium Epulopscium fishelsoni found in intestine of Brown Surgeon Fish is 600 μm long and 80 μm wide. Marine bacterium Thiomargarita ramibiensis is 750 μm long.
Shape. It is of four types:
(i)Coccus:Spherical or oval. Several types-Micrococcus (single), Diplococcus (in twos), Tetracoccus (in fours), Streptococcus (in chains), Staphylococcus (irregular grape-like cluster), Sarcina (cubical).
(ii)Bacillus: Like a rod or cylinder, e.g., Diplobacillus (in twos), Streptobacillus (in chains), Palisade Bacillus (in stacks).
(iii) Spirillum. Spirally coiled, e.g., Spirochaete, Helicobacter.
(iv) Vibrio. Like a comma or single turn of a coil, e.g., VIbrio chloerae (= V. comma).
Some bacteria have more than one form. They are called pleomorphic, e.g., Rhizobium, Azotobacter.
Flagellation.
(i) Atrichous. No flagella.
(ii)Monotrichous. A single polar flgellum, e.g., VIbrio cholerae.
(iii)Amphitrichous. Two flagella (also two groups of flagella), one at each end. e.g., Alkaligenes faecalis.
(iv) Lophotrichous. A group of flagella at one end only, e.g., Spirillum
(v) Peritrichous. Flagella all around, e.g., Escherichia coli. (Thiamann, 1959, has used the term lophotrichous for an amphitrichous bacterium having two groups of flagella. For a single tuft he used the term cephalotrichous). Gram Positive and Gram Negative: Most bacteria can be stained with basic dyes (eg., methylene blue, crystal violet). Gram stain (Gram, 1884) is staining first with weakly alkaline gentian violet or crytal violet, when all get coloured purple. This is followed by treatment With O.5% iodine, washing with water and then alcohol or acetone. Bacteria having blue or purple colouration are Gram positive ( + ) (e.g., Bacillus subtilis or Hay Bacterium) while the bacteria in which stain is lost are Gram negative (-) (e.g., Escherichia coli). Gram ( + ) bacteria are more succeptible to antibiotics than Gram (-) bacteria. Escherichia coli is commonly found as symbiont or commensal in human intestine. It is used for study because it can be easily cultured. Gram Positive Bacteria: Staphylococcus, Streptococcus, Pneumococcus, Bacillus, Clostridium, Mycobacterium, Streptomyces. Gram Negative Bacteria: Salmonella, Pseudomonas, VIbrio, Helicobacter, Haemophilus, Escherichia. Components of Bacterial Cell A bacterial cell is made of a cell envelope, cytoplasm, nucleoid, plasmids, inclusion bodies, pili and fimbriae. nikkua 1. Cell Envelope. It is covering over protoplast of a bacterial cell. Cell envelope has three parts – mucilage, cell wall and plasmalemma (i) Mucilage Sheath (Glycocalyx): On the outside cell is covered by mucilage of non cellulosic polysaccharides and polypeptides. Thick layer of mucilage (more than 0.2 μm) is called capsule. A loose sheath of mucilage is called slime layer. The term microcapsule is used if it is thin. Mucilage protects the cells against desiccation, toxins, phagocytes and viruses. It provides immunogenecity and virulence. Mucilage also possesses ion exchange capacity in the inner layers. (ii) Cell Wall. Cell wall is elastic, porous and freely permeable to solutes less than 10,000 daltons. Cell wall is a single layered and smooth in Gram ( + ) bacteria. It is two layered and wavy in Gram ( - )bacteria. A sol filled space occurs in between inner and outer layers. The wall contains peptidoglycan (= murein = mucopeptide, 70-80% in Gram (+) bacteria and 10-20% in Gram (-) bacteria), diaminopimelic acid, lipid and protein. Peptidoglycan forms several layers in the wall of Gram ( + ) bacteria and only a single layer in the wall of Gram ( - ) bacteria. It has acetyl glucosamine and acetyl muramic acid with small peptide chains (L-alanine, D-alanine, D-glutamic acid and L-lysine in Gram ( + ) bacteria and diaminopimelic acid in Gram ( - ) bacteria. Antibiotic penicillin and cephalosporin prevent cross linking of acetyl glucosamine and acetyl muramic acid. They are, therefore, used in killing bacterial pathogens. Lysozyme present in tears, saliva and gastric juice has similar effect. It hydrolyses peptidoglycan. The wall of Gram ( + ) bacteria contains teichoic acids (acid polymers having glucose phosphate and