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Detailed notes on Movement into and out of Cells for Cambridge IGCSE Biology, covering key concepts, explanations, examples, and exam-focused revision points.
Net movement of particles down a concentration gradient. Cambridge wants the FULL definition: 'net', 'concentration gradient', 'random movement'. Plus the four factors affecting rate.
Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).
NET movement, down a concentration gradient, as a result of random movement of particles.
Cambridge definition (memorise verbatim):
"The NET movement of particles from a region of higher concentration to a region of lower concentration, down a CONCENTRATION GRADIENT, as a result of their RANDOM movement."
Three keywords that Cambridge marks specifically:
Why diffusion happens. All particles are constantly moving randomly (more KE at higher T). With more particles in one region, more move out than in → net flow toward the less crowded region. Eventually concentrations equalise (equilibrium).
Diffusion is PASSIVE. Doesn't use the cell's ATP. Just uses the particles' own random kinetic energy.
Worked qualitative. A drop of perfume in a corner of a room. Within minutes, you can smell it across the room. Why? Perfume molecules have random kinetic energy → they move in all directions; net flow is from the high concentration (corner) to low (rest of room) until well-mixed.
Concentration gradient, surface area, distance, temperature — Fick's law factors.
Four big factors.
1. Concentration gradient. Steeper gradient (bigger difference between regions) → faster diffusion. Maintaining a steep gradient is why blood flow continually carries O₂ away from alveoli.
2. Surface area. More area for particles to cross → faster. The folded surfaces of the small intestine villi, the alveoli, and the gills all maximise SA.
3. Distance to diffuse. Shorter distance → faster. Alveolar walls are ONE CELL thick; capillary walls are ONE CELL thick — so O₂ crosses just two cell membranes, very quickly.
4. Temperature. Higher temperature → more kinetic energy → faster particle motion → faster diffusion.
Fick's law (Extended). rate∝distancesurface area×concentration difference.
Cambridge IGCSE doesn't require the formula but the concept: rate goes UP with SA and concentration difference, DOWN with distance.
Worked qualitative. Why does opening a window speed up the smell of perfume reaching the other end of a room? Air movement carries away nearby molecules → maintains a steep concentration gradient near the source → faster net diffusion.
Gas exchange, gut absorption, water uptake — all rely on diffusion.
1. Gas exchange in alveoli.
2. Gas exchange in fish gills.
3. Absorption in the small intestine.
4. CO₂ uptake by leaves.
5. Excretion in lungs.
6. Active transport vs diffusion. Some substances (e.g. glucose against a gradient in kidney tubules; mineral ions in root hairs) need ATP-powered active transport, not just diffusion.
Worked qualitative. Why are mitochondria found near the surface of cells with high O₂ demand (e.g. muscle fibres)? The cells respire fast; they need a constant O₂ supply. Mitochondria near the surface are closer to incoming O₂ → shorter diffusion distance → can keep up with demand.
Verbatim phrases and definitions Cambridge mark schemes credit.
Diffusion appears every Paper 2 (3-4 marks: define, give factors) and Paper 4 (5-7 marks: explain how an organ structure enables fast diffusion). Examiner reports flag missing 'net' from the definition and confusing diffusion with osmosis.
Sources: Cambridge IGCSE Biology 0610 syllabus 2026-2028 (3.1); 0610/22 May/Jun 2024 — Q5 (diffusion); 0610 Examiner Reports 2022-2024. Last reviewed 2026-05-07.
Step-by-step solutions to past-paper-style questions on diffusion, written exactly the way a tutor would explain them at the board.
Question
Define diffusion using the precise Cambridge mark-scheme wording.
Step-by-step solution
Step 1
Net movement of particles from a region of HIGHER concentration to a region of LOWER concentration.
Step 2
Down a CONCENTRATION GRADIENT.
Step 3
As a result of their RANDOM movement (kinetic energy).
Answer
The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random movement.
Examiner tip
Three marks if you include 'NET', 'concentration gradient', AND 'random movement'.
Question
List FOUR factors that affect the rate of diffusion across a membrane.
Step-by-step solution
Step 1
Concentration gradient — steeper gradient = faster diffusion.
Step 2
Surface area — larger area = faster.
Step 3
Distance / thickness of membrane — shorter distance = faster.
Step 4
Temperature — higher T = particles have more KE = faster diffusion.
