ScienceCBSE Class 9 · Cell: The Building Block of Life

Cell Membrane, Diffusion, Osmosis and Plasmolysis (Class 9)

Updated 17 October 2026 · Fact-checked

The cell membrane is a thin lipid bilayer with proteins (the fluid-mosaic model) that is selectively permeable, so it controls what enters and leaves the cell. Diffusion is the net movement of particles from higher to lower concentration, and osmosis is the movement of water through a selectively permeable membrane from a dilute solution to a more concentrated one. A cell swells in a hypotonic solution, stays the same in an isotonic one, and shrinks in a hypertonic one; in a plant cell the contents shrinking away from the cell wall is called plasmolysis.

Two beakers split by a dashed semipermeable membrane; water rises on the side with more solute particles, labelled 'Semipermeable membrane'. — Notice that water moves across the membrane toward the side with more solute particles, so the liquid level rises there while the solute stays put.
Picture: OpenStax · CC BY 4.0 · Wikimedia Commons

Understand Cell Membrane, Diffusion and Osmosis

The boundary of the cell. Every cell is surrounded by a cell membrane (plasma membrane), only about 7 to 10 nm thick. In the fluid-mosaic model it is a double layer of lipid (fat) molecules, with water-loving heads facing outwards and water-avoiding tails facing inwards, with protein molecules set into it. 'Fluid' because the molecules can move sideways in the layer; 'mosaic' because the lipids and proteins are arranged like tiles. Proteins act as gatekeepers. The membrane is selectively permeable: some substances pass easily, others pass only with help, and some are blocked.

Diffusion. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. No membrane is needed. A drop of ink spreading in water and the smell of cooking reaching another room are examples. It continues until the particles are evenly spread. Cells use diffusion to exchange gases such as oxygen and carbon dioxide.

Osmosis. Osmosis is a special case: the net movement of water through a selectively permeable membrane from a dilute solution (more water, less solute) to a more concentrated solution (less water, more solute). Compare the solution outside the cell with the cell contents. If the outside solution is more dilute it is hypotonic, and water enters the cell, which swells. If both are equal the solution is isotonic and there is no net movement. If the outside is more concentrated it is hypertonic, and water leaves, so the cell shrinks. NCERT's potato activity shows this: a piece in plain water gains mass, and a piece in concentrated salt or sugar solution loses mass. Roots take in water from soil by osmosis.

Cell wall and plasmolysis. Plant cells, fungal cells and bacteria have a cell wall outside the membrane. In plants it is made mainly of cellulose, is rigid, gives shape and support, and is permeable (water and dissolved minerals pass freely), unlike the membrane. If a Rhoeo leaf peel or onion peel is put in a concentrated solution, water leaves by osmosis, the wall keeps its shape, and the contents shrink away from the wall. This is plasmolysis. Placed back in water, the cell takes water in again and fills out. Because the wall resists swelling, a plant cell in plain water becomes firm (turgid) and does not burst; an animal cell has no wall, so it simply shrinks in a hypertonic solution and would swell in a very dilute one.

Reading a data table. Percentage change in mass = (final − initial) ÷ initial × 100. A gain means water entered (outside was hypotonic); a loss means water left (outside was hypertonic); no change suggests an isotonic solution.

Cell Membrane, Diffusion and Osmosis: formulas and key facts

Percentage change in mass: (final mass − initial mass) ÷ initial mass × 100
Hypotonic outside: Outside more dilute
water enters
cell swells (plant cell becomes turgid)
Isotonic outside: Equal concentration
no net water movement
cell unchanged
Hypertonic outside: Outside more concentrated
water leaves
cell shrinks (plant cell: plasmolysis)
Membrane thickness: $A b o u t 7 t o 10 nm ∣1 μ m = 1000 nm$

How to solve Cell Membrane, Diffusion and Osmosis questions

Use this method for osmosis, potato-cylinder and plasmolysis questions.

