NEET Biology Cell The Unit of Life Notes

Cell: The Unit of Life is the foundation chapter of NEET Biology. These notes cover the cell theory, an overview of the cell and its outer envelope, cell size and shape, prokaryotic and eukaryotic cells, the cell wall and plasma membrane (including the fluid mosaic model), protoplasm, the endomembrane system (endoplasmic reticulum, Golgi complex, lysosomes and vacuoles), ribosomes, the cytoskeleton, cilia and flagella, centrosomes and centrioles, microbodies, the nucleus and chromosomes, plastids and mitochondria.

While going through the chapter pay special attention to the following

Terms: (i) Nucleoid (ii) Mesosome (iii) Heterophagosome (iv) Telolysosome (v) Subunit of Parson (vi) Plastidome (vii) Plasmodesmata (viii) Nucleosome (ix) Diplosome (x) Polytene chromosome (xi) Lampbrush chromosome (xii) Chromatophores

Differences: (1) Cytology and cell biology (2) Nucleus and nucleoid (3) Gram positive and Gram negative bacteria (4) Centrosome and centriole

At a Glance

• Cell Theory
• Prokaryotic cell
  • Cell wall and its modification
  • Ribosomes and Inclusion Bodies
  • Genetic material
• Eukaryotic cell
  • Cell wall
  • Cell membrane
  • Endoplasmic reticulum
  • Golgi apparatus
  • Lysosomes
  • Ribosomes
  • Vacuoles
  • Nucleus
  • Plastids
  • Mitochondria
  • Cytoskeleton

Topics Covered

1. Cytology, cell biology and microscopy
2. Cell theory and exceptions
3. An overview of the cell
4. Size and shape of cells
5. Prokaryotic cells
6. Eukaryotic cells and the cell wall
7. Cell membrane and the fluid mosaic model
8. Protoplasm
9. Endomembrane system: endoplasmic reticulum, Golgi complex, lysosomes and vacuoles
10. Ribosomes and the cytoskeleton
11. Cilia and flagella, centrosome and centrioles, microbodies
12. Nucleus, chromatin and chromosomes
13. Plastids and mitochondria

INTRODUCTION TO CELLS

All living organisms are made up of structural and functional units called cells. Cytology is the study of the form and structure of cells and their organelles with the help of a microscope. Today, cytology has been largely replaced by cell biology, the branch of biology that studies the structure, biochemistry, physiology, reproduction, evolution and genetics of cells. Many biochemical techniques are used in cell biology.

Robert Hooke (1665) coined the term 'cell' when he saw dead cells of cork. Anton von Leeuwenhoek was the first to see and describe a live cell. Robert Brown later discovered the nucleus.

Some organisms are unicellular (made of a single cell), while others, like humans, are multicellular. All life begins as a single cell. Cells are grouped into tissues, tissues into organs and organs into organ systems, which results in division of labour. All cells interact and cooperate with each other and remain partly dependent on each other. Every cell has its own life span, and old and worn-out cells are continuously replaced by new cells.

Microscopes

Microscopes are instruments that magnify small things. Resolving power is the ability to distinguish two closely placed objects as separate. The resolving power of the naked eye is 100 µm, of a compound microscope 0.25 to 0.3 µm, and of an electron microscope 2 Å.

- Compound or light microscope: it has a reflector, an objective lens and an eyepiece. Its resolving power depends on the wavelength of light and the numerical aperture of the objective lens. Numerical aperture is the product of the refractive index of the medium (n) and the sine of the angle subtended by the optical axis and the outer ray (sin θ).
- Electron microscope: a large instrument with an internal vacuum, a high voltage (50,000 to 100,000 volts), a beam of electrons, a tungsten cathode filament, electromagnets for focusing, and a fluorescent screen or photographic plate for observation. It has very high resolving power and magnification. Detailed structures of cell organelles can be studied only with an electron microscope.

CELL THEORY

Schleiden and Schwann (1855) proposed the cell theory to explain the cellular nature of living organisms. However, they did not explain how new cells are formed. Rudolf Virchow (1858) made the generalisation omnis cellula e cellula, meaning that all cells come from pre-existing cells.

The three fundamental facts of the cell theory are:
(i) All living organisms (animals, plants and microbes) are made up of cells and cell products. The cell is the structural unit of life.
(ii) All metabolic reactions in unicellular and multicellular organisms take place in cells. The cell is the functional unit of life.
(iii) Cells arise only from pre-existing cells. No cell can originate spontaneously or de novo.

Exceptions to the cell theory

- Viruses are acellular. They have no cellular components but possess genetic material and show multiplication.
- In some organisms the body is not differentiated into cells, though it may have many nuclei (coenocytes, e.g., Rhizopus).
- Some protozoans and thallophytes have a uninucleate body that cannot be divided into cells (e.g., Acetabularia). They are acellular.
- Bacteria and cyanobacteria have no nucleus and no membrane-bound organelles.
- RBCs and sieve tube cells are cells without a nucleus.
- Protoplasm is replaced by non-living materials in surface cells of skin and cork.

Cell theory is one of the fundamental generalisations of biology.

AN OVERVIEW OF THE CELL

A typical cell has three major parts: an outer envelope, genetic material and cytoplasm.

- Outer envelope: the cell is surrounded by an outer membrane called the plasma membrane or plasmalemma, which isolates the cell interior. Plant cells also have a cell wall on the outer side of the plasma membrane. The cell wall gives protection, rigidity and shape.
- Genetic material: this is the hereditary material, DNA, which controls the functioning of the cell and carries the information for forming the whole organism. In eukaryotes it is enclosed inside the nucleus as chromatin, which appears as chromosomes during cell division. In prokaryotes it lies freely in the cytoplasm as a coiled structure called the nucleoid.
- Cytoplasm: a semi-fluid matrix that occupies the interior of the cell between the nuclear region and the outer envelope. It is the site of major cellular activities. Special cytoplasmic structures called organelles carry out specific functions. Organelles are of three types:
(i) Membraneless: ribosomes, centrioles
(ii) Single membrane: endoplasmic reticulum, Golgi complex, lysosomes, microbodies, sphaerosomes
(iii) Double membrane: mitochondria, plastids (in plant cells)

SIZE AND SHAPE OF CELLS

Cells differ greatly in size, shape and activity. Mycoplasma, the smallest cell, is about 0.3 µm in length, while bacteria are 3 to 5 µm. The largest isolated single cell is the egg of an ostrich. Acetabularia, a unicellular green alga, is about 10 cm long, and the alga Caulerpa may be up to one metre long. Among multicellular organisms, human red blood cells are about 7.0 µm in diameter, and nerve fibres are the longest, up to 90 cm to a few metres.

Check Point: Fill in the blanks

1. In prokaryotic cells, ______ chromosome is present in the center of the cell in a region called ______.
2. ______ and ______ are membrane-less organelles.
3. ______ is the smallest cell.
4. Active cells have ______ surface volume ratio.
5. WBCs perform ______.

The upper limit of cell size or volume is determined by:
(i) Metabolic activity: metabolically active cells are small, while less active cells are large (for example, the sperm is active and the egg is passive).
(ii) Nucleocytoplasmic ratio: the nucleus controls the metabolic activities of the cytoplasm, and a higher nucleocytoplasmic ratio provides more efficient metabolic working.
(iii) Surface to volume ratio: active cells have a higher surface to volume ratio. This occurs in small cells, elongated cells and cells with surface infoldings or ingrowths such as microvilli of absorptive cells.

