Cell: The Building Block of Life

The difference comes down to one thing: a plant cannot run away, and an animal can.

Why a fixed plant needs a wall. A plant must stand upright and hold its leaves out towards the light, but it has no skeleton and no muscles. The rigid cell wall, made mainly of cellulose, does that job instead. When water enters the vacuole the cell swells and presses outward against the wall; the wall pushes back, and this pressure is what makes a stem stiff and a leaf flat. A plant also cannot escape wind, heat, or an animal trying to eat it, so each cell needs its own permanent protective layer.

Why a moving animal must not have one. Movement depends on cells that can change shape — muscle cells shorten, white blood cells squeeze between other cells to reach an infection, and cells fold and slide as an embryo develops. A rigid wall would make all of that impossible. An animal gets its support from a skeleton and protects itself by moving away from danger, so a cell wall would cost it mobility while adding nothing it actually needs.

In short, the plant trades flexibility for support and protection; the animal trades support for flexibility, and covers support with a skeleton instead.

The cell wall's whole job is to be rigid. If it became as flexible as the cell membrane, the plant cell would lose the support and protection the wall gives it, and several things would go wrong.

• Loss of shape and support. The rigid wall is what gives a plant cell its fixed shape and lets stems stand upright and leaves stay spread out. A flexible wall could not do this, so the cell — and the whole plant — would become floppy and droop.
• No turgor pressure to push against. When water enters the vacuole the cell swells and presses outward; normally the stiff wall pushes back, and this balance keeps the cell firm. A flexible wall would just stretch instead of pushing back, so the cell could not stay firm.
• Risk of bursting. In dilute surroundings water keeps entering by osmosis. The rigid wall normally stops the cell over-expanding; a flexible wall would let it swell too much, and it could burst.
• Less protection. The wall also shields the cell from injury and from microbes, and a flexible wall would offer far less protection.

In short, the cell would start to behave like an animal cell — but without an animal's skeleton or ability to move away from danger, so the plant would lose its main means of support.

Both steps exist to make the experiment a fair test. The experiment is asking one question — does the surrounding liquid decide whether the potato gains or loses water? — so the liquid must be the only thing that differs between the two set-ups.

• Equal size. A larger piece has more surface area for water to cross and more tissue to hold it, so it would change weight more simply because it is bigger. If the pieces start unequal you can never tell whether the difference you see was caused by the liquid or by the size.
• Initial weight. What actually matters is the change in weight, not the final weight. Without the starting value there is nothing to subtract from, so the reading at the end tells you nothing.

Size is the variable being held constant (the control variable); the initial weight is the baseline the final reading is compared against.

No — white flowers usually do not contain any coloured pigment.

Colour in petals comes from chromoplasts, the plastids that hold yellow, orange or red pigments. A petal with no such pigment has nothing to give it colour — its plastids are colourless leucoplasts.

So why does the flower look white and not transparent? A white petal is full of tiny air spaces between its cells. These scatter and reflect all the wavelengths of light equally, and light reflected as a mixture of every colour is seen as white. The whiteness is therefore caused by the reflection and scattering of light, not by a white pigment.

A plant cell has been drawn here, because it lets all three clues be shown at once (an animal cell would have no chloroplasts).

How each clue is used in the drawing:

• Nucleus — drawn as a large round body, shaded darker than the cytoplasm around it, with a smaller darker nucleolus inside it.
• Endoplasmic reticulum — drawn as a network of channels spreading outward from the nuclear envelope, as though the envelope itself continues into the cytoplasm.
• Mitochondria and chloroplasts — both drawn rod shaped (oval, longer than they are wide). The chloroplasts additionally carry green stacks inside them, which is what distinguishes them from the mitochondria.

Marks in an exam are given for the labels and for the outline being closed and neat, so label every part with a straight line that does not cross any other label line.

Because respiration happens on the inner membrane of a mitochondrion, not in its volume. What the cell needs is as much membrane as possible — and merging many small mitochondria into one big one throws most of that membrane away.

The surface-area argument, with numbers. Surface area grows as the square of the radius while volume grows as the cube. So as something gets bigger, its surface area falls behind its volume. Take eight small mitochondria of radius r and merge them into one:

The single large mitochondrion holds exactly the same volume but has half the membrane area — so it could release only about half as much energy.

Two more practical reasons:

• Delivery. Many small mitochondria can be moved to wherever energy is actually needed — packed into the tail of a sperm cell, or along a nerve fibre. A single giant one would sit in one place and the rest of the cell would be starved.
• Safety. If one small mitochondrion is damaged the cell loses a small fraction of its respiration. If the one giant mitochondrion failed, the cell would lose everything.

