NCERT Solutions for Class 9 Science Chapter 2 Cell the Building Block of Life (PDF) – 2027

Every cell comes from an already existing cell that divides — never from non-living matter. This is the third statement of the Cell Theory, added by Rudolf Virchow in 1855.

Microscopes let us see what the unaided eye cannot (its limit of resolution is 0.1 mm). Robert Hooke’s microscope (1665) revealed cells in cork; light microscopes showed the nucleus, cell wall, chloroplasts and the stages of cell division; and electron microscopes, which use a beam of electrons, revealed structures at the nanometre scale, such as ribosomes and the fine details of organelles. Each improvement in resolution, contrast and magnification created new knowledge about cells.

It is the structural unit because every living body is built out of cells and nothing smaller, and the functional unit because every life process is carried out inside a cell.

• Structural: similar cells form a tissue, tissues form an organ, organs form an organ system — nasal pores, nasal cavity, trachea and lungs form the respiratory system. Take the organisation apart and you always arrive at cells.
• Functional: respiration happens in mitochondria, protein synthesis on ribosomes, photosynthesis in chloroplasts, digestion of waste in lysosomes, storage in vacuoles. All of these sit inside one cell.
• A single cell can be a complete organism — a bacterium or a yeast lives, feeds, grows and reproduces on its own. Nothing smaller than a cell can do that.

By cell division — the parent cell copies its DNA and then splits. There are two types.

It is called the resolution of the eye, and the smallest separation it can still manage is the limit of resolution.

Two dots on paper 0.1 mm apart, held 25 cm from the eye, are just seen as two. Bring them closer than that and they merge into a single point.

By using lenses — a convex lens, or better, a combination of an objective lens and an eyepiece, which is exactly what a microscope is.

• The objective lens forms an enlarged image of the specimen; the eyepiece enlarges that image again. The two magnifications multiply.
• A school light microscope carries several objectives (10X, 40X) so the same slide can be studied at different magnifications under visible light.
• Beyond the reach of light, scientists use an electron microscope, which uses a beam of electrons instead of light and resolves structure at the nanometre scale (1 nm = one-billionth of a metre). Fig. 2.4 in the book is a scanning electron micrograph of stomata on the lower surface of a Colocasia leaf.

Yes. Use the microscope’s own field of view as a ruler.

1. Place a transparent millimetre ruler on the stage and measure the diameter of the circular field of view — say 5 mm.
2. Convert it: 5 mm × 1000 = 5000 µm.
3. Replace the ruler with the specimen slide and count how many cells fit across that same diameter in one straight line — say 25.
4. Divide.

Take the book’s own readings: field diameter 5 mm, and 25 onion peel cells counted along that diameter.

The total magnification is the product of the two lenses.

So the cell that is really only 0.2 mm wide appears about 2 cm wide — roughly the width of your thumbnail.

The cell membrane of the alveolar cells is extremely thin (about 7–10 nm) and is made of a lipid bilayer with proteins embedded in it. It is selectively permeable:

• Oxygen and carbon dioxide pass through it easily by diffusion, from higher to lower concentration — oxygen from the air in the alveoli into the blood, and carbon dioxide from the blood into the alveoli.
• The membrane proteins act as gatekeepers, letting only certain other substances pass.
• The thinness of the membrane keeps the distance short, so the exchange is quick.

The two potato pieces behave in opposite ways.

The weight of the potato piece in Beaker A increases and that of the piece in Beaker B decreases. The cell membrane lets water pass but not salt or sugar molecules. In plain water (A), water moves into the potato cells; in the 20 per cent solution (B), water moves out of the cells. This movement of water through a selectively permeable membrane, from a dilute solution to a concentrated one, is called osmosis.

They will shrink and go wrinkled, and they will not sprout.

1. During the 12 hours in plain water the seeds imbibe water and swell — the seed coat stretches tight and the embryo becomes active.
2. Moved into a concentrated (hypertonic) salt or sugar solution, the solute concentration outside is now far greater than inside the seed cells.
3. Water therefore moves out of the seed cells by osmosis. The swollen seeds lose water, become smaller, softer and wrinkled.
4. Germination stops. Enzymes inside the seed need water to break down stored starch; with the water drawn out, the reactions cannot run.

