BiologyICSE Class 8 · Transport of Food and Minerals in Plants

Diffusion, Osmosis, Root Pressure and Active Transport (ICSE Class 8 Biology)

Updated 17 October 2026 · Fact-checked

Diffusion is the spreading of molecules from higher to lower concentration. Osmosis is the movement of water alone, through a semipermeable membrane, from a dilute to a concentrated solution. Active transport moves ions against their concentration gradient using respiratory energy. Root pressure is the push that water entering roots gives to the xylem sap.

Diffusion and osmosis are passive and go down a gradient; active transport pushes ions from low to high concentration using energy.

Illustration drawn for QuestionBuddy.

Understand Diffusion, Osmosis, Root Pressure and Active Transport

Why plants need transport. Every cell needs water, minerals and gases. A very small plant gets them by simple spreading; a big plant needs help.

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 is passive: the cell spends no energy. Examples: CO₂ moving into a leaf through stomata while O₂ moves out; the smell of incense spreading through a room.

Diffusion in simple plants.

Chlamydomonas is a unicellular green alga. The one cell touches the water all round, so gases and dissolved minerals diffuse in and out through the cell membrane.
Spirogyra is a multicellular alga whose cells are joined in a thread one cell thick, so every cell is in contact with water. Neither needs xylem or phloem. A tree does: the distance from root to leaf is too great for diffusion, and most of its cells touch neither soil nor air.

Osmosis. The movement of water molecules from a region of higher water concentration (a dilute solution) to a region of lower water concentration (a concentrated solution) through a semipermeable membrane. Memory line: water goes where the solute is.

A semipermeable membrane lets water (solvent) pass but holds back the dissolved substance (solute). Examples: egg membrane, cellophane, parchment paper. A living cell membrane is selectively permeable.
Endosmosis: water enters the cell (dry raisins swell in water). Exosmosis: water leaves the cell (salted cucumber slices turn watery).

Potato osmoscope. Hollow out a peeled potato, pour strong sugar solution into the hollow, mark the level and stand it in a dish of water. After a few hours the level rises: water has crossed the living potato cells into the sugar solution. A boiled potato shows no rise because heat has killed the cells and destroyed the membrane's selective action. A control with plain water inside shows no rise either.

Osmosis in roots. The cell sap of a root hair is more concentrated than the soil water, so water enters the root hair by osmosis and then passes cell to cell, from a cell of weaker sap to a cell of stronger sap, towards the xylem.

Root pressure. The pressure built up in the xylem of the root because water keeps entering the root cells by osmosis. It pushes water a short way up the stem.

Evidence: sap exudes from a stem cut close to the ground; guttation, drops of water at leaf edges on humid early mornings when transpiration is very low.
It cannot raise water to the top of a tall tree. The main lifting force is transpiration pull (next topic).

Active transport. The movement of ions or molecules across a cell membrane from lower to higher concentration, against the gradient, using energy from respiration. Root hairs collect nitrate and potassium ions this way even when the soil water holds fewer of them than the cell sap. Hence roots need oxygen: waterlogged soil has little air, roots cannot respire well, uptake of minerals falls and the plant suffers.

	Diffusion	Osmosis	Active transport
What moves	any molecules	water only	ions or molecules
Direction	high to low concentration	high to low water concentration	low to high concentration
Membrane	not needed	semipermeable	living membrane
Energy	none	none	respiratory energy

Flow chart: water from soil to xylem. Soil water → root hair (osmosis, sap stronger than soil water) → cortex cells (osmosis, cell to cell) → xylem → pushed up by root pressure, pulled up by transpiration.

Diffusion, Osmosis, Root Pressure and Active Transport: key facts

Diffusion: Molecules move high to low concentration
Passive
No membrane needed
CO₂ into leaf; No energy used.
Osmosis: Water molecules only
Dilute to concentrated solution
Semipermeable membrane
Endosmosis in, exosmosis out; Water goes where the solute is.
Active transport and root pressure: Active transport low to high concentration
Uses respiratory energy
Root pressure osmotic push in xylem
Evidence exudation, guttation; Roots need oxygen.

Active transport	low to high concentration
	Uses respiratory energy
Root pressure	osmotic push in xylem
Evidence	exudation, guttation

How to answer Diffusion, Osmosis, Root Pressure and Active Transport questions

Questions ask you to define a term, compare processes, predict the result of a set-up such as the osmoscope, or explain why a plant wilts or a root needs air.

