Cell: The Unit of Life Class 11 Notes

Cell theory was independently proposed by Matthias Schleiden (1838, plants) and Theodor Schwann (1839, animals). Rudolf Virchow (1855) added the third tenet. Together, the three tenets form the foundation of cell biology.

• All living organisms are composed of one or more cells.
• The cell is the basic structural and functional unit of life.
• All cells arise from pre-existing cells (Virchow — 'Omnis cellula e cellula').

Prokaryotic cells (bacteria, cyanobacteria) are smaller (1–10 µm) and simpler — no membrane-bound nucleus or organelles. Eukaryotic cells (plants, animals, fungi, protists) are larger (10–100 µm) with a true nucleus and membrane-bound organelles. The key difference is the nuclear envelope and internal compartmentalisation.

• Prokaryote: nucleoid region (circular DNA, no histones), 70S ribosomes, no mitochondria or ER, cell wall of peptidoglycan (bacteria).
• Eukaryote: true nucleus with nuclear envelope, 80S ribosomes (cytoplasm), membrane-bound ER, Golgi, mitochondria, and (in plants) plastids.

• Mesosomes: inward folds of the plasma membrane — involved in cell wall formation, DNA replication and electron transport (analogous to mitochondrial cristae in function).
• Nucleoid: irregularly shaped region containing the single circular, double-stranded DNA molecule — not enclosed by a membrane.
• Plasmids: small, circular, extrachromosomal DNA — carry antibiotic resistance genes, used in genetic engineering.
• Ribosomes: 70S = 50S (large subunit) + 30S (small subunit) — site of protein synthesis.
• Flagella: for locomotion (long, few, made of flagellin protein).
• Pili (fimbriae): short, hair-like appendages for attachment to surfaces and other bacteria; pili also involved in conjugation (DNA transfer).

The cell wall is present in plants, fungi, algae and bacteria — it is absent in animal cells. In plants, it is made of cellulose and gives the cell a definite shape. The plasma membrane (cell membrane) is present in all cells — a selectively permeable phospholipid bilayer that regulates what enters and exits the cell.

• Cytoskeleton: network of protein filaments — microfilaments (actin), intermediate filaments, microtubules (tubulin). Provides shape, motility and intracellular transport.
• Centrioles (centrosome): pair of cylindrical structures at right angles, made of microtubule triplets (9+0 arrangement). Found in animal cells — form the spindle apparatus during cell division. Absent in most higher plant cells.
• Cilia and flagella: hair-like projections for movement. Cilia are short and numerous; flagella are long and few. Eukaryotic flagella have a 9+2 axoneme arrangement (9 doublet microtubules + 2 central singlets). Prokaryotic flagella are made of flagellin and have no internal microtubule structure.

The nucleus is the control centre of the cell — it houses the genetic material and directs all cellular activities. It is bounded by a double membrane (nuclear envelope) perforated by nuclear pores (for RNA and protein transport between nucleus and cytoplasm).

• Nuclear envelope: outer membrane continuous with RER; nuclear pores regulate traffic.
• Nucleoplasm: ground substance inside the nucleus containing chromatin and nucleolus.
• Nucleolus: dense, spherical body — site of rRNA synthesis and ribosome subunit assembly. Multiple nucleoli may be present.
• Chromatin: DNA + histone proteins in a thread-like, dispersed form (interphase). During cell division, chromatin condenses into chromosomes.
• Chromosomes: each has two chromatids joined at a centromere. In humans, 46 chromosomes (23 pairs — 22 autosomes + 1 pair of sex chromosomes).

Example: Explain why the mitochondria and chloroplasts are considered semi-autonomous organelles.

Solution: Both mitochondria and chloroplasts have their own circular DNA and 70S ribosomes (the same size as prokaryotic ribosomes). They can synthesise some of their own proteins independently of the nuclear genome. This, along with their double-membrane structure, supports the endosymbiotic theory — that both organelles evolved from free-living bacteria engulfed by an ancestral eukaryotic cell. Because they depend on the nucleus for most of their proteins but retain partial genetic independence, they are called semi-autonomous.

