Overview
What you’ll learn
- Define diffusion and osmosis precisely, and explain both using concentration gradients and water potential (Supplement).
- Predict the direction net diffusion or osmosis will take in any given situation.
- Describe how surface area, temperature, concentration gradient and distance change the rate of diffusion.
- Use the terms turgid, turgor pressure, flaccid and plasmolysis to explain what happens to plant cells in different solutions (Supplement).
- Contrast active transport with diffusion, and explain why root hair cells need active transport.
Syllabus checklist (0610)
Statements are paraphrased and grouped by tier. Core is studied by everyone; Supplement is examined on the Extended papers only.
Core — every candidate
- 3.1Describe diffusion as the net movement of particles from a region of their higher concentration to a region of their lower concentration (down a concentration gradient), as a result of their random movement.
- 3.1State that the energy for diffusion comes from the kinetic energy of random movement of molecules and ions.
- 3.1State that some substances move into and out of cells by diffusion through the cell membrane.
- 3.1Describe the importance of diffusion of gases and solutes in living organisms.
- 3.1Investigate the factors that influence diffusion, limited to: surface area, temperature, concentration gradient and distance.
- 3.2Describe the role of water as a solvent in organisms with reference to digestion, excretion and transport.
- 3.2State that water diffuses through partially permeable membranes by osmosis.
- 3.2State that water moves into and out of cells by osmosis through the cell membrane.
- 3.2Investigate osmosis using materials such as dialysis tubing.
- 3.2Investigate and describe the effects on plant tissues of immersing them in solutions of different concentrations.
- 3.2State that plants are supported by the pressure of water inside the cells pressing outwards on the cell wall.
- 3.3Describe active transport as the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (against a concentration gradient), using energy from respiration.
Supplement — Extended candidates only
- 3.2Describe osmosis as the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.
- 3.2Explain the effects on plant cells of immersing them in solutions of different concentrations by using the terms turgid, turgor pressure, plasmolysis and flaccid.
- 3.2Explain the importance of water potential and osmosis in the uptake and loss of water by organisms.
- 3.3Explain the importance of active transport as a process for movement of molecules or ions across membranes, including ion uptake by root hairs.
- 3.3State that protein carriers move molecules or ions across a membrane during active transport.
In 30 seconds
- 1Diffusion is the net movement of particles from higher to lower concentration (down the gradient), driven by the kinetic energy of random particle movement — no cell energy is used.
- 2Diffusion speeds up with a larger surface area, a higher temperature, a steeper concentration gradient and a shorter distance.
- 3Osmosis is the diffusion of water through a partially permeable membrane, from a dilute solution (higher water potential) to a concentrated solution (lower water potential) (Supplement wording).
- 4Plant cells pulled in a dilute solution become turgid (water in, turgor pressure pushes on the wall); in a concentrated solution they become flaccid and then plasmolysed (cytoplasm shrinks from the wall).
- 5Active transport moves particles against the gradient, from low to high concentration, using energy from respiration and protein carriers in the membrane.
- 6Root hair cells take up mineral ions from soil where ions are dilute — so they must use active transport, not diffusion.
Notes · 1 of 3
Diffusion
The passive net movement of particles down a concentration gradient, and the four factors that control its rate.
Molecules and ions are in constant random motion because they have kinetic energy. Put a drop of ink in still water and it slowly spreads without anything stirring it. Diffusion is this spreading: the net movement of particles from where they are more concentrated to where they are less concentrated — down a concentration gradient — as a result of that random movement. Because the particles move themselves, diffusion needs no energy from the cell: it is a passive process powered entirely by kinetic energy.
In living organisms, diffusion through the cell membrane moves substances in both directions. The oxygen you inhale diffuses from the air in the alveoli (high concentration) into the blood (lower concentration); carbon dioxide diffuses the opposite way. In a leaf, carbon dioxide diffuses from the air into the palisade cells for photosynthesis, and in the kidney, some solutes diffuse during urine formation. Without diffusion of gases and solutes, none of these exchanges could happen.
| Factor | Change that speeds diffusion up | Why |
|---|---|---|
| Surface area | Larger surface area | More particles can cross at the same time (membranes folded into villi, alveoli, root hairs) |
| Temperature | Higher temperature | Particles have more kinetic energy and move faster |
| Concentration gradient | Bigger difference in concentration | More particles on one side, so the net movement is greater |
| Distance | Shorter distance | Particles reach the other side sooner (membranes for exchange are one cell thick) |
A labelled diagram worth picturing: a beaker split by a dotted membrane, with many dots on the left and few on the right. Arrows show individual particles crossing in both directions — but more cross left-to-right, so the net movement is to the right. Draw that two-way traffic whenever you are asked to explain why the movement is described as "net".
Notes · 2 of 3
Osmosis and water potential
Water diffuses through partially permeable membranes too — with its own vocabulary and its own practicals.
