Overview
What you’ll learn
- Define diffusion and name the three factors that set its rate.
- Calculate surface-area-to-volume ratios and explain why big organisms need exchange surfaces.
- Predict the direction of water movement by osmosis and what happens to animal and plant cells.
- Run the osmosis practical and process the data with percentage change in mass.
- Explain active transport in root hair cells and the gut, and link it to respiration.
- Compare all three transport processes in a table under exam pressure.
Specification checklist (AQA Trilogy 8464 · Edexcel 1SC0 · OCR Gateway J250)
Statements are paraphrased from the shared DfE combined science subject content and grouped by tier. Foundation content is studied by every student; Higher statements are examined on Higher-tier papers only. This chapter covers only what Combined Science (double award) examines — separate-science extras live on the GCSE Biology, Chemistry and Physics hubs. QuizQuip is not affiliated with AQA, Pearson Edexcel or OCR.
Foundation — every student
- 2.1Describe diffusion as the spreading out of particles of a substance from an area of higher concentration to an area of lower concentration, down the concentration gradient, and explain how the rate is affected by the concentration gradient, temperature and membrane surface area.
- 2.2Give examples of substances transported into and out of cells by diffusion, including oxygen and carbon dioxide in gas exchange and the waste product urea from cells into blood plasma, and explain why single-celled organisms can rely on diffusion across their surface.
- 2.3Calculate and compare surface area to volume ratios, and explain how multicellular organisms need exchange surfaces and a transport system because their surface-area-to-volume ratio is too small for diffusion alone.
- 2.4Describe osmosis as the diffusion of water through a partially permeable membrane, from a dilute solution (higher water concentration) to a concentrated solution (lower water concentration), and explain how plant cells are supported by osmotic water uptake.
- 2.5Carry out the required practical: investigate the effect of a range of concentrations of salt or sugar solution on the mass of plant tissue, calculating and graphing percentage change in mass, and use the graph to find the concentration of the plant cell sap.
- 2.6Describe active transport as the movement of substances from a more dilute solution to a more concentrated solution against the concentration gradient, requiring energy from respiration, and explain its role in absorbing mineral ions into plant root hairs and sugar from the gut into the blood.
- 2.7Compare diffusion, osmosis and active transport in terms of direction, membrane requirement and energy use.
Key points
- 1Diffusion = particles spread from higher to lower concentration, down the gradient; faster when the gradient is steeper, the temperature is higher, or the surface area is bigger.
- 2Classic examples: oxygen into the blood at the alveoli, carbon dioxide out, urea from cells into plasma.
- 3Surface area : volume ratio falls as objects grow — so multicellular organisms need exchange surfaces (alveoli, gills, villi) plus a transport system.
- 4Osmosis = water diffuses through a partially permeable membrane from a dilute to a concentrated solution.
- 5In the osmosis practical, plot percentage change in mass — it lets chips of different starting masses be compared fairly.
- 6Where the percentage-change line crosses zero on the concentration axis = the solution with the same concentration as the cell sap.
- 7Active transport moves substances against the gradient and needs energy from respiration — mineral ions into root hairs, sugar from the gut into the blood.
- 8Diffusion and osmosis are passive; active transport is active — the energy distinction is a guaranteed exam question.
Notes · 1 of 5
Diffusion
Particles jostle from crowded to uncrowded — no energy spent, no membrane required.
Diffusion is the spreading out of particles of a gas or a dissolved substance, driven purely by their random movement. The net movement is always from where particles are more concentrated to where they are less concentrated — down the concentration gradient. Across a cell membrane, only small molecules that can slip through the membrane diffuse freely: oxygen and carbon dioxide are the exam favourites.