Example: A cell is treated with a drug that disrupts ribosome function. Predict which organelles and processes will be directly affected.

Solution: Ribosomes are the site of protein synthesis, so the most direct effect is cessation of protein synthesis. This hits the rough ER (which has attached 80S ribosomes) and free cytoplasmic ribosomes first — secretory and membrane proteins stop being made, and cytoplasmic proteins deplete. Over time, the Golgi body (dependent on ER-derived vesicles) is also affected. Mitochondria and chloroplasts have their own 70S ribosomes — they may be partially shielded initially if the drug specifically targets 80S ribosomes, but sustained disruption affects overall ATP production and cell viability.

Exercises (NCERT Q1–Q14)

• 1) Which of the following is not correct? (a) Robert Brown discovered the cell. (b) Schleiden and Schwann formulated the cell theory. (c) Virchow explained that cells are formed from pre-existing cells. (d) A unicellular organism carries out its life activities within a single cell.
• 2) New cells generate from (a) bacterial fermentation (b) regeneration of old cells (c) pre-existing cells (d) abiotic materials
• 3) Match the following Column I Column II (a) Cristae (i) Flat membranous sacs in stroma (b) Cisternae (ii) Infoldings in mitochondria (c) Thylakoids (iii) Disc-shaped sacs in Golgi apparatus
• 4) Which of the following is correct: (a) Cells of all living organisms have a nucleus. (b) Both animal and plant cells have a well defined cell wall. (c) In prokaryotes, there are no membrane bound organelles. (d) Cells are formed de novo from abiotic materials.
• 5) What is a mesosome in a prokaryotic cell? Mention the functions that it performs.
• 6) How do neutral solutes move across the plasma membrane? Can the polar molecules also move across it in the same way? If not, then how are these transported across the membrane?
• 7) Name two cell-organelles that are double membrane bound. What are the characteristics of these two organelles? State their functions and draw labelled diagrams of both.
• 8) What are the characteristics of prokaryotic cells?
• 9) Multicellular organisms have division of labour. Explain.
• 10) Cell is the basic unit of life. Discuss in brief.
• 11) What are nuclear pores? State their function.
• 12) Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.
• 13) Describe the structure of the following with the help of labelled diagrams. (i) Nucleus (ii) Centrosome
• 14) What is a centromere? How does the position of centromere form the basis of classification of chromosomes. Support your answer with a diagram showing the position of centromere on different types of chromosomes.

