Which way does water move in osmosis?

From the higher water potential, the weaker solution, to the lower one.

Why does doubling the diffusion distance take four times as long?

Diffusion time scales with the square of distance, so twice the distance means four times the time.

S4 Biology Membranes and Transport, Paper 2: water potential and transport rate

2026-11-02

Paper 2 follows mass change and diffusion distance through osmosis, active transport and the square law of diffusion time.

Focus　Water moves to the lower water potential, active transport needs ATP, and diffusion time grows with the square of distance.

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)

FoundationIn which direction do particles move net during diffusion?
FoundationIn osmosis, which way does water move?
FoundationBesides a carrier protein, what does active transport require?
FoundationWhat happens to a red blood cell placed in pure water?
IntermediateA potato strip of mass 2.0 g becomes 2.6 g after soaking. What is the percentage gain in mass?
IntermediateBetween a 0.3 M and a 0.1 M sucrose solution, which way does water move net?
IntermediateA substance diffuses at 0.05 mm per second. How long does it take to travel 0.2 mm?
ChallengeCompare the surface area to volume ratios of cubes of side 1 cm and 2 cm.
ChallengeDiffusion time varies with the square of distance. If 0.1 mm takes 2 seconds, how long does 0.4 mm take?
Error redoError: a student says active transport moves substances from high to low concentration. What is correct?

03　Answers and one-line notes

#	Answer	One-line note
1	From high to low concentration	Net movement follows the concentration gradient until the two sides even out.
2	From higher to lower water potential	Water moves from the higher water potential, which is the weaker solution.
3	Energy in the form of ATP	Active transport can go against the gradient, so it needs energy from respiration.
4	It swells and bursts	Pure water has the highest water potential, so water floods in and the membrane bursts.
5	30 per cent	The gain is 0.6 g, which is 30 per cent of the original 2.0 g.
6	Towards the 0.3 M solution	The 0.3 M solution has more solute and lower water potential, so water moves into it.
7	4 seconds	Distance divided by rate gives 0.2 by 0.05, which is 4 seconds.
8	Twice as large for the 1 cm cube	The 1 cm cube gives 6 and the 2 cm cube gives 3, so the smaller one is twice as effective.
9	32 seconds	Four times the distance needs sixteen times the time, so 2 times 16 gives 32.
10	It can move substances against the gradient, so ATP is needed	Diffusion follows the gradient, while active transport can go against it and so 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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