NCERT Solutions for Class 9th Science Chapter 2 Pause and Ponder — Cell wall

Book page 14 Updated on2026-09-08

Q1.

What argument would you give for the necessity of a cell wall in plants usually fixed in one place versus in animals usually moving from one place to the other?

Answer

The argument is about where the support and the escape route come from. A fixed organism must build strength into every cell; a moving organism must instead keep every cell able to change shape.

Problem	How a plant solves it	How an animal solves it
Standing upright, resisting wind and rain	Rigid cellulose wall in every cell; turgid cells press against one another	Internal skeleton and muscles
Too much water entering by osmosis	Wall pushes back and stops further entry — the cell cannot burst	Kidneys and blood keep body fluid isotonic, so the problem does not arise
Escaping drought, heat or a predator	Cannot move; must endure it	Moves away
Movement, growth of an embryo, healing	Not needed — growth is by adding cells at fixed growing points	Wall-free cells change shape, slide past one another and migrate

Why it happens: a wall is not a free gift, it is a trade-off. It buys rigidity and protection from bursting, but it costs mobility and flexibility. For a plant rooted in soil, rigidity is worth far more than mobility, so the trade is a good one. For an animal that must run, swallow, contract a muscle and heal a wound, a rigid box around every cell would make all of that impossible — so animals keep a bare membrane and pay for the strength elsewhere, with bone and cartilage. Notice that fungi and bacteria, which are also non-motile or weakly motile, made the same choice as plants and have walls too.

Q2.

What consequences would you predict for a plant cell if its cell wall were to become as flexible as a cell membrane?

Answer

The cell would behave exactly like an animal cell — and the whole plant would collapse.

In ordinary soil water (hypotonic), the cell would burst. Water keeps entering by osmosis and only the rigid wall stops it. With a flexible wall there is nothing to build up the opposing pressure, so the cell swells until the membrane tears.
The plant would lose its shape. A stem, leaf or petal is held up by millions of turgid walled cells pressing on one another. Remove the rigidity and the plant would flop like a wilted one even when fully watered.
Plasmolysis would become shrivelling. In concentrated solution the cell would shrink as a whole instead of the contents pulling away from an unchanged wall — and it could not be revived simply by adding water.
Root absorption would be less effective. The wall is freely permeable and acts as a wide, open pathway for soil water and minerals to reach the membrane. A soft, membrane-like covering would slow that route down.

Soil water is hypotonic to root cell sap
→ water enters continuously by osmosis
→ pressure builds up inside
rigid wall present: pressure balances, the cell becomes turgid and stops
rigid wall absent: pressure keeps rising → the cell bursts

Why it happens: the wall works as a pressure vessel. Once the cell is full, the wall's inward push equals the osmotic pull inwards, and net water movement stops even though the concentrations are still unequal. That balance is what we call turgor, and turgor is the plant's substitute for a skeleton.

Q3.

Why is it important to cut the two potato pieces in roughly equal size and measure their initial weight before placing them in different liquids?

Answer

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.

change in weight = final weight − initial weight

Beaker A: 25.0 g → 27.2 g gives 27.2 − 25.0 = +2.2 g (water gained)
Beaker B: 25.0 g → 22.6 g gives 22.6 − 25.0 = −2.4 g (water lost)

Why it happens: this is the logic of a controlled experiment. Every factor that could affect the result — size, shape, the same potato, the same time, the same temperature — is deliberately kept the same, so any difference in the outcome must be caused by the one factor that was changed. Without the initial weights the experiment would give only a vague impression of ‘bigger’ and ‘smaller’; with them it gives numbers that can be compared, repeated and checked by someone else.

Tip: pat both pieces dry with the same care before each weighing. Surface water clinging to a piece would be weighed as if it had entered the cells.

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