Cell: The Unit of Life

Every living organism is made of cells. Unicellular organisms such as Amoeba, Paramecium and bacteria carry out all life activities, including independent existence and reproduction, inside one cell. Multicellular organisms are built from many cells that divide the work among themselves. Anything less than a complete cell cannot live independently, which is why NCERT calls the cell the fundamental structural and functional unit of all living organisms. A cell separated from the body, with the right nutrients and conditions, can still perform metabolism and even divide; a nucleus or a mitochondrion taken out of a cell cannot.

The discovery came with the microscope. Anton von Leeuwenhoek was the first to see and describe a live cell. Robert Hooke observed a thin slice of cork and saw compartments, which he called cells. Robert Brown later discovered the nucleus (1831). The detailed structure of the cell that this chapter describes was revealed only after 1940, when the electron microscope made it possible to see the fine internal organisation that a light microscope cannot resolve. When a question asks why the cell was studied in such detail only in the twentieth century, this is the answer: the ultrastructure of organelles is beyond the resolving power of the light microscope.

The examiner's favourite two-mark question from this section is why the cell is the basic unit of life. Give three points: all organisms are made of cells and their products; the cell is the smallest unit that shows all the properties of life, such as metabolism, growth, response and reproduction; and in multicellular organisms the activities of the whole body are the sum of the activities of its cells, with the cells showing division of labour.

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 that form the tissues of the plant. In 1839 Theodore Schwann, a British zoologist, studied animal cells and reported that they have a thin outer layer, known today as the plasma membrane. From his study of plant tissues he also concluded that the cell wall is a unique feature of plant cells. On the basis of this Schwann proposed the hypothesis that the bodies of animals and plants are composed of cells and the products of cells.

Schleiden and Schwann together formulated the cell theory, but their theory did not explain how new cells are formed. Rudolf Virchow (1855) first explained that cells divide and new cells are formed from pre-existing cells, in his famous phrase Omnis cellula-e cellula. He modified the hypothesis of Schleiden and Schwann to give the cell theory its final shape.

The cell theory as it is understood today states two things, and the examiner wants both: (i) all living organisms are composed of cells and products of cells, and (ii) all cells arise from pre-existing cells. If you are asked for the contributions of the three scientists separately, write Schleiden for plants, Schwann for animals (plasma membrane, cell wall unique to plants) and Virchow for the origin of new cells from pre-existing cells. Exemplar questions also ask what was missing from the original theory; the answer is that it did not explain the origin of new cells, which Virchow supplied.

A quick check on the word 'products of cells': bone matrix, cartilage matrix, the cuticle of a leaf and the secondary cell wall are non-living products of cells, and the cell theory covers them through this phrase. Viruses do not fit the cell theory because they are acellular and cannot be called cells at all.

Under a microscope an onion peel shows plant cells and a human cheek scraping shows animal cells. Each cell has an outer cell membrane; plant cells have a rigid cell wall outside it as well. Inside is a dense, membrane-bound structure, the nucleus, which contains the chromosomes, which in turn contain the genetic material, DNA. The semi-fluid matrix filling the cell is the cytoplasm, the main arena of cellular activities in both plant and animal cells, where the chemical reactions that keep the cell alive take place.

Cells that have a membrane-bound nucleus are eukaryotic; cells that lack one are prokaryotic. Both types have the cytoplasm, but eukaryotic cells also have other membrane-bound distinct structures called organelles: endoplasmic reticulum, Golgi complex, lysosomes, mitochondria, microbodies and vacuoles. Prokaryotic cells lack such membrane-bound organelles. Ribosomes are non-membrane-bound organelles found in all cells, both eukaryotic and prokaryotic; in a eukaryotic cell they occur free in the cytoplasm, on the rough endoplasmic reticulum, and inside the chloroplasts and mitochondria. Animal cells contain a further non-membranous organelle, the centrosome, which helps in cell division.

Cells vary enormously in size, shape and activity. Mycoplasmas, the smallest cells, are only about 0.3 µm in length; bacteria may 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 and nerve cells are among the longest cells. In shape cells may be disc-like (RBC), polygonal (many epithelial and plant cells), columnar (columnar epithelium), cuboid, thread-like (some fungal hyphae) or irregular (white blood cells, nerve cells with long processes). The exemplar asks you to arrange cells in increasing order of size; the order is Mycoplasma, bacteria, human RBC, ostrich egg.