1Identify the two solutions or regions and decide which is more dilute (more water) and which is more concentrated.
2Check that a selectively permeable membrane separates them; without one it is diffusion, not osmosis.
3State the direction of net water movement: from the dilute side to the concentrated side.
4Say what happens to the cell: swells, stays the same or shrinks (and for a plant cell, turgid or plasmolysed).
5For data, calculate the percentage change in mass using the initial mass as the base.
6Use technical words: hypotonic, hypertonic, isotonic, selectively permeable, turgid, plasmolysis.

Cell Membrane, Diffusion and Osmosis in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Osmosis in soaked and shrivelled fruit

Suppose you soak a dried raisin of mass 2.0 g in plain water for six hours and it becomes plump with a mass of 2.9 g. You then place a different 2.0 g raisin in a thick sugar syrup, and after six hours it weighs 1.6 g.

(a) Find the percentage change in mass in each case. (b) Name the type of solution outside the raisin in each case. (c) Explain what the membranes of the raisin cells are doing.

Try it first, then show the solutionSolution

(a) In water: (2.9 − 2.0) ÷ 2.0 × 100 = 45% gain. In syrup: (1.6 − 2.0) ÷ 2.0 × 100 = −20%, a 20% loss.
(b) Plain water is hypotonic to the raisin; the thick syrup is hypertonic.
(c) The selectively permeable membranes let water pass but not the sugar and other solutes, so water moves from where it is more plentiful to where it is less.

Answer: (a) +45% and −20%; (b) hypotonic and hypertonic; (c) water moves by osmosis through the selectively permeable membranes.

Memory hook: Water follows the saltier or sweeter side.

Wilting after too much fertiliser

Suppose a gardener mixes a very strong fertiliser solution and pours it on a potted plant. Within hours the leaves droop, though the pot is full of water.

Explain this with osmosis, and say what the gardener could do to help the plant recover.

Try it first, then show the solutionSolution

The strong fertiliser makes the soil water more concentrated than the cell sap of the root cells, so it is hypertonic to them.
Water leaves the root cells by osmosis instead of entering, so the cells lose turgor and the plant wilts.
Flushing the soil with plenty of plain water dilutes the soil solution, making it hypotonic again, so water can enter the root cells and the plant recovers.

Answer: The hypertonic soil solution draws water out of the roots; flush with plain water to restore inward osmosis.

Memory hook: Too much food outside makes the cell thirsty inside.

Tricks to remember Cell Membrane, Diffusion and Osmosis

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

Hypo = less outside, so water goes in
Hypotonic means the outside has fewer solutes. 'Hypo' sounds like 'hippo': the cell gulps water and gets big.
Use it when: Predicting swelling or shrinking.
Water goes toward the salt
In osmosis water moves toward the side with more dissolved solute (the more concentrated side).
Use it when: Quickly deciding the direction of net water movement.
Wall holds, contents shrink
In plasmolysis the wall stays where it is and the protoplast pulls away from it, like a deflating balloon inside a glass jar.
Use it when: Describing plasmolysis in a diagram question.
Percentage change uses the start
Divide the change by the starting mass, never by the final mass.
Use it when: Potato and raisin numericals.

Cell Membrane, Diffusion and Osmosis: solved examples

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

Example 1: Raisins and osmosis in words · 3 marks · Short answerBoard level

A dried raisin placed in plain water becomes plump, and another placed in concentrated sugar solution becomes smaller.
(a) Name the process responsible. (1)
(b) Explain the plumping in plain water in terms of concentrations. (1)
(c) Explain the shrinking in the sugar solution and name the type of solution. (1)

Try it first, then show the solutionSolution

(a) Osmosis. (1)
(b) Plain water is more dilute than the cell contents (hypotonic), so water moves into the raisin cells through the selectively permeable membrane and they swell. (1)
(c) The sugar solution is more concentrated than the cell contents (hypertonic), so water leaves the cells and the raisin shrinks. (1)

Answer: (a) osmosis; (b) hypotonic water enters; (c) hypertonic solution draws water out.