A cut heals when new skin cells are made to replace the ones that were lost. That repair depends on mitosis, which makes two daughter cells that are identical to the parent — same type, same full number of chromosomes. If skin cells divided by meiosis instead, healing would fail.

• Meiosis makes four daughter cells, each with only half the chromosome number (haploid) and each genetically different from the parent.
• Such half-set cells are not proper skin cells and cannot do a skin cell's job, so they could not correctly rebuild the damaged tissue.
• The wound would therefore not heal properly — it would stay open or repair abnormally, because the new cells are neither identical to, nor as complete as, the cells they are meant to replace.

This is exactly why the body uses mitosis, not meiosis, for growth and repair — meiosis is kept for making gametes in the reproductive organs.

(i) Cell membrane and cell wall — permeability

(ii) RER and SER — structure

(iii) Chloroplasts and chromoplasts — pigments

Answer: option (iii) is correct.

Both cells are showing osmosis — the movement of water across the selectively permeable cell membrane, from where water is more concentrated to where it is less concentrated.

• Cell X is in pure water, which is more dilute than the cell's contents, so water moves into the cell and it swells.
• Cell Y is in concentrated salt solution, which is more concentrated than the cell's contents, so water moves out of the cell and it shrinks.

Statement (iii) says exactly this. The others are wrong: (i) it is water leaving, not salt entering, that shrinks Cell Y; (ii) the membrane holds the salt back, so it is not that the salt solution entered; and (iv) osmosis is the movement of water, not of solute.

The cell shown is a plant cell — you can tell because it has a cell wall, chloroplasts and one large central vacuole, none of which an animal cell would have.

Watch the two labels on the edge of the cell. (e) and (f) both point at the boundary, and they are easy to swap. The outer layer is the cell wall — thick, rigid, and the one that gives the cell its shape. The inner layer, just inside it, is the cell membrane — thin, and the one that actually controls what enters and leaves. Rigidity goes with the outer layer; separating the contents goes with the inner one.

Answer: only option (i) is correct.

For an option to be correct both entries must be true — the first must be present in plant cells, and the second must be absent in animal cells. Checking each:

Renu is correct.

Rohit has mixed up plastids with chloroplasts. Plastids are a whole family of organelles, and they are present in all parts of a plant, including the roots. What changes from one part to another is the type of plastid:

• Chloroplasts (green, for photosynthesis) are found in parts exposed to light — leaves and green stems.
• Leucoplasts (colourless, for storing starch, oils and proteins) are found in roots and other underground or non-green parts.
• Chromoplasts (yellow, orange, red) are found in petals and fruits.

So roots do contain plastids — leucoplasts, not chloroplasts. Rohit is right that roots have no chloroplasts (there is no light for photosynthesis), but wrong to say they have no plastids at all.

Mitochondria and chloroplasts are both the plant cell's energy organelles, and they share a striking set of features — yet they do opposite jobs.

How they are similar:

• Both are enclosed by a double membrane.
• Both have their own DNA and their own ribosomes, so each can make some of its own proteins.
• Both handle the cell's energy transformations, and both are believed to have descended from once free-living bacteria.

How they differ:

In short, the chloroplast stores energy by building food, while the mitochondrion releases that energy by breaking food down.

Answer: option (ii) — Mitochondria and Nucleus.

DNA is found in the nucleus (the cell's main store of genetic material) and, in smaller amounts, inside the two organelles that carry their own DNA — mitochondria and chloroplasts. Checking each pair:

(i) The hypothesis being tested. That the concentration of the surrounding solution decides whether plant tissue gains or loses water by osmosis — that a carrot stays firm in plain water but loses water and goes limp in a solution more concentrated than its own cell sap.

(ii) How to improve the experiment.

• Weigh (or measure) each carrot before and after, so the change is recorded as a number instead of just ‘looks limp’.
• Use carrots of genuinely equal size and firmness to begin with.
• Keep both at the same temperature and for the same time, and repeat with more carrots so the result can be trusted.

(iii) Why one stays crunchy and the other goes limp. In plain water the surroundings are more dilute than the carrot's cell sap, so water enters the cells by osmosis. The cells become turgid (swollen and pressing on their walls), keeping the carrot stiff and crunchy. In concentrated salt solution the surroundings are more concentrated than the cell sap, so water leaves the cells; they become flaccid, and the carrot turns rubbery and limp.

A bacterial cell is prokaryotic, so it has no true (membrane-bound) nucleus and no membrane-bound organelles — hence mitochondria and the Golgi complex are absent — though its genetic material is still present as a single chromosome in the nucleoid region. Chromoplasts are plastids found only in plant cells, so they are absent from both bacterial and animal cells.

This is the classic osmosis experiment. Everything in it turns on two requirements: a concentration difference across the potato wall, and a living, selectively permeable membrane for the water to cross.