| Factor | Faster when… | Everyday example |
|---|---|---|
| Concentration gradient | The difference between the two sides is bigger | A strong-smelling spill spreads across a room faster than a faint one |
| Temperature | Particles move with more kinetic energy | Sugar dissolves and spreads faster in hot tea |
| Surface area of the membrane | More membrane is available for crossing | Millions of alveoli multiply the lung’s surface |
| Distance (thickness) | The barrier is thinner | Alveoli and capillary walls are one cell thick |
Worked example — comparing surface area to volume ratios
Two cube-shaped cells have sides of 6 mm and 2 mm. For the 6 mm cube: surface area = 6 × (6 × 6) = 216 mm²; volume = 6³ = 216 mm³; ratio = 216 : 216 = 1 : 1. For the 2 mm cube: surface area = 6 × (2 × 2) = 24 mm²; volume = 2³ = 8 mm³; ratio = 24 : 8 = 3 : 1. The smaller cell has the bigger surface-area-to-volume ratio — that is why big organisms need exchange organs.
Notes · 2 of 5
Exchange surfaces and size limits
Why single cells cope alone and your body needs lungs, gills and a gut.
A single-celled organism absorbs everything it needs straight across its membrane — its surface-area-to-volume ratio is huge, so diffusion alone keeps up. As organisms grow, volume rises much faster than surface area, so the ratio crashes. A whale’s interior cells are far from the outside world: diffusion across the skin could never feed them. Multicellular organisms solve this with exchange surfaces that multiply area and keep distances short, plus a circulatory system that carries substances between the exchange surface and every cell.
Alveoli (lungs)
Huge area, thin walls, rich blood supply
Oxygen diffuses into the blood; carbon dioxide diffuses out. Moist surfaces and steep gradients keep the exchange fast.
Gills (fish)
Feathered filaments in a one-way flow
A large area draws oxygen out of water, with blood flowing opposite to the water to keep the gradient steep along the whole filament.
Villi (small intestine)
Finger folds inside folds
Villi multiply the gut’s absorbing surface; digested food diffuses into the blood, and active transport tops up glucose.
Every efficient exchange surface shares the same three tricks: a large surface area, a thin barrier so diffusion distances are short, and a way of maintaining the gradient — fresh supply on one side, removal on the other.
Notes · 3 of 5
Osmosis
Water’s own diffusion, through a partially permeable membrane.
Osmosis is the movement of water through a partially permeable membrane — one that lets water through but holds back dissolved solutes — from a dilute solution (more water molecules) to a concentrated solution (fewer free water molecules). Water keeps moving until the water concentration is balanced on both sides. Osmosis needs no energy: it is diffusion, just for water.
| Solution compared with the cell | Animal cell | Plant cell |
|---|---|---|
| More dilute (hypotonic) | Water enters; a cell with no wall may swell and burst | Water enters; the vacuole swells and the cell becomes turgid — the wall stops bursting |
| Same concentration (isotonic) | No net movement; the cell keeps its shape | No net movement; the cell is flaccid-ish but supported |
| More concentrated (hypertonic) | Water leaves; the cell shrinks and shrivels | Water leaves; the cytoplasm pulls away from the wall (the cell is plasmolysed) |
Turgid cells are what keep lettuce crisp and seedlings upright — the vacuoles press outward against the cellulose walls. Wilted plants have lost that water pressure; watering them lets osmosis refill the vacuoles.
Notes · 4 of 5
The osmosis required practical
Potato (or other plant tissue) in sugar solutions — method, controls and the graph trick.
- 11. Cut and weigh
Cut potato cylinders of equal size, blot dry, and record each chip’s starting mass to 0.1 g or better.
- 22. Set up the range
Place one chip in each of a range of sugar (or salt) concentrations — for example 0, 0.25, 0.5, 0.75 and 1.0 mol/dm³ — plus distilled water.
- 33. Control the conditions
Use the same volume of solution in each tube and leave every tube for the same time at the same temperature.
- 44. Reweigh
Remove the chips, blot them dry the same way, and record each final mass.
- 55. Process the data
Calculate percentage change in mass for each chip, plot it against concentration, and read off where the line crosses zero change.
