NCERT
Every NCERT book that matters for UPSC — open a book, then a chapter. Learn each chapter's concepts, then how UPSC tests them — with diagrams, flowcharts and the past questions to attempt.
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← Exploration — all chaptersCell: The Building Block of Life
Start here: why this chapter matters
In simple wordsA house is built from bricks, but a cell is a strange brick: it eats, makes its own parts, throws out rubbish, talks to its neighbours and builds copies of itself. Every living thing — a bacterium, a neem tree, you — is made of these living bricks. This chapter opens one up and names its rooms.
This is Chapter 2 of Exploration, NCERT's new Class 9 science textbook for NCF-2023 (book pages 8–27; the contents page gives the same title, and Chapter 3, Tissues in Action, starts on page 28). It opens with life's possible origin in hot springs and the idea of levels of organisation (cells → tissues → organs → organ systems), then covers how cells are studied (2.1), the membrane and wall (2.2), the interior and its organelles (2.3), cell division (2.4) and Cell Theory with cell life span and cancer (2.5). It has 22 figures, 2 tables, 5 activities, four ‘Threads of Curiosity’, four ‘Ready to Go Beyond’, three ‘Meet a Scientist’ boxes, one ‘Bridging Science and Society’ box, two ‘What if …’ boxes, seven ‘Pause and Ponder’ questions and 16 end exercises. This page has nine concept sections.
- Neighbouring chapters on this site: Exploration: Entering the World of Secondary Science (Class 9, Ch 1) for estimation and models, which Activity 2.1 uses; Life Processes (Class 10) for cellular respiration, ATP and photosynthesis; How do Organisms Reproduce and Heredity (Class 10) for DNA copying and chromosomes; Biological Classification (Class 11) for Monera, viruses, viroids and prions; Human Reproduction (Class 12) for meiosis in making gametes; Human Health and Disease (Class 12) for cancer; Biotechnology: Principles and Processes and Biotechnology and its Applications (Class 12) for cell culture, tissue culture and Haberlandt. The Class 11 NCERT chapters Cell: The Unit of Life and Cell Cycle and Cell Division take this chapter further.
- UPSC relevance: one of the most-asked school biology topics in Prelims science — which organelles carry DNA, which make proteins, plant vs animal cells, osmosis, acellular agents, programmed cell death (questions from 1987 to 2020 are linked below). It is the base for GS3 Science and Technology (stem cells, cell culture and vaccines, synthetic biology, cancer therapy, tissue-culture plants) and touches GS3 agriculture (salt and sugar preservation against post-harvest loss, exercise 16) and GS2 health.
| Word | What it means here |
|---|---|
| Cell | the basic structural and functional unit of every living organism |
| Unicellular / multicellular | made of one cell (bacteria, yeast) / of many cells working together (plants, fish, humans) |
| Limit of resolution | the smallest separation at which two points are still seen as two; 0.1 mm for the human eye at 25 cm |
| Selectively permeable | lets some substances through and blocks others (the cell membrane) |
| Osmosis | diffusion of water across a selectively permeable membrane, from the dilute to the concentrated side |
| Organelle | a sub-cellular part with a specific job; ‘membrane-bound’ ones have their own membrane |
| Prokaryotic / eukaryotic | without / with a well-defined nucleus and membrane-bound organelles (karyon = nucleus) |
| Chromosome, DNA, gene | rod-shaped carriers of heredity / the molecule holding genetic information / a functional segment of DNA |
| Mitosis / meiosis | division giving two identical cells / two divisions giving four cells with half the chromosomes |
| Cell Theory | all organisms are made of cells; the cell is the basic unit; all cells come from pre-existing cells |
Reading NCERT's chapter opening and ‘Think It Over’ (page 8)An illustration of a student in a lab coat at a light microscope; a magnified circle shows tiny organisms and cells, with arrows leading to cut-away models of a plant cell, a rod-shaped bacterium and an animal cell; a QR code labelled 0906CH02 sits by the title. ‘Think It Over’ asks: where does a cell come from? how has technology helped us see beyond the naked eye? how is the cell the structural and functional unit of life? how does a cell multiply? The point: the chapter's four sections answer these four questions in turn.
The opening text adds a story about life's beginnings: life is widely thought to have begun in water, and some researchers think in small pools with changing conditions such as hot springs rather than oceans. The Puga Valley hot springs in Ladakh stay nearly at the boiling point of water in a cold climate, conditions thought similar to the early Earth about 3.5 billion years ago; they hold mostly unicellular heat-loving bacteria, thermophiles. Scientists of the Birbal Sahni Institute of Palaeosciences, Lucknow, found calcium carbonate forming rapidly around these springs; such deposits may have shielded early organic molecules from radiation and helped form the first protective membrane — the barrier that defines a cell. Cells group into tissues, tissues into organs, organs into organ systems (nostrils, nasal cavity, trachea and lungs form the respiratory system), but the cell stays the fundamental unit.
Common confusions
- The hot-spring origin is presented as what some researchers believe — a hypothesis, not a settled fact; ‘life began in water’ is the widely accepted part.
- Levels of organisation run cell → tissue → organ → organ system → organism; a tissue is a group of similar cells doing similar work.
- NCERT's ‘nasal pores’ means the nostrils.
Beyond NCERT
- Heat-loving microbes matter for technology: the heat-stable DNA polymerase used in PCR (Taq polymerase) comes from the thermophilic bacterium Thermus aquaticus — see Biotechnology: Principles and Processes (Class 12) on this site.
At a glance — the six ideas to carry away
- Seeing cells: the eye resolves 0.1 mm at 25 cm; cells need microscopes. Robert Hooke (1665) named ‘cells’ in cork. Total magnification = eyepiece × objective (10X × 10X = 100X). Activity 2.1: 5,000 µm field ÷ 25 cells = 200 µm onion cell. Electron microscopes use electrons, not light.
- Cell membrane: in every cell; selectively permeable; 7–10 nm lipid bilayer with proteins — the fluid-mosaic model. Osmosis = diffusion of water across such a membrane towards the more concentrated side: potato swells in water, shrinks in 20% salt.
- Cell wall (plants, fungi, bacteria) is rigid but permeable; plant walls are cellulose. In strong sugar solution a plant cell's contents pull away from the wall but the cell keeps its shape; a cheek cell simply shrinks.
- Prokaryotic (bacteria, 1–10 µm): nucleoid, no membrane-bound organelles. Eukaryotic (10–100 µm): nucleus + organelles. Plant cells: wall, plastids, one big vacuole. Viruses, viroids and prions are acellular.
- Organelles: nucleus (chromosomes, DNA, genes; nucleolus makes ribosome parts) · ribosomes (protein) · RER/SER · Golgi (‘post office’) · lysosomes (clean-up) · mitochondria (ATP; cristae) · plastids (chloro-, chromo-, leucoplasts) · vacuole. Mitochondria and plastids carry their own DNA and ribosomes.
- Division and theory: mitosis → 2 identical cells; meiosis → 4 cells with half the chromosomes (gametes). Cell Theory: Schleiden 1838, Schwann 1839, Virchow 1855 (cells from cells). Contact inhibition fails in cancer; programmed cell death shapes fingers; Haberlandt (1902): totipotency.
1. How to study cells: resolution, magnification and microscopes
In simple wordsFrom across the road, a car's two headlights look like one blur; walk closer and they split into two. Your eye has a limit to how close two things can be before they merge. A cell is far below that limit, so we need a microscope — not just to make things bigger, but to keep tiny neighbouring things separate.
Draw two dots and move them closer: at some point they look like one. Viewed from about 25 cm (the near point of the eye), two points about 0.1 mm apart can just be told apart — the limit of resolution of the human eye. Most cells are smaller, so they are studied with lenses: a convex lens, or an objective lens and an eyepiece together, produces magnification. Robert Hooke was the first to observe a cell, in 1665, with a self-designed microscope magnifying about 200–300X: a thin slice of cork showed small box-like compartments he called ‘cells’. School light microscopes use visible light and objectives such as 10X and 40X. Total magnification = eyepiece × objective: 10X × 10X = 100X, so a 200 µm cell looks 100 times larger. Microscopes have improved in three features — resolution (clarity), contrast (difference in brightness between parts) and magnification. Electron microscopes use a beam of electrons instead of light and show cell structure at the nanometre scale (1 nm = one-billionth of a metre).
