Why does a plant cell not burst in pure water?

The cell wall is strong enough to bear the turgor pressure, which animal cells lack.

Why does facilitated diffusion saturate?

Carrier proteins are finite, so once all are occupied a steeper gradient adds no speed.

S4 Biology Membranes and Transport, Paper 4: the four transport types

2026-11-02

Paper 4 separates the four transport types using gas exchange, pure water and carrier saturation, with mass change, distance and size ratio figures.

Focus　Simple diffusion and osmosis are passive, facilitated diffusion saturates, active transport goes uphill with ATP and water moves to lower water potential.

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)

FoundationHow do oxygen and carbon dioxide mainly move in and out of a cell?
FoundationWhat direction and energy demand does active transport have?
FoundationWhat happens to a plant cell placed in pure water?
FoundationWhat happens to the rate of facilitated diffusion when all carrier proteins are occupied?
IntermediateA potato strip of mass 2.5 g becomes 3.0 g after soaking. What is the percentage change in mass?
IntermediateDiffusion time varies with the square of distance. If 0.1 mm takes 5 seconds, how long does 0.3 mm take?
IntermediateCompare the surface area to volume ratios of cubes of side 4 cm and 2 cm.
ChallengeHow do root hair cells mainly take up water from the soil?
ChallengeA 0.2 M and a 0.5 M solution are separated by a partially permeable membrane. Which way does water move?
Error redoError: a student says diffusion needs energy from the cell. What is correct?

03　Answers and one-line notes

#	Answer	One-line note
1	Simple diffusion straight through the phospholipid bilayer	Both gases are small and lipid-soluble, so they cross the bilayer freely without a carrier.
2	Against the concentration gradient, using ATP	Active transport moves substances from dilute to concentrated using carrier proteins and ATP.
3	It takes up water, becomes turgid and the wall prevents bursting	Water enters the vacuole and the tough wall bears the pressure, so the cell swells without bursting.
4	It levels off at a maximum	Carriers are limited, so once saturated a steeper gradient no longer speeds uptake.
5	A 20 per cent increase	The gain is 0.5 g, which is 20 per cent of the original 2.5 g.
6	45 seconds	Three times the distance needs nine times the time, so 5 times 9 gives 45.
7	Twice as large for the 2 cm cube	The 4 cm cube gives 1.5 and the 2 cm cube gives 3, so the smaller one is twice as effective.
8	By osmosis, down the water potential gradient	Soil water potential is higher than that of the cell sap, so water enters through the membrane by osmosis.
9	Towards the 0.5 M side, which has the lower water potential	The stronger solution has the lower water potential, so water moves from 0.2 M to 0.5 M.
10	It needs none, since diffusion is passive and driven by random motion	Net movement from high to low concentration happens on its own without any ATP.

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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