alcohol) which act as receptor sites and surface antigens. They also attract chemicals that provide protection from thermal (e.g., Mg) and pH changes. Mycolic acid is present in the wall of actinomycetes ,e.g., Mycobacterium, Nocardia, Corynebacterium. It is long fatty acid which is responsible for acid fast staining reaction. The inner wall layer of Gram ( - ) bacteria has peptidoglycan while the outer one has lipopolysaccharides, proteins and phospholipids. It is known as outer membrane. Endotoxin and antigenic properties reside in it. Porins (protein trimers with central channels) occur in the outer wall layer or outer membrane. They allow passage of low molecular weight hydrophilic substances. Periplasmic space occurs between plasma membrane and cell wall. (iii) Plasmalemma (Plasma Membrane). It is thin selectively permeable quasifluid membrane which forms the innermost component of cell envelope and outer covering of cytoplasm. Plasma membrane has typical lipoprotein structure as visualised by fluid mosaic model. There is a lipid bilayer with extrinsic, intrinsic and transmembrane proteins. The lipid bilayer is stabilised by other pentacyclic sterols called hopanoids instead of cholesterol. Plasma membrane holds semifluid cytoplasmic contents, separates them from surroundings, serves as selectively permeable barrier and having' receptor molecules for detection and responding to chemicals in the surroundings. It contains enzymes for respiration, lipid synthesis, cell wall constituents and photosynthesis. Some ribosomes are also attached to it. Mesosome is an extension of plasmalemma. 2. Cytoplasm. It is granular, crystallo-colloidal complex that fills the cell protoplast but excludes nucleoid. Sap vacuoles, cytoplasmic streaming and membrane bound organelles are absent. (i) Mesosomes. A convoluted multilaminated membranous structure called mesosome is formed by invagination of plasma membrane. It is of two types, septal and lateral. Septal mesosome is in contact with nucleoid. It takes part in separation of replicated nucleoids and septum formation. Lateral or peripheral mesosome is not connected with nucleoid. It is equivalent to mitochondrion and is called chondrioid. Respiratory enzymes occur over it as well as the plasma membrane. (ii) Ribosomes. They are 70 S in nature which measure 20 nm X 14-15 nm. Each ribosome has two subunits, 50S and 30S (showing that S-values or Svedberg units are not proportionate to molecular weight). Ribosomes take part in protein synthesis. 4-8 ribosomes are often found attached to a single mRNA strand for forming copies of the same polypeptide. Ribosome aggregates are called polysomes or polyribosomes. Bacteria have two types of 70 S ribosomes, fixed (attached to plasma membrane) and free (matrix ribosomes). Matrix or free ribosomes synthesise proteins for intracellular use while fixed or plasmamembrane ribosomes form proteins for transport to outside. (iii) Chromatophores. They are pigment containing complexes of photoautotrophic bacteria and cyanobacteria. In cyanobacteria (blue green algae) thylakoid occur in the outer part of cytoplasm called chromoplasm. They bear chlorophyll a and carotenoids. Attached to thylakoids are minute granules called phycobilisomes. They possess phycobilins. In purple bacteria thylakoids or chromatophores can be lamellar, vesicular or tubular. Their membranes bear photosynthetic pigments - bacteriochlorophyll, bacteriophaeophytin (= bacterioviridin) and carotenoids. In green bacteria photosynthetic pigments lie inside chromatophores called chlorosomes. They are covered by non-unit, non-lipid protein membrane. Membranous sacs also occur in nitrifying bacteria. 3. Nucleoid. It is folded single circular strand of DNA double helix which is also called genophore, prochromosome or chromoneme. DNA strand is 250- 700 times the length of the cell. It is folded with the help of non-histone proteins or polyamines and RNA. Prokaryotic DNA is called naked due to its nonassociation with histone proteins and absence of nuclear envelope. Nucleoid lies directly inside the cytoplasm. It is in contact with cell membrane either directly or through mesosome. 