⤵ Estimating the size of a cell (Activity 2.1)
Measure the fieldtransparent mm ruler on the stage; read the diameter of the circular field: say 5 mm
Convert units1 mm = 1,000 µm, so 5 mm = 5,000 µm
Swap in the specimenonion peel slide on the stage, same objective
Countcells in one straight line along the diameter: say 25
Divide5,000 µm ÷ 25 = 200 µm per cell
Reading NCERT's Fig. 2.1 — Size of objects and their visibility (page 9)A tall vertical scale from 0.1 nm to 1 km with small pictures beside it: atoms, small molecules, proteins and lipids, viruses, ribosomes and the smallest bacteria, mitochondria, most bacteria, the nucleus, most plant and animal cells, Amoeba, a fish egg, a chicken egg, the Great Indian Bustard, human height, a neem tree and a rocket. Three coloured bars show the ranges of the unaided eye, the light microscope and the electron microscope. The point: cells sit between about 1 and 100 µm, below the eye's reach. Our horizontal version is above.
Reading NCERT's Fig. 2.2 — Structure of a light microscope (page 10)A photograph of a school compound microscope labelled eyepiece, body tube, objective lens, stage, mirror, coarse and fine adjustment knobs, handle (arm) and base. The point: two lens systems (eyepiece and objective) multiply their magnifications; the mirror lights the specimen and the knobs focus it.
Reading NCERT's ‘Ready to Go Beyond’ — electron microscope, and Fig. 2.3 (page 10)Fig. 2.3 is a photograph of a large electron microscope cabinet beside a computer screen showing an image. The box: electron microscopes use a beam of electrons instead of light and reveal cell structure at the nanometre scale. The point: organelles such as ribosomes are mostly seen only with electron microscopes.
Reading NCERT's Activity 2.1 — Let us estimate the size of a cell (page 10)Measure the field of view with a mm ruler, convert to micrometres, count onion cells across the diameter and divide. NCERT's worked numbers: 5 mm = 5,000 µm; 25 cells; 200 µm per cell; at 10X × 10X = 100X it appears 100 times larger. The point: a microscope lets you measure, not just look — an estimation habit from Chapter 1.
Reading NCERT's Fig. 2.4 — Electron micrograph of a <i>Colocasia</i> leaf (page 11)A grey scanning-electron-microscope image of the lower surface of a Colocasia (taro) leaf, rough with ridges, with arrows to several stomata and a scale bar of 0.05 mm. The point: an electron microscope shows surface detail far beyond a light microscope.
Common confusions
- Magnification ≠ resolution: enlarging a blur gives a bigger blur; resolution is the clarity that separates close points.
- Total magnification is the product (10X × 40X = 400X), not the sum.
- Hooke saw the walls of dead cork cells, not living cells; NCERT's ‘first person to observe a cell’ is true in that sense. Living single cells were first seen by Antonie van Leeuwenhoek in the 1670s (Beyond NCERT).
- 1 mm = 1,000 µm; 1 µm = 1,000 nm; 1 nm = 0.000001 mm.
- NCERT's Fig. 2.2 prints ‘Course adjustment knob’ — it is the coarse adjustment knob.
Beyond NCERT
- Hooke's observations appeared in his book Micrographia (1665).
- A light microscope cannot separate points closer than about 0.2 µm, whatever its magnification, because of the wavelength of visible light; electrons have far shorter wavelengths, which is why electron microscopes resolve so much more (the reasoning behind UPSC Prelims 1988, a physics question not linked here).
- Two kinds of electron microscope: transmission (thin slices, internal detail) and scanning (surfaces, like Fig. 2.4).
2. The cell membrane, diffusion and osmosis
In simple wordsThink of a school gate with a guard: students in uniform walk through, but outsiders with large bags are stopped. The cell membrane is that gate. Water slips through easily; salt and sugar are turned back. So when the water outside is ‘saltier’ than inside, water walks out of the cell; when it is purer, water walks in.
The cell membrane (plasma membrane) is the thin boundary around every cell; it protects the contents and defines the cell's individuality. It is selectively permeable: some substances pass, others are blocked. Even single-celled organisms exchange materials and respond to surroundings through it. Diffusion is the net movement of particles from a region of higher to lower concentration — it happens even without a membrane (dye spreading in water, fragrance in air). Osmosis is the diffusion of water across a selectively permeable membrane, from the dilute side (more water, less solute) to the concentrated side (less water, more solute), until the concentrations are equal. Root cells take water from the soil this way.
Structurally the membrane is about 7–10 nm thick and made of lipids (fats) and proteins. The fluid-mosaic model explains it: a lipid bilayer — two layers of special fat molecules with water-attracting heads outwards and water-repelling tails inwards — with proteins embedded in it; the molecules can move sideways, flip and rotate (fluid); the molecules are arranged like tiles (mosaic); membrane proteins act as gatekeepers helping substances across.
| Solution outside the cell | Compared with inside | Water moves | Animal cell | Plant cell |
|---|---|---|---|---|
| Isotonic | equal solute concentration | no net movement | unchanged | unchanged |
| Hypotonic | less concentrated | into the cell | swells | swells against the wall, stays firm |
| Hypertonic | more concentrated | out of the cell | shrinks | contents pull away from the wall; shape kept |
⤵ Activity 2.2: potato pieces in water and in salt solution
Cut and weightwo roughly equal potato pieces; record initial weights
Set upBeaker A: plain water; Beaker B: 20 per cent salt or sugar solution
Waitabout an hour, or until a visible change
Weigh againA has swollen and gained weight; B has shrunk and lost weight
Explainthe membranes let water through but not salt or sugar: water moves towards the more concentrated side — osmosis
Reading NCERT's Activity 2.2 and Fig. 2.5 — potato pieces (page 11)Fig. 2.5 is three photographs: two beakers with a potato piece in each (Beaker A plain water, Beaker B salt solution), the two pale pieces before, and after — the piece from water larger and firm, the one from salt solution smaller and wrinkled. The point: water, not salt, crosses the membrane, and its direction depends on concentration. Pause and Ponder 3 asks why the pieces must be similar and pre-weighed (answered below).
Reading NCERT's ‘What if …’ box and Fig. 2.6 — cells in solutions of different concentrations (page 12)A three-row panel: in each row a dish of red cells is placed in a solution and the result shown — isotonic (extracellular solute concentration equal to intracellular): cells unchanged; hypotonic (outside less concentrated): cells larger; hypertonic (outside more concentrated): cells smaller and shrivelled. The point: name the solution by comparing outside with inside, and predict water's direction from it.
Reading NCERT's ‘What if …’ — soaked mung beans (page 12)What happens to mung bean seeds soaked in water for 12 hours and then put in a concentrated solution? The point: the swollen seeds lose water by osmosis and shrink and wrinkle (outline in the intext answers).
Reading NCERT's Fig. 2.7 — Structure of a cell membrane (page 12)A 3-D cut-away of a sheet of purple ball-headed lipids in two layers with tails inward, blue protein channels passing through it, smaller proteins on the surface and chains of beads (sugar chains, unlabelled) sticking out above. Labels: lipid bilayer, various types of proteins. The point: proteins float in a lipid sea. Our schematic is above.
Common confusions
- Osmosis is about water (the solvent) moving towards the more concentrated solution — not solute moving, and not ‘from concentrated to dilute’ (UPSC 2005).
- Diffusion needs no membrane; osmosis needs a selectively permeable one.
- ‘Hypo-’ and ‘hyper-’ describe the outside solution compared with the cell's inside.
- The membrane is selectively permeable; the plant cell wall is freely permeable — exercise 1(i).
- ‘Fluid’ means the molecules move within the sheet, not that the membrane is a liquid that leaks.
Beyond NCERT
- Reverse osmosis (RO) water purifiers push water the ‘wrong’ way by applying pressure on the concentrated side, leaving salts behind (UPSC 2005 tested this).
- The fluid-mosaic model was proposed by S. J. Singer and Garth Nicolson in 1972.
- Oral rehydration solution and intravenous saline are made close to isotonic with body fluids so that blood cells neither swell nor shrink.
3. The cell wall: why plant cells keep their shape
In simple wordsPut a balloon inside a cardboard box. Let some air out of the balloon and it shrinks away from the box walls — but the box keeps its shape. That is a plant cell losing water: the membrane (balloon) pulls in, the cell wall (box) stays. An animal cell is a balloon with no box: it just shrinks.
Cells of plants, fungi and bacteria have an extra layer outside the cell membrane: the cell wall. Plants cannot move away from wind and rain, so their cells need a rigid wall; it keeps leaves and flowers firm and helps plants stay upright. Though rigid, the wall is permeable — water and some dissolved minerals pass through — and together with the selectively permeable membrane it lets roots absorb water and nutrients. The plant cell wall is mainly cellulose, a carbohydrate made of many glucose units linked together; in our diet cellulose is roughage that helps digestion. In fungi and bacteria the wall gives protection and support. Animal cells have no wall, so they change shape easily — a flexibility that supports movement and the working of animal tissues.