Three outcomes are possible, and which one occurs depends only on how the outside solute concentration compares with the inside.

The cell wall is a rigid outer covering outside the cell membrane. It is needed because:

• it gives the cell a definite shape and strength — plants cannot move from place to place, so they need it to withstand wind and rain and to stay upright;
• it keeps leaves and flowers firm;
• it gives protection and structural support (this is its role in fungi and bacteria too);
• being permeable, it lets water and dissolved minerals pass, which helps roots absorb them.

The plant cell wall is made mainly of cellulose.

Because plant cells have a rigid cell wall and animal cells do not. The wall gives each onion or Rhoeo cell a fixed, box-like shape and holds neighbouring cells together in a regular pattern. Cheek cells have only a thin, flexible cell membrane, so they have no fixed shape and lie irregularly.

Both cells lose water, but they look completely different afterwards.

• Rhoeo (plant) cells: the outer boundary stays exactly where it was, but the inner contents shrink away from it. A clear gap appears between the inner and the outer boundary.
• Cheek (animal) cells: the whole cell shrinks considerably and looks shrivelled, because there is no outer wall to hold the shape.

Plants are usually fixed in one place, so they cannot escape wind, rain or other stresses. A rigid cell wall around every cell gives them shape, strength and support, keeps leaves and flowers firm and helps the plant stay upright. Animals move from place to place, so their cells need to be flexible and change shape; this supports the movement and functioning of their tissues. A rigid wall would prevent this, and animals get support from a skeleton instead.

The plant cell would lose the support and protection that the rigid wall gives:

• It would lose its fixed shape and could not stay firm, so the plant would droop and could not stand upright against wind and rain.
• In plain (dilute) water it would keep swelling as water enters by osmosis, and it could burst, because nothing would resist the swelling.
• In a concentrated solution the whole cell would shrink, like a cheek cell, instead of only its contents pulling away from the wall.

So that the only difference between the two set-ups is the liquid, and so that the change in weight can actually be calculated.

• Equal size — a fair test. Osmosis depends on surface area and on the amount of tissue. A bigger piece has more membrane area and more cells, so it would exchange more water for the same reason of size alone. Matching the sizes removes size as an explanation and leaves the liquid as the only variable.
• Initial weight — a baseline. Weight gained or lost cannot be read from the final weight by itself. You need the difference.

Here is Table 2.1 completed from Fig. 2.10.

Two more structures are visible in Fig. 2.10 and worth noting: the bacterial cell has an appendage for locomotion and free ribosomes; the plant cell alone has chloroplasts and a large central vacuole.

The bacterial cell (Fig. 2.10a) is prokaryotic. The plant cell (Fig. 2.10b) and the animal cell (Fig. 2.10c) are eukaryotic.

What you can actually see in the figure that settles it:

• Fig. 2.10a shows a nucleoid — genetic material lying naked in the cytoplasm — with ribosomes scattered around and an appendage for locomotion. There is no nuclear membrane and no organelle bounded by its own membrane.
• Fig. 2.10b and 2.10c both show a nucleus with a nucleolus inside a nuclear membrane, together with mitochondria, endoplasmic reticulum, Golgi body, lysosome and vacuole — every one of them membrane-bound.

Yes. Other examples of cells (or cell parts) without a nucleus are platelets in human blood, the sieve tube cells of phloem in plants, mature xylem vessels (which are dead) and the mature lens fibre cells of the eye.

It digests them, using lysosomes — single membrane-bound sacs filled with powerful enzymes.

• The enzymes can break down proteins, carbohydrates, fats and even worn-out parts of the cell itself, keeping the cell clean and healthy.
• The products of the breakdown are not thrown away — they are released into the cytoplasm and used again as raw material.

Plants make food in chloroplasts, a type of plastid, and then release energy from that food in mitochondria, exactly as animals do.