Answer
Concentration gradient, surface area, distance (thickness), temperature.
Question
Explain why diffusion is faster when (a) the temperature is higher and (b) the concentration gradient is steeper.
Step-by-step solution
Step 1
(a) At a higher temperature the particles have more kinetic energy, so they move faster and more randomly.
Step 2
This means they spread out more quickly, so the net movement from high to low concentration happens faster.
Step 3
(b) A steeper concentration gradient means a bigger difference in concentration between the two regions.
Step 4
With a bigger difference, proportionally more particles move from the high to the low side per second, so the net rate of diffusion is greater.
Answer
Higher temperature → more kinetic energy → particles move faster → faster diffusion. Steeper gradient → bigger concentration difference → more net movement per second → faster diffusion.
Examiner tip
Always link the factor to a REASON: temperature → kinetic energy; gradient → size of the concentration difference.
Question
Explain how the structure of an alveolus is adapted for fast diffusion of oxygen into the blood.
Step-by-step solution
Step 1
Many alveoli → very large surface area for gas exchange.
Step 2
Thin walls (one cell thick) → short diffusion distance.
Step 3
Moist surface → oxygen dissolves before crossing the membrane.
Step 4
Surrounded by capillaries → blood removes O₂ quickly, maintaining a steep concentration gradient.
Answer
Large surface area, thin walls, moist surface and a rich blood supply all keep the rate of diffusion high (large SA × steep gradient ÷ short distance).
Question
Explain why diffusion alone can supply the needs of a single-celled organism but not those of a large active animal.
Step-by-step solution
Step 1
A single-celled organism is very small, so it has a large surface area relative to its volume (a high SA:V ratio).
Step 2
Its surface is also close to every part of the cell, so the diffusion distance is short. Oxygen and food can diffuse in, and waste out, fast enough to meet its needs.
Step 3
A large animal has a small surface area relative to its volume (low SA:V), and its inner cells are far from the surface, so the diffusion distance is large.
Step 4
Diffusion would be far too slow to supply the inner cells, so a large animal needs specialised exchange surfaces (e.g. lungs) and a transport system (e.g. blood) to move substances quickly.
Answer
Small organisms have a high SA:V ratio and short diffusion distances, so diffusion is fast enough. Large animals have a low SA:V ratio and large diffusion distances, so they need exchange surfaces and transport systems instead.
Examiner tip
The crucial idea is the SA:V ratio falling as size increases, plus the increasing distance to inner cells.
Question
Agar blocks are dyed with a coloured indicator and cut into cubes of different sizes. They are placed in dilute acid, which diffuses in and changes the colour. Describe how this models the effect of surface area to volume ratio on diffusion, and what result you would expect.
Step-by-step solution
Step 1
Each agar cube models a cell or organism; the acid diffusing in models a substance entering by diffusion.
Step 2
Smaller cubes have a larger surface area to volume ratio, so the acid has a shorter distance to diffuse to the centre relative to their volume.
Step 3
To make it a fair test, keep the acid concentration, temperature and time the same, and use the same shape (cubes) so only size varies.
Step 4
Expected result: the smallest cube changes colour all the way through first, because diffusion reaches its centre soonest — showing a high SA:V ratio speeds up the supply of substances by diffusion.
Answer
The smallest cube (highest SA:V ratio) is fully penetrated by the acid first, modelling how small cells/organisms exchange substances by diffusion quickly while large ones cannot. Keep acid concentration, temperature and time constant for a fair test.
Examiner tip
Paper 6 expects you to identify the variable changed (cube size/SA:V), the variables controlled, and a prediction linked to diffusion distance.
High-scoring sample answers for diffusion on the Cambridge IGCSE 0610 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
State where the energy for diffusion comes from. (1 mark)
Model answer
From the kinetic energy of the random movement of the particles (molecules and ions) themselves.
Why this scores
One mark for 'kinetic energy of random movement'. Diffusion does NOT use energy from respiration.
State two factors that affect the rate of diffusion. (2 marks)
Model answer
Any two of: the concentration gradient (a steeper gradient gives faster diffusion); the surface area (a larger area gives faster diffusion); the distance the particles must travel (a shorter distance gives faster diffusion); and the temperature (a higher temperature gives faster diffusion).