Answers at a glance

1. 1
   Q1 → The incorrect statement is (a), since Robert Brown did not discover the cell. The chapter credits Antonie von Leeuwenhoek, who first saw and described a live cell, whereas the contribution of Robert Brown was a different one, for in 1831 he described the nucleus. The other three statements are correct. In 1838 Matthias Schleiden, a German botanist, examined a large number of plants and observed that all plants are composed of different kinds of cells which form the tissues of the plant, and at about the same time Theodore Schwann (1839), a German zoologist, studied different types of animal cells, and Schwann proposed the hypothesis that the bodies of animals and plants are composed of cells and products of cells, so Schleiden and Schwann together formulated the cell theory. That theory did not explain as to how new cells were formed, and Rudolf Virchow in 1855 first explained that cells divided and new cells are formed from pre-existing cells, Omnis cellula-e cellula, modifying the hypothesis of Schleiden and Schwann to give the cell theory its final shape. And a unicellular organism does carry out all its life activities within the single cell that constitutes its entire body, which is precisely why the cell is both the structural and the functional unit of life.
2. 2
   Q2 → (c) Pre-existing cells. This is the second statement of the cell theory as it is now understood, that all cells arise from pre-existing cells, and it was supplied by Rudolf Virchow in 1855, who first explained that cells divided and new cells are formed from pre-existing cells, Omnis cellula-e cellula, and in doing so modified the hypothesis of Schleiden and Schwann to give the cell theory a final shape; the first statement, that all living organisms are composed of cells and products of cells, came from Schleiden and Schwann but did not explain as to how new cells were formed. The alternatives are wrong: abiotic materials are contradicted directly by the second statement, since a cell is never built up out of non-living matter; bacterial fermentation is merely a metabolic process of certain bacteria and has no bearing on cell origin; and regeneration of old cell substance is not what happens, since a cell does not rebuild itself but divides, and by that division each cell gives new cells, so that the increase in cell number is always the result of the division of cells already present.
3. 3
   Q3 → (a) Cristae — (ii) Infoldings in mitochondria, since the cristae, singular crista, are the infoldings of the inner mitochondrial membrane that project towards the matrix and increase the surface area, giving more membrane to carry the specific enzymes associated with the mitochondrial function, in which aerobic respiration produces ATP. (b) Cisternae — (iii) Disc-shaped sacs in Golgi apparatus, since the cisternae are the stacks of flattened disc-like sacs that constitute the Golgi body, in which the proteins synthesised by ribosomes on the endoplasmic reticulum are modified before they are released from the trans face. (c) Thylakoids — (i) Flat membranous sacs in stroma, since the thylakoids are the organised flattened membranous sacs lying within the stroma of the chloroplast, arranged in stacks like piles of coins called grana, singular granum, or the intergranal thylakoids, and connected to one another by the flat membranous tubules called the stroma lamellae, with their membrane enclosing a space called a lumen, and they carry the chlorophyll and carotenoid pigments that trap the light energy essential for photosynthesis. The three are best kept apart by remembering that the crista is a fold of the mitochondrion, the cisterna a sac of the Golgi and the thylakoid a sac of the chloroplast, and that the mitochondrion is the only one of the three in which the named part is an infolding rather than a sac.
4. 4
   Q4 → The correct statement is (c), that in prokaryotes there are no membrane-bound organelles. The nuclear membrane is found in eukaryotes, and no organelles like the ones in eukaryotic cells are found in prokaryotic cells except for ribosomes, so the prokaryotic cell has no endoplasmic reticulum, no Golgi apparatus, no mitochondria, no lysosomes, no chloroplasts and no nucleus, and it therefore lacks the extensive compartmentalisation of cytoplasm that characterises a eukaryotic cell. The other three are incorrect. Statement (a) is wrong because there is no well-defined nucleus in the prokaryotic cell, its genetic material being basically naked and not enveloped by a nuclear membrane but lying directly in the cytoplasm, so an organised nucleus with a nuclear envelope belongs only to the eukaryotic cell. Statement (b) is wrong because only the plant cell of the two has a well defined cell wall, the animal cell having no cell wall at all and being bounded only by a plasma membrane, since cell walls, plastids and a large central vacuole are present in plant cells and absent in animal cells. Statement (d) is wrong because all cells arise from pre-existing cells, which is the second statement of the cell theory.
5. 5
   Q5 → A mesosome is a special membranous structure of the prokaryotic cell, formed by the extensions of the plasma membrane into the cell, and these extensions are in the form of vesicles, tubules and lamellae, so the mesosome is essentially an infolding of the cell membrane and is the characteristic of prokaryotes; the plural of the word is mesosomata. It performs four main functions. First, it helps in respiration, so that the prokaryotic cell respires without any membrane-bound organelle. Second, it helps in cell wall formation. Third, it helps in DNA replication and in the distribution of the DNA to the daughter cells. Fourth, it helps in the secretion processes, and it also helps to increase the surface area of the plasma membrane and its enzymatic content.
6. 6