Activity 2.3 shows the difference. Onion or Rhoeo (cradle lily) leaf peel stained with safranin shows box-shaped, regularly arranged cells; cheek cells scraped with a cotton swab and stained with methylene blue are irregular. In 20 per cent sugar solution for half an hour, the plant cells' outer boundaries stay the same while the inner contents shrink and the space between them grows; the cheek cells shrink considerably.
⤵ A plant cell in concentrated sugar solution
Solution outside is hypertonic20 per cent sugar is more concentrated than the cell sap
Water leaves by osmosisout through the membrane and the permeable wall
Contents shrinkthe membrane with the cytoplasm pulls away from the wall
Wall holds its shapethe cell's outline is unchanged; a gap opens inside
Back in water, it recoverswater returns and the contents press against the wall again (Beyond NCERT)
Reading NCERT's Activity 2.3 and Fig. 2.8 — onion peel and cheek cells (page 13)Fig. 2.8 is two round micrographs: (a) onion peel cells as neat brick-like rectangles with dark nuclei, stained pink-brown; (b) human cheek cells as flat, irregular blue-stained cells scattered unevenly. The point: a wall gives plant cells a fixed, regular shape; animal cells without one are irregular.
Reading NCERT's Fig. 2.9 — Cradle lily leaf peel in water and in sugar solution (page 13)Two micrographs of Rhoeo leaf peel with pink-purple cell contents: (a) in water, the colour fills each cell; (b) in 20 per cent sugar solution, the coloured contents have shrunk into the middle of many cells while the cell outlines stay put. The point: this is what a plant cell losing water looks like under a school microscope.
Reading NCERT's ‘Pause and Ponder’ 1–3 (page 14)Why do fixed plants need a wall more than moving animals? What would happen if a plant cell wall became as flexible as a membrane? Why cut the potato pieces equal and weigh them first? The point: students argue from function to structure, and design a fair experiment (outlines in the intext section).
Common confusions
- The cell wall is outside the membrane; plant cells have both (UPSC 2020's statement that plant cells lack a plasma membrane is false).
- The wall is permeable; it does not choose what passes — the membrane does.
- Fungi and bacteria have walls too, but not of cellulose (Beyond NCERT: fungal walls contain chitin; bacterial walls peptidoglycan).
- Humans cannot digest cellulose; for us it is roughage, not an energy source.
Beyond NCERT
- The shrinking of a plant cell's contents away from its wall is called plasmolysis; its reversal in water, deplasmolysis. The water pressure that keeps a cell pressed against its wall is turgor, which is why well-watered plants stand firm and dry ones wilt.
4. Prokaryotic and eukaryotic cells; plant and animal cells
In simple wordsA one-room hut does cooking, sleeping and storage in the same space; a big house has separate rooms with doors. A bacterium is the hut — everything happens in one shared space. Plant and animal cells are the house — a nucleus and many walled-off rooms (organelles), each with its own job.
Most cells have three basic parts: a plasma membrane, a semi-fluid, jelly-like cytoplasm, and a nucleus. The cytoplasm holds organelles, most visible only with an electron microscope. A bacterial cell lacks a well-defined nucleus and membrane-bound organelles; its genetic material lies in a region called the nucleoid, and most activities happen directly in the cytoplasm. Such cells are prokaryotic (pro = primitive, karyon = nucleus). Plant and animal cells have a well-defined nucleus and several membrane-bound organelles: eukaryotic (eu = true). In eukaryotic cells a network of fine fibres, the cytoskeleton, supports the cell, keeps its shape and enables movement and internal transport. The cytoplasm may also hold cell inclusions: starch (plant cells), or crystals of calcium oxalate or silica (some plant cells).
| Structure (Table 2.1) | Bacterial cell | Plant cell | Animal cell |
|---|---|---|---|
| Cell membrane | present | present | present |
| Cell wall | present | present | absent |
| Cytoplasm | present | present | present |
| Well-defined nucleus (membrane around DNA) | absent | present | present |
| Nucleoid (DNA without a membrane) | present | absent | absent |
| Membrane-bound organelles | absent | present | present |
| Characteristic (Table 2.2) | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Primitive nucleus (nucleoid) | present | absent |
| Diameter of a typical cell | 1 to 10 µm | 10 to 100 µm |
| Number of cells in an organism | usually unicellular | unicellular or multicellular |
| Membrane-bound organelles | absent | present |
| Membrane-bound nucleus | absent | present |
| Feature | Plant cell | Animal cell |
|---|---|---|
| Cell wall | present (cellulose) | absent |
| Shape | fixed, often box-like | flexible, often irregular |
| Plastids | present (chloro-, chromo-, leucoplasts) | absent |
| Vacuole | usually one large central vacuole | sometimes present, small |
| Lysosomes | not shown in NCERT's plant cell | present |
| Contact inhibition | not shown (rigid walls) | shown by many cells |
Reading NCERT's Activity 2.4, Fig. 2.10 and Tables 2.1–2.2 (pages 14–15)Fig. 2.10 is three coloured cut-away drawings: (a) a green rod-shaped bacterium with a long whip-like ‘appendage for locomotion’, pink coiled nucleoid, ribosomes, cytoplasm and cell wall; (b) a box-shaped plant cell with a thick green wall, a large pale-blue vacuole, nucleus with nucleolus, chloroplasts, mitochondria, ER, Golgi body and ribosomes; (c) a round red animal cell cut open to show the nucleus, ER, Golgi body, mitochondria, lysosome, a small vacuole and ribosomes. Activity 2.4 asks students to tick structures in the blank Table 2.1; Table 2.2 compares prokaryotes and eukaryotes. The point: the nucleus and membrane-bound organelles divide the living world's cells into two kinds. Our filled tables are above.
Reading NCERT's ‘Ready to Go Beyond’ — viruses, viroids and prions (page 15)Viruses, viroids and prions are acellular (no cells) infectious agents too small for a light microscope: viruses are genetic material in a protein coat; viroids lack the protein coat; prions are misfolded proteins with no genetic material. The point: not every infectious agent is a cell — these sit outside Cell Theory. Biological Classification (Class 11) on this site treats them in detail.
Reading NCERT's ‘Ready to Go Beyond’ — cytoskeleton and cell inclusions (page 15)Eukaryotic cells have a network of fine fibres, the cytoskeleton, for support, shape, movement and internal transport, visible as a separate entity only under an electron microscope; the cytoplasm may also store starch or crystals of calcium oxalate or silica, called cell inclusions. The point: the cytoplasm is organised, not a bag of soup.
Common confusions
- Prokaryotes do have DNA and ribosomes — they lack a nuclear membrane and membrane-bound organelles, not genetic material.
- Bacteria have a cell wall: ‘wall’ does not mean ‘plant’.
- Plastids and the cell wall are found in plant cells but not animal cells (UPSC 1987); mitochondria, Golgi and chromosomes are in both.
- ‘Eukaryotic’ includes unicellular organisms such as Amoeba and yeast; ‘prokaryotic’ is not the same as ‘unicellular’.
- Viruses, viroids and prions are not cells; prions are proteins with no genetic material (UPSC 2002).
Beyond NCERT
- Viroids are small circular RNA molecules without a protein coat (UPSC 2002); Mycoplasma are bacteria with no cell wall at all; rickettsias have one.
- Fungal cell walls contain chitin; bacterial walls peptidoglycan.
5. The nucleus: house of coded instructions
In simple wordsA school office keeps the master timetable and rulebook locked in one room, but sends photocopies out through its windows to every classroom. The nucleus is that office: DNA is the rulebook, the nuclear pores are the windows, and the nucleolus is the corner where the parts of the cell's protein-making machines are assembled.
Eukaryotic cells run many life processes at the same time in different organelles — a cell is like a tiny living factory with each part doing a specific job. The nucleus has a double-layered nuclear membrane with pores that let material pass between nucleus and cytoplasm. The nucleolus, a dense round body, is where ribosomal subunits are made; they leave the nucleus, and one large and one small subunit join in the cytoplasm to form a ribosome. The nucleus contains chromosomes, rod-shaped and visible only when the cell is about to divide; they carry information for inheriting characters from parents to the next generation as DNA (deoxyribonucleic acid). Chromosomes are made of DNA and specific proteins; the functional segments of DNA are genes. In a non-dividing cell, DNA is part of chromatin, an entangled mass of threads that organises into chromosomes before division. In prokaryotes, DNA is a single circular molecule with proteins in the nucleoid.
