NCERT Solutions for Class 9th Science Chapter 2 Activity 2.2: Let us experiment — Cell membrane
Book page 11 Updated on2026-09-08
Q1.
What do you observe?
Answer
The two potato pieces behave in opposite ways.
Beaker
Liquid
Observation after about an hour
A
Plain water
The potato piece swells; it feels firmer
B
20 per cent salt or sugar solution
The potato piece shrinks; it feels soft and floppy
Water always moves towards the more concentrated solution, so the same potato gains water in beaker A and loses it in beaker B.
Tip: both pieces must be cut to roughly the same size and weighed before they go in, otherwise there is nothing to compare the final weights with.
Q2.
What do you infer? What do you expect in terms of changes in their weight?
Answer
The weight of the piece in Beaker A increases and the weight of the piece in Beaker B decreases. The process responsible is osmosis.
gain in weight (A) = final weight − initial weight → a positive number
loss in weight (B) = final weight − initial weight → a negative number
Example: A 25.0 g → 27.2 g, change = +2.2 g
B 25.0 g → 22.6 g, change = −2.4 g
Since only water crossed the membrane, the change in weight is a direct measure of how much water moved in or out.
Why it happens: the cell membrane of the potato cells is selectively permeable — it lets water through but not the sugar or salt molecules. In beaker A the water outside is more dilute (more water, less solute) than the cell sap inside, so water moves inwards until the two concentrations approach each other. In beaker B the 20 per cent solution outside is far more concentrated than the cell sap, so water moves outwards. This net movement of water through a selectively permeable membrane is osmosis, which is simply the diffusion of water. Ordinary diffusion needs no membrane at all — you saw it in Grade 8 when dye spread through water (Activity 7.8) and fragrance spread through air (Activity 7.9). Osmosis is the special case where the moving particles are water molecules and a selectively permeable membrane stands in the way.
Did you know? This is exactly how a plant drinks. Soil water is dilute, root cell sap is concentrated, so water enters root cells by osmosis — no pump and no energy needed.