   Q6 → Neutral solutes move across the plasma membrane by the process of simple diffusion along the concentration gradient, that is from higher concentration to the lower, and this is a passive transport, since many molecules can move briefly across the membrane without any requirement of energy. Water may also move across the membrane from higher to lower concentration, and the movement of water by diffusion is called osmosis. The polar molecules cannot move across it in the same way, because they cannot pass through the nonpolar lipid bilayer; instead they require a carrier protein of the membrane, which facilitates their transport across it. So the neutral solutes diffuse down the gradient on their own, while the polar molecules are ferried across with the help of a carrier protein. The same membrane also carries out the opposite kind of transport, for a few ions or molecules are moved across it against their concentration gradient, from lower to the higher concentration, and since this is an energy dependent process in which ATP is utilised, it is called active transport, the example being the Na+/K+ pump.
7. 7
   Q7 → The two organelles that are double membrane bound are the mitochondrion and the chloroplast. (i) The mitochondrion is a double membrane-bound structure with an outer membrane and an inner membrane dividing its lumen distinctly into two aqueous compartments, the outer compartment and the inner compartment, the inner compartment being filled with a dense homogeneous substance called the matrix while the outer membrane forms the continuous limiting boundary of the organelle, and the inner membrane forms a number of infoldings called the cristae, singular crista, towards the matrix, the cristae increasing the surface area. The two membranes have their own specific enzymes associated with the mitochondrial function, and the mitochondria are the sites of aerobic respiration, producing 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 of the 70S type and the components required for the synthesis of proteins, and the mitochondria divide by fission. In shape and size it shows considerable variability and is typically sausage-shaped or cylindrical, with a diameter of 0.2 to 1.0 micrometre, the average 0.5 micrometre, and a length of 1.0 to 4.1 micrometres, and it is not easily visible under the microscope unless specifically stained; the longitudinal section diagram should label the outer membrane, the inner membrane, the inter-membrane space, the matrix and a crista. (ii) The chloroplast is also double membrane bound, the inner chloroplast membrane being relatively less permeable, and the space limited by it is called the stroma, in which a number of organised flattened membranous sacs called the thylakoids are present, arranged in stacks like piles of coins called grana, singular granum, or the intergranal thylakoids, with flat membranous tubules called the stroma lamellae connecting the thylakoids of the different grana and the membrane of the thylakoid enclosing a space called a lumen. The chloroplasts contain chlorophyll and carotenoid pigments which are responsible for trapping the light energy essential for photosynthesis, and the majority of the chloroplasts of the green plants are found in the mesophyll cells of the leaves, these organelles being lens-shaped, oval, spherical, discoid or even ribbon-like, with a variable length of 5 to 10 micrometres and a width of 2 to 4 micrometres, and their number varying from 1 per cell of Chlamydomonas to 20 to 40 per cell in the mesophyll. On the type of pigments, plastids are classified into chloroplasts, chromoplasts and leucoplasts, the chromoplasts carrying the fat soluble carotenoid pigments like carotene and xanthophylls that give a part of the plant a yellow, orange or red colour, and the colourless leucoplasts storing nutrients, the amyloplasts storing carbohydrates such as starch as in potato, the elaioplasts storing oils and fats and the aleuroplasts storing proteins. The sectional diagram of the chloroplast should label the outer membrane, the inner membrane, the stroma, a granum, the stroma lamellae and the lumen.
8. 8
   Q8 → The major characteristics of a prokaryotic cell are these. The prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma and PPLO, Pleuro Pneumonia Like Organisms, and they are generally smaller and multiply more rapidly than the eukaryotic cells, and they may vary greatly in shape and size, the four basic shapes of bacteria being bacillus, rod like, coccus, spherical, vibrio, comma shaped, and spirillum, spiral, with the shape of the cell varying with the function it performs. Most of them, particularly the bacterial cells, have a chemically complex cell envelope consisting of a tightly bound three layered structure, the outermost glycocalyx followed by the cell wall and then the plasma membrane, and all prokaryotes have a cell wall surrounding the cell membrane except in mycoplasma. The glycocalyx differs in composition and thickness among different bacteria, being a loose sheath called the slime layer in some and thick and tough, called the capsule, in others. The cell wall determines the shape of the cell and provides a strong structural support to prevent the bacterium from bursting or collapsing, and the plasma membrane is selectively permeable and interacts with the outside world. On the basis of the differences in the cell envelopes and the manner in which they respond to the staining procedure developed by Gram, those that take up the gram stain are Gram positive and the others that do not are Gram negative bacteria. There is no well-defined nucleus, and the genetic material is basically naked, not enveloped by a nuclear membrane, lying directly in the semi-fluid matrix that fills the cell, the cytoplasm, and small circular DNA molecules called plasmids are also present in the cytoplasm. A specialised differentiated form of cell membrane called the mesosome is the characteristic of prokaryotes, being essentially an infolding of the cell membrane, and it helps in cell wall formation, in DNA replication and distribution to daughter cells, in respiration and in secretion processes, while increasing the surface area of the plasma membrane and its enzymatic content; in cyanobacteria there are other membranous extensions into the cytoplasm called chromatophores, which contain pigments. The ribosomes are associated with the plasma membrane, are about 15 nanometres by 20 nanometres in size, and are made of the 50S and 30S subunits which together form the 70S prokaryotic ribosomes, being the site of protein synthesis, and several may attach to a single mRNA to form a polyribosome or polysome. Reserve materials are stored as inclusion bodies, which are not bound by any membrane system and lie free in the cytoplasm, such as phosphate granules, cyanophycean granules and glycogen granules, and gas vacuoles are found in blue green and in purple and green photosynthetic bacteria. Bacterial cells may be motile or non-motile, and if motile they have thin filamentous extensions from the cell wall called flagella, composed of the filament, the hook and the basal body, the filament being the longest portion; pili and fimbriae are also surface structures but do not play a role in motility, and in some bacteria they help attach the bacteria to rocks in streams and to the host tissues.