Cells also vary in shape. They may be disc-like, polygonal, columnar, cuboid, thread-like or irregular. Shape is related to function:
- RBCs are biconcave so they can pass through capillaries and carry O₂.
- WBCs are irregular to perform phagocytosis.
- Nerve cells are long to conduct impulses.
- Sperms have a tail for motility.

There are two basic types of cells: prokaryotic and eukaryotic. They differ in the organisation of biomembranes, the variety of cytoplasmic organelles and the complexity of nuclear material.

PROKARYOTIC CELLS

Prokaryotic cells are the most primitive cells. A single membrane surrounds the cell, and there is no membrane-bound nucleus. They lack membrane-bound organelles such as plastids and mitochondria, and they lack advanced (9 + 2) flagella. They reproduce asexually by transverse binary fission or budding.

Prokaryotes are bacteria, cyanobacteria (blue-green algae), Mycoplasma and PPLO (pleuro-pneumonia like organisms).

Bacteria are unicellular organisms with prokaryotic organisation. Anton van Leeuwenhoek first discovered them (1676) in pond water and tartar of teeth. They are among the simplest living organisms, reproduce by fission and are mostly pathogenic to human beings. They are found almost everywhere, including hot springs, beneath icebergs, in the human intestine, deep in soil and in sea water.

Bacteriology is the study of bacteria. Robert Koch (1843 to 1910) is known as the father of bacteriology. Bacteria were earlier called schizomycetes or fission fungi.

Size and shape

Prokaryotic cells range from 1 to 10 µm in diameter. The smallest bacterium is Dialister pneumosintes (0.15 to 0.3 µm in diameter), and the largest is Spirillum volutans (500 µm). The four basic shapes of bacteria are:
- coccus (spherical)
- bacillus (rod-shaped)
- vibrio (comma-shaped)
- spirillum (spiral-shaped)

Some bacteria change their form during their life cycle and are called pleomorphic.

Outer coverings

A bacterial cell is covered by (A) an outer slime layer or capsule, (B) a middle cell wall and (C) an inner cell membrane.

- Slime layer or capsule: a viscous, gelatinous sheath outside the cell wall of most bacteria. When its components are compactly arranged, it is called a capsule. It protects the cell. Capsulated bacteria are usually non-flagellated. Capsulation is genetically fixed. Capsulated bacteria form smooth colonies (S-type) and are highly virulent. Non-capsulated bacteria form rough colonies (R-type).
- Cell wall: a strong, rigid layer made of polysaccharides, proteins and lipids. It protects the cell, prevents rupture due to high osmotic and turgor pressure, and contains two sugar derivatives not found anywhere else in the biological world: NAG (N-acetyl glucosamine) and NAM (N-acetyl muramic acid).
- Gram staining: the Gram stain, developed by C. Gram in 1884, separates bacteria into two classes.

Gram-positive bacteria take up the Gram stain (for example Diplococcus pneumoniae, Pneumococcus, Streptococcus, Bacillus, Clostridium, Mycobacterium and Streptomyces). Their cell wall is thick, homogeneous and single layered, contains 60 to 90 percent peptidoglycan (the murein network), and has almost no lipids. It contains teichoic acid in its outer layer. These bacteria are destroyed by penicillin.

Gram-negative bacteria do not take up the Gram stain (for example Escherichia coli, Salmonella, Pseudomonas, Haemophilus, Helicobacter, Vibrio and Rhizobium). Their cell wall is thin, less homogeneous and multi-layered, contains about 10 percent peptidoglycan, and has an outer layer of lipopolysaccharides and protein-polysaccharide complexes. The wall is more complex, and it is not destroyed by penicillin.

In both gram-positive and gram-negative bacteria, murein (a constituent of the cell wall) can be dissolved by lysozyme.

Gas vacuoles (pseudovacuoles or air vacuoles) occur in prokaryotes. A gas vacuole is a group of many submicroscopic polygonal gas vesicles, each surrounded by an extra-thin protein membrane. Gas vacuoles store metabolic gases, provide mechanical strength, regulate buoyancy and reduce the intensity of harmful radiation.

Plasma membrane

The plasma membrane of prokaryotes is semi-permeable and interacts with the outside world. It is structurally similar to the membrane of eukaryotes.

Fig. 8.1 A prokaryote cell of bacteria

Ribosomes and inclusion bodies

The cytoplasm of bacteria lacks most membrane-bound organelles such as endoplasmic reticulum, Golgi complex, mitochondria, lysosomes and centrioles.

- Ribosomes: bacterial ribosomes are 70S and lie freely in the cytoplasm. They are made of two subunits, 50S and 30S. Several ribosomes can attach to one mRNA to form a chain called a polyribosome or polysome.
- Mesosomes: in some bacteria (e.g., Bacillus subtilis) the plasma membrane folds inwards to form mesosomes. They have respiratory and secretory functions, help separate the two daughter nucleoids, and help form the new cell plate during cell division.
- Chromatophores: in some prokaryotes, such as cyanobacteria, membranous extensions called chromatophores carry pigments. Bacteriochlorophyll in them differs somewhat from eukaryotic chlorophyll.
- Vacuoles: there are no true fluid-filled vacuoles.
- Inclusion bodies: reserve materials are stored as inclusion bodies. They are not bounded by a membrane and lie free in the cytoplasm, for example phosphate granules, cyanophycean granules and glycogen granules.

Genetic material of prokaryotes

- A true nucleus with nuclear envelope, nucleolus and histone proteins is absent. The genetic material is a single naked, double-stranded, circular, ring-like, highly folded and supercoiled DNA with no free ends. This nucleus without a nuclear envelope is called a nucleoid. It is also called a nuclear body, prochromosome, chromatin body, prokaryotic chromosome, genophore, or incipient nucleus.
- Prokaryotic cells are haploid and contain much less DNA.
- Many prokaryotes have extra-chromosomal, circular DNA molecules called plasmids. Plasmid DNA carries traits such as fertility factors, nif genes, resistance factors and colicinogenic factors. Plasmids that temporarily associate with the nucleoid are called episomes. Plasmid DNA is used as a vector in genetic engineering.
- Respiration may be aerobic or anaerobic. Nutrition may be autotrophic or heterotrophic.
- Locomotion occurs by flagella that protrude from an intracellular basal granule through the plasma membrane and cell wall. A bacterial flagellum has three parts: filament (the longest part, projecting outside the cell), hook and basal body.
- Many gram-negative bacteria have short, tubular, non-motile structures on the cell wall called pili and fimbriae. Fimbriae are small bristle-like fibres. Pili are elongated tubular structures made of a special protein. Pili that take part in conjugation are called sex pili. In some pathogenic strains, pili help the bacteria recognise and attach to the host, and to rocks in streams.
- No mitotic apparatus is formed during cell division. Transcription and translation both occur in the cytoplasm, and mRNA does not need processing.

Bacteria classified by respiration

(1) Obligate aerobes respire only aerobically and die without oxygen, e.g., Bacillus subtilis.
(2) Obligate anaerobes respire only anaerobically and cannot tolerate oxygen, e.g., Clostridium acetobutylicum.
(3) Facultative anaerobes are normally aerobic but switch to anaerobic respiration when oxygen is not available, e.g., Clostridium tetani.
(4) Facultative aerobes are normally anaerobic but can respire aerobically in the presence of oxygen, e.g., purple non-sulphur bacteria like Rhodobacter.
(5) Aerotolerant anaerobes continue anaerobic respiration or fermentation even in the presence of oxygen, e.g., lactic acid bacteria.
(6) Anaerotolerant aerobes respire aerobically even without free oxygen, using oxygen from oxidised salts such as nitrate, e.g., denitrifying bacteria.