1Identify what is moving: any molecules (diffusion), water only (osmosis), ions against a gradient (active transport).
2Compare the concentrations to fix the direction: osmosis sends water towards the more concentrated solution.
3Check the membrane: osmosis needs a semipermeable one, and a dead (boiled) one does not work.
4Ask whether energy is used: only active transport spends it, which is why oxygen supply matters.
5Close with the effect on the plant (swelling, shrinking, wilting, uptake of minerals).

Diffusion, Osmosis, Root Pressure and Active Transport 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.

Salt on a cut vegetable

Suppose a cook sprinkles a spoonful of salt over sliced brinjal and, after twenty minutes, finds a pool of water in the dish.

Explain where the water came from.

Try it first, then show the solutionSolution

Salt dissolving on the surface makes a concentrated solution outside the cells.
The cells hold a more dilute sap, so water molecules move out across the cell membranes.
This outward movement of water is exosmosis, and it collects as the pool.

Answer: Exosmosis: water left the cells towards the concentrated salt solution outside.

Memory hook: Salt pulls water.

A plant in a pot with no drainage

Suppose a gardener keeps watering a potted plant in a pot with no drainage hole, and after two weeks the leaves turn yellow and droop although the soil is wet.

Why does the plant suffer in wet soil?

Try it first, then show the solutionSolution

Water fills the air spaces, so little oxygen reaches the roots.
Root cells cannot respire well and make less energy.
Active transport of mineral ions needs that energy, so uptake of minerals falls and the leaves yellow.

Answer: Lack of oxygen reduces respiration in roots, so active uptake of minerals fails.

Memory hook: No air, no energy, no minerals.

Tricks to remember Diffusion, Osmosis, Root Pressure and Active Transport

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

Water follows the salt
In osmosis water moves towards the stronger solution.
Use it when: Fixing the direction of osmosis.
DOA
Diffusion Downhill, Osmosis for water only, Active Against (uphill).
Use it when: Telling the three processes apart.
Boiled means broken
Boiling kills cells, so the osmoscope stops.
Use it when: Control experiment answers.

Diffusion, Osmosis, Root Pressure and Active Transport: solved examples

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

Example 1: Energy needed · 1 mark · MCQBoard level

Which of these processes needs energy from the cell?
(a) diffusion of CO₂ into a leaf (b) osmosis in a raisin (c) uptake of nitrate ions by a root hair against a gradient (d) smell of agarbatti spreading

Try it first, then show the solutionSolution

Diffusion and osmosis are passive.
Moving ions from low to high concentration is active transport and uses respiratory energy.

Answer: (c).

Example 2: Diffusion versus osmosis · 2 marks · Short answerBoard level

Give two differences between diffusion and osmosis.

Try it first, then show the solutionSolution

Diffusion can involve any molecules, but osmosis involves only water molecules (1 mark).
Osmosis needs a semipermeable membrane; diffusion does not (1 mark).

Answer: Any molecules versus water only; no membrane versus a semipermeable membrane.

Example 3: Boiled potato osmoscope · 3 marks · Short answerHarder board level

Two potato cups are set up in water, each holding 10 cm³ of the same strong sugar solution. The level in cup P (fresh potato) rose from 3.0 cm to 4.2 cm in 4 hours. Cup Q is made from a boiled potato. (a) Calculate the average rate of rise in P. (b) Predict the result in Q and explain both results.

Try it first, then show the solutionSolution

(a) Rise = 4.2 - 3.0 = 1.2 cm; rate = 1.2 ÷ 4 = 0.3 cm per hour (1 mark).
(b) Q shows no rise (1 mark).
In P the living cells act as a semipermeable membrane, so water enters the strong sugar solution by osmosis; boiling killed the cells in Q, so there is no selective membrane and no osmosis (1 mark).

Answer: (a) 0.3 cm per hour. (b) No rise in Q: dead cells are not semipermeable.

Example 4: Potassium in a root hair · 4 marks · Long answerHarder board level

Suppose the sap of a root hair has 15 units of potassium ions and the soil water has 3 units. The root hair still takes in more potassium. (a) Name the process. (b) Why is diffusion ruled out? (c) Why does this stop if the root is deprived of oxygen?

Try it first, then show the solutionSolution

(a) Active transport (1 mark).
(b) Diffusion moves ions from high to low concentration; here the movement is into the cell, which already has more (1 mark).
(c) Moving ions against the gradient needs energy released by respiration (1 mark).
Without oxygen respiration slows, so less energy is available and uptake stops (1 mark).

Answer: Active transport; it works against the gradient and needs energy from respiration, which needs oxygen.