9. 9
   Q9 → Division of labour is the phenomenon in which the cells of a multicellular organism are grouped into specialised units, and each group performs one particular function, so that the work of the organism is divided among the groups and the whole organism carries out its life activities more efficiently than a single cell could, whereas in a unicellular organism all the metabolic, excretory and reproductive activities are performed within the one cell. It is achieved in a particular way. A multicellular organism arises by the repeated mitotic division of a single cell, the zygote, and the cells so produced do not all remain alike, for while a part of them keeps dividing and retains the capacity to divide, forming the meristematic tissue, a part stops dividing, changes its shape and takes on a particular function, forming the permanent differentiated tissue. The differentiated cells of one kind form a tissue, tissues combine to form an organ, and the organs work together as an organ system, so the division of the cells is differential and the cells end up specialised, and this is possible because all the cells descend from the one original zygote and share the same genetic constitution. The consequences are several. Each specialised group performs its own work far more efficiently, as in the human body where the muscle cells contract, the nerve cells conduct, the lung tissue respires and the kidney tissue excretes. Division of labour is responsible for the differentiation and specialisation that produce the complex organisation of the body and its organ systems. It raises the efficiency of the organism, since a specialised cell does one thing better than a general cell, which allows a larger body and a higher rate of activity. It also integrates the organism, since the specialised groups do not act independently but are co-ordinated so that the whole is a co-ordinated whole and not a collection of parts. Thus division of labour is the process by which the single cell becomes a functional unit of a larger integrated organism.
10. 10
    Q10 → The cell is called the basic unit of life for several reasons, and they are the reasons the cell theory was formulated to express. First, the cell is the basic structural unit, for all living organisms are composed of cells and their products, which was the first proposition of Schleiden and Schwann, and in a multicellular organism the cells are not a loose collection but are arranged into tissues, organs and organ systems, so the whole structure of the body is built on the cell. Second, the cell is the basic functional unit, since all the life activities of the organism, the metabolic and physiological processes by which it lives, are performed within the cell, and the cell is the smallest unit that can carry out all of them, including ingestion, digestion, respiration, excretion, synthesis, growth and reproduction; this is why the cell is called both the structural and the functional unit of life. Third, the cell is the basic unit of reproduction, since every cell reproduces and the division of a cell is the basis of all reproduction, in a unicellular organism the one cell dividing to give two new organisms and in a multicellular organism the cells dividing to build the body and to produce the gametes, so that no new organism arises except by the division and growth of cells. Fourth, the cell is the basic unit of metabolism and of heredity, being the site of all the chemical reactions of the organism and the place where the DNA, the chromosomes and the machinery of protein synthesis lie, so the cell carries both the information and the means of expressing it, and the hereditary characters pass from cell to cell and from parent to offspring through the cell. Fifth, the cell is the basic unit of growth and development, since an organism grows by an increase in the number of its cells and not by an increase in the size of its material, and every organism begins as one cell, the zygote, which divides to form the whole. Finally, the cell is the smallest unit that can exhibit all the characters of life, because structures smaller than a cell, such as the organelles, cannot live independently, while a cell can, as it does in a unicellular organism. These facts are summed up in the two sentences of the cell theory: that the cell is the basic structural and functional unit of life, and that every new cell arises only from a pre-existing cell.
11. 11
    Q11 → A nuclear pore is one of the minute pores by which the nuclear envelope is interrupted, for at a number of places the envelope is interrupted by minute pores which are formed by the fusion of its two membranes, the envelope itself consisting of two parallel membranes with a space between called the perinuclear space and forming a barrier between the materials present inside the nucleus and that of the cytoplasm. Its function is that the nuclear pores are the passages through which movement of RNA and protein molecules takes place in both directions between the nucleus and the cytoplasm, so that the RNA made in the nucleus passes out to the cytoplasm and the proteins made in the cytoplasm pass into the nucleus. The outer membrane of the envelope usually remains continuous with the endoplasmic reticulum and also bears ribosomes on it, and it is at the pores that the two membranes of the envelope are fused.
12. 12