⤵ From cell to gene (Fig. 2.12, in words)
Cella eukaryotic cell with a nucleus
Nucleusdouble membrane with pores; nucleolus inside
Chromatinin a non-dividing cell, a tangle of threads of DNA and proteins
Chromosomebefore division, chromatin condenses into rod-shaped chromosomes
DNA and geneseach chromosome carries DNA; its functional segments are genes
Reading NCERT's Figs. 2.11 and 2.12 — nucleus, and from cell to DNA (page 16)Fig. 2.11 is a purple sphere cut open to show the double-layered nuclear membrane with pores, the dark nucleolus and chromatin, with pink folded ER attached below. Fig. 2.12 is a vertical chain: an animal cell, its nucleus enlarged with chromatin, an X-shaped blue chromosome, and a long twisting DNA ladder unwinding from it. The point: heredity is packed at four scales — nucleus, chromatin, chromosome, DNA.
Reading NCERT's ‘Threads of Curiosity’ — red blood cells without a nucleus (page 16)Mature human RBCs have no nucleus (enucleate), leaving more space for haemoglobin to carry oxygen; without a nucleus they cannot repair or divide, so they live only about 120 days. It asks for other cells without a nucleus. The point: structure is traded for function — and the nucleus is what keeps a cell repairable.
Common confusions
- Nucleolus ≠ nucleus: the nucleolus is inside the nucleus and makes ribosome subunits.
- Chromatin and chromosomes are the same material in two states — loose in a resting cell, condensed before division.
- Chromosomes are DNA plus proteins; a gene is a segment of DNA, not a whole chromosome.
- The nuclear membrane is double and has pores; it is not a sealed wall.
- ‘Enucleate’ RBCs are mature human RBCs; they are made from nucleated precursor cells (Beyond NCERT).
Beyond NCERT
- Mature mammalian RBCs also lack mitochondria. Other cells without a nucleus at maturity: sieve-tube cells of phloem in flowering plants; platelets are cell fragments without nuclei.
- Human body cells carry 46 chromosomes (23 pairs); see Principles of Inheritance and Variation and Molecular Basis of Inheritance (Class 12) on this site.
6. Ribosomes, ER, Golgi and lysosomes: make, pack, ship, recycle
In simple wordsThink of a sweet shop's kitchen: cooks (ribosomes) make sweets on a long work-table (rough ER); a packing room (Golgi) puts them in boxes and labels them; delivery boys (vesicles) carry boxes to the counter (cell membrane); and a cleaning crew (lysosomes) breaks down broken trays and scraps so the material can be used again.
Ribosomes — the protein factories — are tiny structures free in the cytoplasm or attached to the ER; they are the sites of protein synthesis. The endoplasmic reticulum (ER) is a large network spreading through the cytoplasm, continuous with the outer nuclear membrane, that makes and transports proteins, lipids and some hormones. Rough ER (RER) carries ribosomes on its surface and is mainly for protein synthesis and secretion (e.g. gland cells such as pancreatic cells); smooth ER (SER) has no ribosomes and makes and stores fats and hormones. The Golgi apparatus — stacks of flattened sacs — is the cell's post office: functionally linked to the ER, the membrane and other organelles, it modifies, sorts and packages proteins and lipids into vesicles for transport, secretion or lysosome formation. Lysosomes are single-membrane sacs of enzymes that break down unwanted proteins, carbohydrates, fats and damaged parts; the products return to the cytoplasm for reuse.
⤵ The route of a secreted protein
Madeby ribosomes on the rough ER
Movedthrough the ER and out in small vesicles
Processedin the Golgi apparatus: modified, sorted and packaged
Shippedin new vesicles from the Golgi
Deliveredsecreted at the cell membrane, added to the membrane, or packed into lysosomes
| Organelle | Membrane | Main job (NCERT) | Nickname |
|---|---|---|---|
| Ribosome | none | protein synthesis | protein factory |
| Rough ER | single | protein synthesis and secretion | manufacturing factory |
| Smooth ER | single | synthesis and storage of fats and hormones | — |
| Golgi apparatus | single (stacked sacs) | modify, sort, package into vesicles; form lysosomes | post office / shipping centre |
| Lysosome | single | enzymes break down wastes and worn-out parts | clean-up system |
Reading NCERT's Fig. 2.13 — ER and Golgi apparatus (page 17)A cut-away nucleus with folded pink ER sheets attached: the part dotted with ribosomes labelled rough ER, the tubular part smooth ER; below, a curved stack of flattened Golgi sacs budding round vesicles, one fusing with the plasma membrane and releasing its contents. The point: ER and Golgi form one pathway for protein processing and secretion.
Reading NCERT's ‘Meet a Scientist’ — Camillo Golgi (page 17)With a portrait of a moustached man in a dark suit. The Italian scientist Camillo Golgi first saw the structure in 1898 in the nerve cells of a barn owl using special stains, as a thread-like network; early microscopes could not resolve it, so many doubted it existed, until electron microscopy confirmed it decades later; it was named the Golgi apparatus in his honour. The point: new instruments settle old doubts.
Reading NCERT's ‘Threads of Curiosity’ — lysosomal enzymes in sperm (page 18)Human sperm cells contain lysosomal enzymes that help break down the egg's outer layer so that fertilisation can occur (taken up in Chapter 11 of the book). The point: lysosome-type enzymes can work outside the cell's own clean-up — see Human Reproduction (Class 12) on this site.
Common confusions
- Ribosomes have no membrane and are found in prokaryotes too; they are not ‘membrane-bound organelles’.
- Rough ER is rough because of ribosomes: proteins. Smooth ER: lipids and hormones.
- Protein synthesis = ER and ribosomes (UPSC 2001), not Golgi or lysosomes; Golgi packages, it does not make proteins from scratch.
- Lysosomes are single-membrane sacs; mitochondria and chloroplasts are double.
- Exercise 11's book key names (iii) lysosome–digestion of foreign agents as the wrong pair; by standard biology the mismatch is (ii) — cellulose is not made in SER. See the exercise outline.
Beyond NCERT
- Golgi shared the 1906 Nobel Prize in Physiology or Medicine with Santiago Ramón y Cajal for work on the structure of the nervous system.
- Lysosomes are often nicknamed ‘suicide bags’: if they burst, their enzymes digest the cell itself. They also digest bacteria and other material a cell engulfs — white blood cells rely on this.
- Cellulose is made at the cell membrane, by enzyme complexes in it, not in the ER.
7. Mitochondria, plastids and vacuoles
In simple wordsA house has a kitchen where raw food is cooked (chloroplast — only in green plants, which ‘cook’ sugar from sunlight), a generator that turns fuel into usable electricity (mitochondria — in nearly every plant and animal cell), and a water tank on the roof (vacuole) whose pressure keeps the taps running. Empty the tank and everything goes limp.
Mitochondria, the ‘powerhouses of the cell’, supply energy for most activities. Each has two membranes: a smooth, porous outer membrane, and an inner membrane folded into finger-like cristae that increase the surface for the reactions that release energy. In mitochondria glucose and other molecules are broken down in cellular respiration; the energy is stored as ATP (adenosine triphosphate), the cell's energy currency. Plastids make and store food in plants. Chloroplasts hold the green pigment chlorophyll that absorbs sunlight for photosynthesis; they are double-membraned like mitochondria, with a semi-fluid stroma containing disc-shaped membranes that carry chlorophyll; sugars made are stored in the stroma with starch granules. Chromoplasts (chroma = colour) carry yellow, orange or red pigments that colour petals and fruits, attracting pollinators and seed-dispersing animals. Leucoplasts (leukos = white) are colourless and store starch, oils or proteins — some in potato and taro (Colocasia) cells store starch.
Mitochondria and plastids resemble certain bacteria: both have their own DNA and ribosomes and make some of their own proteins, which suggests a shared evolutionary history with single-celled organisms. A mature plant cell usually has one large central vacuole bounded by a single selectively permeable membrane and filled with cell sap; it stores water, minerals, sugars and wastes and, by holding water, keeps the cell firm. Short of water, the vacuole shrinks, cells lose firmness and the plant wilts. Animal cells sometimes have vacuoles, smaller, for temporary storage.
| Plastid | Pigment | Job | Example (NCERT) |
|---|---|---|---|
| Chloroplast | chlorophyll (green) | photosynthesis; temporary food storage | green leaf cells |
| Chromoplast | yellow, orange or red | colour petals and fruits — attract pollinators and seed dispersers | flower petals, fruits |
| Leucoplast | none (colourless) | store starch, oils or proteins | potato, taro (Colocasia) |
| Mitochondrion | Chloroplast | |
|---|---|---|
| Found in | nearly all eukaryotic cells, plant and animal | green plant cells only |
| Membranes | two; inner folded into cristae | two; disc-shaped membranes stacked in the stroma |
| Does | releases energy from food as ATP (cellular respiration) | captures light energy to make sugar (photosynthesis) |
| Own DNA and ribosomes | yes | yes |
| Pigment | none | chlorophyll |
Reading NCERT's Figs. 2.14 and 2.15 — mitochondrion and chloroplast (page 18)Fig. 2.14: an orange bean-shaped cut-away with labels outer membrane, inner membrane, intermembrane space, cristae, DNA and ribosomes. Fig. 2.15: a green oval cut-away with stacks of coin-like discs, labelled outer and inner membrane, stroma, DNA and ribosomes. The point: the two energy organelles look alike in plan — two membranes, own DNA — but do opposite jobs. Our schematic is above.
