Why is there no net movement in an isotonic solution?

Water enters and leaves at equal rates, so mass and shape stay the same.

What separates facilitated diffusion from active transport?

Facilitated diffusion follows the gradient without energy, while active transport can go against it using ATP.

S4 Biology Membranes and Transport, Paper 3: transport types and environment

2026-11-02

Paper 3 compares simple diffusion, facilitated diffusion, osmosis and active transport with mass change and diffusion distance figures.

Focus　Each transport type has its own requirements, warmth speeds diffusion, water follows water potential and active transport works uphill.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S4 Biology: Membranes and Transport
Level	S4

02　Questions (10 items)

FoundationWhat effect does a rise in temperature have on the rate of diffusion?
FoundationWhat happens to a red blood cell in 0.9 per cent saline?
FoundationWhat happens to a plant cell placed in concentrated sugar solution?
FoundationWhat does facilitated diffusion need, though it needs no energy?
IntermediateA potato strip of mass 3.0 g becomes 2.4 g after soaking. What is the percentage change in mass?
IntermediateCompare the surface area to volume ratios of cubes of side 3 cm and 1 cm.
IntermediateDiffusion time varies with the square of distance. If 0.2 mm takes 8 seconds, how long does 0.6 mm take?
ChallengeRoot hair cells take up mineral ions from more dilute soil. Which transport process is this?
ChallengeHow does lipid-soluble glycerol mainly enter a cell?
Error redoError: a student says osmosis needs ATP. What is correct?

03　Answers and one-line notes

#	Answer	One-line note
1	It increases, because particles gain kinetic energy	Higher temperature gives particles more kinetic energy, so diffusion speeds up.
2	Little change, because the solution is isotonic	The 0.9 per cent saline is isotonic with the cytoplasm, so water enters and leaves at the same rate.
3	Plasmolysis, as the cell loses water	The outside has lower water potential, so water leaves the vacuole and the membrane pulls away from the wall.
4	A carrier protein	Facilitated diffusion follows the gradient through carrier proteins and uses no ATP.
5	A 20 per cent decrease	The loss is 0.6 g, which is 20 per cent of the original 3.0 g.
6	Three times larger for the 1 cm cube	The 3 cm cube gives 2 and the 1 cm cube gives 6, so the smaller one is three times as effective.
7	72 seconds	Three times the distance needs nine times the time, so 8 times 9 gives 72.
8	Active transport, against the gradient and using ATP	Uptake continues even when the soil is more dilute, so carriers and ATP are needed.
9	Simple diffusion straight through the phospholipid bilayer	Glycerol dissolves in lipid and crosses the phospholipid bilayer directly without a carrier.
10	It needs none, since osmosis is passive movement down a water potential gradient	Osmosis follows the water potential gradient passively; only active transport needs energy.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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