EUKARYOTIC CELLS

Eukaryotic cells are characterised by extensive compartmentalisation of the cytoplasm through the presence of membrane-bound organelles, and a true nucleus. They are found in all protists, fungi, plants and animals. A eukaryotic cell is made up of a plasma membrane, cytoplasm with its organelles (mitochondria, endoplasmic reticulum, Golgi complex, etc.), and a true nucleus.

Plant and animal cells differ. Plant cells have a cell wall, plastids and a large central vacuole, which animal cells lack. Animal cells have centrioles, which are absent in almost all plant cells.

[Figure 8.2: Eukaryotic cell]

A eukaryotic cell has three main components: (A) cell wall and plasma membrane, (B) cytoplasm and (C) nucleus.

Cell wall

The cell wall is a non-living, semi-rigid, external protective covering. Plant cells have a cell wall, while animal cells lack one. The cell wall is made of cellulose secreted by the cell itself. Primary and secondary walls are mainly cellulose polysaccharide. Lignin and suberin may be present in the secondary wall.

The cell wall has two parts: microfibrils (cellulose and rarely chitin) and a ground substance or matrix. It has three layers:

- Middle lamella: the outermost layer, which acts as a cement between adjacent cells. It is mainly calcium pectate and is highly lignified in woody tissue.
- Primary wall: thin, single layered and the first wall formed. It lies on both sides of the middle lamella, can grow and extend, and contains pectic substances, hemicellulose (glucose, xylulose, mannose, glucuronic acid), lipids and a loose network of cellulose microfibrils.
- Secondary wall: very thick, rigid and loses the capacity to grow. It is formed in mature cells and has three layers, S1, S2 and S3. S2 is the thickest and S3 the thinnest. It is mainly hemicellulose and cellulose. Its cellulose microfibrils are embedded in a matrix of hemicellulose, lignin and pectin.
- Tertiary wall: present in the tracheids of gymnosperms. It is purely cellulosic.

Plasmodesmata are cytoplasmic bridges that connect neighbouring cells through pits. They are characteristic of multicellular plants and let solutes move over considerable distances without crossing differentially permeable membranes. Together they form a living system called the symplast.

Functions of the cell wall

- Gives mechanical strength.
- Acts as a barrier against viral, bacterial and fungal pathogens.
- Helps in transport of materials and metabolites into and out of the cell.
- Withstands the turgor pressure that develops within a cell due to high osmotic pressure.

Deposits in the cell wall

- Lignin: a polysaccharide deposited mostly in xylem cells, making them hard and lignified. Xylem tracheids take different forms due to lignin deposition, such as annular, spiral and scalariform thickenings.
- Suberin: a derivative of fatty acids deposited on cork cells. It is impermeable to water.
- Cutin: a wax-like fatty substance deposited on epidermal cells as a cuticle, which reduces water loss. The cuticle is very thick in xerophytes, thin in mesophytes and absent in hydrophytes.
- Silica: deposited in some plants, giving a rough touch, e.g., Equisetum and Saccharum munja.
- Mucilage: secreted by some cells, making them slippery, e.g., blue-green algae. These cells can withstand extremes of temperature.
- Calcium oxalate and calcium carbonate crystals: deposited in the cell wall of members of Cucurbitaceae and Moraceae.

The cell wall varies in thickness from 0.1 µm to 10 µm in different cells. In most cells the wall is present from the very beginning. In slime fungi (myxomycetes) it is present only for a very short time. In fungi, the cell wall is made of chitin, a polymer of N-acetyl glucosamine.

Lomasomes are vesicular and membranous structures found between the cell wall and plasmalemma of plant cells, mainly in fungi. Their exact function is not known, but they probably help in cell wall formation. In fleshy fruits during ripening, proteolytic enzymes dissolve the pectic substances of the middle lamella. The cells loosen and the fruit becomes soft.

CELL MEMBRANE

The term cell membrane was coined by Nageli and Cramer (1855). C.B. Plowe (1931) gave the term plasma membrane or plasmalemma. The cell membrane is a living, thin, elastic and semi-permeable membrane covering plant and animal cells. It is also called the biological membrane or biomembrane.

Plasmalemma occurs in both prokaryotes and eukaryotes. Intracellular biomembranes are found in eukaryotes around most organelles and inside some of them, for example the thylakoids of chloroplasts. The endoplasmic reticulum is also lined by membranes. Some membranes have special names: the tonoplast surrounds the vacuole and the nuclear envelope surrounds the nucleus. A double membrane surrounds the nucleus, mitochondria and plastids.

The membrane is composed of lipids arranged in a bilayer, with the polar heads facing outward and the hydrophobic tails facing inward. This keeps the non-polar tails of saturated hydrocarbons away from the watery environment. The lipid component mainly consists of phosphoglycerides. Membranes also contain proteins and carbohydrates. The ratio of protein to lipid varies among cell types. In human erythrocytes, the membrane has about 52 percent protein and 40 percent lipid. The membrane is about 75 to 100 Å thick.

Unit membrane model

Robertson (1959) proposed the unit membrane model. All biomembranes from prokaryotes to eukaryotes are assumed to have a similar structure. The unit membrane is 75 Å thick and consists of a bimolecular lipid leaflet sandwiched between outer and inner layers of protein arranged in a beta-pleated sheet configuration (protein-phospholipid-phospholipid-protein). Its drawback is that it does not explain the permeability and elasticity of the plasma membrane.

Fluid mosaic model

S.J. Singer and G. Nicolson (1972) proposed the most accepted model of membrane structure. The lipid bilayer is semi-fluid, and proteins are dispersed in it, giving a mosaic appearance. The components (lipids, proteins and oligosaccharides) are held in place by non-covalent interactions.

- Fluidity is due to phospholipids rich in unsaturated fatty acids. Phospholipids form the main component of the membrane and provide its structural framework.
- Lipids are amphipathic, with polar hydrophilic heads and non-polar hydrophobic tails. Lipids with sugar chains on their polar heads are called glycolipids.
- Cholesterol makes eukaryotic membranes more stable, because it is more rigid than phospholipids. Prokaryotic membranes lack cholesterol, although some bacteria contain pentacyclic sterol-like molecules called hopanoids, which stabilise the bacterial membrane.
- Two kinds of proteins are present. Peripheral (extrinsic) proteins are loosely attached to the polar heads on either surface. They can move freely in the membrane, act as carriers for transport, and make up 20 to 30 percent of membrane proteins. Integral (intrinsic) proteins are tightly held in the phospholipids. Small ones partly project from one or both surfaces, while larger ones are embedded through the whole thickness. Intrinsic proteins form channels that allow water-soluble materials to pass quickly. They make up about 70 percent of membrane proteins.

[Figure 8.3: Fluid mosaic model of the cell membrane]

Fig. 8.3 Fluid-mosaic model of cell membrane

Functions of the membrane

The membrane is selectively permeable. Some molecules move across it without energy, which is called passive transport.

Check Point: Name the following organelle. (i) Ribosomal factory (ii) Power house of the cell (iii) Scavenger of cell (iv) Kitchen of the cell

Check Point: State True or False.

1. Acetabularia is unicellular organism.
2. Golgi complex is double membrane organelle.
3. Flagella in prokaryotes has (9+0) arrangement while those in eukaryotes has (9+2) arrangement.
4. RBCs and sieve tube cells are without nucleus.
5. Unicellular prokaryotic cells are more advanced.

