Practise 34 exam-style questions on Diffusion for AQA GCSE Biology. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.
34 questions
2–4 marks each
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20 questions, marked as you go
Sample questions with model answers
Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.
Question 1
2 marks
State one limitation of simple diffusion for transporting large polar molecules across membranes and name one transport mechanism that cells use instead.
Model answer
Limitation: large polar molecules cannot pass through lipid bilayer; Mechanism: facilitated diffusion via protein channels or active transport via carrier proteins (pumps).
Explanation
Large or charged molecules need membrane proteins to cross; small nonpolar molecules diffuse, but others require facilitated diffusion or active transport.
Mark scheme
1 mark
large polar molecules cannot diffuse through lipid bilayer / membrane impermeable to them
1 mark
facilitated diffusion (channels) or active transport (carrier proteins / pumps)
Question 2
3 marks
A cell is placed in a solution and gains water by osmosis. Explain the difference between osmosis and diffusion.
Model answer
Diffusion is movement of particles from higher to lower concentration; osmosis is the diffusion of water across a partially permeable membrane from a region of higher water potential (or lower solute concentration) to lower water potential (or higher solute concentration).
Explanation
Osmosis is a special case of diffusion limited to water molecules and requires a selectively permeable membrane.
Mark scheme
1 mark
diffusion: movement of particles from high to low concentration
1 mark
osmosis: movement of water only
1 mark
osmosis across a partially permeable membrane / down water potential gradient / from dilute to concentrated solution
Question 3
3 marks
A cell is placed into an isotonic solution. Predict what will happen to the cell's volume and explain why.
Model answer
No net change in cell volume because water potential is equal inside and outside so there is no net movement of water by osmosis.
Explanation
Isotonic means same osmotic concentration so water movements in and out are balanced.
Mark scheme
1 mark
no net change in volume
1 mark
because water potential / solute concentration is equal inside and outside
1 mark
so no net osmosis
Question 4
4 marks
A dialysis membrane allows small solutes and water through but not large proteins. Describe an experiment using dialysis tubing to model dialysis in kidneys and what results you would expect when tubing containing starch and glucose is placed in distilled water.
Model answer
Place dialysis tubing with starch and glucose in distilled water. After time, test outer water for glucose (Benedict’s positive) but not for starch (iodine negative) because glucose diffuses out while starch does not. Measure decrease of glucose inside or increase outside.
Explanation
The membrane simulates selective permeability: small sols like glucose diffuse out but large molecules like starch remain.
Mark scheme
1 mark
dialysis tubing containing starch + glucose placed in distilled water
1 mark
test outside water for glucose (Benedict’s) and for starch (iodine)
1 mark
expect glucose to appear in outside water (positive Benedict’s)
1 mark
starch should not appear in outside water (iodine negative) as it is too large
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