Reading NCERT's ‘Pause and Ponder’ 4–5 (page 19)Do white flowers contain any pigment? And draw a labelled plant or animal cell from clues: the nucleus as a dark round body, ER as a network extending the nuclear envelope, mitochondria and chloroplasts rod-shaped. The point: link colour to plastids, and practise drawing from description (outlines in the intext section).
Common confusions
- Organelles with their own DNA: nucleus, mitochondria and chloroplasts — not Golgi, lysosomes, ribosomes or centrioles (UPSC 2001).
- Plants have mitochondria too: chloroplasts make food, mitochondria still release its energy.
- ‘Powerhouse’ does not mean mitochondria make energy; they release energy stored in food and hold it as ATP. Glucose is first split in the cytoplasm before mitochondria finish the job (Life Processes, Class 10).
- Chromoplasts ≠ chloroplasts: carrot-orange and tomato-red come from chromoplasts.
- Potato and taro are underground stems, not roots (Beyond NCERT) — remember this for exercise 5.
- Pause and Ponder 5 calls chloroplasts ‘rod shaped’; NCERT's own Fig. 2.15 and standard texts show them lens- or disc-shaped.
Beyond NCERT
- The idea that mitochondria and chloroplasts descend from bacteria engulfed by an ancestral cell is the endosymbiotic theory, championed by Lynn Margulis from the late 1960s.
- Mitochondrial DNA is inherited from the mother (asked in UPSC 1991); ‘three-parent’ mitochondrial replacement therapy (UPSC 2021) aims to prevent mitochondrial diseases.
- The disc-shaped membranes of the chloroplast are thylakoids, stacked into grana.
8. How cells divide: mitosis and meiosis
In simple wordsPhotocopying a notebook gives two identical notebooks — that is mitosis, used to grow and repair. Making a ‘half-notebook’ with one page from each pair, to be joined later with someone else's half, is meiosis — used only to make sex cells, so that children get a new mix from both parents.
A cut heals and hair grows back because cells grow and divide to replace old, dead or damaged cells. Cells grow only up to a certain size; the body grows mainly because cells divide. In a growing onion root tip cells divide continuously, so a squash shows them in different stages. Every day an estimated hundreds of billions of cells in our body are replaced — almost 1 per cent of the total. Both prokaryotic and eukaryotic cells divide; eukaryotic cells do so in a controlled, orderly sequence called the cell cycle. Cell division forms new cells from pre-existing ones and allows growth, repair and reproduction; skin cells, for example, divide continuously.
Mitosis, the most common division, produces two genetically identical daughter cells with the same DNA and chromosome number as the parent; a single fertilised egg becomes trillions of cells this way. It serves growth, repair, maintenance and asexual reproduction. Meiosis produces gametes and occurs only in reproductive organs — testes (sperm) and ovaries (eggs) in animals; anthers (pollen grains, which later produce sperm cells) and ovaries (egg cells) in plants. The parent cell divides twice to form four daughter cells: the first division halves the chromosome number, the second is like mitosis. Fertilisation of two gametes restores the original number; the reshuffling creates variation, so children resemble but are not identical to their parents. Errors in mitosis can lead to uncontrolled division, tumours and abnormal chromosome numbers in body cells; errors in meiosis may cause genetic disorders, early pregnancy loss or reduced fertility.
| Mitosis | Meiosis | |
|---|---|---|
| Where | body cells (e.g. skin, root tip) | only cells of reproductive organs |
| Divisions | one | two |
| Daughter cells | 2 | 4 |
| Chromosome number | same as parent | half of parent |
| Genetically | identical to parent | varied (gametes) |
| Purpose | growth, repair, maintenance, asexual reproduction | gametes for sexual reproduction; genetic diversity |
| If it goes wrong | uncontrolled division, tumours | genetic disorders, pregnancy loss, reduced fertility |
⤵ Activity 2.5: onion root-tip squash
Grow rootsonion base dipped in a jar of water for 5–6 days
Fixcut 2–3 cm root tips into fresh aceto-alcohol (acetic acid : ethanol = 1 : 3) for 24 hours; store in 70 per cent ethanol
Softena drop of dilute hydrochloric acid for 10–15 minutes, then rinse
Stain2–3 drops of aceto-carmine for 5–10 minutes; warm gently over a spirit lamp
Squash and observecut the tip, cover, press gently with the thumb, look for cells in different stages
Synthetic DNA and cell culture. In 2010 J. Craig Venter's team read the full DNA sequence of the simple bacterium Mycoplasma mycoides by computer, chemically synthesised an exact copy, removed the DNA from a closely related bacterium (keeping its cytoplasm and membrane) and inserted the synthetic DNA; the cell grew and divided following the new DNA — showing that DNA controls the structure and activities of a cell. NCERT stresses the limit: only the DNA was synthetic; the rest came from a living cell. In cell culture, plant or animal cells taken from an organism grow and multiply in a nutrient-rich medium under sterile conditions with the right temperature, acidity or alkalinity and moisture; it is used to study cells and to make biochemicals, food, medicines and vaccines.
| Position on synthetic cells (attributed) | What it emphasises |
|---|---|
| NCERT (Threads of Curiosity box, page 20; The Quest Continues, page 27) | the 2010 work showed DNA directs a cell, but no cell was made from scratch; asks students what the future of synthetic cells is and what ethical issues a fully synthetic cell would raise |
| Supporters (many synthetic-biology researchers and biotechnology firms) | designed microbes could make medicines, vaccines, fuels and materials, clean pollutants, and show the minimum set of genes life needs |
| Critics (some environmental and civil-society groups, some ethicists and religious voices) | escape or misuse of designed organisms (biosafety and biosecurity), ‘creating life’ as a moral line, patents concentrating control over living systems |
| Regulators and advisory bodies (e.g. the US Presidential Commission for the Study of Bioethical Issues, 2010; in India, rules of 1989 under the Environment (Protection) Act, 1986, applied through the RCGM and GEAC) | case-by-case oversight of genetically engineered organisms and cells rather than a ban; the US commission's phrase was ‘prudent vigilance’ |
The table records positions, not a verdict; the exercise outline for ‘The Quest Continues’ follows the same balance.
Reading NCERT's ‘Threads of Curiosity’ — Venter's synthetic DNA (page 20)The 2010 experiment described above, step by step, ending with the caution that scientists did not create a completely new cell. The point: DNA is the cell's instruction set — and claims of ‘artificial life’ need careful reading.
Reading NCERT's Activity 2.5, Figs. 2.16 and 2.17 — onion root tips (pages 20–21)Fig. 2.16 is a photograph of a red onion sitting on a glass jar of water with a tuft of white roots. Fig. 2.17 joins that photograph to two micrographs: a root tip of small, closely packed rectangular cells, and a magnified band of pink-stained cells — most with a round nucleus, several with dark threads lined up or pulling apart. The book asks which stage comes first. The point: a growing tip is a live record of division; the stage names come in higher classes (‘Next Level Up’).
Reading NCERT's Figs. 2.18 and 2.19 — mitosis and meiosis (pages 21–22)Fig. 2.18: a cell with a red and a blue chromosome, an arrow to the same cell with the chromosomes doubled, then two daughter cells each with one red and one blue. Fig. 2.19: a cell with two pairs, divided into two cells each with one chromosome, then into four cells each with one. The point: count the chromosomes per cell — same after mitosis, halved after meiosis.
Reading NCERT's ‘Meet a Scientist’ — Arun Kumar Sharma, and ‘Next Level Up’ markers (page 22)A portrait of a smiling man in glasses. Arun Kumar Sharma, an Indian botanist known for work on chromosomes, plant taxonomy, evolution and development, invented laboratory methods to study plant chromosomes and received the Shanti Swarup Bhatnagar award and the Padma Bhushan. Small ‘Next Level Up’ tags beside the cell cycle (page 21) and meiosis (page 22) signal topics taken further in later classes. The point: Indian cytology has a place in the story.
Reading NCERT's ‘Bridging Science and Society’ — cell culture, and ‘Pause and Ponder’ 6–7 (page 22)The box (with a small globe-in-hands icon) describes growing cells outside the body and their uses in research, biochemicals, food, medicines and vaccines. Pause and Ponder asks why a cell has many small mitochondria rather than one giant one (surface area), and what would happen to a cut if skin cells divided by meiosis. The point: cell biology turns into industry, and division type decides function (outlines in the intext section).