Prokaryotic cells are represented by bacteria, blue-green algae (cyanobacteria), mycoplasma and PPLO (pleuro-pneumonia-like organisms). They are generally smaller than eukaryotic cells and multiply much faster. Bacteria come in four basic shapes: bacillus (rod-like), coccus (spherical), vibrio (comma-shaped) and spirillum (spiral).

The organisation of the prokaryotic cell is fundamentally similar in all of them, even though they vary greatly in shape and function. All have a cell wall surrounding the cell membrane, except in mycoplasma, which has no wall. The fluid matrix filling the cell is the cytoplasm. There is no well-defined nucleus. The genomic DNA, a single circular chromosome, is not enclosed by a nuclear membrane; NCERT says the genetic material is basically naked. In addition to the genomic DNA many bacteria have small circular DNA molecules outside the genomic DNA, called plasmids. Plasmids confer certain unique phenotypic characters on the bacterium, the most familiar being resistance to antibiotics. You meet plasmids again in Class 12 as the vectors of recombinant DNA technology; the same word is also used in the exemplar for extra-genomic DNA, along with the DNA of mitochondria and chloroplasts in eukaryotes.

Prokaryotes do not have membrane-bound organelles, but they do have a special membranous structure, the mesosome, formed by extensions of the plasma membrane into the cell in the form of vesicles, tubules and lamellae. Mesosomes help in cell wall formation, in DNA replication and its distribution to daughter cells, in respiration and secretion, and they increase the surface area of the plasma membrane and its enzymatic content. In some prokaryotes, such as cyanobacteria, there are other membranous extensions into the cytoplasm called chromatophores, which contain pigments.

Prokaryotes have ribosomes (70S) and reserve material stored as inclusion bodies, which are the subject of the next two sections. When a question asks for the characteristics of prokaryotic cells (NCERT exercise 8), list: small size and rapid multiplication; cell wall present (absent in mycoplasma); no nuclear membrane, naked circular genomic DNA; plasmids; no membrane-bound organelles; mesosomes; 70S ribosomes; inclusion bodies; flagella of a simple filament-hook-basal body type; pili and fimbriae.

Most prokaryotic cells, particularly bacteria, have a chemically complex cell envelope. It consists of a tightly bound three-layered structure: the outermost glycocalyx, then the cell wall, and innermost the plasma membrane. Each layer performs a distinct function, but the three act together as a single protective unit.

Bacteria are classified into two groups on the basis of the differences in their cell envelopes and the way they respond to Gram staining, a procedure developed by Gram: those that take up the stain are gram positive and those that do not are gram negative. (The chemical basis, a thick peptidoglycan wall in gram positive bacteria and a thin wall with an outer membrane in gram negative bacteria, is not in NCERT but is worth one line if a 3-mark answer needs it.)

The glycocalyx differs in thickness and composition among bacteria. In some it is a loose sheath called the slime layer; in others it is thick and tough, called the capsule. The cell wall determines the shape of the cell and provides strong structural support to prevent the bacterium from bursting or collapsing. The plasma membrane is selectively permeable and interacts with the outside world; structurally it is similar to that of eukaryotes.

Bacterial cells may be motile or non-motile. If motile, they have thin filamentous extensions from the cell wall called flagella. Bacteria show a range in the number and arrangement of flagella. A bacterial flagellum has three parts: the filament, the hook and the basal body. The filament is the longest portion and extends from the cell surface to the outside. Note that the bacterial flagellum is structurally completely different from the eukaryotic flagellum: it has no 9+2 axoneme and is not covered by plasma membrane.

Besides flagella, pili and fimbriae are surface structures of bacteria that do not play a role in motility. Pili are elongated tubular structures made of a special protein. Fimbriae are small bristle-like fibres sprouting out of the cell; in some bacteria they are known to help attach the bacterium to rocks in streams and to host tissues.

In prokaryotes the 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, which together form a 70S ribosome. The letter S stands for the Svedberg unit or sedimentation coefficient, a measure of how fast a particle sediments in an ultracentrifuge; it depends on both size and density, which is why 50S + 30S gives 70S and not 80S. Ribosomes are the site of protein synthesis. Several ribosomes may attach to a single mRNA and form a chain called a polyribosome or polysome. The ribosomes of a polysome translate the mRNA into proteins, so many copies of the same polypeptide are made from one mRNA at the same time.

Inclusion bodies are the reserve material of prokaryotic cells stored in the cytoplasm. They are not bound by any membrane and lie free in the cytoplasm; this is the key phrase the marking scheme looks for. Examples are phosphate granules, cyanophycean granules and glycogen granules. Gas vacuoles are found in blue-green, purple and green photosynthetic bacteria; they are gas-filled structures that give buoyancy, keeping the cell at the depth of water where light and other conditions suit it.

