BiologyICSE Class 10 · Absorption by Roots

Osmosis, Diffusion and Imbibition (ICSE Class 10 Biology)

Updated 15 October 2026 · Fact-checked

Diffusion is the spread of molecules from a region of higher to lower concentration. Osmosis is the movement of water molecules from a dilute solution to a more concentrated one through a selectively permeable membrane. Imbibition is the absorption of water by dry solids such as seeds and wood, which swell. All three are passive and need no energy.

Understand Osmosis, Diffusion and Imbibition

Three ways water and molecules move without the cell spending energy. These physical processes explain how a seed swells, how a root hair drinks and how gases enter a leaf.

1. Imbibition. The absorption of water by dry, solid, water-loving (hydrophilic) substances such as cellulose, starch and proteins, so that they swell. It can produce a large pressure (imbibition pressure).

Dry seeds swell when soaked; the swelling bursts the seed coat during germination.
Wooden doors and windows stick in the monsoon.
Dry raisins and chickpeas swell in water. (Raisins also take in water by osmosis.)
The wall of a root hair first takes up water by imbibition.

2. Diffusion. The movement of molecules of a substance from a region of their higher concentration to a region of their lower concentration, until they are evenly spread. It occurs in gases, liquids and dissolved solutes, and needs no membrane.

The scent of incense spreads through a room.
A crystal of potassium permanganate colours the whole beaker of water.
CO₂ diffuses into a leaf through stomata; O₂ diffuses out.

3. Osmosis. The movement of water molecules from a region of their higher concentration (a dilute solution or pure water) to a region of their lower concentration (a more concentrated solution) through a selectively permeable membrane. It is a special kind of diffusion: only the solvent (water) moves, and a membrane is essential.

Endosmosis: water enters a cell placed in a dilute (hypotonic) solution.
Exosmosis: water leaves a cell placed in a concentrated (hypertonic) solution.

Kinds of membrane (examiners asked that students know these apart).

Membrane	What passes	Example
Freely permeable	water and solutes	cellulose cell wall
Selectively (semi-) permeable	water, but not most solutes	plasma membrane, egg membrane, parchment paper
Impermeable	neither	cuticle-covered or cork layer, polythene

Osmotic pressure. The pressure that must be applied to a solution to stop water entering it by osmosis across a selectively permeable membrane. The more concentrated the solution, the higher its osmotic pressure, and the more strongly it draws water in. Root hair sap has a higher osmotic pressure than soil water, so water enters the root.

Active and passive transport (in brief).

	Passive transport	Active transport
Direction	along the concentration gradient	against the gradient
Energy	not needed	needed (ATP from respiration)
Examples	diffusion, osmosis, imbibition	uptake of mineral ions by root hairs

Osmosis experiment: the potato osmometer. Peel a large potato, cut a flat base and scoop a deep cavity in it. Half fill the cavity with strong sugar solution and mark the level with a pin. Stand the potato in a dish of water. After a few hours the level in the cavity rises: water has moved from the dish, through the living potato cells (acting as a selectively permeable membrane), into the sugar solution. A control with a boiled potato shows no rise, because heat kills the cells and their membranes stop being selectively permeable.

Flow chart. Dilute solution (more water molecules) → selectively permeable membrane → concentrated solution (fewer water molecules): net water flow until concentrations equalise or pressure stops it.

Draw-and-label checklist: potato osmometer. Beaker or dish of water; peeled potato with a flat base standing in it; cavity holding sugar solution; pin marking the initial level and the raised level; labels: water, potato tissue (living cells), sugar solution, initial level, final level.

Osmosis, Diffusion and Imbibition: key facts

Three processes compared: Imbibition water absorbed by dry hydrophilic solids, they swell
Diffusion any molecules, high to low concentration, no membrane
Osmosis water only, dilute to concentrated, through a selectively permeable membrane; All three are passive.
Membranes: Freely permeable cell wall
Selectively permeable plasma membrane
Impermeable cuticle, cork, polythene; Osmosis needs a selectively permeable membrane.
Endosmosis and exosmosis: Endosmosis: water enters the cell, outside solution hypotonic | Exosmosis: water leaves the cell, outside solution hypertonic; Name the outside solution compared with the cell sap.

Imbibition	water absorbed by dry hydrophilic solids, they swell
Diffusion	any molecules, high to low concentration, no membrane
Osmosis	water only, dilute to concentrated, through a selectively permeable membrane

Freely permeable	cell wall
Selectively permeable	plasma membrane
Impermeable	cuticle, cork, polythene

How to answer Osmosis, Diffusion and Imbibition questions

ICSE asks you to define and distinguish these processes, pick the right membrane, explain everyday examples and interpret osmosis experiments.

1Decide what moves: only water (osmosis) or any molecule (diffusion), or water into a dry solid (imbibition).
2Check whether a selectively permeable membrane is present; without one it is not osmosis.
3Give the direction: from higher to lower concentration of the moving molecules.
4For experiments, state the observation, the reason, and the role of the control.
5For mass or length data, a gain means the outside was hypotonic; a loss means it was hypertonic.