4. Plasmids (Hayes and Lederberg, 1952). They are additional or extra-chromosomal small rings of DNA having a few useful but non-vital genes, e.g., F or fertility factor, Nif or nitrogen fixing genes, R-factors/resistance to common antibiotics like chloramphenicol, tetracycline, streptomycin, sulphonamide). Colicinogenic factors (producing colicins/hemolytic chemicals for other bacteria). Some plasmids do not confer any useful trait to bacteria. They are called cryptic plasmids. Plasmids replicate independent of genophore. At times plasmids associate temporarily with nucleoid. They are called episomes (Jacob and Wollmann). Plasmids have these days become an important tool in genetic engineering as vectors of genes. 5. Inclusion Bodies. They are non-living structures present inside the cytoplasm either directly (e.g., cyanophycean, volutin and glycogen granules) or covered by 2-4 nm thick non-unit, non-lipid, protein membrane (e.g., gas vacuoles, carboxysomes, sulphur granules, PHB granules). (i) Gas Vacuoles. They. occur in cyanobacteria, purple bacteria, green bacteria and a few other planktonic bacteria for buoyancy regulation as well as protection from harmful radiations. A gas vacuole is made up of a large number of minute, hexagonal, hollow and cylindrical gas vesicles each surrounded by a ribbed nonunit, nonlipid, protein membrane. The protein membrane is permeable to atmospheric gases but impermeable to water. (ii) Inorganic Inclusions. Granules of inorganic substances occur in many bacteria, e.g., volutin granules, sulphur granules, iron granules, magnetite granules. These granules are also called metachromatic granules because of their ability to pick up different colours with basic dyes. Volutin granules are polyphosphates. They are storage reserve of phosphate. Sulphur granules occur in those bacteria which metabolic H2S for either obtaining hydrogen required in photosynthesis or energy as well as for chemosynthesis. Iron granules occur, similarly, in bacteria that metabolise iron compounds for obtaining energy. Aquaspirillum magnetotacticum contains magneto somes or magnetic particles that help the bacterium in orientating along geomagnetic lines (Balkwill et al, 1980). (iii) Food Reserve. Cyanobacteria possess three types of food reserves - α-granules (= cyanophycean starch), β-granules (= lipid globules) and protein or cyanophycin granules. Common food reserve of bacteria is glycogen. Neutral fats are absent. Instead many bacteria possess poly β- hydroxybutyrate or PRB granules. A biodegradable plastic can be prepared from them. Proteins occurs as protein granules. Carboxysomes are Rubisco containing particles which occur in photosynthetic prokaryotes. 6. Flagella. They occur in some bacteria. Bacterial flagella are unistranded, equivalent to a single microtubular fibre. Diameter is about 20 nm (= O. 02μm). Length is variable, 1- 70 μm. Bacterial flagellum has three parts - basal body, hook and filament. Basal body is rod like basal part of flagellum which occurs in cell envelope and is attached to it by means of two pairs of rings in Gram (-) bacteria and only one pair of rings in Gram (+) bacteria. Hook is curved and thickest part. It is made up of protein units different from those of filament. Filament is long tubular structure of flagellum. Its wall is formed of 3-5 spiral rows of globular protein molecules called flagellin. Bacterial flagellum rotates by 360 degree (Lowey and Spencer, 1968) that brings about backward pushing of water. 7. Pili (singular pilus, Brinton, 1959). They are 1-4 long hollow tubes (18-20 μm long, 30-35 nm diameter) which are also called sex pili or F-pili because they take part in conjugation. Pili develop in Gram (-) bacteria. Similar structures present in Gram (+) bacteria called spinae. Pili are made of a distinct protein called pilin. 8. Fimbriae (Duguid et al, 1955). They are numerous (300-400 per cell), small fibrous outgrowths from the cell surface which reach a length of 0.1-1.5 μm and 3 -10 nm in diameter. Fimbriae help in attaching bacteria to solid surfaces, e.g., Neisseria gonorrhoeae to urinary tract.