• Inside the chloroplast is a semi-fluid substance, the stroma. Within it are disc-shaped membrane structures containing chlorophyll, the green pigment that absorbs sunlight.
• The sugars made during photosynthesis are stored in the stroma along with starch granules.
• Chloroplasts are double-membrane-bound, just like mitochondria.
• Other plastids do other jobs: chromoplasts carry yellow, orange or red pigments; leucoplasts have no pigment and store starch, oils or proteins.

Yes. Chromoplasts contain pigments other than the green chlorophyll — yellow, orange or red. They give colour to flower petals and fruits.

The Greek roots make the names easy: chroma means colour, leukos means white or colourless.

From the pigments held in their chromoplasts — pigments other than chlorophyll, which may be yellow, orange or red.

• In the petals of a flower and in the flesh and skin of a fruit, the plastids carry these coloured pigments instead of chlorophyll.
• The bright colour is not decoration. It attracts pollinators to flowers so that pollination happens, and it attracts fruit-eating animals, which then carry the seeds away and help in seed dispersal.

In the vacuole. In a mature plant cell there is usually one large central vacuole, surrounded by a single selectively permeable membrane and filled with a watery fluid called cell sap.

• It stores water, minerals, sugars and waste material.
• By holding a large volume of water it maintains pressure inside the cell, which keeps the plant cell firm.
• Animal cells sometimes have vacuoles too. They are much smaller, and are used for the temporary storage of materials.

When a plant does not get enough water, the vacuoles of its cells lose water. The pressure inside the cells falls, so the cells become less firm and can no longer keep the leaves and stem stiff. The leaves and stem droop, and the plant looks wilted.

White flowers do not contain coloured pigments such as chlorophyll or the yellow, orange and red pigments of chromoplasts. The plastids in their petals are leucoplasts, which lack pigment and are colourless. The petals look white because they reflect all the colours of the light that falls on them.

Here is a schematic plant cell drawn to those three clues. Copy this layout, then label it.

Because cells in the body grow and divide to replace old, dead or damaged cells.

• Skin cells and the cells at the base of a hair follicle divide continuously, because these tissues are worn away all the time.
• The daughter cells are genetically identical to the parent, so the new skin is real skin and the new hair is the same kind of hair.
• Division stops once the gap is filled and the new cells touch their neighbours — this is contact inhibition.

Yes. Fresh white roots grow out from the base of the onion bulb into the water, usually a few centimetres in 5–6 days.

• The base of the bulb bearing the roots must stay immersed; the rest of the bulb stays above the water.
• Growth is fastest at the very tip. Cut 2–3 cm of the freshly grown roots for the slide, because it is the tip that contains dividing cells.

The root tip cells are visible, and they are not all alike. Different cells show different arrangements of stained material inside them.

• Some cells show an ordinary round nucleus with a diffuse, thread-like chromatin network — these are not dividing at that moment.
• Others show deeply stained, rod-shaped chromosomes, sometimes lined up along the middle of the cell, sometimes being pulled towards the two ends, and sometimes gathered into two separate groups.

The reason is that the cells of a growing onion root tip divide continuously. At the moment the tissue was fixed, each cell happened to be at a different point in the process, so the slide is a snapshot of many stages of cell division at once — which is exactly what Fig. 2.17 shows.

The first stage is the one in which the cell still has a round, complete nucleus and the thread-like chromatin inside it begins to organise into chromosomes. Only after this are the chromosomes separated.

Reading Fig. 2.17 in order:

1. Cells with a round nucleus (most cells in the figure) — division has not yet begun or is just beginning.
2. The cell in the bottom row whose two sets of chromosomes have only just separated near the middle.
3. The last cell of the top row, where the two sets have moved farther apart.
4. The fourth cell of the top row, where the two sets have reached the two ends — the cell is about to form two daughter cells.

Errors in these divisions cause problems in the body:

• Errors in mitosis lead to uncontrolled cell division, which can form tumours, and to an abnormal number of chromosomes in body cells.
• Errors in meiosis may cause genetic disorders, which may be linked with developmental problems or distinctive physical features. Faulty meiosis may also cause early pregnancy loss or reduced fertility.