- Simple diffusion: neutral molecules move along a concentration gradient, from higher to lower concentration. Movement of water by diffusion is called osmosis.
- Facilitated transport: polar molecules cannot pass the non-polar lipid bilayer, so they need carrier proteins to cross the membrane.
- Active transport: some ions or molecules move against their concentration gradient, from lower to higher concentration. This is energy dependent and uses ATP, for example the Na⁺/K⁺ pump.

PROTOPLASM

Protoplasm is present in all living cells and performs all vital functions. J. Huxley called it the 'physical basis of life'. Dujardin (1835) discovered it and named it 'sarcode', and Purkinje (1837) renamed it 'protoplasm'.

Protoplasm is a complex, granular, elastic, viscous and colourless substance. It is a polyphasic colloidal system:
- Water is its main constituent. Glucose, minerals and other soluble substances dissolve in water to form a true solution, which acts as the dispersion medium.
- Larger insoluble organic compounds, such as fatty acids, proteins and some carbohydrates, are suspended as colloidal particles, which form the dispersed phase.
- Enzymes and other colloidal particles give a large surface area for chemical reactions and show adsorption. Large protein molecules are hydrophilic and adsorb water.
- Protoplasm shows sol-gel interconversion.
- Some proteins coagulate at about 60 degrees Celsius or above, which kills the cell.
- Its exact chemical composition cannot be determined, because it changes continuously.

Approximate composition of plant protoplasm:

- Water: about 90 percent
- Proteins: 7.2 percent
- Carbohydrates: 2.0 percent
- Fats: 1.0 percent
- Minerals: 1.0 percent
- DNA and RNA: in traces

Water content is highest in hydrophytes (95 percent) and lowest in seeds and spores (10 to 15 percent). In animals, water is about 65 percent and proteins about 15 percent.

Inorganic matter is found in ionic form. Main cations are calcium, potassium, magnesium and iron. Main anions are nitrates, sulphates, bicarbonates and chlorides. Glucose, fructose and sucrose are the main carbohydrates and provide energy. Lipids release energy and form part of membranes. Proteins form about 64 percent of the total solids and most enzymes are proteins. Pigments, latex, vitamins, growth regulators, enzymes and alkaloids are also present.

The pH of protoplasm varies from 5.2 to 8.5 among cells, but the pH of a normal cell is considered to be 7.

ENDOMEMBRANE SYSTEM

Several membranous organelles work in a coordinated way and are together called the endomembrane system. It includes the endoplasmic reticulum, Golgi complex, lysosomes and vacuoles.

GERL: a complex of Golgi apparatus, endoplasmic reticulum and lysosome.

Endoplasmic Reticulum (ER)

Garnier (1897) first observed the ER and named it ergastoplasm. Porter and Kallman (1952) coined the term endoplasmic reticulum. ER is also called the endoskeleton of the cell or the endomembranous system. It is found in all eukaryotic cells.

The ER is bounded by a single unit membrane, which is believed to arise by infolding of the plasma membrane. It is continuous with the nuclear membrane and the plasma membrane. ER is a network of interconnected tubules and vesicles that divides the intracellular space into two compartments: the luminal compartment (inside the ER) and the extra-luminal compartment (cytoplasm).

The ER has three forms:
- Cisternae: long, flattened, unbranched units arranged in stacks, 40 to 50 nm thick.
- Tubules: long, isolated, branched units, 50 to 200 nm thick.
- Vesicles: 25 to 500 nm in diameter.

[Figure 8.4: Endoplasmic reticulum]

[Figure 8.5: Endoplasmic reticulum showing cisternae, vesicles and tubules]

In ER, the cisternae, tubules and vesicles are isolated in the cytoplasm and do not form a complex as in the Golgi body.

Types of ER

- Smooth or agranular ER: has no ribosomes on its surface. It is well developed in cells that synthesise and secrete steroid hormones, and it is involved in lipid synthesis. It is present in liver cells that store glycogen, intestinal cells, adipose cells, spermatocytes, leucocytes and muscle cells. In muscle cells it is called sarcoplasmic reticulum.
- Rough or granular ER: has ribosomes attached to its surface by their larger subunits. Cells that are active in protein synthesis have abundant RER.

Functions of ER

- Mechanical support: ER forms the framework of the cytoplasmic matrix.
- Intracellular exchange: ER acts as an intracellular circulatory system and moves substances into and out of the cell through membrane flow.
- Lipid secretion: lipids are synthesised by SER, stored in the Golgi complex, and then released into the cytoplasm as lipid droplets and sent out of the cell. The major lipids made by SER are phospholipids and cholesterol.
- Detoxification: SER detoxifies drugs and steroids.
- Formation of nuclear membrane: fragmented vesicles of the disintegrated nuclear membrane and ER elements arrange around chromosomes to form a new nuclear membrane.

Check Point: Fill in the blanks with suitable word/words

1. The longest cell of human body is ______.
2. Movement of ______ across the selective permeable membrane is osmosis.
3. Chromatophores containing ______ found in ______.
4. ______ can dissolve the bacterial cell wall.
5. The thickness of biological membrane is ______.

Golgi Apparatus (Golgi Complex)

The Golgi apparatus is a stack of flattened, membrane-bound, parallel organelles that lie close to the ER in the cytoplasm. Camillo Golgi (1898) first detected these structures as densely stained reticular bodies near the nucleus. In plants, Golgi bodies are called dictyosomes because of their net-like appearance. Each Golgi complex consists of lamellae, tubules, vesicles and cisternae 0.5 to 1.0 µm in diameter. Their membranes are made of lipoproteins and are thought to originate from the ER membrane.

Enzymes found in the Golgi body include transferase, adenosine diphosphatase, thiamine phosphatase and glucose-6-phosphatase. Thiamine phosphatase is the marker enzyme of the Golgi body. The Golgi body is present in all eukaryotic cells but absent in mature mammalian RBCs, sieve tubes and bryophytes.

Structure of the Golgi body

- Cisternae (saccules): the most conspicuous part. They are plate-like, unbranched, slightly concave membranous structures lying in parallel arrays. An animal cell usually has 3 to 8 cisternae in a stack, and a plant cell has 10 to 20. Fungi have a mono-cisternal Golgi body. Each cisterna has a convex forming face and a concave maturing face. The two faces are different but interconnected.
- Tubules: parallel, fibrous intercisternal elements, about 70 to 80 Å in diameter, that connect the cisternae.
- Vesicles: spherical structures, about 600 Å in diameter, that arise by budding from tubules and are filled with secretory material.

The area around the Golgi body that lacks ribosomes is called the 'zone of exclusion'.

Biogenesis of Golgi apparatus: the nucleus is essential for maintenance and formation of the Golgi apparatus. In all probability, a new Golgi apparatus develops from division of the pre-existing one. Other possibilities are invagination of plasmalemma, evagination of the nuclear envelope and formation from the endoplasmic reticulum.

[Figure 8.6: Golgi complex]

Functions of the Golgi apparatus

- It packages materials to be delivered inside the cell or secreted outside.
- Materials are carried from the ER in vesicles that fuse with the forming face and move towards the maturing face. This explains the close association between the Golgi apparatus and the ER.
- Proteins made by ribosomes on the ER are modified in the cisternae before they are released from the maturing face.
- It is the site of formation of glycoproteins and glycolipids.
- It secretes materials needed for cell wall formation during cell division. In plant cells, its vesicles secrete Ca²⁺ and Mg²⁺ pectates that form the middle lamella.
- It forms the acrosome during spermiogenesis.
- Lysosomes are formed by the combined function of the Golgi body and the ER.
- It recycles and reprocesses membrane fragments that enter the cytoplasm by endocytosis.

Connecting concept: the maturing face of the Golgi apparatus has acid phosphatase and produces lysosomes.