Common confusions
- Meiosis halves the chromosome number; mitosis keeps it. Meiosis has two divisions and gives four cells.
- Meiosis occurs only in reproductive organs; gametes are not made by mitosis (exercise 15).
- Growth is mainly by more cells, not bigger cells.
- Venter's team synthesised a genome, not a whole cell (UPSC questions on ‘artificial life’ use this distinction).
- Cell culture (cells in a medium) is not the same as tissue culture of plants from explants, though both rest on cells growing outside the body.
Beyond NCERT
- Human body cells have 46 chromosomes and gametes 23; fertilisation restores 46 (asserted in the source file). An extra copy of chromosome 21 from faulty meiosis causes Down's syndrome.
- One of the most famous human cell lines, HeLa, came from a tumour sample taken from Henrietta Lacks in 1951 without her consent — a case often cited in debates on consent and cell-culture ethics.
- The 2010 cell is known as JCVI-syn1.0; the synthetic genome was transplanted into Mycoplasma capricolum.
9. Cell Theory, life span, cancer and totipotency
In simple wordsIn a well-run school, students join, learn, do their job and graduate; new students take their places, and nobody stays in Class 9 forever. Cells are the same: they divide, work and then die on schedule. A cancer cell is a student who refuses to leave and keeps bringing in copies of itself until the classroom overflows.
In 1838 the German botanist Matthias Schleiden reported that all plants are made of cells; in 1839 the German zoologist Theodor Schwann found that all animals are too; in 1855 the German scientist Rudolf Virchow added that new cells form only from pre-existing cells. The classical Cell Theory: (1) all living organisms are made of one or more cells; (2) the cell is the basic unit of structure and function; (3) all cells arise from pre-existing cells. It unifies biology from bacteria to humans and explains life's continuity through division.
Cells grow and divide in a controlled way, stay in place, work, and die when no longer needed, replaced by new cells: every cell has a definite life span. Problems arise if cells do not die when they should, or die too early. In many animal cells division stops when cells touch their neighbours — contact inhibition. Cancer cells lose this control and keep dividing, forming tumours, which may be benign or malignant; cancerous tumours can invade nearby tissue and spread to form new tumours. Plant cells, with rigid walls, do not show contact inhibition and grow in a different pattern. Programmed cell death (PCD) is a genetically regulated, organised destruction of selected cells, needed for development, quality control and immunity — it removes the cells between developing fingers, without which hands would be webbed.
⤵ How Cell Theory was built
1665: Hookenames box-like compartments in cork 'cells'
1838: Schleidenall plants are made of cells
1839: Schwannall animals are made of cells
1855: Virchownew cells arise only from pre-existing cells
Classical Cell Theoryall life is cellular; the cell is the unit of structure and function; cells come from cells
Reading NCERT's ‘Meet a Scientist’ — Gottlieb Haberlandt (page 23)With a portrait of a bearded man. In 1902 the Austrian botanist Gottlieb Haberlandt proposed that any living plant cell, even a mature cell of a permanent tissue, can grow into a complete plant given suitable nutrients and conditions — totipotency — laying the foundation of plant tissue culture. The point: plant cells keep the ability to become other kinds of cells; see Biotechnology and its Applications (Class 12) on this site.
Reading NCERT's ‘Ready to Go Beyond’ — programmed cell death, and ‘Threads of Curiosity’ — cancer (page 23)The first box: cells have natural, genetically regulated ways of dying (PCD) that matter for development, quality control and immunity — fingers form by removing cells between digits; explore other ways cells maintain themselves. The second: normal cells grow, age and die in a controlled manner; when this breaks down, abnormal cells divide uncontrollably into benign or malignant tumours, and cancerous ones invade and spread. The point: dying on time is as important as dividing.
Common confusions
- Schleiden = plants (botanist, 1838); Schwann = animals (zoologist, 1839); Virchow (1855) = cells from cells. Hooke (1665) is not a founder of Cell Theory.
- Viruses are acellular — they do not contradict Cell Theory because they are not counted as cells.
- Benign tumours stay put; malignant ones invade and spread (spreading is metastasis, Beyond NCERT; see Human Health and Disease, Class 12).
- Programmed cell death is orderly and planned; it is not the same as cells dying from injury.
- Totipotency was proposed for plant cells; NCERT does not say animal cells are totipotent.
Beyond NCERT
- Programmed cell death is better known as apoptosis (UPSC 2001 asked for the name); cell death from injury is necrosis.
- Virchow's summary is often quoted in Latin: omnis cellula e cellula — every cell from a cell.
- Plants can form tumour-like growths: crown gall is caused by the bacterium Agrobacterium tumefaciens, whose ability to transfer DNA into plant cells is used in genetic engineering (Biotechnology: Principles and Processes, Class 12).
NCERT's activities, ‘What if …’ and ‘Pause and Ponder’ — short answers
The five activities, the two ‘What if …’ boxes, the seven ‘Pause and Ponder’ questions and the main in-text questions. Outlines:
Activity 2.1 (page 10): Estimate the size of an onion peel cell.
Measure the field of view with a mm ruler (say 5 mm = 5,000 µm), count cells along the diameter (say 25): 5,000 ÷ 25 = 200 µm. With a 10X eyepiece and 10X objective the cell is magnified 100X, so it looks about 20 mm wide.
Activity 2.2 (page 11): What happens to potato pieces in plain water and in 20 per cent salt or sugar solution?
In water the piece swells and gains weight; in salt or sugar solution it shrinks and loses weight. The cell membranes let water through but not salt or sugar, so water moves towards the more concentrated side — osmosis.
Activity 2.3 (page 13): Compare onion or Rhoeo peel with cheek cells, then put both in 20 per cent sugar solution.
Plant cells are box-shaped and regular because of the cell wall; cheek cells are irregular. In sugar solution both lose water: plant cells keep their outline while the contents shrink away from the wall; cheek cells, with no wall, shrink.
Activity 2.4 (page 14): Study Fig. 2.10 and fill Table 2.1. Which cells are prokaryotic and which eukaryotic?
See the filled Table 2.1 in section 4. The bacterial cell (nucleoid, no membrane-bound organelles) is prokaryotic; the plant and animal cells are eukaryotic.
Activity 2.5 (pages 20–21): What do you see in a stained onion root-tip squash, and why do cells differ?
Cells of similar size but different appearances — some with a plain round nucleus, others with visible thread-like chromosomes lined up or moving apart — because root-tip cells divide continuously and are caught at different stages. Cells with an intact nucleus and no visible chromosomes have not yet started dividing; condensed chromosomes appear first (stage names come in higher classes).
What if … (page 12): Mung bean seeds soaked in water for 12 hours are kept in a concentrated solution. What happens?
The concentrated solution is hypertonic to the swollen seeds' cells, so they lose water by osmosis: the seeds shrink and wrinkle (and would stop germinating if left long).
What if … (page 12): A cell is kept in salt or sugar solutions of different concentrations?
Isotonic: no net change. Hypotonic (outside less concentrated): water enters, the cell swells. Hypertonic (outside more concentrated): water leaves, the cell shrinks — a plant cell's contents pull away from the wall.
Pause and Ponder 1 (page 14): Why is a cell wall necessary for plants fixed in one place but not for animals that move?
A plant cannot move away from wind, rain or trampling, so it must withstand stress where it stands and hold itself upright and firm — a rigid wall in every cell does this. Animals avoid stress by moving and need flexible cells for muscles and body movement; a wall would hinder that.
Pause and Ponder 2 (page 14): What would happen to a plant cell if its wall became as flexible as a membrane?
It would lose its fixed shape; in a hypotonic solution water would keep entering and it could swell and burst like an animal cell; leaves and stems would sag and the plant could not stay upright.
Pause and Ponder 3 (page 14): Why cut the two potato pieces roughly equal and weigh them before placing them in different liquids?
To make it a fair test: equal size gives equal surface and volume, so differences come only from the liquid; the initial weight is the baseline against which gain or loss of water is measured.
Pause and Ponder 4 (page 19): Do white flowers contain any pigment?
White petals have no coloured pigment in their plastids — they contain leucoplasts (colourless) rather than chromoplasts; they look white because they reflect all colours of light (air spaces between cells add to this, Beyond NCERT).
Pause and Ponder 5 (page 19): Draw a labelled plant or animal cell from the clues given.
Draw a boundary (add a thick wall for a plant cell), a dark round nucleus, the ER as a network extending from the nuclear envelope, rod- or oval-shaped mitochondria, oval chloroplasts (plant), a large central vacuole (plant) and a few small dots for ribosomes; label each. Use section 4's schematic as a guide.