6 ↔

SVT1108.01

1. Draw well labeled diagrams of the following
a. Plant cell
b. Animal cell
c. Nucleus
d. Types of chromosomes

2. Write the names of organelles with following features
a. double membrane

Chloroplast and Mitochondrion

b. single membrane

lysosome, Spherosomes

c. No membrane

Centrosome, Ribisome

3. Define totipotency?

3. Each vegetative plant cell has the potential to grow into a whole plant. The term totipotency refers to a plant's ability to reproduce itself.

4. Name two cell organelles that contain their own DNA?

4. In a cell, two organelles that contain their own DNA are mitochondria and the chloroplast.

5. Which cell organelle functions as a "segregation apparatus"?

5. Endoplasmic Reticulum (ER) is the one cell organelle present in the cytoplasm that functions as the segregation apparatus.

6. Which structure is called a little nucleus?

6. The nucleolus is a structure within the cell nucleus that is responsible for ribosome production and assembly.

7. What is the function of a contractile vacuole?

7. The contractile vacuole maintains the osmoregulation aka water balance.

8. Who gave the statement “Omnis cellula cellula"?

8. Rudolf Virchow gave the statement ‘Omnis cellula cellula’ which means new cells are generated from the pre-existing ones.

9. Which organelle is called the engine of the cell?

9. Ribosomes aid in protein synthesis, which is why they are referred to as the cell's engine.

10. What is mycoplasma?

10. Mycoplasma is a prokaryote that lives in an aerobic environment. They don't have a cell wall, but they do have a nucleoid.

11. Expand PPLO.

11. The full form of PPLO is ‘pleuropneumonia-like organisms’.

12. Name the parts of bacterial flagella.

12. A filament, a hook, and a basal body make up the bacterial flagellum.

13. Who first saw and described a live cell?

13. Anton Van Leeuwenhoek was the first to notice a live cell.

14. Which is the largest single cell?

14. The ostrich egg is the world's largest single cell.

15. Who first explained that cells arose from pre-existing cells?

15. Rudolf Virchow was the first person who explained the pre-existence of the cells.

16. Eukaryotic ribosomes are the 80S. What does 'S' stand for.

16. In the 80S eukaryotic ribosome, the S stands for sedimentation coefficient.

SVT021108.2 12 Jun 2023

1. Draw well labeled diagram of Golgi bodies.
2. What are the similarities between Golgi Bodies and Dictyosomes?
3. What is the difference between Golgi Bodies and Dictyosomes?
4. Draw a detailed structure of cilium ( T.S).
5. Mention three types of proteins present in Cilium/ Flagellum.
6. How cilium and flagellum are different?
7. Draw well labeled diagram of Chloroplast and Mitochondrion.
8. Explain proto filaments in cilium.
SVT1108.03

1. What is called quantasome?

Quantasomes are particles found in the thylakoid membrane of chloroplasts in which photosynthesis takes place. They are embedded in a paracrystalline array on the surface of thylakoid discs in chloroplasts. They are composed of lipids and proteins that include various photosynthetic pigments and redox carriers. Proteins, lipids, and a combination of photosynthetic pigments and redox mediators make up quantasomes.

2. What are the different types of chlorophyll?

There are 5 types of chlorophyll found in plants - chlorophyll a, b, c , d and e. Chlorophyll a & b are the most abundant forms of chlorophyll. chlorophylls c and d are found, often with a, in different algae; chlorophyll e is a rare type found in some golden algae; and bacterio-chlorophyll occurs in certain bacteria. Chlorophyll A has s central role in an electron donor in the electron transport chain.

3. What are carotenoids?

Carotenoids are tetraterpene pigments, which exhibit yellow, orange, red and purple colors. Carotenoids are the most widely distributed pigments in nature and are present in photosynthetic bacteria, some species of archaea and fungi, algae, plants, and animals.