Worked example — percentage change in mass
A chip starts at 6.4 g and reaches 7.2 g after 30 minutes. Percentage change = (change ÷ starting mass) × 100 = (0.8 ÷ 6.4) × 100 = +12.5%. A second chip falls from 8.0 g to 7.6 g: (−0.4 ÷ 8.0) × 100 = −5.0%. Always divide by the starting mass, and keep the sign — losing mass gives a negative change.
| Design point | Why it matters |
|---|---|
| Independent variable | The concentration of the sugar or salt solution |
| Dependent variable | The percentage change in mass of the plant tissue |
| Control variables | Starting size/mass of chips, solution volume, temperature, time, blotting method |
| Why percentage change? | Chips start at slightly different masses, so relative change is the fair comparison |
| Where the line crosses zero | The concentration where no water moves — the estimated concentration of the cell sap |
| Main sources of error | Surface drying varies; chips cut unevenly; tubes at different temperatures |
Risks and improvements
Low risks: clean up spills promptly, and never eat the experiment. Improvements worth a mark: repeat each concentration and average the results; use a cork borer so all chips share a diameter; blot each chip with the same number of paper towel presses; use a water bath to hold temperature constant.
Notes · 5 of 5
Active transport
The energy-consuming pump that moves substances the wrong way — on purpose.
Active transport moves substances from a more dilute solution to a more concentrated one — against the concentration gradient. That cannot happen by diffusion, so the membrane’s carrier proteins burn energy from respiration to do it. Two places matter at GCSE: root hair cells pull mineral ions in from soil water that is far more dilute than the cell sap, and the gut sweeps the last traces of sugar from the intestine into the blood even when the blood already holds more.
Worked example — spotting active transport
A student measures 0.3 g of nitrate ions in a plant after a night in dilute soil solution, but the sap already contained 0.9 g. Uptake still happened — so it cannot have been diffusion, which only runs downhill. The cell must have used active transport, powered by respiration — which is exactly why root hair cells are packed with mitochondria.
| Process | Direction | Membrane needed? | Energy? |
|---|---|---|---|
| Diffusion | Higher → lower concentration | Not always | No — passive |
| Osmosis | Dilute → concentrated solution (water only) | Yes — partially permeable | No — passive |
| Active transport | Dilute → concentrated (against the gradient) | Yes — carrier proteins | Yes — from respiration |
Revise
Key terms
The words examiners expect you to use precisely.
- Concentration gradient
- The difference in concentration between two areas; particles diffuse down it.Steeper gradient = faster diffusion.
- Partially permeable membrane
- A membrane with tiny holes that let small molecules such as water pass but block larger solutes.
- Osmosis
- Diffusion of water through a partially permeable membrane from a dilute to a concentrated solution.
- Turgid
- Swollen and firm because a plant cell’s vacuole is full of water pressing against the wall.
- Active transport
- Movement of substances against the concentration gradient using energy from respiration.
- Surface area to volume ratio
- How much absorbing surface an organism has compared with the volume that needs serving; it falls as size grows.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Saying diffusion moves substances “up” the gradient
Diffusion runs only from higher to lower concentration.
Instead: Net diffusion is down the concentration gradient; active transport goes up it.
Mistake: Defining osmosis as “water moving from weak to strong” without the membrane
The partially permeable membrane is the defining feature — full marks need it.
Instead: Osmosis is water diffusion through a partially permeable membrane.
Show 10 more mistakesShow fewer mistakes
Mistake: Dividing by the final mass in percentage-change questions
The percentage change is measured against the start.
Instead: Percentage change = (change ÷ starting mass) × 100.
Mistake: Dropping the minus sign when tissue loses mass
Loss and gain mean opposite water movements; the sign carries the marks.
Instead: Report negative percentage change for mass loss.
Mistake: Claiming plant cells burst in pure water
The inelastic cellulose wall resists the swelling.
Instead: Plant cells become turgid; it is wall-less animal cells that can burst.
Mistake: Saying active transport happens without energy
Against-the-gradient movement needs respiration.