Water is the solvent of life: digested food is dissolved in it for absorption, wastes travel dissolved in it for excretion, and blood and sap carry substances dissolved in water around organisms. Because water molecules are also in constant random motion, water diffuses too — but through a partially permeable membrane, which lets water molecules through while holding back larger dissolved particles. The diffusion of water this way is called osmosis.
Water moves into and out of cells by osmosis through the cell membrane. Supplement: the direction is described using water potential: water moves by osmosis from a region of higher water potential (a dilute solution, more "free" water molecules) to a region of lower water potential (a concentrated solution, where solutes have tied up water molecules), through a partially permeable membrane. Pure water has the highest water potential of all.
Two standard practicals test osmosis. Dialysis tubing: a sac of partially permeable membrane filled with concentrated sugar solution, tied and weighed, then placed in water. Water enters by osmosis and the sac gains mass and swells. Plant tissue: potato strips or chips are weighed or measured, left in solutions of different concentrations, then re-measured.
Supplement: in a plant cell bathed in a dilute solution, water enters, pushing the cytoplasm against the cell wall; the swollen cell is turgid, and the outward push of water inside the cell is the turgor pressure. Turgor is what keeps soft plant tissues upright — a plant is supported by the pressure of water inside its cells pressing outwards on the cell wall. In a concentrated solution, water leaves: first the cell becomes flaccid (soft, less firm), and if water keeps leaving, the cytoplasm pulls away from the wall — plasmolysis. A wilting plant has lost turgor across many cells.
| Solution | Net water movement | Appearance of the cell |
|---|---|---|
| Dilute (higher water potential outside) | Into the cell | Turgid — cytoplasm pressed against the wall, wall prevents bursting |
| Same concentration (isotonic) | No net movement | Neither turgid nor plasmolysed — firmness unchanged |
| Concentrated (lower water potential outside) | Out of the cell | Flaccid, then plasmolysed — cytoplasm shrinks away from the wall |
The cell wall is the plant cell’s safety net: animal cells in pure water have no wall, so they simply swell and may burst, which is why water balance matters so much to animals — kidneys (Topic 11) fine-tune it constantly.
Notes · 3 of 3
Active transport
The uphill, energy-hungry route across membranes — and why root hair cells cannot live without it.
Diffusion and osmosis only ever move substances down their gradients. But cells often need to do the opposite — absorb a substance that is already more concentrated inside the cell than outside. Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration — against the concentration gradient — using energy from respiration.
Supplement: the particles do not squeeze through the membrane on their own: protein carriers in the membrane grab specific molecules or ions, change shape using energy, and release them on the other side. The energy comes from respiration — which is why actively transporting cells are packed with mitochondria and why cyanide, which stops respiration, stops active transport but not diffusion.
| Property | Diffusion and osmosis | Active transport |
|---|---|---|
| Direction | Down the concentration gradient (high → low) | Against the gradient (low → high) |
| Energy | None needed — kinetic energy of the particles | Energy from respiration required |
| Membrane proteins | Not required | Protein carriers required (Supplement) |
| Example | Oxygen into the blood; water into root cells | Mineral ions into root hair cells; glucose absorption in the gut |
Revise
Key terms
The words examiners expect you to use precisely.
- Diffusion
- The net movement of particles from a region of higher concentration to a region of lower concentration, as a result of their random movement.
- Concentration gradient
- The difference in concentration between two regions; particles diffuse down it.
- Osmosis
- The diffusion of water molecules through a partially permeable membrane from a dilute solution to a concentrated solution.Supplement wording: from higher to lower water potential.
- Partially permeable membrane
- A membrane with tiny holes that lets small molecules such as water pass but blocks larger dissolved particles.
- Water potential
- A measure of how freely water molecules can move; water moves from higher to lower water potential by osmosis.Supplement term.
- Turgid
- Describes a plant cell that has absorbed water by osmosis and is swollen firm because the cytoplasm presses on the cell wall.
Show 4 more key termsShow fewer key terms
- Turgor pressure
- The outward pressure of the cell contents on the cell wall in a turgid cell; it supports soft plant tissue.
- Plasmolysis
- The shrinkage of cytoplasm away from the cell wall when a plant cell loses water in a concentrated solution.A plasmolysed cell first passes through being flaccid.
- Active transport
- The movement of particles through a cell membrane from a lower to a higher concentration, against the gradient, using energy from respiration.
- Protein carrier
- A protein in the cell membrane that moves specific molecules or ions across it during active transport.Supplement term.
Revise
Common mistakes
Where students most often lose marks in this topic — and what to do instead.
Mistake: Saying diffusion "needs energy from the cell".
Diffusion is passive; the kinetic energy of the particles themselves does the work.
Instead: Respiration energy is only used in active transport.
Mistake: Confusing osmosis with diffusion of any substance.
Osmosis is specifically the movement of WATER through a partially permeable membrane.