4. What are the 2 types of carotenoids?

There are primarily two classes of carotenoids: carotenes and xanthophylls. Carotenes are hydrocarbon carotenoids, and xanthophylls contain oxygen in hydroxyl, methoxyl, carboxyl, keto, or epoxy groups. 1. What is called quantasome? 2. What are the different types of chlorophyll?
3. What are carotenoids?
4. What are the 2 types of carotenoids? 5. What is the importance of a vacuole in a plant cell? 5. The vacuole is a membrane-bound space in the cytoplasm of a plant cell. It contains sap, water, excretory products and other materials not useful for the cell. Vacuoles occupy 90% of the cell volume during osmosis. They maintain the turgor pressure against the cell wall thereby maintaining the shape of the cell and cell fluid balance. 6. What is a satellite chromosome? 6. The chromosomes that have an additional or secondary constriction at the distal part of the arm formed by a chromatin thread are known as satellite chromosomes. These appear as an outgrowth or a small fragment. These are also known as marker chromosomes. The chromosomes 13, 14, 15, 16, 21, and 22 are satellite chromosomes. 7. State the characteristics of prokaryotic cells. 7. The characteristics of a prokaryotic cell are as follows: A prokaryotic cell is surrounded by a cell membrane. Mitochondria, chloroplast and nucleus are not present. The DNA is circular and not associated with basic proteins. The cytoplasm is filled with dense granules most of which are ribosomes. The thylakoids are scattered in the chloroplast, and not placed in the form of stacks. 8. Describe the cell theory in brief. 8. The cell theory is based on the following postulates: The cell is the basic structural and functional unit of life. All living organisms are made up of cells. All cells arise from pre-existing cells. 9. Define tonoplast? 9. Tonoplast, also called as the vacuolar membrane is the cytoplasmic membrane filled with cell sap and functions as a membrane boundary of the vacuole of plant cells 10. What is ATP? 10. ATP – Adenosine triphosphate is the organic molecules, which provide energy for various biochemical processes in the body. Therefore, these molecules are called the energy currency of the cell.
MCQs
https://forms.gle/5xyASqWN59H9Y76e6
021108.001. The part/parts of a cell that can be seen with an electron microscope, but never with a light microscope is / are the:
(A) nucleus (B) golgi bodies (C) chloroplasts (D)membrane separating the nucleus from the cytoplasm.
C
021108.001. 1A is equal to :
(A) 10-8cm (B) 10-4 cm (C) 10-6 cm (D) 10-3 cm.
A 021108.002. Euglena reaction of DNA is due to : (A) aldehyde produced by acid hydrolysis (B) removal of RNA but not DNA (C) phosphoric acid, carbohydrates and nitrogen bases (D)phosphoric acid.
A
021108.003. A human egg is very large as comparedto human sperm. Most of this size differential is due to the difference in their :
(A) nucleus (B) membranes (C) cytoplasm (D) Both (A) and (C).
C
021108.004. The correct order of sedimentation of subcellular structures during differential centrifugationis :
(A)Lysosome Mitochondria Nucleus Ribosome (B) Mitochondria Nucleus Lysosome Ribosome (C) Nucleus Mitochondria LysosomeRibosome (D)Lysosome RibosomesMitochondria Nucleus.
C
021108.005. Leaving aside water, which of the following constitutes the bulk of an active living cell ?
(A)Ribose nucleic acid (B) Deoxyribose nucleic acid (C) Proteins (D)Carbohydrates.
C
021108.006. Match the following columns and select the correct option:
Column-I Column-II (a) Smooth endoplasmic (i) Protein synthesis Reticulum
(b) Rough endoplasmic (ii) Lipid synthesis Reticulum
(c) Golgi complex (iii) Glycosylation
(d) Centriole (iv) Spindle formation
(A)(a)-(ii), (b)-(i), (c)-(iii), (d)-(iv)
(B) (a)-(iii), (b)-(i), (c)-(ii), (d)-(iv)
(C) (a)-(iv), (b)-(ii), (c)-(i), (d)-(iii)
(D) (a)-(i), (b)-(ii), (c)-(Ui), (d)-(iv).
A
021108.007. Which of the following are prokaryotes ?
(A) Viruses and Rickets
(B) Bacteria and Archaebacteria
(C) Cyanobacteria and Mycoplasma
(D)Both (B) and (C).
D
021108.008. Many ribosomes may associate with a single mRNA to form multiple copies of a polypeptide simultaneously. Such strings of ribosomes are termed as
(A) Polysome (B) Polyhedral bodies
(C) Plastidome (D) Nucleosome.
A
021108.009. Which of the following is widely distributed in a cell ? (A)Chromoplasts (B) Chloroplast (C) RNA (D) DNA. C