[Figure 8.7: Secretion by the Golgi complex]

Fig. 8.7 Secretion of Golgi complex

Lysosomes

Lysosomes were discovered by Christian de Duve (1955) in liver cells, where he called them 'pericanalicular dense bodies'. Novikoff later named them lysosomes. They are tiny, spheroid or irregular, membrane-bound vesicles originating from the Golgi complex. They contain about 40 hydrolytic enzymes (acid hydrolases such as amylases, lipases, proteases and carbohydrases), which are active at acidic pH. These enzymes can digest carbohydrates, proteins, lipids and nucleic acids. Acid hydrolases are divided into six categories: protease, galactosidase, phosphatase, nuclease, sulphatase and esterase.

Lysosomes are called 'suicide bags', lytic bags, scavengers of the cell or disposal units. They occur in all animal cells except RBCs.

Stabilisers and labilisers of the lysosomal membrane
Stabilisers such as cortisone, cholesterol and chloroquine protect the lysosomal membrane from rupture and autolysis. Labilisers, such as progesterone, testosterone, vitamins A, D, E and K, and UV radiation, weaken the membrane and increase the chance of autolysis.

Types of lysosomes

- Primary lysosomes (storage granules or protolysosomes): vesicles from the Golgi body that store enzymes in an inactive form, which become active in acidic medium.
- Secondary lysosomes (heterophagosomes or digestive vacuoles): formed when a primary lysosome fuses with a phagosome.
- Autophagic vacuoles (cytolysosomes): primary lysosomes that engulf and digest the cell's own organelles. They form during starvation.
- Residual bodies (telolysosomes): secondary lysosomes or autophagic vacuoles that contain undigested matter. They may be removed by exocytosis.
- Spherosomes (plant lysosomes): lysosomes of plants that contain several hydrolytic enzymes. The term was given by Parner. They are the main site of lipid synthesis and storage in plant cells (about 98 percent). Spherosomes arise directly from the ER.

Functions of lysosomes

Lysosomes are mainly meant for digestion, which may be intracellular or extracellular.

- Heterophagy: digestion of food material taken into the cell by pinocytosis and phagocytosis.
- Autophagy: during starvation, lysosomes engulf and digest old or worn-out organelles.
- Cellular digestion (autolysis): the whole cell is digested by lysosomal enzymes. Sometimes all lysosomes of a cell burst and dissolve the cell completely. Old cells and unwanted organs of the embryo are removed this way.
- Extracellular digestion: lysosomes of osteoclasts (bone-eating cells) dissolve unwanted parts of bone. Extracellular digestion by lysosomes also occurs in fungi. Cathepsin from lysosomes digests the tail of a tadpole during metamorphosis.
- Crinophagy: lysosomes digest excess secretory granules of hormones. In thyroid follicles, a lysosomal protease releases thyroxine from thyroglobulin.
- Defence: lysosomes are abundant in leucocytes, which defend against microbes.

Lysozyme is an enzyme with antiseptic properties because it acts on bacterial cell walls. It is present in saliva, tears, nasal mucus, gastric juice, milk, egg white and many tissues.

Anthocyanins are pigments that give flowers their violet, purple, blue, red or brown colours. They also occur in coloured roots (beetroot), coloured stems (balsam) and some variegated leaves (Croton, Amaranthus). They protect chloroplasts and protoplasm from strong sunlight and attract insects for pollination. They are soluble in water and alcohol, and they change colour with the pH of the cell sap: red in acid and blue in alkali.

[Figure 8.8: Formation of lysosomes and intracellular digestion in them]

Fig. 8.8 Formation of lysosomes and intracellular digestion in them.

Vacuoles

The cytoplasm of many plant cells and some animal cells (ciliate protozoans) contains hollow, liquid-filled structures called vacuoles. The fluid inside is the vacuolar sap. It contains organic acids, soluble carbohydrates, nitrates, enzymes, tannins, chlorides, phosphates, amino acids, alkaloids and anthocyanin pigments. In plant cells, vacuoles can occupy up to 90 percent of the cell volume. A vacuole is bounded by a single membrane called the tonoplast.

Young cells have small vacuoles or none. As the cell enlarges, these vacuoles fuse to form one large central vacuole. In a mature cell, the protoplasm forms a thin layer pressed against the cell wall. This layer is called the primordial utricle.

Functions of vacuoles are storage, transmission of materials and maintenance of internal pressure (turgidity). Vacuoles store water, phenols, flavonoids, anthocyanins, alkaloids, sugars and proteins. In Amoeba, the contractile vacuole regulates osmotic balance. In protists, food vacuoles form by engulfing food particles.

RIBOSOMES

Ribosomes are small, spherical, non-membranous organelles, the smallest of the cell organelles, visible only under an electron microscope. Robinson and Brown first observed them (1953). Palade observed them in animal cells (1953) and named them ribosomes (1955). Ribosomes are present in all living cells except mammalian RBCs. They may be attached to the ER membrane (rough ER) or float freely in the cytoplasm. Protein synthesis takes place on both forms.

Ribosomes are made of RNA and protein, so they are ribonucleoprotein particles (RNP). Two semi-autonomous organelles have their own ribosomes: mitoribosomes in the matrix of mitochondria and plastidoribosomes in the stroma of plastids.

Types of ribosomes

- Prokaryotic or 70S ribosomes: found in prokaryotes, chloroplasts and the mitochondria of higher organisms. The complete ribosome has a sedimentation coefficient of 70S, made of a smaller 30S subunit and a larger 50S subunit.
- Eukaryotic or 80S ribosomes: each ribosome is 80S, made of a smaller 40S subunit and a larger 60S subunit. Ribosomes attach to the ER membrane through their 60S subunit with the help of ribophorins. The 40S subunit sits on the larger subunit like a cap.

The two subunits join only during protein synthesis and are normally separate. Their union depends on Mg²⁺ concentration. They join at 1 to 2 mM, form dimers above this level and dissociate below it (0.5 mM in prokaryotes and 0.3 mM in eukaryotes). The subunits may break apart if Mg²⁺ is reduced further.

Centrifugation and sedimentation

Particles can be separated by centrifugation because they settle at different rates depending on their density, shape and size. The sedimentation rate is proportional to the applied centrifugal field. Theodor Svedberg developed analytical ultracentrifugation and won the Nobel Prize in 1926. He estimated the mass of haemoglobin as 68,000 daltons. The unit of sedimentation coefficient, the Svedberg unit (S), is named after him. It is related to the molecular weight of sedimenting particles.

In ribosomes, 60 percent of the RNA is double-stranded or helical. Of the total RNA in a cell, 70 to 75 percent is found in ribosomes.

[Figure 8.9: Form of ribosomes and their subunits]

Fig. 8.9 Form of ribosomes and its subunits

Synthesis of ribosomes In prokaryotes, ribosomes are synthesised in the cytoplasm. In eukaryotes, they are synthesised in the nucleolus. Special proteins called chaperones help newly synthesised polypeptides fold correctly, and they help transport proteins into organelles such as mitochondria, the nucleus and chloroplasts.

Functions of ribosomes

- Ribosomes are the sites of protein synthesis.
- Free cytoplasmic ribosomes make structural proteins (peripheral proteins of the plasma membrane), proteins of microbodies, chloroplasts and mitochondria, cytosolic enzymes such as those of glycolysis, and cytoskeletal proteins. These are used inside the cell.
- Ribosomes attached to the ER make proteins for transport out of the cell, integral proteins of membranes, and proteins of organelles such as the Golgi complex, lysosomes and plant vacuoles.