Example 2: Potato cylinders in three solutions · 3 marks · Short answerBoard level

Three potato cylinders, each of initial mass 5.0 g, are left for two hours in (A) distilled water, (B) 20% salt solution and (C) a solution of salt in which the mass does not change. After blotting the final masses are A 5.6 g, B 4.3 g, C 5.0 g.
(a) Find the percentage change in mass for A and B. (1)
(b) Classify each solution as hypotonic, hypertonic or isotonic to the potato cells. (1)
(c) Why is blotting dry done before weighing? (1)

Try it first, then show the solutionSolution

(a) A: (5.6 − 5.0) ÷ 5.0 × 100 = 12% gain. B: (4.3 − 5.0) ÷ 5.0 × 100 = −14%, a 14% loss. (1)
(b) A is hypotonic (water entered), B is hypertonic (water left), C is isotonic (no net change). (1)
(c) Water sticking to the outside of the cylinder would be weighed with it and would make the mass look higher; only water inside the cells should count. (1)

Answer: (a) +12% and −14%; (b) A hypotonic, B hypertonic, C isotonic; (c) to remove surface water.

Example 3: Plasmolysis data from onion peel · 5 marks · Long answerHarder board level

A student mounts onion peel in four sugar solutions and counts plasmolysed cells in a fixed field of 50 cells each: 0% sugar: 0 cells; 5%: 4 cells; 10%: 21 cells; 15%: 45 cells.
(a) Calculate the percentage of plasmolysed cells in each. (1)
(b) State the trend, with a reason. (1)
(c) Estimate by straight-line interpolation the concentration at which half the cells plasmolyse. (2)
(d) State why the cell wall does not shrink with the contents. (1)

Try it first, then show the solutionSolution

(a) 0 ÷ 50 = 0%; 4 ÷ 50 = 8%; 21 ÷ 50 = 42%; 45 ÷ 50 = 90%. (1)
(b) The percentage of plasmolysed cells rises as the concentration of sugar solution rises: the stronger solution is more hypertonic, so more cells lose water. (1)
(c) 50% lies between 10% sugar (42%) and 15% sugar (90%). The rise is 48 percentage points over 5% concentration, i.e. 9.6 per 1%. We need 8 more points: 8 ÷ 9.6 = 0.83. Concentration ≈ 10 + 0.83 = 10.8%, about 11% (an estimate, since the graph is not exactly a straight line). (2)
(d) The cell wall is rigid cellulose, so it holds its shape while the protoplast (the living contents inside the membrane) shrinks away from it. (1)

Answer: (a) 0%, 8%, 42%, 90%; (b) more plasmolysis at higher concentration; (c) about 10.8% (roughly 11%); (d) the cellulose wall is rigid.

Example 4: Two chambers and a selectively permeable membrane · 5 marks · Long answerHarder board level

A container is divided by a membrane that lets only water pass. Chamber A holds 100 mL of solution with 2 g of dissolved solute; Chamber B holds 100 mL of solution with 8 g of dissolved solute.
(a) In which direction is the net movement of water, and why? (1)
(b) Which chamber is hypertonic to the other? (1)
(c) In the ideal case, where the container can stretch freely and nothing opposes the flow, the solutions end up with equal concentration. Find the final volume in each chamber. (2)
(d) Why can a plant cell not follow the ideal case indefinitely? (1)

Try it first, then show the solutionSolution

(a) From A to B, because A is more dilute (2 g per 100 mL) than B (8 g per 100 mL). (1)
(b) B is hypertonic to A (and A is hypotonic to B). (1)
(c) The solute cannot cross, so A keeps 2 g and B keeps 8 g. Total solute 10 g in 200 mL gives 5 g per 100 mL at equal concentration. Volume of A = 2 ÷ 5 × 100 = 40 mL. Volume of B = 8 ÷ 5 × 100 = 160 mL. Check: 40 + 160 = 200 mL. (2)
(d) The rigid cell wall resists swelling, so pressure builds up inside the cell and stops more water from entering; the cell becomes turgid but does not burst. (1)

Answer: (a) A to B; (b) B; (c) A 40 mL, B 160 mL; (d) the cell wall stops further swelling.