    Q12 → Both are endomembrane structures, since the endomembrane system includes the ER, the golgi complex, the lysosomes and the vacuoles, and both are bounded by a single membrane, but they differ sharply in function. A lysosome is a membrane bound vesicular structure formed by the process of packaging in the golgi apparatus, and the isolated lysosomal vesicles have been found to be very rich in almost all types of hydrolytic enzymes, the hydrolases such as the lipases, proteases and carbohydrases, which are optimally active at the acidic pH and are capable of digesting carbohydrates, proteins, lipids and nucleic acids, so the lysosome is the digestive compartment of the cell and its function is intracellular digestion. A vacuole, on the other hand, is the membrane-bound space found in the cytoplasm, bound by a single membrane called the tonoplast, and it contains water, sap, excretory product and other materials not useful for the cell; in plant cells the vacuoles can occupy up to 90 per cent of the volume of the cell, and in plants the tonoplast facilitates the transport of a number of ions and other materials against concentration gradients into the vacuole, hence their concentration is significantly higher in the vacuole than in the cytoplasm, which makes the vacuole a storage and regulatory compartment rather than a digestive one. The vacuole also has specialised forms, for in Amoeba the contractile vacuole is important for osmoregulation and excretion, and in many cells, as in protists, food vacuoles are formed by engulfing the food particles. Thus the lysosome digests and the vacuole stores and regulates, and that is the functional difference between two structures of the same endomembrane system.
13. 13
    Q13 → (i) The nucleus. The nucleus as a cell organelle was first described by Robert Brown as early as 1831, and the material of the nucleus stained by the basic dyes was later given the name chromatin by Flemming. Electron microscopy has revealed that the nuclear envelope consists of two parallel membranes with a space between them, of 10 to 50 nanometres, called the perinuclear space, and that it forms a barrier between the materials present inside the nucleus and that of the cytoplasm; the outer membrane usually remains continuous with the endoplasmic reticulum and also bears ribosomes on it, and at a number of places the envelope is interrupted by minute pores formed by the fusion of its two membranes. Within it the nuclear matrix, or the nucleoplasm, contains the nucleolus and the chromatin, the interphase nucleus having highly extended and elaborate nucleoprotein fibres called chromatin along with one or more spherical bodies called nucleoli, singular nucleolus; the nucleoli are not membrane bound, their content is continuous with the rest of the nucleoplasm, and the nucleolus is a site for active ribosomal RNA synthesis, larger and more numerous nucleoli being present in cells actively carrying out protein synthesis. Chromatin contains DNA and some basic proteins called histones, some non-histone proteins and also RNA, and during different stages of cell division the cells show structured chromosomes in place of the nucleus. Normally there is only one nucleus per cell, though variations in the number are frequently observed, and some mature cells even lack a nucleus, such as the erythrocytes of many mammals and the sieve tube cells of the vascular plants. The labelled diagram must show the nuclear envelope as a double line, the perinuclear space, a nuclear pore, the nucleoplasm, the chromatin and the nucleolus. (ii) The centrosome. A centrosome is an organelle usually containing two cylindrical structures called centrioles, surrounded by amorphous pericentriolar materials, and both the centrioles in a centrosome lie perpendicular to each other, in which each has an organisation like the cartwheel, being made up of nine evenly spaced peripheral fibrils of tubulin protein, each of the peripheral fibrils being a triplet, with the adjacent triplets also linked, while the central part of the proximal region of the centriole is proteinaceous and called the hub, which is connected with the tubules of the peripheral triplets by radial spokes made of protein. 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, the centrosome carrying no enclosing membrane of its own, and the animal cells have centrioles which are absent in almost all plant cells. The labelled diagram should show the two rings of the paired centrioles at right angles, each with the nine peripheral triplets in a cartwheel around the central hub, surrounded by the pericentriolar material.
14. 14
    Q14 → A centromere is the primary constriction of a chromosome, for every chromosome, which is 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, and the centromere holds the two chromatids of a chromosome together. Based on the position of the centromere the chromosomes are classified into four types. Metacentric: the centromere is in the middle of the chromosome, forming two equal arms of the chromosome, so the diagram is a straight rod with the centromere marked exactly at the centre and the two arms labelled as equal. Sub-metacentric: the centromere is slightly away from the middle of the chromosome, resulting in one shorter arm and one longer arm, so the diagram is a rod with the centromere marked a little to one side of the centre. Acrocentric: the centromere is situated close to its end, forming one extremely short and one very long arm, so the diagram is a rod with the centromere marked almost at the tip. Telocentric: the centromere is terminal, at the very end of the chromosome, so the diagram is a straight rod with the centromere marked at the very tip and a single arm. Thus, keeping the length of the chromosome the same in each case and moving only the centromere, one obtains the metacentric, sub-metacentric, acrocentric and telocentric types, and it is the position of the centromere that forms the basis of the classification of chromosomes. A related detail worth adding is that sometimes a few chromosomes have non-staining secondary constrictions at a constant location, which gives the appearance of a small fragment called the satellite, and this secondary constriction must not be confused with the primary constriction, the centromere.