CYTOSKELETON

The cytoskeleton is an elaborate network of filamentous, proteinaceous structures in the cytoplasm. It is made of microtubules, microfilaments and intermediate filaments, together with the ER. It gives mechanical support, supports motility and maintains cell shape.

Microtubules

Microtubules were discovered by De Robertis and Franchi (1953). They are hollow structures made of the protein tubulin. Each protein dimer has an α and a β monomer. Their diameter is about 25 nm. They are found in centrioles, cilia, flagella and spindle fibres. In plants, they are often associated with the cell wall.

Functions: they form the cytoskeleton, maintain cell shape and support the cell, carry cell wall material from the Golgi body to the cell surface, form spindle fibres during cell division, and form the motile elements of cilia and flagella.

Microfilaments

Microfilaments are solid, non-living structures made of the protein actin. They were discovered by Pelvitz (1974). They are 4 to 6 nm in diameter and lie next to the plasma membrane, where they form a web in the ectoplasm. They help in cyclosis (streaming movement of cytoplasm), in gliding amoeboid movement of Amoeba, leucocytes and macrophages, and in muscle contraction.

Intermediate filaments are unbranched and made of structural proteins such as keratin, vimentin and desmin. They help the nucleus keep its position in the cell and often connect the nuclear envelope with the cell membrane.

CILIA AND FLAGELLA

Cilia and flagella are contractile, filamentous or hair-like outgrowths of the cytoplasm. They help in cell movement, excretion, secretion and transport. Both have a similar structure, but cilia are smaller than flagella. They are found in eukaryotic cells (except slime moulds) and in prokaryotic cells. Both are microtubular structures enclosed by an extension of the plasma membrane.

Each has two parts:
- Shaft: projects out of the cell surface. It has an outer sheath and an inner core called the axoneme. The axoneme has microtubules running parallel to its long axis.
- Basal body (kinetosome): both cilia and flagella arise from a centriole-like basal body. They contain tubulin, dynein proteins and ATP.

The axoneme usually has nine pairs of peripheral microtubule doublets arranged in a circle, and a pair of central microtubules. This is called the 9 + 2 array.

Cilia or flagella may fail to vibrate if the central fibrils are absent or dynein arms are deficient. In humans, this immotile cilia syndrome causes chronic bronchitis, sinusitis and infertility.

Prokaryotic flagella have a different structure from eukaryotic ones. Prokaryotic flagella lack the 9 + 2 array.

CENTROSOME AND CENTRIOLES

A centrosome contains two cylindrical structures called centrioles, surrounded by amorphous pericentriolar material. The two centrioles lie at right angles to each other in a cartwheel arrangement. The cytoplasm around the centrioles is called the centrosphere or kinoplasm. The centriole and centrosphere together form the centrosome.

In anaphase and telophase, the centrioles appear as two cylinders at right angles. A pair of centrioles forms a diplosome, which lies at one pole of an interphase cell outside the nuclear envelope in animal cells. Centrioles duplicate before cell division so that each pole of a dividing cell has a pair.

Each centriole is made of nine evenly spaced peripheral fibrils of tubulin. Each fibril is a triplet, and adjacent triplets are linked. The central hub is proteinaceous and is connected to the peripheral triplets by radial spokes made of protein. Centrioles form the basal bodies of cilia and flagella and the spindle fibres of the spindle apparatus in animal cells. A centriole has a (9 + 0) pattern and is not covered by any membrane.

[Figure 8.10: Diagrammatic representation of the internal structure of a centriole, showing peripheral microtubule doublets, central sheath, interdoublet bridge, central microtubule and radial spokes]

Fig. 8.10 Diagrammatic representation of internal structure of centriole

Check Point

1. Give the specific scientific terms for the following: (a) Cluster of ribosomes found in cytoplasm. (b) Extensive infoldings of inner membranes of mitochondria. (c) Stacks of closely packed thylakoids. (d) Stalked particles on the inner membrane of mitochondria.

MICROBODIES

Microbodies are single-membrane-bound, minute, spherical structures containing enzymes. They are present in both plant and animal cells. There are two types: peroxisomes and glyoxysomes.

Peroxisomes are the smallest membrane-bound organelles, about 0.1 µm in diameter. Tolbert discovered them. They are named peroxisomes because they produce hydrogen peroxide (H₂O₂), a highly reactive and toxic oxidising agent. They are found only in C₃ plants, where photorespiration occurs, and also in animal liver and kidney cells. They contain several enzymes, including amino transferase, glycolate oxidase, glyoxylate reductase, peroxidase and catalase. Catalase destroys the H₂O₂ formed when glycolate is converted to glyoxylate, and it is present in high concentration. Luciferase, the enzyme that makes light in fireflies, is also found in peroxisomes.

Glyoxysomes were discovered by Beevers (1961) in castor bean endosperm. They are small spherical vesicles enclosed by a single unit membrane. They are found in yeast, Neurospora and germinating fatty seeds, where fat is converted into carbohydrates through the glyoxylate cycle (gluconeogenesis).

NUCLEUS

The nucleus is centrally located, spherical and the largest component of a eukaryotic cell. Robert Brown (1831) first described it. The material of the nucleus that stains with basic dyes was named chromatin by Flemming. Every living cell has a nucleus except mature mammalian RBCs and sieve cells of phloem.

Evidence that the nucleus is the controlling centre of the cell:
- J. Hammerling (1934) showed this through grafting experiments on two species of the unicellular green alga Acetabularia, A. mediterranea and A. crenulata. The body of the alga has three parts: rhizoid, stalk and cap. The nucleus is in the rhizoid. The cap of A. crenulata has loose rays, while the cap of A. mediterranea is umbrella-like. When the stalk of one species was grafted onto the rhizoid of another, the cap that formed resembled the species to which the rhizoid belonged. This shows that the nucleus in the rhizoid determines the shape of the cap.

Coenocytes (many nuclei in one cytoplasm) form in plants by repeated karyokinesis. A syncytium forms in animals by fusion of cells. A multinucleate plasmodium occurs in slime moulds. Paramecium is binucleate.

Structure of the nucleus has four parts: (i) nuclear membrane, (ii) nucleoplasm, (iii) chromatin and (iv) nucleolus.

- Nuclear membrane (karyotheca, nuclear envelope or nucleolemma): made of two unit membranes separated by a perinuclear space 100 to 500 Å wide. The outer membrane is continuous with the ER and bears ribosomes. At many places the envelope has minute nuclear pores, formed by fusion of the two membranes. Pores allow movement of RNA and protein molecules between the nucleus and cytoplasm in both directions. Below the nuclear envelope, a filamentous network of proteins called the nuclear lamina attaches chromosomes to the envelope and helps the envelope grow.
- Nucleoplasm (karyolymph): the fluid inside the nucleus. It is chemically similar to the cytoplasm but has a high concentration of nucleotides, DNA and RNA. It is the site of enzyme activity and of synthesis of DNA, RNA and ribosomal subunits.

[Figure 8.11: Nucleus]

Chromatin

In an interphase nucleus, the DNA appears as thread-like chromatin fibres. During mitosis and meiosis, chromatin condenses into chromosomes. E. Strasburger (1875) discovered the distinct chromosome structures during cell division. Waldeyer (1888) named them chromosomes (chroma: colour, soma: body), because they stain with basic dyes such as basic fuchsin and acetocarmine.