Pause and Ponder 6 (page 22): Why does a cell have many small mitochondria rather than one giant one?
Many small bodies have more surface area than one big body of the same volume — splitting one sphere into eight half-radius spheres keeps the volume but doubles the surface (asserted). More membrane surface means more room for energy-releasing reactions, and small mitochondria can be spread to wherever energy is needed.
Pause and Ponder 7 (page 22): If skin cells divided by meiosis instead of mitosis, what would happen to a cut?
Meiosis would give cells with half the chromosomes and a new genetic mix — not proper skin cells. They would be unable to function or divide normally, so the cut would not heal properly; repair needs identical cells from mitosis.
In-text (page 16): Do you know other cells without a nucleus?
Mature human red blood cells (NCERT). Beyond NCERT: sieve-tube cells of phloem in flowering plants at maturity; platelets are cell fragments without nuclei.
In-text (page 19): How do flowers, fruits and vegetables get their varied colours?
From chromoplasts, plastids with yellow, orange or red pigments; green comes from chlorophyll in chloroplasts.
Revise, Reflect, Refine — answer outlines
Reading NCERT's ‘At a Glance’, ‘The Journey Beyond’ and ‘The Quest Continues’ (pages 24, 27)‘At a Glance’ lists the chapter's ten main points (cell as basic unit; prokaryotes with nucleoid; eukaryotes larger with organelles; membrane in all, wall in plants, fungi and bacteria; chromosomes of DNA; organelles; plastids; mitosis; meiosis; cancer). ‘The Journey Beyond’ suggests projects: digital animations of cell division, a low-cost model of a ‘synthetic cell’, a class mitosis or meiosis model, and a nukkad natak (street play) on organelle functions. ‘The Quest Continues’ asks about the future and ethics of synthetic cells. The book's Answer Key (page 271) gives 2 (iii), 4 (i), 7 (ii), 11 (iii) and 16 (i) osmosis.
Reading NCERT's Figs. 2.20–2.22 (pages 25–26)Fig. 2.20: a coloured plant cell with arrows (a)–(g) to a mitochondrion, the nucleus, a Golgi body, a chloroplast, two outer boundary layers and the large vacuole. Fig. 2.21: two beakers with a carrot — A in plain water, B in salt solution. Fig. 2.22: four hollowed potato cups in beakers of water — A empty, B with sugar, C with salt, D a boiled potato with sugar. The point: exercises test identification, osmosis and experimental design.
1. Differentiate: (i) cell membrane and cell wall (permeability); (ii) RER and SER (structure); (iii) chloroplasts and chromoplasts (pigments).
(i) Membrane: selectively permeable; wall: permeable (lets water and dissolved minerals through freely). (ii) RER: ribosomes on its surface, looks rough; SER: no ribosomes, looks smooth. (iii) Chloroplasts: green chlorophyll; chromoplasts: yellow, orange or red pigments, no chlorophyll.
2. Cell X in pure water swells; Cell Y in concentrated salt solution shrinks. Which explanation is correct: (i) salt moved into Y; (ii) water moved into X and more water left Y than salt solution entered; (iii) water moved into X and out of Y through the membrane; (iv) solute movement caused osmosis?
(iii) (NCERT key). Osmosis is the movement of water across the membrane: into X (hypotonic outside), out of Y (hypertonic outside). (i) and (iv) wrongly make the solute move; (ii) wrongly implies salt solution enters the cell.
3. Identify parts (a)–(g) of the cell in Fig. 2.20 and match them with: (i) controlling all activities; (ii) cellular respiration; (iii) storage and rigidity; (iv) separates contents from surroundings; (v) structural rigidity; (vi) packs materials from ER; (vii) manufacturing food.
(a) mitochondrion → (ii); (b) nucleus → (i); (c) Golgi apparatus → (vi); (d) chloroplast → (vii); (e) and (f), the two boundary layers — the thin cell membrane → (iv) and the thick outer cell wall → (v) (the two arrows end very close together in the print); (g) vacuole → (iii).
4. Which option correctly places organelles under ‘present in plant cells’ and ‘absent in animal cells’: (i) leucoplast, cell wall; (ii) mitochondria, ribosome; (iii) cell wall, Golgi apparatus; (iv) lysosome, endoplasmic reticulum?
(i) (NCERT key): leucoplasts are present in plant cells and animal cells have no cell wall. Ribosomes, Golgi and ER are all present in animal cells.
5. Renu says all plant parts, even roots, contain plastids; Rohit says roots have none because they do not photosynthesise. Who is correct?
Renu. Plastids are not only chloroplasts: roots have leucoplasts that store starch and may have chromoplasts — the orange of a carrot root comes from pigment in chromoplasts (Beyond NCERT). Roots lack chloroplasts because they get no light, not plastids altogether. (NCERT's potato and taro examples are underground stems, so prefer a true root such as carrot.)
6. How are mitochondria and chloroplasts structurally and functionally similar, and how do they differ?
Similar: both have two membranes, their own DNA and ribosomes and make some of their own proteins; both have folded or stacked inner membranes that increase surface; both handle energy; both suggest a bacterial ancestry. Different: mitochondria occur in nearly all eukaryotic cells, chloroplasts only in green plant cells; mitochondria release energy from food as ATP (respiration), chloroplasts capture light to make sugar (photosynthesis); cristae vs chlorophyll-bearing discs in a stroma; only chloroplasts have chlorophyll. (See the comparison table in section 7.)
7. Which pair of organelles contains DNA: (i) chloroplasts, ribosomes; (ii) mitochondria, nucleus; (iii) Golgi bodies, ribosomes; (iv) nucleus, lysosomes?
(ii) (NCERT key). Ribosomes, Golgi bodies and lysosomes do not contain DNA.
8. One carrot in plain water, one in concentrated salt solution, for 24 hours. (i) What hypothesis is being tested? (ii) How could the experiment be improved? (iii) Why is one carrot stiff and the other limp?
(i) That water moves into or out of plant cells by osmosis depending on the concentration of the surrounding solution (salt solution draws water out). (ii) Weigh or measure each carrot before and after; use several carrots per treatment, of equal size; keep volume, temperature and time the same; try a range of salt concentrations; include a third, isotonic-looking control. (iii) In water, cells take in water, vacuoles fill and press against the walls, so the carrot is stiff and crunchy; in salt solution water leaves, contents shrink from the walls and the cells lose firmness — rubbery and limp.
9. Show presence or absence in bacterial and animal cells: chromosome, nucleus, mitochondria, Golgi complex, chromoplasts.
Chromosome — bacterial: a single circular DNA molecule with proteins in the nucleoid (no chromosomes in the eukaryotic sense; often called a ‘bacterial chromosome’, Beyond NCERT); animal: present. Nucleus — bacterial: absent; animal: present. Mitochondria — absent; present. Golgi complex — absent; present. Chromoplasts — absent; absent.
10. Four potato cups in water: A empty, B sugar, C salt, D boiled potato with sugar. (i) Why does water gather in B and C? (ii) Why is Cup A needed? (iii) Why does no water gather in A and D?
(i) Sugar or salt dissolves in the cup's moisture to make a concentrated solution; water from the beaker moves through the living potato cells by osmosis into the hollow. (ii) A is the control: it shows water does not collect without solute. (iii) A has no concentration difference; in D, boiling has killed the cells, so their membranes are no longer selectively permeable and osmosis does not occur.
11. Identify the pair that incorrectly matches organelle and function: (i) ribosome — protein synthesis; (ii) SER — lipid and cellulose synthesis; (iii) lysosome — digestion of foreign agents.
NCERT's Answer Key gives (iii), probably because this chapter describes lysosomes digesting only the cell's own wastes. By standard biology, however, lysosomes do digest foreign material a cell engulfs, while cellulose is not made in SER (it is made at the cell membrane, Beyond NCERT), so (ii) is the mismatch. Safe line: in a board exam write the book's key; for competitive exams remember SER = lipids and some hormones, and lysosomes = digestion of both worn-out parts and engulfed foreign material.
12. What would happen if all the mitochondria were removed from a eukaryotic cell?
Cellular respiration in mitochondria would stop, so very little ATP would be made; energy-demanding work — making proteins, transport, division, movement — would fail, and the cell would eventually die. (Only a little energy comes from splitting glucose in the cytoplasm, Beyond this chapter.)
13. Which phenomenon inhibits tumour formation in the human body? Can plants also develop tumours?
Contact inhibition: many animal cells stop dividing when they touch their neighbours (with programmed cell death removing unwanted cells). Cancer cells lose it. Plant cells, with rigid walls, do not show contact inhibition (NCERT), but plants can form tumour-like growths — e.g. crown gall caused by Agrobacterium tumefaciens (Beyond NCERT).