021108.010. In the nucleonema of nucleus, particles 150--- 200A are seen which resemble :
(A)ribosomes (B) lysosomes (C) mitochondria (D) sphaerosomes. A
SV021108.0112030 Cell https://forms.gle/tor1MaQZDesq5Ejn7 Cell https://docs.google.com/spreadsheets/d/1M--5ezq9O71LE2QyGsR5EPywAUn7Y45pFApqR4xqSFA/edit?resourcekey#gid=1758236251 021108.011 Fundamentally a dead cell differs from a living cell because :
(A)it has become separated from other cells (B) its vital forces have been destroyed (C) a change of its surrounding environment has occurred (D) a change in its specific organization has occurred.
B
021108.012 Plant cell differs from animal cell by :
(A) cell wallabsentin animalcellbut chloroplast present (B) cell wall and chloroplast absent in animal cell (C) vacuoles only few and that too contractile;are present in plant cell (D)cell wall present in animal cell.
B
4. The organic molecules present in traces in living cells can be detected and isolated by :
(A) Centrifugation (B) Tracer technique (C) Chromatography (D) Microscopy.
C
021108.013. The cell theory was proposed by :
(A)Robert Hooke (B) Leuwenhoek (C) Schleiden and Schwann (D)Purkinje.
C
021108.014. Which of the following sets resemble in their basic structure and function :
(A)Centrioles, cilia and flagella (B) DNA, mRNA and tRNA (C) ER, Golgi complex and lysosome (D)Leucoplast, chloroplast and chromoplasts.
A
021108.015. How many membranes comprise the nuclear envelope?
(A)One (B) Two (C) Three (D) None.
B
021108.016. Which of the following biomolecules regularly moves from nucleus to cytoplasm ?
(A) Glycogen (B) Cholesterol (C) RNA (D) DNA.
C
021108.017. Which of the following cellular organelles breaks down complex macromolecules such as polysaccharides and proteins?
(A) Golgi complex (B) Lysosome (C) Mitochondria (D)Rough endoplasmic reticulum.
B
021108.018. Which one of the following does not lose living nature even after crystallization ?
(A) Protista (B) Bacteria (C) Viruses (D) Parazoa.
C
021108.019. In prokaryotic cells, the enzymes involved In the oxidation of metabolites are associated with :
(A)nucleoid (B) plasma membrane (C) ribosomes (D) plasmosome.
B
021108.020. Which of the following is an exception to cell theory ? (A)Bacteria (B) Protozoans (C) Protista (D) Viruses.
D
021108.021. Which of the following relationships between cell structure and their respective function is not correct?
(A) Cell wall - Support, protection (B) Cilia - Site for diffusion (C) Chromosome - Carrier of heredity material (D)Mitochondria - Power house of cell.
B 021108.022. Nucleoli are rich in : (A)ribose nucleic acid
(B) deoxyribose nucleic acid (C) proteins and RNA (D) carbohydrates.
C
021108.023. The size of Pleuropneumonia - like Organism (PPLO) is :
(A) 0.02 ,um (B) 1-2,um (C) 10-20,um (D) 0.1 ,um.
B
021108.024. The activities of all living cells are controlled by :
(A) chloroplasts (B) auxins (C) nucleus (D) tonoplast.
C 021108.025. Which one of the following is incorrect ?
(A)All cells do not contain a true nucleus (B) All living plant cells contain chlorophyll (C) Cell walls are generally made up of cellulose (D)Respiration occurs in mitochondria.
B
021108.026. Endoplasmic reticulum often contains :
(A) ribosomes (B) golgi bodies (C) centrioles (D) lysosomes.
A
021108.027. The fine network of membrane distributed extensively throughout the cytoplasm in a cell is referred to as :
(A)golgi bodies (B) peroxisome (C) lysosome (D) endoplasmic reticulum.
D
021108.028. The endoplasmic reticulum occurs in the form of:
(A)cisternae only (B) vesicles only (C) tubules only (D) All the above.
D
021108.029. The principal site of the synthesis of ribosomal RNA is the:
(A) mitochondria (B) golgi bodies (C) nucleolus (D) lysosomes.
C
021108.030. During active protein synthesis, some ribosomes seemto occurin groupsand are collectivel yknown as :
(A)bound ribosomes (B) polyribosomes (C) lysosomes (D) dictyosomes.
B