Instead: Diffusion: any particles, down the gradient. Osmosis: water only, through a partially permeable membrane.
Show 10 more mistakesShow fewer mistakes
Mistake: Saying water moves "from strong to weak solution".
The solute moves nowhere; the WATER moves towards the concentrated (strong) solution.
Instead: Water diffuses from the dilute side to the concentrated side — towards the salt, not away from it.
Mistake: Writing that plasmolysis means the whole cell shrinks to nothing.
Only the living contents shrink away from the wall; the rigid cellulose wall keeps the cell’s shape.
Instead: In plasmolysis the cytoplasm and membrane pull away from the unchanged cell wall.
Mistake: Forgetting the minus sign in percentage-change answers.
The sign shows the direction of water movement — a marker cannot award the direction point without it.
Instead: Final − original, divided by original, × 100: shrinkage gets a minus.
Mistake: Dividing by the final length instead of the original.
Percentage change is always measured against the starting value.
Instead: Percentage change = (final − original) ÷ original × 100.
Mistake: Saying active transport moves particles "from high to low concentration".
That is diffusion; active transport is defined by going the opposite way.
Instead: Active transport moves particles from LOW to HIGH concentration, against the gradient.
Mistake: Claiming root hair cells absorb ions by osmosis.
Ions are not water, and soil ions are less concentrated than the cell contents anyway.
Instead: Root hair cells absorb mineral ions by active transport; they absorb water by osmosis.
Mistake: Describing a partially permeable membrane as "not permeable at all".
Partially permeable means SOME molecules pass — small ones like water.
Instead: Water passes; larger dissolved solutes are held back.
Mistake: Using "turgid" for animal cells.
Animal cells have no cell wall, so they have no turgor and can burst instead.
Instead: Turgid, flaccid and plasmolysed are wall words — plant cells only.
Mistake: Reversing the surface area to volume ratio rule.
As an object gets bigger, its volume grows faster than its surface area, so big things have the smaller ratio.
Instead: Small objects have a **larger surface area to volume ratio**; exchange surfaces and thin shapes exist to restore a large ratio.
Mistake: Saying plants get all their mineral ions by diffusion.
Mineral ions are usually at a lower concentration in the soil than inside the root cells, so diffusion would move ions the wrong way.
Instead: Root hair cells absorb ions **against the concentration gradient by active transport**, using energy from respiration and carrier proteins.
Check yourself
Quick quiz
26 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
Define diffusion down a concentration gradient
Diffusion is best described as the net movement of particles
Ready for more practice on Movement in & out of cells?
Generate a fresh 5-question adaptive drill grounded in this topic’s 0610 key points. Free with your study account.
Practise
Topical questions
14 exam-style questions with model answers and mark schemes.
A set of structured and calculation questions matched to this topic, in three tiers, each with a model answer, mark scheme and examiner tip.
Practise topical questionsExam practice
Practice paper
A 30-mark paper set out like a Cambridge paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Topic 3 Practice Paper
30 marks · 40 min · 5 pages
10 multiple-choice questions and structured questions with answer lines. Topic 3: diffusion and its rate factors, osmosis and water potential, plant cell turgor and plasmolysis, active transport.
Mark scheme
PROTopic 3 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
Which paper covers this topic?
Paper 1 / 2
Multiple choice (45 min, 40 marks)
Core and Extended versions
Paper 3 / 4
Theory, short and structured (1 h 15, 80 marks)
Core and Extended versions
Paper 5 / 6
Practical test or Alternative to Practical (40 marks)
Experimental skills
Want a full paper? Try the Paper 2-style multiple choice, Paper 4-style structured or Paper 6-style practical sets.
Questions
Frequently asked questions
What is the difference between diffusion and osmosis?
Diffusion moves any particles down their concentration gradient. Osmosis is the special case of WATER diffusing through a partially permeable membrane — usually described as movement from dilute to concentrated solution, or higher to lower water potential at Supplement.
How do I calculate percentage change in the osmosis practical?
Percentage change = (final value − original value) ÷ original value × 100. Keep the minus sign for a decrease — the sign tells the marker which way the water moved.
Why do root hair cells need active transport for ions but not for water?
Water is more concentrated in dilute soil than inside the root, so it diffuses in by osmosis. Ions are MORE concentrated inside the cell, so the gradient points outwards — the ions must be pumped in against the gradient using energy from respiration.
Does active transport happen in animal cells too?
Yes — nerve cells pump ions to reset their impulses, and the gut absorbs glucose against its gradient. Any cell with many mitochondria is probably doing a lot of active transport.
Is QuizQuip connected to Cambridge?
No. Cambridge IGCSE and Cambridge are trademarks of Cambridge University Press & Assessment. QuizQuip is independent — not affiliated with, endorsed by, or connected to Cambridge. All our notes and questions are original material written against the published 0610 syllabus.
Up next
Topic 4: Biological molecules and enzymes