Instead: Active transport requires energy released by respiration, hence the mitochondria.
Mistake: Mixing up turgid and flaccid
The two words describe opposite water states.
Instead: Turgid = swollen with water; flaccid = lacking water, soft.
Mistake: Making the dependent variable “the concentration”
That is what the experimenter changes.
Instead: Independent variable = solution concentration; dependent variable = percentage change in mass.
Mistake: Forgetting to control time or temperature
Unequal soaking times change the result independent of concentration.
Instead: Time, temperature, volume and chip size are the classic controls here.
Mistake: Ignoring the zero-crossing point on the graph
It is the whole point of plotting percentage change against concentration.
Instead: The concentration at zero change estimates the cell-sap concentration.
Mistake: Claiming larger cells have a larger surface-area-to-volume ratio
Volume grows faster than surface area as size increases.
Instead: The ratio falls as objects get bigger.
Mistake: Using diffusion alone to explain mineral uptake
Soil water is more dilute than cell sap, so diffusion would push ions out.
Instead: Root hair cells use active transport for ions — that is why they need mitochondria.
Check yourself
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Explain diffusion and its rate factors
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Practise
Topical questions
12 exam-style questions with model answers and mark schemes.
A set of structured and calculation questions matched to this chapter, in three tiers, each with a model answer, mark scheme and examiner tip. Higher-tier questions are tagged.
Practise topical questionsExam practice
Practice paper
A 30-mark paper in the style of a Combined Science paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Chapter 2 Practice Paper
30 marks · 40 min · 5 pages
6 multiple-choice questions and structured questions with answer lines. Transport in cells: diffusion, surface-area-to-volume ratio, exchange surfaces, osmosis, the plant-tissue osmosis required practical, and active transport.
Mark scheme
PROChapter 2 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.
How does this chapter fit the real exams?
Biology Paper 1
1 h 15 · 70 marks (Trilogy)
Cell biology, organisation, infection and response, bioenergetics (chapters 1–7)
Biology Paper 2
1 h 15 · 70 marks (Trilogy)
Homeostasis, inheritance variation and evolution, ecology (chapters 8–10)
Tiers
Foundation (grades 1-1 to 5-5) · Higher (4-3 to 9-9)
Two grades are awarded together; the 21 required practicals are examined on the written papers
Want a full paper? Try the Biology Paper 1-style (Foundation), Biology Paper 1-style (Higher), Biology Paper 2-style (Foundation), Biology Paper 2-style (Higher), Chemistry Paper 1-style (Foundation), Chemistry Paper 1-style (Higher), Chemistry Paper 2-style (Foundation), Chemistry Paper 2-style (Higher), Physics Paper 1-style (Foundation), Physics Paper 1-style (Higher), Physics Paper 2-style (Foundation), Physics Paper 2-style (Higher), Required practicals practice sets.
Questions
Frequently asked questions
Is transport in cells in Combined Science (Trilogy)?
Yes. Diffusion, surface-area-to-volume ratio, osmosis and active transport are all core Combined Science content in the DfE subject content shared by AQA Trilogy (8464), Pearson Edexcel (1SC0) and OCR Gateway (J250).
Which required practical belongs to this chapter?
The plant-tissue osmosis practical (AQA Trilogy required practical activity 2): plant tissue in a range of sugar or salt concentrations, with percentage change in mass plotted to find the cell-sap concentration. Edexcel’s core practical and OCR’s PAG cover the same skills.
Do Foundation students need percentage-change maths?
Yes — percentage change in mass appears at both tiers. Higher papers usually add a graph-interpolation step, such as reading the zero-crossing concentration between two points.
What is the most common exam mistake here?
Dividing by the final mass in percentage-change calculations. Always divide by the starting mass, keep the sign for mass loss, and quote the % symbol.
How do I remember which processes need energy?
Only active transport spends energy — it is the one that moves particles against the gradient, and it is why the cells doing it are stuffed with mitochondria.
Up next
Chapter 3: Digestion and enzymes