Why this scores
Two distinct factors from the syllabus list (surface area, temperature, concentration gradient, distance). Naming alone scores; a direction of effect is not required here.
Define diffusion. (3 marks)
Model answer
Diffusion is the net movement of particles from a region of their higher concentration to a region of their lower concentration (i.e. down a concentration gradient), as a result of their random movement.
Why this scores
Three marks: (1) net movement of particles; (2) from higher to lower concentration / down a gradient; (3) due to random movement. 'NET' is essential — particles move both ways.
Explain how the structure of the alveoli is adapted for the rapid diffusion of gases. (4 marks)
Model answer
The alveoli provide a very large surface area because there are millions of them, which allows more gas to diffuse at once. Their walls are only one cell thick, giving a short diffusion distance for gases to cross. The surface is moist, so oxygen can dissolve before diffusing across. Finally, each alveolus is surrounded by a dense network of capillaries that constantly carries oxygen away, maintaining a steep concentration gradient for fast diffusion.
Why this scores
Four marks for four adaptations, each linked to its effect on diffusion: large surface area; thin walls/short distance; moist surface; good blood supply/steep gradient.
Explain why a single-celled organism can rely on diffusion to exchange substances with its surroundings, but a large multicellular animal cannot. (5 marks)
Model answer
A single-celled organism is very small, so it has a large surface area compared with its volume (a high surface area to volume ratio). Its surface is also very close to every part of the cell, so substances have only a short distance to diffuse. This means oxygen and nutrients can diffuse in, and waste products can diffuse out, fast enough to meet its needs. A large animal, however, has a small surface area compared with its volume and its inner cells are a long way from the surface, so the diffusion distance is large. Diffusion would therefore be far too slow to supply the inner cells, so large animals need specialised exchange surfaces, such as lungs, and a transport system, such as the blood, to move substances quickly.
Why this scores
Five marks across: high SA:V in small organisms; short diffusion distance; diffusion fast enough; low SA:V / long distance in large animals; need for exchange surfaces and a transport system.
Describe an investigation to find out how temperature affects the rate of diffusion. (6 marks)
Model answer
Place equal-sized agar cubes (or equal volumes of water) at different temperatures, for example by using water baths at 10°C, 20°C, 30°C and 40°C measured with a thermometer. Add the same volume and concentration of a coloured substance (such as a drop of dye, or acid into dyed agar) to each at the start, and time how long it takes for the colour to spread a fixed distance, or to fully penetrate the cube. Keep all other factors the same to make it a fair test: the same size of cube/volume of water, the same amount and concentration of dye, and the same starting conditions. Repeat each temperature several times and calculate a mean time, then work out the rate as 1÷time. The expected result is that the higher the temperature, the faster the diffusion (shorter time), because the particles have more kinetic energy and move faster.
Why this scores
Up to 6 marks for: the variable changed (temperature, with values and a thermometer); how diffusion is measured (time for colour to spread); a measured result; controlled variables for a fair test; repeats/mean; and a valid prediction linked to kinetic energy.
Definitions to memorise and the exact keywords mark schemes credit for diffusion answers — sharpened from recent examiner reports for the 2026 0610 sitting.
The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random movement.
The difference in concentration between two regions. Steeper gradient = faster diffusion.
The state in which net movement stops because concentrations are equal — particles still move, but no net change.
The surface area of an object divided by its volume. Smaller objects have a higher ratio, so they exchange substances by diffusion more easily.
The traps other students keep falling into on diffusion questions — taken from recent Cambridge IGCSE 0610 examiner reports and mark schemes — and how to avoid them.
Why it happens
Particles move both ways at all times. The word 'NET' is the key.
How to avoid it
Always say NET MOVEMENT. Particles move both ways randomly; the OVERALL flow is from high to low.
Why it happens
Confused with active transport.
How to avoid it
Diffusion is PASSIVE — it uses the random kinetic energy of the particles, not metabolic ATP.
Why it happens
Net movement stops, so it feels like 'stopping'.
How to avoid it
Particles KEEP MOVING — equilibrium just means the net movement is zero (equal numbers moving each way).
Why it happens
Students learn the list of factors but not the mechanism.
How to avoid it
Link each factor to a reason: temperature → kinetic energy; surface area → more room for particles to cross; distance → how far particles must travel.
The things students keep getting wrong in this sub-topic, answered.