14. Which organelles help make the cell membrane's proteins and lipids? Trace their route to the membrane with a labelled diagram.
Proteins: ribosomes on the rough ER → through the ER → vesicles → Golgi apparatus (modified, sorted, packed) → vesicles → fuse with the cell membrane. Lipids: made in the smooth ER → vesicles → Golgi → membrane. The production-line diagram in section 6 shows this numbered route.
15. What would happen if gametes were formed by mitosis?
Gametes would carry the full chromosome number; fertilisation would double it every generation (in humans 46 + 46 = 92, then 184 …) and the offspring would not develop normally; gametes would also be genetically identical to the parent's cells, losing the variation meiosis creates.
16. Deepa preserved surplus amla and lemons as pickles, murabba and sharbat with salt, sugar or jaggery. (i) Which concept? (ii) How do high salt or sugar stop spoilage microbes? (iii) Suggest a healthy recipe. (iv) What scientific values are shown?
(i) Osmosis (NCERT key). (ii) Concentrated salt or sugar is hypertonic to bacterial and fungal cells; water leaves them by osmosis, so they dry out and cannot grow. (iii) e.g. amla pieces in jaggery syrup or a lemon–ginger pickle with salt and spices, in clean sterilised glass jars, with moderate salt and sugar for health. (iv) Applying science to daily life, evidence-based method, reducing waste, sustainability, enterprise and food security (she could sell only a quarter; three-quarters was at risk).
The Quest Continues (page 27): What is the future of developing synthetic cells from non-living chemicals, and what ethical issues would a synthetic cell raise?
Future: so far only a synthetic genome has been placed into an existing cell (2010); building a whole cell from chemicals remains a research goal — progress may give designed microbes for medicines, vaccines, fuels and pollution clean-up, and insight into life's minimum needs. Ethical issues, as attributed positions (section 8 table): biosafety (escape into the environment), biosecurity (misuse), the moral status of ‘creating life’, ownership and patents, and how to regulate — India applies its 1989 rules under the Environment (Protection) Act through RCGM and GEAC. No verdict is offered.
What to remember
Numbers
Chapter pages8–27 (20 pages; Chapter 3 starts on page 28)
Figures / tables / activities22 / 2 / 5
Pause and Ponder / end exercises7 / 16
Limit of resolution of the human eye0.1 mm (= 100 µm) at about 25 cm
Hooke's cork cells; his microscope1665; about 200–300X
Activity 2.15 mm = 5,000 µm field ÷ 25 cells = 200 µm
Total magnificationeyepiece × objective: 10X × 10X = 100X
Unit ladder1 mm = 1,000 µm; 1 nm = 0.000001 mm = one-billionth of a metre
Cell membrane thickness7–10 nm
Prokaryotic / eukaryotic cell diameter1–10 µm / 10–100 µm
Salt or sugar solution in Activities 2.2–2.320 per cent
Life span of a human red blood cellabout 120 days
Body cells replaced dailyhundreds of billions — almost 1 per cent
Meiosis2 divisions → 4 cells, half the chromosomes
Aceto-alcohol (Activity 2.5)glacial acetic acid : ethanol = 1 : 3
Cell TheorySchleiden 1838 · Schwann 1839 · Virchow 1855
Golgi / Haberlandt / Venter1898 / 1902 / 2010
Early Earth (Puga analogy)about 3.5 billion years ago
One-liners
- The cell is the basic structural and functional unit of life.
- The human eye resolves about 0.1 mm.
- Robert Hooke named cells in cork in 1665.
- Total magnification is eyepiece times objective.
- Electron microscopes use electrons instead of light.
- Every cell has a selectively permeable cell membrane.
- The membrane is a lipid bilayer with proteins: the fluid-mosaic model.
- Osmosis is diffusion of water across a selectively permeable membrane.
- Plant, fungal and bacterial cells have a cell wall.
- The plant cell wall is made mainly of cellulose.
- Prokaryotes lack a nucleus and membrane-bound organelles.
- Prokaryotic DNA lies in the nucleoid as a single circular molecule.
- The nucleolus makes ribosomal subunits.
- Genes are functional segments of DNA.
- Ribosomes are the sites of protein synthesis.
- Rough ER has ribosomes; smooth ER makes lipids and hormones.
- The Golgi apparatus modifies, sorts and packages.
- Lysosomes are sacs of digestive enzymes.
- Mitochondria release energy as ATP; cristae increase surface.
- Mitochondria and chloroplasts have their own DNA and ribosomes.
- Chromoplasts colour fruits and petals; leucoplasts store food.
- The large central vacuole keeps a plant cell firm.
- Mitosis gives two identical cells; meiosis four with half the chromosomes.
- All cells arise from pre-existing cells (Virchow, 1855).
- Cancer cells lose contact inhibition.
- Haberlandt proposed totipotency of plant cells in 1902.
Traps to avoid
- Magnification is not resolution.
- Osmosis moves water towards the more concentrated solution.
- The cell wall is permeable; the membrane is selectively permeable.
- Plant cells have both a cell wall and a plasma membrane.
- Bacteria have a cell wall but no nucleus.
- Ribosomes have no membrane and occur in prokaryotes too.
- Nucleolus is inside the nucleus; it is not the nucleus.
- Plants have mitochondria as well as chloroplasts.
- Golgi, lysosomes and ribosomes carry no DNA.
- Meiosis halves the chromosome number; mitosis keeps it.
- Schleiden = plants (1838); Schwann = animals (1839).
- Hooke saw dead cork cell walls, not living cells.
- Viruses, viroids and prions are acellular.
- Venter's team made synthetic DNA, not a whole synthetic cell.
- Potato and taro are stems, not roots.
How to strategise
Prelims — what UPSC asks
- Organelles and their jobs are UPSC's favourite cell questions: which organelle other than the nucleus contains DNA (mitochondrion, 2001); which organelles matter most for protein synthesis (ER and ribosomes, 2001); matching photosynthesis–chloroplast, mineral uptake–plasma membrane, respiration–mitochondria, protein synthesis–ribosomes (1996). Learn section 6–7 tables by heart, including membranes (none, single, double).
- Plant vs animal cells: plastids and cell wall in plant cells only (1987); cellulose walls, plasma membrane in both, one large vacuole vs small ones (2020). Statement questions turn on ‘both have a plasma membrane’ and ‘bacteria and fungi have walls too’.
- Osmosis and beyond: 2005 asked the direction of osmosis and where pressure is applied in reverse osmosis; 2002 tested acellular agents (viroids as single-stranded RNA; prions are not cells); 2001 asked the name for programmed cell death (apoptosis). Add the Beyond NCERT terms — plasmolysis, apoptosis, endosymbiosis, metastasis — to your notes.
- Likely next: stem cells and totipotency, synthetic genomes and minimal cells, cell-culture vaccines and lab-grown meat, mitochondrial replacement, cancer immunotherapy and contact inhibition, electron microscopy (cryo-EM), thermophile enzymes in PCR.
Mains — where it appears and how to answer
- No GS Mains question in the bank is built on this chapter's core content; the 2017 GS3 question on stem cell therapy draws on biotechnology chapters, not this one. Use the chapter as the first paragraph of GS3 Science and Technology answers: define the cell-level mechanism in one line (totipotency for tissue culture, contact inhibition for cancer, osmosis for food preservation, DNA as the instruction set for synthetic biology), then move to application and policy.
- Answer frame for an emerging-biotech question (synthetic biology, cell culture, gene editing): the cell biology in one line → the technology → uses in health, agriculture and industry → concerns as attributed positions (biosafety, biosecurity, ethics, patents, consent) → India's framework (1989 rules under the Environment (Protection) Act, 1986; RCGM; GEAC) → balanced conclusion.
- Exercise 16 is a ready GS3 agriculture example: salt and sugar preservation uses osmosis to cut post-harvest loss and turn farmers into agro-processors — useful in answers on food processing and farm incomes.
Self-check
Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.
1. Viewed from about 25 cm, the smallest separation at which the human eye can still see two points as separate is about:
2. A microscope's field of view is 5 mm across and 25 onion cells fit along its diameter. Each cell is about:
3. A potato piece placed in 20 per cent salt solution shrinks because:
4. Which of these cell organelles contains its own DNA?
5. In concentrated sugar solution, a Rhoeo leaf cell keeps its outline while its contents shrink. This is because of the:
6. Which statement about meiosis is correct?
7. ‘New cells are formed only from pre-existing cells’ was added to Cell Theory by:
UPSC questions from this chapter
Open any question to attempt it in the Question bank — your attempt is saved to your progress.