MCQ Questions

1 Mark Questions

1. 1
   What is a plasmid? (Ans: extrachromosomal circular DNA found in prokaryotes)
2. 2
   Where does protein synthesis occur? (Ans: ribosomes (free or on rough ER))
3. 3
   Name the nucleoid. (Ans: the region in prokaryotes containing the genetic material, not enclosed by a membrane)
4. 4
   Write the pigment present in chloroplasts. (Ans: chlorophyll)

2 Mark Questions

1. 1
   Differentiate between prokaryotic and eukaryotic cells on any two points.
2. 2
   Why is mitochondria called the power house of the cell? (Ans: produces most ATP through aerobic respiration)
3. 3
   Name two organelles present in plant cells but not in animal cells. (Ans: plastids/chloroplasts, and a large central vacuole; cell wall)

3 Mark Questions

1. 1
   Describe the fluid mosaic model of the plasma membrane. (Ans: phospholipid bilayer with proteins (integral/ peripheral), cholesterol, carbohydrates as glycocalyx)
2. 2
   Describe the structure and functions of the Golgi apparatus. (Ans: stacks of flattened cisternae; modifies, packages and transports proteins/lipids; forms lysosomes)
3. 3
   Give the ultrastructure of a chloroplast. (Ans: double membrane, stroma, grana with thylakoids, DNA and ribosomes, enzymes for Calvin cycle)

5 Mark Questions

1. 1
   Describe the structure and function of the nucleus, including the nuclear envelope, nucleolus, chromatin and nucleoplasm. (Ans: controls all cell activities; site of DNA/RNA synthesis; nuclear pores for exchange)
2. 2
   Give a comparative account of prokaryotic and eukaryotic cells covering: nucleus, organelles, ribosomes (70S vs 80S), cell wall and membrane systems, with examples. (Ans: bacteria/cyanobacteria vs protists, fungi, plants, animals)