Example 5: Over-fertilised plants · 3 marks · Case-basedHOTS / competency

A farmer dissolves a large quantity of fertiliser in the water for a small bed of seedlings, hoping to feed them well. Within a day the seedlings wilt. Using the idea of osmosis, explain what has gone wrong and what the farmer should do.

Try it first, then show the solutionSolution

The soil water became more concentrated than the sap of the root hairs (1 mark).
Water therefore left the root cells by exosmosis instead of entering, so the cells lost firmness and the seedlings wilted (1 mark).
Fix: water the bed well to dilute the fertiliser, and use smaller amounts in future (1 mark).

Answer: Soil water became stronger than the cell sap, so exosmosis occurred; dilute it with plain water.

Where students lose marks in Diffusion, Osmosis, Root Pressure and Active Transport

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

Mistake: Defining osmosis as the movement of salt or sugar
Students mix up the solute with the solvent; in osmosis only water crosses the membrane.
Do this: Always write water molecules, and name the membrane.
Usually costs: 1 mark.
Mistake: Reversing the direction of osmosis
Many write that water moves from the concentrated to the dilute solution.
Do this: Water moves from the dilute to the concentrated solution, towards the solute.
Usually costs: 1 mark.
Mistake: Saying active transport is movement from high to low concentration
It is confused with diffusion because both move ions.
Do this: Active transport is against the gradient and needs energy.
Usually costs: 1 mark.
Mistake: Not explaining why the boiled potato fails
Candidates write only that nothing happens.
Do this: Say boiling killed the cells, so the membrane is no longer semipermeable.
Usually costs: 1 mark.

Exam tips

Osmosis: water only, semipermeable membrane, dilute to concentrated.
Active transport needs energy and works against the gradient.
Quote evidence for root pressure: exudation and guttation.
Spirogyra and Chlamydomonas use diffusion; no xylem or phloem.

Practice questions on Diffusion, Osmosis, Root Pressure and Active Transport

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 · Very short answerBoard level
What is guttation?
Hint
Early morning, leaf edges.
Answer
The oozing of drops of water from the edges of leaves, seen when transpiration is low, which is evidence of root pressure.
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): A dead root hair cannot absorb water by osmosis.
Reason (R): Osmosis needs a semipermeable membrane, which only living cell membranes provide in a root.
(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
Boiled potato.
Answer
(c). Death destroys the selective action of the membrane, so osmosis in the root hair stops.
Q3 · 2 marks · Short answerHarder board level
Why do Spirogyra and Chlamydomonas need no conducting tissue, while a tall tree does? Give two points.
Hint
Distance and contact.
Answer
In both algae every cell is in direct contact with water, so materials move in and out by diffusion over very short distances. In a tree, distances from root to leaf are long and most cells touch neither soil nor air, so diffusion is far too slow and xylem and phloem are needed.
Q4 · 3 marks · Long answerHarder board level
Explain why a stem cut near the ground on a damp morning oozes sap, and why this cannot be the main way a tall tree gets water to its top.
Hint
Root pressure and its limit.
Answer
Water entering root cells by osmosis builds pressure in the xylem and forces sap out of the cut stump (exudation). This push is small and can raise water only a short way, while the main lifting force for a tall tree is the pull created by transpiration from the leaves.
Q5 · 3 marks · Case-basedHOTS / competency
Suppose two identical dry raisins are left overnight, one in pure water and the other in a thick sugar syrup.
(a) State what you would see in each. (1)
(b) Name the process in each. (1)
(c) If the syrup were gradually diluted to the strength of the raisin's sap, what would you expect? (1)
Hint
Direction of water.
Answer
(a) The raisin in water swells; the one in syrup shrinks. (b) Endosmosis in water; exosmosis in syrup. (c) Water would move in and out at the same rate, so the raisin would show no net change in size.

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 kind of membrane does osmosis need?Show answer
A semipermeable membrane.
2.Name a unicellular alga that uses diffusion for transport.Show answer
Chlamydomonas.
3.What is the entry of water into a cell called?Show answer
Endosmosis.
4.Which process uses respiratory energy?Show answer
Active transport.
5.Give one piece of evidence for root pressure.Show answer
Exudation of sap from a cut stem (or guttation).

Diffusion, Osmosis, Root Pressure and Active Transport: frequently asked questions

Is osmosis a kind of diffusion?

It is the diffusion of water across a semipermeable membrane, which is why it is passive too, but in answers treat it as a separate process with its own definition.

Do I need to draw the osmoscope?

Be ready to label potato cup, sugar solution, level mark and water in the dish.

Why do roots need oxygen?

For respiration, which supplies energy for active uptake of minerals.

Where is transpiration covered?

In the next topic, transpiration, xylem and phloem.