Example 5: Why the lettuce goes limp in dressing · 4 marks · Case-basedHOTS / competency

A student puts 10.0 g of crisp lettuce strips in plain water and another 10.0 g in salty salad dressing. After 30 minutes the strips in water weigh 10.8 g and are firmer; those in dressing weigh 8.6 g and are limp.
(a) Find the percentage change in each case. (1)
(b) Explain the firmness and the limpness using the words turgid and plasmolysed. (1)
(c) Explain why an animal cell and a plant cell behave differently in plain water. (1)
(d) Suggest a simple way to make the limp strips crisp again, with a reason. (1)

Try it first, then show the solutionSolution

(a) In water: (10.8 − 10.0) ÷ 10.0 × 100 = 8% gain. In dressing: (8.6 − 10.0) ÷ 10.0 × 100 = −14%, a 14% loss. (1)
(b) In water the cells take in water and become turgid, pressing on their walls, so the strips are firm. In the salty dressing water leaves, the contents shrink away from the walls (plasmolysis) and the strips lose firmness. (1)
(c) A plant cell has a rigid cell wall, which resists swelling, so it becomes turgid without bursting. An animal cell has no wall, so in a very dilute solution it would swell and might burst. (1)
(d) Put the strips in plain water. Water is hypotonic to the cells, so water re-enters by osmosis and they become turgid again. (1)

Answer: (a) +8% and −14%; (b) turgid in water, plasmolysed in dressing; (c) the plant cell has a cell wall; (d) soak in plain water.

Where students lose marks in Cell Membrane, Diffusion and Osmosis

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

Mistake: Reversing the direction of water movement in osmosis.
Concentration of water and concentration of solute are opposite, and students lose track of which is meant.
Wrong working
Water moves from the concentrated solution to the dilute solution.
Right working
Water moves from the dilute solution (less solute) to the concentrated solution (more solute) through a selectively permeable membrane.
Do this: State the direction with solute: toward more solute.
Usually costs: 1 mark.
Mistake: Calling the cell wall selectively permeable.
The wall and the membrane are confused.
Wrong working
The cell wall controls what enters and leaves the cell.
Right working
The cell wall is permeable; the cell membrane is selectively permeable.
Do this: Write 'wall: rigid and permeable; membrane: selectively permeable'.
Usually costs: 1 mark.
Mistake: Using the final mass as the base for percentage change.
Students divide by the number in the last column.
Wrong working
(5.6 − 5.0) ÷ 5.6 × 100 = 10.7%
Right working
(5.6 − 5.0) ÷ 5.0 × 100 = 12%
Do this: Percentage change always divides by the initial mass.
Usually costs: 1 to 2 marks in a data question.
Mistake: Describing plasmolysis as the cell wall shrinking.
The whole cell looks smaller.
Wrong working
In plasmolysis the cell wall shrinks.
Right working
The wall keeps its shape; the cell contents (protoplast) shrink away from the wall.
Do this: Say 'contents shrink away from the wall'.
Usually costs: 1 mark.
Mistake: Calling diffusion of water 'osmosis' without a membrane.
The definition of osmosis needs the selectively permeable membrane.
Wrong working
Sugar spreading in water is osmosis.
Right working
Sugar spreading in water is diffusion; osmosis is water moving across a selectively permeable membrane.
Do this: Check for a membrane before using the word osmosis.
Usually costs: 1 mark.

Exam tips

Define osmosis with all three parts: water, selectively permeable membrane, dilute to concentrated.
For any swelling or shrinking question, name the outside solution (hypotonic, isotonic or hypertonic) before giving the result.
Draw plasmolysis with the wall unchanged and the protoplast pulled away from it, and label wall, cell membrane and cell sap.
In a data table, write the percentage-change formula once and then substitute.
In a comparison table, give at least two points of difference for each pair, such as diffusion and osmosis or cell wall and cell membrane.