Compare this with the eukaryotic cell while it is fresh in your mind: eukaryotic ribosomes are 80S (60S + 40S), but the ribosomes inside mitochondria and chloroplasts are the prokaryotic 70S type, which is one of the pieces of evidence that these organelles evolved from free-living prokaryotes (endosymbiosis). A question that asks for 'a feature common to prokaryotes and the mitochondria of eukaryotes' wants 70S ribosomes and circular naked DNA.

Eukaryotes include all the protists, plants, animals and fungi. Their cells possess an organised nucleus with a nuclear envelope, and the cytoplasm is divided into compartments by an extensive endomembrane system. The genetic material is organised into chromosomes. Eukaryotic cells are not all identical: plant cells have cell walls, plastids and a large central vacuole, which animal cells lack; animal cells have centrioles, which are absent in almost all plant cells.

The detailed structure of the cell membrane was studied only after the advent of the electron microscope in the 1950s. Chemical studies, especially of the human erythrocyte (RBC) membrane, showed that it is composed of lipids arranged in a bilayer, with proteins. The lipids are mainly phosphoglycerides, and the membrane also contains cholesterol. In the erythrocyte membrane there is approximately 52 per cent protein and 40 per cent lipid. The lipid molecules are arranged with their polar heads towards the outer sides and the hydrophobic tails towards the inner part, so the non-polar tails are never exposed to the aqueous environment. The membrane proteins are of two kinds: integral proteins that are partially or totally buried in the membrane, and peripheral proteins that lie on the surface.

The fluid mosaic model, proposed by Singer and Nicolson in 1972, is the widely accepted model. According to it the quasi-fluid nature of the lipid bilayer enables the lateral movement of proteins within the bilayer; the proteins are like tiles in a mosaic floating in the lipid sea. The ability to move within the membrane is measured as its fluidity. This fluid nature is important for functions such as cell growth, formation of intercellular junctions, secretion, endocytosis and cell division.

The most important function of the plasma membrane is the transport of molecules across it. The membrane is selectively permeable. Some molecules move across it without any requirement of energy, which is passive transport: neutral solutes move by simple diffusion along the concentration gradient (high to low), and water moves by osmosis, also a passive process. Polar molecules cannot pass through the non-polar lipid bilayer, so they require a carrier protein to facilitate their transport across the membrane (facilitated diffusion, still passive). A few ions or molecules are transported against their concentration gradient, from lower to higher concentration; this is active transport, an energy-dependent process using ATP, such as the Na⁺/K⁺ pump. NCERT exercise 6 is exactly this paragraph and it is a standard 3-mark item.

As Schwann pointed out, the cell wall is a feature of plant cells (and fungi), not animal cells. It is a non-living rigid structure outside the plasma membrane. It gives shape to the cell, protects it from mechanical damage and infection, helps in cell-to-cell interaction and provides a barrier to undesirable macromolecules.

The chemistry differs between groups, and this is asked as a 1-mark or 2-mark question. In algae the cell wall is made of cellulose, galactans, mannans and minerals like calcium carbonate. In other plants it consists of cellulose, hemicellulose, pectins and proteins. In fungi the wall is made of chitin, and in bacteria it is peptidoglycan (from Chapter 2, Biological Classification); the exemplar asks which of these is not true of eukaryotes, and the answer is a peptidoglycan wall.

The cell wall of a young plant cell, the primary wall, is capable of growth; it gradually diminishes as the cell matures, and the secondary wall is formed on the inner (inside) side of the cell, between the primary wall and the plasma membrane. The middle lamella is a layer mainly of calcium pectate that holds or glues the different neighbouring cells together. The cell wall and the middle lamella may be traversed by plasmodesmata, cytoplasmic strands that connect the cytoplasm of neighbouring cells so that materials can pass from cell to cell; this is how the symplast path of Chapter 11 works.

Do not describe the cell wall as 'selectively permeable'. It is freely permeable; selective permeability belongs to the plasma membrane. And never write cellulose for fungi or chitin for plants, a slip that costs the whole mark.

Each membranous organelle of the eukaryotic cell has its own structure and function, but the functions of some of them are coordinated, and these are considered together as the endomembrane system: the endoplasmic reticulum (ER), Golgi complex, lysosomes and vacuoles. Since the functions of the mitochondria, chloroplasts and peroxisomes are not coordinated with these, they are not considered part of the endomembrane system. The examiner often asks precisely this: name the components and state which organelles are excluded and why.