1. Draw the following (Fig 8.1) Red blood cells (round and biconcave), White blood cells (amoeboid), Columnar epithelial cells (long and narrow) Nerve cell (Branched and long), A tracheid (elongated) and Mesophyll cells (round and oval) 2. Write shapes of the following cells- Red blood cells, White blood cells, Columnar epithelial cells, Nerve cell, A tracheid and Mesophyll cells 3. Draw the following giving the size of the cells. (Fig 8.2) A typical eukaryotic cell (10-20 mm), Typical bacteria (1-2 µm), PPLO (about 0.1 mm) and Viruses (0.02-0.2 mm). 4. Write size of the following cells. Mycoplasmas, largest isolated single cell of ostrich and human red blood cells 5. Give names of the groups represented by Prokaryotes. 6. Which are four basic shapes of bacteria. 7. Which group of prokaryotes lack a cell wall? 8. Fill in the blanks (Prokaryotes) a. The fluid matrix filling the cell is the ........ There is no well-defined ......... -The genetic material is basically ........, not enveloped by a ......... -In addition to the genomic DNA (the single chromosome/circular DNA), many bacteria have small circular DNA outside the genomic DNA. These smaller DNA are called ......... -The plasmid DNA confers certain unique phenotypic characters to such bacteria. One such character is resistance to ......... ........ DNA is used to monitor bacterial transformation with foreign DNA. 9. Name the organelles found in prokaryotic cells. 10. Name three layers of cell envelope found in prokaryotes. 11. What are mesosomes? 12. What are gram positive and gram negative bacteria. 13. Which layer of cell envelope in bacteria provides a strong structural support to prevent the bacterium from bursting or collapsing. 14. Write three functions of mesosome. 15. Which are three parts of bacterial flagellum? 16. Give names of two surface structures besides flagellum of the bacteria which do not play a role in motility. Also give their structure and functions. 17. Explain the following about Ribosomes- a. size b. names of sub units c. functions 18. Name three inclusion bodies in prokaryotes. 19. Name any four groups of Eukaryotes. 20. Write names of various organelles found in Eukaryotes. 21. Write three differences between Plant and animal cells. 22. Which organelle is present in animal cells but is absent in almost all plant cells? 23. Fill in the blanks Cell membrane is mainly composed of ........... and lipids. The major lipids are ........... that are arranged in a ............ Lipids are arranged within the membrane with the ........... head towards the ........... sides and the ........... tails towards the ........... part. This ensures that the nonpolar tail of saturated hydrocarbons is protected from the ........... (Figure). The lipid component of the membrane mainly consists of ............ -The ratio of protein and lipid varies considerably in different cell types. In human beings, the membrane of the erythrocyte has approximately ...........per cent protein and ...........per cent lipids. 24. Depending on the ease of extraction, name two proteins present in the cell membrane. 25. Write any three important functions of the plasma membrane. 26. Name the type of transport process taking place in the following- a. Many molecules can move briefly across the membrane without any requirement of energy. b. movement of Water c. polar molecules 27. Write three functions of Cell Wall. 28. Which layer glues two neighboring cells? Also give its composition. 29. What is endomembrane system? Write its three components. Also write names of three organelles which do not form part of endomembrane system. 30. What are Endoplasmic Reticulum (ER). Give names of its two types with their functions. 31. Where lipid-like steroidal hormones are secreted in animal cells? 32. Write following about Golgi apparatus a. Name of discoverer b. Structure c. Size and shape d. cis and trans faces e. why golgi apparatus remains in close association with ER? 33. Write following about Lysosomes a. Number of membranes present b. names of three hydrolases c. functions 33. Write following about Mitochondria. a. size b. name the dense homogeneous substance present in inner compartment c. cristae d. Type of DNA 34. Write following about plastids a. names of pigments in chloroplasts b. names of pigments in chromoplasts c. Fill in the blanks Amyloplasts store ............ (starch), elaioplasts store ............ and ............ whereas the aleuroplasts store ............. Majority of the chloroplasts of the green plants are found in the ............ cells of the leaves. d. size and shape of chloroplasts e. structure of granum f. structure of ribosomes present in chloroplast 35. Who gave the name ribosomes? 36. Name the units of ribosomes of prokaryotic and eukaryotic cells.