Osmosis, Diffusion and Imbibition in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Soaking chana overnight

Suppose your mother soaks a cup of dry chickpeas in water at night. By morning they have doubled in size and the water level in the bowl has fallen.

Which processes made the chickpeas swell, and why do they not keep swelling for ever?

Try it first, then show the solutionSolution

The dry seed coat and stored starch and proteins absorb water by imbibition and swell.
Living cells inside also take in water by osmosis, as their contents are more concentrated than the water.
As cells fill, wall pressure rises to balance turgor, and imbibition stops once the materials are saturated.
So the seeds reach a maximum size.

Answer: Imbibition (and osmosis in living cells); swelling stops when the materials are saturated and the cells are fully turgid.

Memory hook: Dry things drink by imbibition.

Tricks to remember Osmosis, Diffusion and Imbibition

Short memory aids. Say each one aloud once, then use it on a question the same day.

Osmosis = water only, membrane must
O for 'only water', M for 'membrane must'.
Use it when: Separating osmosis from diffusion.
Water runs to the salt
In osmosis water always moves towards the saltier or sweeter side.
Use it when: Predicting the direction of water flow.
Imbibe = drink in
A dry seed 'imbibes' water just as a thirsty sponge drinks.
Use it when: Recognising imbibition examples.

Osmosis, Diffusion and Imbibition: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Which membrane · 1 mark · MCQBoard level

A student wants water to pass by osmosis from a beaker of pure water into a sugar solution. The two liquids must be separated by:
(a) a freely permeable membrane (b) a cellulose cell wall (c) a selectively permeable membrane (d) a sheet of polythene
(The 2024 paper tested this idea.)

Try it first, then show the solutionSolution

Osmosis needs a membrane that lets water through but not most solutes: a selectively permeable membrane.

Answer: (c) a selectively permeable membrane.

Example 2: Osmosis versus diffusion · 3 marks · Short answerHarder board level

Give three differences between osmosis and diffusion.

Try it first, then show the solutionSolution

Osmosis needs a selectively permeable membrane; diffusion does not (1 mark).
In osmosis only water (solvent) moves; in diffusion molecules of any substance, solute or gas, move (1 mark).
Osmosis occurs only in liquids; diffusion occurs in gases, liquids and solutes (1 mark).

Answer: Membrane needed; only water moves; only in liquids (osmosis) versus no membrane, any molecules, gases and liquids (diffusion).

Example 3: Potato osmometer · 3 marks · Short answerHarder board level

In a potato osmometer the level of sugar solution in the cavity rose after two hours, but in a similar set-up using a boiled potato it did not. Explain both observations.

Try it first, then show the solutionSolution

Water from the dish passed through the living potato cells into the more concentrated sugar solution by osmosis (endosmosis), raising the level (1 mark).
The living plasma membranes acted as selectively permeable membranes (1 mark).
Boiling killed the cells; their membranes became freely permeable, so no osmosis occurred and the level did not rise (1 mark).

Answer: Osmosis through living, selectively permeable membranes raised the level; dead cells in the boiled potato cannot carry out osmosis.

Example 4: Mass change of potato cubes · 3 marks · Case-basedHOTS / competency

Suppose three 20 g potato cubes are kept for an hour in solutions A, B and C, and a fourth cube of 25 g in solution D. Final masses: A 22.4 g, B 20.0 g, C 17.0 g, D 23.5 g. (a) Calculate the percentage change in each. (b) Which solution is most concentrated? (c) Which is isotonic to the potato cells?

Try it first, then show the solutionSolution

(a) A: (22.4 − 20.0)/20.0 × 100 = +12%; B: 0%; C: (17.0 − 20.0)/20.0 × 100 = −15%; D: (23.5 − 25.0)/25.0 × 100 = −6% (1 mark).
(b) C: the largest percentage loss shows the strongest exosmosis, so C is the most hypertonic (1 mark).
(c) B: no change in mass means no net osmosis (1 mark).

Answer: (a) +12%, 0%, −15%, −6%. (b) C. (c) B.

Example 5: Imbibition in a jammed door · 2 marks · Short answerBoard level

Why does a wooden door that closes easily in winter jam in the rainy season? Name the process.

Try it first, then show the solutionSolution

Wood contains hydrophilic cellulose that absorbs moisture from humid air (1 mark).
It swells by imbibition, so the door no longer fits its frame (1 mark).

Answer: Imbibition of moisture makes the wood swell.

Example 6: Active uptake · 3 marks · Case-basedHOTS / competency

Suppose the cell sap of a root hair contains 50 times more potassium ions than the soil water, yet potassium continues to enter. (a) Can this be diffusion? (b) Name the process and its requirement. (c) What would happen if the root were treated with a poison that stops respiration?

Try it first, then show the solutionSolution

(a) No: diffusion moves ions only from higher to lower concentration (1 mark).
(b) Active transport, which needs energy (ATP) from respiration (1 mark).
(c) Without ATP, active uptake of potassium would stop (1 mark).

Answer: (a) No. (b) Active transport using ATP. (c) Uptake would stop.