Many small mitochondria together have a much larger surface area than one giant mitochondrion of the same total volume. When the same volume is divided into smaller pieces, the total surface increases. Energy is released by reactions on the mitochondrial membranes, so more surface means more energy can be produced. Small mitochondria can also be spread throughout the cell, close to where energy is needed, and substances reach their inside quickly.

The cut would not heal properly. Meiosis halves the chromosome number, so the new cells would have only half the chromosomes (half the DNA) of normal skin cells, and they would not be identical to the parent cell. Such cells could not work as normal skin cells, and the wound would not be repaired with proper skin. Healing needs mitosis, which gives identical daughter cells with the full number of chromosomes.

They grow because they have lost the controls that stop normal cell division, and they spread by invading nearby tissues and travelling to other parts of the body.

By balancing the making of new cells against the deliberate, controlled death of old ones. The main mechanism is Programmed Cell Death (PCD).

• PCD is a genetically regulated and organised process of selective cell destruction — the cell dismantles itself neatly on instruction, instead of bursting and damaging its neighbours.
• It is essential for three things: normal development, cellular quality control, and immune function.
• Contact inhibition works alongside it: in many animal cells, division stops when a cell comes into contact with its neighbours, so a tissue stops growing once it is complete.
• Every cell also has a definite life span. Mature red blood cells, for example, survive about 120 days and are then replaced.

A cell stays healthy by running several housekeeping systems at once. Here is what to explore, with the part of the chapter each connects to.

(i) Cell membrane and cell wall — permeability

(ii) RER and SER — structure

(iii) Chloroplasts and chromoplasts — pigments

The correct explanation is (iii) Water moved into Cell X and moved out of Cell Y through the cell membrane.

Fig. 2.20 is a plant cell — you can tell from the thick outer wall, the chloroplasts and the large vacuole.

Only option (i) is correct: Leucoplast is present in plant cells and Cell wall is absent in animal cells.

Renu is correct. Roots do contain plastids. Rohit has confused plastids with one particular kind of plastid, the chloroplast.

• Plastids are a family of organelles with three members: chloroplasts (green, for photosynthesis), chromoplasts (yellow, orange or red) and leucoplasts (colourless, for storage).
• Roots lack chloroplasts because there is no light underground and no photosynthesis to do — that part of Rohit’s reasoning is right.
• But roots are full of leucoplasts, which store starch, oils and proteins. Storing food is one of the main jobs of a root.
• Some roots even have chromoplasts. A carrot is orange because of the pigment in the chromoplasts of its root.

Similarities

• Both are double-membrane-bound organelles.
• Both have their own DNA and their own ribosomes, so both can make some of their own proteins.
• Both share features with certain bacteria, which suggests both share an evolutionary history with single-celled organisms.
• Both are energy organelles: one captures energy, the other releases it. Both increase their internal membrane surface — cristae in one, disc-shaped membranes in the other.
• Both are found in plant cells.

Differences

The correct pair is (ii) Mitochondria, Nucleus.

Options (i), (iii) and (iv) each pair one DNA-containing organelle with one that has none, so all three are wrong. Note that chloroplasts do contain DNA — option (i) fails only because ribosomes do not.

(i) Hypothesis: Water moves into or out of plant cells by osmosis depending on the concentration of the surrounding solution.

(ii) Improvements:

• Use carrot pieces of the same size and weigh them before and after the 24 hours.
• Keep the volume of liquid and the temperature the same in both beakers.
• Repeat with several carrots in each liquid and take the average.

(iii) In plain water, water enters the carrot cells by osmosis; the cells fill up and press firmly against their rigid walls, so the carrot stays stiff and crunchy. In concentrated salt solution, water leaves the cells by osmosis; the cells lose their firmness, so the carrot becomes rubbery and limp.

Note: a bacterial cell has no nucleus; its genetic material is a single circular DNA molecule with proteins, lying in the nucleoid — often called the bacterial chromosome. Chromoplasts are plastids, which are found only in plant cells.

(i) The sugar (Cup B) or salt (Cup C) dissolves in the moisture in the hollow and makes a concentrated solution, while the water in the beaker is dilute. The living potato cells act as a selectively permeable membrane, so water moves from the beaker, through the potato cells, into the hollow by osmosis and gathers there.