- In all eukaryotes, the nucleus has a definite number of chromosomes with a definite size and shape.
- Electron microscopy shows that chromatin fibres are strings of bead-like repeating units called nucleosomes. A nucleosome is a length of DNA coiled around a core of eight histone molecules (an octamer). Neighbouring nucleosomes are joined by linker or spacer DNA, which is more exposed to enzymes. Nucleosomes are similar in all eukaryotes.
- Chromatin consists mainly of DNA and nucleoproteins. The proteins are basic histones (structural) and non-histone proteins (functional, with great variability and regulatory roles). Histones are rich in basic amino acids. DNA and histones together form a nucleoprotein complex. There are five types of histones: H1 (lysine rich), H2A and H2B (slightly lysine rich), H3 and H4 (arginine rich).
- A chromosome can be measured at metaphase. The smallest chromosome, about 0.25 µm, is found in fungi and birds, and the largest, about 30 µm, is found in some plants such as Trillium.

Units: the picogram (10^-12 g) is the unit of DNA measurement. A diploid human cell has about 5 to 6 picograms of DNA, about 174 cm long. The total DNA in a human body is estimated at about 100 g, with a length of about 2.5 × 10¹⁰ km.

Types of chromosomes based on the position of the centromere

- Metacentric: the centromere is in the middle, forming two equal arms.
- Sub-metacentric: the centromere is nearer one end, forming one shorter and one longer arm.
- Acrocentric: the centromere is close to one end, forming one extremely short arm and one very long arm.
- Telocentric: the centromere is terminal.

[Figure 8.12: Types of chromosomes based on the position of the centromere: (a) telocentric, (b) acrocentric, (c) sub-metacentric, (d) metacentric]

Fig. 8.12 Types of chromosomes based on the position of centromere (a) Telocentric (b) Acrocentric (c) Sub-metacentric (d) Metacentric

Structure of a chromosome

- Each chromosome has two coiled filaments called chromonemata, which carry bead-like structures called chromomeres. Chromomeres are thought to be the gene-bearing parts of the chromosome.
- The chromosome has two arms called chromatids, joined at the centromere, or primary constriction. Each chromosome has a kinetochore, a disc-shaped structure on either side of the centromere.
- The chromonemata are covered by a sheath called the pellicle, which is present only at metaphase when the nucleolus disappears.
- Some chromosomes have a secondary constriction, which pinches off a small portion called a satellite or trabant. Such chromosomes are called SAT chromosomes (SAT: 'Sine Acido Thymonucleinico'). Two satellites on one chromosome are called tandem SAT chromosomes. SAT chromosomes are used as marker chromosomes.
- The ends of chromosomes are called telomeres. They do not join with other structures and help maintain the individuality of the chromosome.

Heterochromatin and euchromatin were named by Emil Heitz (1928). With basic dyes, two regions are seen:
- Heterochromatin: stains darkly during interphase. It lies near the ends of chromosomes and near the centromere. It is genetically inactive and has less DNA.
- Euchromatin: stains lightly during interphase. It is genetically active and rich in DNA.

Giant chromosomes are of two types: polytene and lampbrush chromosomes.

Polytene chromosomes were first reported by E.G. Balbiani (1881). They are common in the salivary glands of insects and are therefore called salivary gland chromosomes. They also occur in other insect organs and in antipodal cells of the embryo sac.

- They can reach a length of 2000 µm and contain 1000 times (in Drosophila) to 16,000 times (in Chironomus) more DNA than ordinary somatic chromosomes.
- They are multistranded and are permanently in prophase. They form by somatic pairing of homologous chromosomes and repeated replication of their chromonemata (endomitosis).
- All polytene chromosomes may stay attached at a common point called the chromocentre.
- Polytene cells cannot divide further and eventually die. Adult organs develop from small diploid cells nearby.
- At certain developmental stages, polytene chromosomes show swellings called chromosome puffs or Balbiani rings. In a puff, DNA uncoils, becomes active and produces many copies of mRNA, which may be stored temporarily in the puff. Puffs are not permanent. Different bands puff at different stages.
- By relating puffs to developmental processes, scientists have located genes on polytene chromosomes and prepared chromosome maps.

Lampbrush chromosomes occur in the diplotene stage of vertebrate oocytes, spermatocytes of Drosophila, and the giant nucleus of the unicellular alga Acetabularia. They are larger than polytene chromosomes and occur in homologous pairs joined at contact points called chiasmata. Each chromosome has a main axis of DNA bearing rows of chromomeres. Many chromomeres give out lateral loops, which give the chromosome its lampbrush appearance. The loops are uncoiled parts of chromomeres, each with one or more transcription units. Lateral loops rapidly transcribe mRNA for yolk and other substances needed for the growth of meiocytes.

Chromosome terms

- Autosomes: chromosomes that are more numerous and not related to sex, but determine other body characters.
- Sex chromosomes (heterosomes, idiosomes): concerned with sex and of X and Y types. In human males there is one X and one Y chromosome, while females have two X chromosomes. The X chromosome is partly euchromatic and partly heterochromatic, while the Y is heterochromatic.
- Genome: the haploid set of chromosomes, i.e., the number of chromosomes in a gamete. For example, if a pollen grain of an angiosperm has 8 chromosomes and its stem, leaf and root cells have 16, its genome is 8.
- Karyotype: the morphology of the chromosomes of a species considered as a whole. It includes the number of chromosomes, the diameter and length of each chromosome, the total length, the ratio of short to long arms, and the contribution of each chromosome to the total length.
- Idiogram: a diagram that represents a species' karyotype.

Nucleolus

The nucleolus is a conspicuous, darkly stained spherical body in the nucleoplasm. It is not membrane-bound, so its content is continuous with the rest of the nucleoplasm. Fontana (1781) first described it, and Bowman gave it its name. The nucleolus is attached to the nucleolar organiser region (NOR) of a particular chromosome. It is made of ribosomal proteins and ribosomal RNA, and it stores rRNA synthesised from the nucleolar organiser region of DNA (rDNA). It is the site of active rRNA synthesis, which makes it the 'ribosomal factory'. Cells that are actively synthesising proteins have larger and more numerous nucleoli. The nucleolus is absent in lower genera such as yeast and some algae, in undifferentiated embryonic cells, and in some mammalian cells (reticulocytes, erythrocytes and spermatozoa).

Functions of the nucleus

- It is the controlling centre of the cell.
- It contains genetic information for reproduction and development of the organism.
- It maintains cell structure and growth by directing the synthesis of structural proteins.
- It brings about cell multiplication when needed.
- It takes part in the formation of ribosomes.
- It regulates cell metabolism by directing the synthesis of functional proteins (enzymes).

PLASTIDS

Plastids are found in plants and a few protists such as Euglena. They may be colourless (leucoplasts) or coloured (chromoplasts and chloroplasts). Haeckel first used the term plastid. Plastids range from 4 to 6 µm in diameter, and the size depends on the light available. All plastids arise from protoplastids, which are sac-like, non-lamellar structures. Plastids can change from one type to another, but chromoplasts never change into chloroplasts. Plastid inheritance is independent of nuclear inheritance, as seen in Mirabilis jalapa (four o'clock plant).

Example of plastid change in tomato: a young ovary is colourless (leucoplast), young fruits are green (chloroplasts), and mature fruits are red (chromoplasts). In carrot root, leucoplasts change into chromoplasts.