The endoplasmic reticulum is a network or reticulum of tiny tubular structures scattered in the cytoplasm. It divides the intracellular space into two distinct compartments: the luminal (inside the ER) and the extra-luminal (cytoplasm) compartments. When ribosomes are attached to the outer surface the ER is called rough endoplasmic reticulum (RER); in the absence of ribosomes it appears smooth and is called smooth endoplasmic reticulum (SER). RER is frequently observed in cells actively involved in protein synthesis and secretion; it is extensive and continuous with the outer membrane of the nucleus. SER is the major site for synthesis of lipids; in animal cells lipid-like steroidal hormones are synthesised in the SER. That is why the exemplar says RER is easily seen in secretory cells, and why liver cells and cells of the adrenal cortex and testis are rich in SER.

The Golgi apparatus was first observed by Camillo Golgi in 1898 as densely stained reticular structures near the nucleus. It consists of many flat, disc-shaped sacs or cisternae of 0.5 µm to 1.0 µm diameter, stacked parallel to each other. Varied numbers of cisternae are present in a Golgi complex. The cisternae are concentrically arranged near the nucleus with distinct convex cis (forming) and concave trans (maturing) faces; the two faces are entirely different but interconnected. The Golgi apparatus principally performs the function of packaging materials, to be delivered either to intracellular targets or secreted outside the cell. Materials to be packaged in the form of vesicles from the ER fuse with the cis face and move towards the maturing face. This explains why the Golgi apparatus remains in close association with the ER. A number of proteins synthesised by ribosomes on the ER are modified in the cisternae of the Golgi before release from the trans face. The Golgi apparatus is the important site of formation of glycoproteins and glycolipids (glycosylation).

Lysosomes 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 and carbohydrases, which are optimally active at acidic pH. These enzymes are capable of digesting carbohydrates, proteins, lipids and nucleic acids. Because they can digest the cell's own material when released, lysosomes are popularly called suicide bags of the cell (the phrase is from Class 9, but examiners still reward it). They also digest worn-out organelles and material brought in by endocytosis, and they play a role in the defence of white blood cells.

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

NCERT exercise 12 asks you to comment on how lysosomes and vacuoles differ although both are endomembrane structures. Answer in a table: lysosomes are small vesicles formed from the Golgi, contain hydrolytic enzymes active at acidic pH, and digest macromolecules and worn-out organelles (intracellular digestion, defence, autolysis); vacuoles are larger single-membrane spaces bound by the tonoplast, contain water, sap and waste, store materials and ions moved in against a gradient, and maintain turgidity; in protists they act as contractile or food vacuoles.

Mitochondria, unless specifically stained, are not easily visible under the microscope; the vital stain for them is Janus green. The number of mitochondria per cell is variable and depends on the physiological activity of the cell; a muscle cell or a liver cell has many. In shape and size too they show considerable variation. Typically a mitochondrion is sausage-shaped or cylindrical, 0.2 to 1.0 µm in diameter (average 0.5 µm) and 1.0 to 4.1 µm in length.

Each mitochondrion is a double membrane-bound structure, with the outer membrane and the inner membrane dividing its lumen distinctly into two aqueous compartments: the outer compartment (intermembrane space) and the inner compartment. The inner compartment is called the matrix. The outer membrane forms the continuous limiting boundary of the organelle. The inner membrane forms a number of infoldings called cristae towards the matrix. The cristae increase the surface area; this is where the electron transport chain and ATP synthesis of Chapter 12 (Respiration in Plants) sit. The two membranes have their own specific enzymes associated with mitochondrial function.

Mitochondria are the sites of aerobic respiration. They produce cellular energy in the form of ATP, hence they are called the power houses of the cell. The matrix also possesses a single circular DNA molecule, a few RNA molecules, ribosomes (70S) and the components required for the synthesis of proteins. Mitochondria divide by fission. Because they have their own DNA, ribosomes and protein-synthesising machinery and divide independently, mitochondria (and chloroplasts) are called semi-autonomous organelles; they are not fully autonomous because most of their proteins are coded by nuclear genes.

For the 5-mark version of NCERT exercise 7, draw the mitochondrion and the chloroplast side by side (see the diagrams section), list the shared characteristics (double membrane, own circular DNA, 70S ribosomes, division by fission, semi-autonomy) and then give each organelle's function.