Where students lose marks in Osmosis, Diffusion and Imbibition

The slips examiners see most often, and what to write instead.

Mistake: Confusing the three kinds of membrane
CISCE examiners' suggestions (2024): on the 'osmosis takes place when there is' MCQ, teachers were asked to differentiate selectively permeable, freely permeable and impermeable membranes with examples, as candidates chose the wrong one.
Do this: Learn one example each: cell wall, plasma membrane, cuticle.
Usually costs: 1 mark.
Mistake: Defining osmosis without the membrane or the water
'Movement from higher to lower concentration' defines diffusion, not osmosis.
Wrong working
Osmosis is the movement of molecules from higher to lower concentration.
Right working
Osmosis is the movement of water molecules from their higher to lower concentration through a selectively permeable membrane.
Do this: Always include 'water' and 'selectively permeable membrane'.
Usually costs: 1 mark.
Mistake: Writing 'solute moves into the solution' in osmosis
In osmosis the solvent (water) moves; the solute stays behind the membrane.
Do this: Say water moves towards the more concentrated solution.
Usually costs: 1 mark.

Exam tips

Write definitions with three parts: what moves, which direction, through what.
In experiment questions, mention the control and why it is needed.
For data, show the percentage change working: (final − initial) ÷ initial × 100.
Use 'hypotonic' and 'hypertonic' for the outside solution.

Practice questions on Osmosis, Diffusion and Imbibition

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Dry soybeans left in a bowl of water overnight swell to about twice their size. The process mainly responsible is:
(a) diffusion (b) imbibition (c) plasmolysis (d) transpiration
Hint
Dry solids absorbing water.
Answer
(b) imbibition.
Q2 · 1 mark · Very short answerBoard level
Name a freely permeable part of a plant cell.
Hint
Outermost layer.
Answer
The cell wall.
Q3 · 2 marks · Short answerHarder board level
Define osmotic pressure. How does it change as a sugar solution is made more concentrated?
Hint
Pressure to stop water entering.
Answer
The pressure that must be applied to a solution to prevent water from entering it by osmosis through a selectively permeable membrane. It increases as the solution becomes more concentrated.
Q4 · 2 marks · Short answerHarder board level
Distinguish between endosmosis and exosmosis with one example each.
Hint
Direction of water.
Answer
Endosmosis: water enters a cell kept in a hypotonic solution, e.g. raisins swelling in water. Exosmosis: water leaves a cell kept in a hypertonic solution, e.g. grapes shrinking in strong sugar syrup.
Q5 · 2 marks · Short answerHarder board level
Give one difference between active transport and passive transport, and one example of each in plants.
Hint
Energy and gradient.
Answer
Active transport needs energy and works against the concentration gradient (uptake of mineral ions by root hairs); passive transport needs no energy and works along the gradient (osmosis of water into root hairs).
Q6 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Diffusion of CO₂ into a leaf can occur without any membrane.
Reason (R): Diffusion is the movement of molecules from a region of higher to lower concentration.
(a) (A) is true and (R) is false.
(b) (A) is false and (R) is true.
(c) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(d) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
Hint
Does a gradient alone explain it?
Answer
(c).
Q7 · 2 marks · Short answerHarder board level
A student sets up two potato osmometers side by side, one with a raw potato and one with a potato boiled for ten minutes. What is the boiled potato for, and what would it show?
Hint
Living versus dead membranes.
Answer
Boiling kills the cells, so their membranes are no longer selectively permeable. If no rise occurs with it, the rise in the test potato is shown to be due to osmosis through living cells.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose an egg's shell is dissolved in vinegar, leaving the egg in its membrane. The egg is weighed (60 g), kept in pure water overnight (it becomes 72 g), and then kept in strong salt solution (it becomes 54 g).
(a) Calculate the percentage change after the water. (1)
(b) Name the process in each step. (1)
(c) What kind of membrane does the egg membrane behave as? (1)
Hint
(72 − 60)/60.
Answer
(a) (72 − 60)/60 × 100 = +20%. (b) Water: endosmosis; salt solution: exosmosis. (c) A selectively permeable membrane.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Which process needs a selectively permeable membrane?Show answer
Osmosis.
2.What makes wooden doors swell in the rainy season?Show answer
Imbibition.
3.Does diffusion need energy?Show answer
No, it is passive.
4.Give an example of a selectively permeable membrane.Show answer
The plasma membrane (also egg membrane, parchment paper).
5.What is the uptake of mineral ions against a gradient called?Show answer
Active transport.

Osmosis, Diffusion and Imbibition: frequently asked questions

Is osmosis a type of diffusion?

Yes. It is the diffusion of water alone through a selectively permeable membrane.

What is the difference between semi-permeable and selectively permeable?

A semi-permeable membrane in theory lets only the solvent pass. Living membranes are called selectively permeable because they let water and some chosen solutes pass. ICSE accepts both terms for osmosis.

Does imbibition need living cells?

No. Dead wood and dry seeds imbibe water; it depends only on hydrophilic materials.

Why do roots need oxygen to absorb minerals?

Most minerals are taken in by active transport, which uses energy from respiration, and respiration needs oxygen.