Check Point

1. Where would the following structures be found in a cell? (a) Microtubules (b) Thylakoids (c) F₀-F₁ complex (d) Ribosomes (e) Nucleolus.
2. Which of the following are found exclusively in plant cells, animal cells and in both? (i) Nucleus (ii) Centrosomes (iii) Golgi Bodies (iv) Leucoplasts (v) Cell coat

Leucoplasts are found in underground parts, internal parts of herbaceous stems and deep tissues where sunlight is absent. They lack pigments and internal lamellae, and usually have a storage function. Depending on what they store, they are called:
- amyloplasts (starch), e.g., potato tuber, wheat and rice grain
- aleuroplasts (proteins), e.g., maize
- elaioplasts (oils and fats), e.g., endosperm cells of castor seed

Chromoplasts contain pigments such as carotenes and xanthophylls. They give colour to flowers, fruits and some algae. In flowers they help in pollination, and in fruits they help in seed dispersal because bright colours attract insects and birds. Lycopene, a type of carotene, gives red colour to red chillies and tomatoes. Yellow-orange colours come from alpha-, beta- and gamma-carotene. Beta-carotene is the precursor of vitamin A, and carrot is the richest source of it.

Chromoplasts are of three types:
- Rhodoplasts in red algae (Rhodophyceae). They contain chlorophyll-a, chlorophyll-d, R-phycoerythrin and R-phycocyanin (phycobilins).
- Phaeoplasts in brown algae (Phaeophyceae). They contain fucoxanthin (a brown pigment), chlorophyll-a and chlorophyll-c.
- Chromatophores in blue-green algae and bacteria. In blue-green algae they contain chlorophyll-a, C-phycocyanin and C-phycoerythrin. In photosynthetic bacteria they contain bacteriochlorophyll or chlorobium chlorophyll.

Chloroplasts are green plastids, called chloroplastids by Schimper (1883). The term chloroplast was proposed by Erera. In plants, chloroplasts are the largest organelles of the cell. Most green-plant chloroplasts are in the mesophyll cells of leaves.

- They are lens-shaped, oval, spherical, discoid or ribbon-like, about 5 to 10 µm long and 2 to 4 µm wide.
- The number varies from one per cell in Chlamydomonas to 20 to 40 per cell in the mesophyll cells of higher plants.
- Size is generally 4 to 8 µm. Shade plants (sciophytes) have larger chloroplasts than light plants (heliophytes).
- Shapes vary between species: cup-shaped (Chlorella, Chlamydomonas), girdle-shaped (Ulothrix), star-shaped or stellate (Zygnema), spiral or scalariform (Spirogyra), reticulate or net-like (Cladophora, Oedogonium), and discoid (Vaucheria).
- Number per cell: one single chloroplast in many algae (Chlamydomonas, Ulothrix, Chlorella), two in Zygnema, two to fourteen in Spirogyra, and 20 to 40 in higher plants.

[Figure 8.15: Ultrastructure of a chloroplast]

Structure of the chloroplast

- The chloroplast is double membranous. The outer membrane has integral proteins called porins, which let solutes up to about 10,000 daltons pass. The inner membrane is relatively impermeable.
- Stroma: the matrix of the chloroplast, which is the enzymatic part. It contains proteins, DNA, ribosomes and fewer enzymes. The dark reaction of photosynthesis occurs here.
- Grana: stacks of 20 to 50 membranous sac-like thylakoids, arranged like a pile of coins. A chloroplast has 40 to 100 grana. The light reaction of photosynthesis occurs in the granal thylakoids because chlorophyll is present in them.
- Granal thylakoids produce 4 to 8 projections called stroma thylakoids or fret channels. Weier proposed the concept of grana with an interconnecting fret system.
- Photosynthetic pigments form units called quantosomes (Park and Biggins). Each quantosome contains 230 chlorophyll molecules. The quantasome of bacteria has only 50 chlorophyll molecules.
- The stroma contains osmiophilic droplets called plastoglobuli, which store lipids.
- The genetic system of the chloroplast is called the plastidome. Its DNA, called ctDNA or plastidial DNA, is naked.

Pigments of chloroplasts: chlorophyll-a (C₅₅H₇₂O₅N₄Mg), chlorophyll-b (C₅₅H₇₀O₆N₄Mg), chlorophyll-c (C₃₅H₃₀O₅N₄Mg), chlorophyll-d (C₅₄H₇₀O₆N₄Mg), chlorophyll-e, carotenoids (carotenes and xanthophylls).

Functions: chloroplasts make food by photosynthesis and help maintain the balance of CO₂ and O₂ in the atmosphere.

MITOCHONDRIA

Kolliker (1850) first described muscle mitochondria of insects as sarcosomes. Altmann (1886) called them bioplasts. Benda (1897) introduced the term 'mitochondrion' for thread-like granules. Michaelis (1900) found that they could be stained with Janus Green B. Mitochondria are called the powerhouse of the cell. They are large, rounded or rod-like, double-membranous structures. They occur singly or in groups. Their shape and size (0.2 to 0.5 µm) vary with cell type and physiological activity. All the mitochondria of a cell are collectively called the chondriome.

Mitochondria are semi-autonomous, because:
(1) they have their own naked, circular DNA, and can replicate and transcribe it;
(2) they make some of their own proteins, enzymes and biochemicals, because they have 70S ribosomes that translate the coded information in mRNA transcribed from mitochondrial DNA;
(3) new mitochondria arise either by division of pre-existing ones or from precursors called promitochondria.

Structure

- Outer membrane: forms the continuous outer boundary. It has many non-stalked particles called subunits of Parson. It also has a carrier protein similar to the bacterial porins found in gram-negative bacteria.
- Inner membrane: folds inward into finger-like projections called cristae, which increase the surface area. In animal cells, cristae are plate-like. In plant cells they are tubular and called tubuli. The inner membrane has less phospholipid and cholesterol than the outer membrane. It carries many enzymes for ATP synthesis (oxidative phosphorylation), which depends on O₂. The cristae carry components of the respiratory chain, including dehydrogenases and cytochromes. The inner membrane bears large numbers of oxysomes (F₀-F₁ particles) facing the matrix. ATP is made in the head region of oxysomes, where ATPase is found.

[Figure 8.16: Three-dimensional structure of a mitochondrion]

The inner membrane divides the mitochondrion into two compartments:
- Perimitochondrial space: the fluid-filled space between the outer and inner membranes.
- Matrix (inner chamber): a dense material containing double-stranded circular DNA, RNA and 70S ribosomes.

Number and size of mitochondria

- Mitochondria number 50 to 50,000 per cell. In some protozoans the number can reach 5,00,000.
- A single mitochondrion is found in Chlorella fusca, Microsterias, Trypanosoma and Microsomonas.
- The largest mitochondria are found in Rana pipens, 20 to 40 µm.
- The smallest are in yeast, less than 1 µm.
- In the human body, the largest mitochondria are in pancreatic cells, about 10 µm.

Functions of mitochondria

- They produce about 95 percent of the cell's ATP through glycolysis, oxidative decarboxylation, the Krebs cycle and oxidative phosphorylation (the respiratory chain). This is why they are called the powerhouse of the cell.
- They are the only major site of oxygen consumption in the cell. Eukaryotic cells without mitochondria, such as mature mammalian RBCs, cannot use oxygen.
- They contain circular DNA and ribosomes and can synthesise certain proteins.
- They provide many intermediates for synthesis of biochemicals such as cytochromes, chlorophylls, alkaloids and steroids. Many amino acids are also synthesised in mitochondria.
- About 70 percent of the enzymes in a cell are found in mitochondria alone. The enzymes of the Krebs (TCA or citric acid) cycle are in the matrix, except succinic dehydrogenase, which is attached to the inner membrane. The electron transport system and oxidative phosphorylation are in the cristae or oxysomes.

Mitochondrial inheritance, independent of nuclear inheritance, occurs in some cases. Examples are the petite character in yeast and cytoplasmic male sterility in maize.