Plastids are found in all plant cells and in euglenoids. They are easily observed under the microscope because they are large. They bear specific pigments, which impart specific colours to the plants. Based on the pigments they contain, plastids are of three types.

Chloroplasts contain chlorophyll and carotenoid pigments, which are responsible for trapping light energy for photosynthesis. Chromoplasts contain fat-soluble carotenoid pigments such as carotene and xanthophylls, giving the part of the plant a yellow, orange or red colour (the ripe tomato, the carrot root). Leucoplasts are colourless plastids of varied shapes and sizes that store nutrients: amyloplasts store carbohydrates (starch, as in the potato tuber), elaioplasts store oils and fats, and aleuroplasts store proteins. The exemplar asks whether plastids are interchangeable, and the answer is yes: chloroplasts turn into chromoplasts when a tomato ripens, and the leucoplasts of a potato tuber turn green (chloroplasts) when exposed to light.

Chloroplasts of the majority of green plants are found in the mesophyll cells of leaves. They are lens-shaped, oval, spherical, discoid or even ribbon-like organelles, 5 to 10 µm in length and 2 to 4 µm in width. Their number varies from one per cell in the green alga Chlamydomonas to 20 to 40 per cell in the mesophyll. Like mitochondria, chloroplasts are double membrane-bound; of the two, the inner membrane is relatively less permeable. The space limited by the inner membrane is the stroma. A number of organised flattened membranous sacs called thylakoids are present in the stroma. Thylakoids are arranged in stacks like piles of coins called grana (singular granum), and there are about 40 to 60 grana per chloroplast. Flat membranous tubules called stroma lamellae connect the thylakoids of different grana. The membrane of the thylakoids encloses a space called the lumen. The stroma contains the enzymes required for the synthesis of carbohydrates and proteins (the Calvin cycle of Chapter 11), a double-stranded circular DNA and 70S ribosomes. The chlorophyll pigments are present in the thylakoid membranes.

The difference the exemplar tests: plastids differ from mitochondria in having thylakoids (and pigments). Everything else on the list, double membrane, own DNA, 70S ribosomes, is shared.

Ribosomes were first observed under the electron microscope as dense particles by George Palade (1953). They are granular structures composed of ribonucleic acid (RNA) and proteins, and they are not surrounded by any membrane. The eukaryotic ribosomes are 80S, made of two subunits, 60S and 40S; the prokaryotic ribosomes are 70S (50S + 30S). Here S denotes the sedimentation coefficient, which is indirectly a measure of density and size, so the values of the subunits are not additive. Ribosomes are the site of protein synthesis in every cell. In eukaryotes they occur free in the cytoplasm, bound to the RER, and inside mitochondria and chloroplasts (where they are 70S). Several ribosomes on one mRNA form a polysome, as in prokaryotes.

The cytoskeleton is an elaborate network of filamentous proteinaceous structures present in the cytoplasm: microtubules (tubulin), microfilaments (actin) and intermediate filaments. NCERT gives its functions in one line, and you should give all three: mechanical support, motility, and maintenance of the shape of the cell. Microtubules also build the spindle, the axoneme of cilia and flagella and the centrioles, which is why the next section follows.

The exemplar asks which is not a function of the cytoskeleton and the trick is that all the listed functions (intracellular transport, support, shape, motility) are genuine. It also asks what is common to ribosomes and centrioles; the answer is that neither has a membrane.

Cilia (singular cilium) and flagella (singular flagellum) are hair-like outgrowths of the cell membrane. Cilia are small structures that work like oars, causing the movement of either the cell or the surrounding fluid (the ciliated epithelium of the trachea moves mucus; Paramecium swims with cilia). Flagella are comparatively longer and responsible for cell movement (sperm, Euglena). The prokaryotic bacteria also possess flagella, but these are structurally different from the eukaryotic flagella.

The electron microscope shows that a cilium or flagellum is 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 doublets of radially arranged peripheral microtubules and a pair of centrally located microtubules. This arrangement of the axonemal microtubules is referred to as the 9+2 array. The central tubules are connected by bridges and are also enclosed by a central sheath, which is connected to one of the tubules of each peripheral doublet by a radial spoke; thus there are nine radial spokes. The peripheral doublets are also interconnected by linkers (interdoublet linkers). Both the cilium and the flagellum emerge from centriole-like structures called basal bodies.