(ii) Cup A is the control. It is the same as Cups B and C except that nothing is added, so it shows that water does not gather in the hollow by itself — the water in B and C is due to the sugar or salt, that is, the difference in concentration.

(iii) Why no water gathers in Cups A and D:

The incorrectly matched pair is (ii) SER — Lipid and cellulose synthesis.

The cell would run out of usable energy and die, because nothing else in it can release the energy stored in glucose and package it as ATP.

Every energy-requiring activity would stop — making proteins, transport across the membrane, cell division and movement.

The phenomenon is contact inhibition — in many animal cells, cell division stops when a cell comes into contact with its neighbouring cells.

Can plants develop tumours? Yes, plants can form tumour-like abnormal growths, for example crown gall, caused by a bacterium. Plant cells do not show contact inhibition — because of their rigid cell walls they follow a different pattern of growth. But the walls also hold every cell in place, so the abnormal cells cannot move and spread to other parts of the plant the way cancer cells spread in animals.

Four organelles are involved: ribosomes, the Rough ER, the Smooth ER and the Golgi apparatus.

The path:

The chromosome number would double in every generation, and the species could not survive.

(i) Osmosis. High concentrations of salt or sugar make a concentrated (hypertonic) medium, which draws water out of cells.

(ii) The salt or sugar solution around the food is much more concentrated than the inside of bacterial and fungal cells, so water moves out of them by osmosis. Having lost water, they cannot grow or multiply, and the food does not spoil.

(iii) Sample answer — amla murabba with jaggery: cook pricked amla in a thick jaggery syrup with a little cardamom, cool, and store in a clean, dry glass jar with the fruit covered by syrup.

(iv) Applying science to daily life; respect for traditional knowledge; preventing waste (sustainability); food security; self-reliance and higher income; hygiene and safety.

Build the animation around what the chromosomes are doing, because that is the only thing that really changes from stage to stage.

Sample plan:

1. Choose a free tool — Scratch, an animation feature in Google Slides or Canva, or a stop-motion phone app with clay models.
2. Storyboard on paper. For mitosis: chromatin → chromosomes → chromosomes lined up in the middle → pulled to opposite ends → two identical daughter cells.
3. For meiosis, show two divisions giving four cells, each with half the number of chromosomes.
4. Write the chromosome number on every frame, add a short commentary and present it in class.

A ‘synthetic cell’ model works best when every material you choose says something true about the part it stands for.

How to build it: make chromosomes from coloured pipe cleaners, wool or clay (two colours for the chromosomes from the two parents) and cells from paper plates or trays. Lay out the stages from left to right, with the chromosome number written under each.

Sample answer: Teamwork helped because we divided the work — making chromosomes, writing captions, checking chromosome numbers and presenting — and one member caught a mistake in our meiosis model before the exhibition. Yes, it changed my understanding: I saw that mitosis gives each daughter cell a full, identical set of chromosomes, while meiosis halves the number so that fertilisation can restore it.

Cast each organelle as a worker in a factory or a resident of a village, and let the plot be what happens when one of them stops working.

Plot in three scenes: an ordinary day in the ‘cell village’ → one worker (say the lysosome or the mitochondrion) stops working and everything goes wrong → the nucleus explains that every part depends on the others.

The honest answer is that a fully synthetic cell has not yet been built, and the chapter is careful to say so. What has been achieved, and what may come next, is this.

If it is achieved, the possible uses are large: bacteria designed to produce medicines and vaccines cheaply, to digest oil spills or plastic waste, to fix nitrogen for crops, or to make fuels. It would also let biologists test what life actually requires, by building it rather than only taking it apart.

The questions are as serious as the science, and they fall into a few clear groups.

• Safety: a laboratory-made organism might escape and harm natural species or the environment.
• Misuse: the same methods could be used to make harmful organisms.
• Ownership: should anyone be allowed to patent a living organism?
• Fair access: medicines made this way must be affordable for everyone.
• Respect for life: many people feel that making life from chemicals crosses a line.
• Transparency: society should be informed and allowed to debate before such work is done.