Practice questions on Cell Membrane, Diffusion and Osmosis

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

Q1 · 2 marks · Very short answerBoard level
Write two differences between diffusion and osmosis.
Hint
Think of what moves and whether a membrane is needed.
Answer
Diffusion can happen for any particles (gases, solutes) and needs no membrane, while osmosis is the movement of water only and needs a selectively permeable membrane (1 mark). Diffusion is movement from higher to lower concentration of the diffusing particles; osmosis is movement of water from a dilute to a concentrated solution (1 mark).
Q2 · 3 marks · Short answerHarder board level
Two raisins of mass 2.0 g each are placed in (i) plain water and (ii) 20% sugar solution. After six hours they weigh 2.9 g and 1.6 g respectively. Find the percentage change in each case and explain both results.
Hint
Percentage change = change ÷ initial mass × 100.
Answer
(i) (2.9 − 2.0) ÷ 2.0 × 100 = +45%; water is hypotonic, so water enters the cells (1 mark). (ii) (1.6 − 2.0) ÷ 2.0 × 100 = −20%; the sugar solution is hypertonic, so water leaves the cells (1 mark). Both changes are osmosis through the selectively permeable cell membranes (1 mark).
Q3 · 2 marks · Assertion-reasonBoard level
Assertion (A): The plant cell wall is selectively permeable. Reason (R): The plant cell wall is made mainly of cellulose. Choose: (a) A and R both true, R explains A; (b) A and R both true, R does not explain A; (c) A true, R false; (d) A false, R true.
Hint
Which structure is selectively permeable, the wall or the membrane?
Answer
(d). The cell wall is permeable to water and dissolved minerals, so A is false; it is made mainly of cellulose, so R is true. It is the cell membrane that is selectively permeable.
Q4 · 3 marks · Short answerHarder board level
The cell membrane is about 8 nm thick. (a) How many such layers laid side by side would make a thickness of 1 micrometre (1 μm = 1000 nm)? (b) Explain the words 'fluid' and 'mosaic' in the fluid-mosaic model of the membrane.
Hint
Convert units before dividing.
Answer
(a) 1000 nm ÷ 8 nm = 125 layers (1 mark). (b) Fluid: the lipid and protein molecules can move sideways within the membrane (1 mark). Mosaic: the lipids and proteins are arranged next to one another like the tiles of a mosaic (1 mark).
Q5 · 4 marks · Case-basedHOTS / competency
A student plants a seedling in soil that has been irrigated with very salty water and it wilts, though the soil is wet. Using the terms hypertonic and osmosis, (a) explain the wilting, (b) say why a plant cell in plain water does not burst, and (c) name the process observed in an onion peel placed in strong salt solution and describe what is seen under the microscope.
Hint
Think about which way water moves at the root hairs.
Answer
(a) The soil water is hypertonic to the root cells, so water leaves the roots by osmosis instead of entering; the cells lose turgor and the plant wilts (1 mark). (b) The rigid cell wall resists swelling, so the cell becomes turgid but does not burst (1 mark). (c) Plasmolysis; the cytoplasm and membrane shrink away from the cell wall while the wall keeps its shape (2 marks).

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.What is the thickness of the cell membrane?Show answer
About 7 to 10 nm.
2.Define osmosis in one line.Show answer
Net movement of water through a selectively permeable membrane from a dilute solution to a more concentrated one.
3.What happens to a cell in a hypertonic solution?Show answer
Water leaves and the cell shrinks.
4.What is the main component of the plant cell wall?Show answer
Cellulose.
5.What is plasmolysis?Show answer
Shrinking of the cell contents away from the cell wall in a hypertonic solution.
6.Why is the model of the membrane called 'fluid'?Show answer
Its lipid and protein molecules can move sideways within the layer.

Cell Membrane, Diffusion and Osmosis: frequently asked questions

Is a hypertonic solution the same as a concentrated solution?

It is hypertonic relative to something else. A solution is hypertonic to the cell if it has more solute than the cell contents.

Why do plant cells not burst in water?

The rigid cell wall resists swelling, so the cell becomes turgid and firm instead of bursting.

Does osmosis stop when the cell is turgid?

Water entry slows and stops when pressure from the wall balances the pull of water in, so there is no more net movement.

Do gases enter cells by osmosis?

No. Gases such as oxygen and carbon dioxide move by diffusion. Osmosis is only for water.

Can animal cells show plasmolysis?

No. Plasmolysis is the pulling away from a cell wall, and animal cells have no wall; they simply shrink.