The centrosome is an organelle usually containing two cylindrical structures called centrioles, surrounded by amorphous pericentriolar material. Both the centrioles in a centrosome lie perpendicular to each other, and each has an organisation like a cartwheel. They are made up of nine evenly spaced peripheral fibrils of tubulin protein. Each of the peripheral fibrils is a triplet, and the adjacent triplets are also linked. The central part of the proximal region of the centriole is also proteinaceous and is called the hub, which is connected with the tubules of the peripheral triplets by radial spokes made of protein. There are no central microtubules, so the arrangement is described as 9+0 with triplets. The centrioles form the basal body of cilia or flagella and the spindle fibres that give rise to the spindle apparatus during cell division in animal cells. Note the contrast that the examiner loves: axoneme = 9 doublets + 2 central singlets, covered by plasma membrane; centriole = 9 triplets + 0 central, hub and spokes, no membrane.

The nucleus, as a cell organelle, was first described by Robert Brown in 1831. Later the material of the nucleus stained by basic dyes was given the name chromatin by Flemming. The interphase nucleus (the nucleus of a cell when it is not dividing) has highly extended and elaborate nucleoprotein fibres called chromatin, a nuclear matrix, and one or more spherical bodies called nucleoli (singular nucleolus).

Electron microscopy shows that the nuclear envelope consists of two parallel membranes with a space between them (10 to 50 nm) called the perinuclear space; it forms a barrier between the materials present inside the nucleus and 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, formed by the fusion of its two membranes; these nuclear pores are the passages through which the movement of RNA and protein molecules takes place in both directions between the nucleus and the cytoplasm (NCERT exercise 11). Normally there is only one nucleus per cell, but variations are found: some mature cells lack a nucleus altogether, such as the erythrocytes of many mammals and the sieve tube cells of vascular plants (the exemplar's 'common feature'), although they had one during development.

The nucleoplasm contains the nucleolus and chromatin. The nucleoli are spherical structures present in the nucleoplasm, and their content is continuous with the rest of the nucleoplasm because they are not membrane-bound. The nucleolus is the site of active ribosomal RNA synthesis, so larger and more numerous nucleoli are present in cells actively carrying out protein synthesis.

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 metres 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 (where spindle fibres attach). Based on the position of the centromere, chromosomes are of four types. The metacentric chromosome has a middle centromere forming two equal arms. In the sub-metacentric chromosome the centromere is slightly away from the middle, resulting in one shorter arm and one longer arm. In the acrocentric chromosome the centromere is situated close to its end, forming one extremely short and one very long arm. The telocentric chromosome has a terminal centromere. Besides the primary constriction a few chromosomes have non-staining secondary constrictions at a constant location, which give the appearance of a small fragment called the satellite (a satellite chromosome; the exemplar calls it a marker chromosome).

Microbodies are many membrane-bound minute vesicles that contain various enzymes; they are present in both plant and animal cells. NCERT gives only this one line. The peroxisomes (which contain catalase and break down hydrogen peroxide, and take part in photorespiration in leaves) and glyoxysomes (in germinating fatty seeds) are the well-known examples, and a line on them earns credit in a 3-mark answer on organelles that are not part of the endomembrane system.

Prokaryotic versus eukaryotic cell. Size: prokaryotes generally smaller (1 to 10 µm), eukaryotes larger (10 to 100 µm). Nucleus: absent, naked circular DNA in the cytoplasm versus a true nucleus with a nuclear envelope, nucleolus and linear chromosomes with histones. Membrane-bound organelles: absent (mesosomes and chromatophores only) versus present (ER, Golgi, lysosomes, mitochondria, plastids, vacuoles, microbodies). Ribosomes: 70S versus 80S in the cytoplasm. Cell wall: peptidoglycan versus cellulose (plants) or chitin (fungi) or absent (animals). Flagella: filament, hook, basal body versus 9+2 axoneme covered by plasma membrane. Cell division: fission versus mitosis and meiosis. Examples: bacteria, cyanobacteria, mycoplasma, PPLO versus protists, fungi, plants, animals.

Plant cell versus animal cell. Cell wall of cellulose present in plants, absent in animals. Plastids present in plants, absent in animals. Vacuole: a large central vacuole occupying up to 90 per cent of the volume in plants; small, few or absent in animals. Centrioles: absent in most plants, present in animals. Lysosomes: rare in plants, common in animals. Shape: fixed (polygonal or rectangular) in plants because of the wall, variable in animals. Storage material: starch in plants, glycogen in animals. Plasmodesmata in plants; none in animals. Write these as a table with two columns; five differences are enough for a 3-mark question and eight for 5 marks with diagrams.