(i) Why water gathers in Cups B and C

The sugar in Cup B and the salt in Cup C dissolve in the small amount of moisture in the hollow, making a concentrated solution. The beaker outside holds plain water, which is far more dilute. Water therefore moves by osmosis from the beaker, through the living potato tissue, into the hollow — always from where water is more concentrated to where it is less concentrated. Over two hours enough water collects for the level in the hollow to visibly rise.

(ii) Why Cup A is necessary

Cup A is the control. Its hollow is empty, so there is no concentration difference and no water should collect. It is what proves the water in B and C really was drawn in by the sugar and salt — and not by the potato leaking, or water splashing over the rim, or the tissue simply soaking through. Without Cup A you could not rule those out, and the experiment would prove nothing.

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

Read together, the four cups isolate the two requirements one at a time: A shows a membrane alone is not enough without a gradient, and D shows a gradient alone is not enough without a living membrane.

Answer: pair (ii) is the incorrect match.

The smooth endoplasmic reticulum (SER) does synthesise lipids — but it does not make cellulose. Cellulose is built at the cell membrane by enzymes there and is used to form the plant cell wall; it is not a product of the SER. So ‘SER — Lipid and cellulose synthesis’ is wrong.

The other two are correct: ribosomes are the site of protein synthesis, and lysosomes contain digestive enzymes that digest foreign particles and worn-out parts of the cell.

Mitochondria are the site of aerobic respiration and produce most of the cell's usable energy as ATP — which is why they are called the ‘powerhouse of the cell’. If every mitochondrion were removed:

• The cell could no longer carry out aerobic respiration, so its ATP supply would fall drastically.
• All work that needs energy — active transport, movement, building new molecules, growth and cell division — would slow down and then stop.
• The cell would be left only with the small amount of energy from the anaerobic breakdown of glucose, which is nowhere near enough.

Starved of energy, the cell would be unable to keep itself running and would eventually die.

The phenomenon is contact inhibition. In the human body, normal cells stop dividing once they come into contact with their neighbouring cells. This ‘contact inhibition’ keeps cell division under control and prevents cells from piling up into a tumour. When cells lose this control and keep dividing uncontrollably, tumours form.

Can plants develop tumours? Not in the same way. Plant cells are enclosed in rigid cell walls and are fixed in place, so they do not show contact inhibition and instead grow in an organised, controlled pattern. Because of this, plants generally do not form the kind of spreading tumours seen in animals. (They can sometimes form localised swellings or galls when infected by certain microbes, but the rigid walls stop such growths from spreading through the plant the way an animal tumour can.)

Organelles involved: the ribosomes on the rough endoplasmic reticulum (RER) make the proteins, the smooth endoplasmic reticulum (SER) makes the lipids, and the Golgi apparatus then modifies, sorts and packs them for delivery.

The path the proteins and lipids follow:

• Made at the ER — proteins on the RER, lipids on the SER.
• Packed into vesicles that bud off the ER and carry them to the Golgi apparatus.
• Processed at the Golgi — it modifies and re-packs them, then buds off fresh vesicles.
• Delivered to the cell membrane — these vesicles move to the cell surface and fuse with the membrane, adding their proteins and lipids to it.

Gametes are normally made by meiosis, which halves the chromosome number so that each gamete carries only half the chromosomes of a body cell (it is haploid). This is what keeps the chromosome number constant from one generation to the next.

If gametes were instead formed by mitosis, each gamete would keep the full chromosome number of a body cell (diploid). Then, at fertilisation, two such gametes would fuse and the zygote would have double the normal number of chromosomes — and with every generation the number would keep doubling.

This ever-increasing chromosome number would disturb the genetic make-up of the offspring, cause serious abnormalities, and the species could not survive normally. This is exactly why gametes must be formed by meiosis, not mitosis.

(i) The scientific concept applied. Osmosis — preserving food by surrounding it with a high concentration of salt or sugar so that spoilage microbes cannot survive.

(ii) Why high salt or sugar stops microbes. A strong salt or sugar solution is far more concentrated than the inside of a bacterial or fungal cell, so water moves out of the microbes by osmosis. Losing their water, the microbes become dehydrated (plasmolysed) and cannot carry out their life processes or multiply, so the food does not spoil.

(iii) A healthy recipe of this kind. Amla murabba: lightly steam whole amla until soft, prick them, and steep them in a thick syrup made from jaggery (in place of refined sugar) with a pinch of salt, cardamom and a little dried ginger; store in a clean, dry glass jar. The high sugar concentration preserves the amla, while jaggery keeps it healthier than refined sugar.

(iv) The scientific values shown. Applying scientific knowledge (osmosis) to a real problem; reducing food wastage and working sustainably; supporting food security; resourcefulness and self-reliance; and blending traditional knowledge with science.