S4 Biology: Membrane Transport and Osmosis prep guide | Diffusion, active transport and plasmolysis

2026-11-01

Tutor lesson-prep guides · Secondary Biology

Students chant that water moves from high to low water potential, then guess wildly once sucrose concentrations and mass changes appear. This lesson makes osmosis a pressure difference with numbers.

Objectives — Students can: (1) use surface-area-to-volume ratio to explain folded membranes and transport proteins; (2) compare water potentials and state the direction of water movement; (3) separate diffusion, facilitated diffusion, active transport and bulk transport.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Diffusion	What happens to ink dropped in water	It spreads from high to low concentration
Cell structure	What the membrane is made of	A phospholipid bilayer with proteins
Comparing concentrations	Which is stronger, 0.30 M or 0.25 M	0.30 M

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Surface area to volume	A 1 mm cube against a 10 mm cube	6.0 against 0.6
10-24 min	Diffusion and distance	Time scales with distance squared, 746 seconds at 1 mm	About 12.4 minutes
24-40 min	Water potential	Minus CRT for 300 and 250 mol per cubic metre	-743 and -619 kPa
40-50 min	Active transport and bulk transport	Against the gradient needs ATP and a carrier	Accumulation against the gradient
50-56 min	Reading the experiment	Potato strip 5.20 g to 5.72 g	Plus 10.0 per cent
56-60 min	Exit check	Three questions collected and marked	Independent work

03 Teaching script (read it out)

Scene 1: Why cells must stay small

A cube of side 1 mm has 6 square millimetres of surface and 1 cubic millimetre of volume, so the ratio is 6.0. At 10 mm the surface is 600 and the volume 1000, so the ratio falls to 0.6, a factor of 10.
Diffusion time scales with distance squared. With glucose diffusing at 6.7 times ten to the minus six square centimetres per second, crossing 10 micrometres takes 0.075 seconds, while crossing 1 mm takes 746 seconds, which is 12.4 minutes.
That is why large organisms need a circulatory system and folded surfaces such as villi and alveoli.

Task: work out the ratio for a 2 mm cube, then say how many times worse it is than 1 mm.

Scene 2: Water potential decides the direction

Water potential combines solute potential and pressure potential. With R equal to 8.314 and T equal to 298 kelvin, a 300 mol per cubic metre sucrose solution has a solute potential of -743 kPa while cell sap at 250 has -619 kPa.
The sap is less negative, so its water potential is higher and water leaves the cell for the sucrose solution, driven by a gap of 124 kPa.
A plant cell shrinks to plasmolysis, an animal cell shrinks and crenates, and in pure water an animal cell bursts while a plant cell merely becomes turgid because the wall supplies pressure potential.

Task: explain why a higher concentration means a lower water potential.

Scene 3: Three ways across the membrane

Facilitated diffusion runs down the gradient through a carrier and needs no ATP, as with glucose entering red cells. Active transport runs against the gradient and needs both a carrier and ATP, as with the sodium potassium pump or root hairs absorbing mineral ions. Bulk transport wraps material in membrane and costs ATP, as with phagocytosis or insulin secretion.
Three questions settle any example: is it down the gradient, does it need ATP and a carrier, and is the material wrapped in membrane?
Carriers are finite, so the rate plateaus when every carrier is saturated, which is a favourite graph question.

Warn: oxygen and carbon dioxide cross by simple diffusion, so never write that they need carriers.

04 Question types and worked steps

Type	Example	Steps	Answer
Data	Diffusion time	Distance squared over twice the diffusion coefficient	About 12.4 minutes at 1 mm
Comparison	Direction of water movement	Compare solute potentials first	From -619 to -743 kPa
Experiment	Mass change	Gain means uptake, loss means loss of water	Plus 10.0 per cent is uptake
Classification	Transport mechanism	Gradient, ATP and carrier	Oxygen simple, ions active

Sanity check (teach this): The time must be quoted in the right unit: 746 seconds is not 746 minutes. Water potential can never exceed zero, so a positive answer means a sign error. A gain of 10.0 per cent must agree with a dilute bathing solution, and a 0.30 M solution would make the strip lose mass instead. The surface-area ratio is dimensionless, so answering six square millimetres forgets the division by volume.

05 Independent practice (3 questions)

Question	Answer	What to watch
Surface area to volume ratio of a 2 mm cube	3.0	24 over 8
Solute potential of 300 mol per cubic metre sucrose	-743 kPa	Minus CRT
Percentage mass change from 5.20 to 5.72 grams	Plus 10.0 per cent	0.52 over 5.20
Fate of a red cell in pure water	It swells and bursts	No wall to supply pressure potential

06 Common mistakes and what to say

Mistake	Student writes	Say this
Reversing the direction	Saying water moves from low to high potential	“Water moves to the lower potential, which is the stronger solution.”
Giving oxygen a carrier	Listing oxygen under facilitated diffusion	“Oxygen crosses the bilayer directly.”
Forgetting to square the distance	Assuming double the distance doubles the time	“Time goes with distance squared, so double means four times.”
Confusing plasmolysis with bursting	Saying a plant cell bursts in a strong solution	“A plant cell only plasmolyses; the wall prevents bursting.”

07 Homework (paper, section and question numbers)

Secondary biology chapter index — All chapters
DSE biology practice paper — Cell transport questions 5 to 9

08 Exit check (last 5 minutes)

Question	Answer	Decision
Time for diffusion across 1 mm	About 12.4 minutes	All correct: move to enzymes
Which way does water move	From the sap to the sucrose solution	Wrong: redo minus CRT
Two requirements for active transport	Against the gradient and ATP	Two wrong: restart act three

09 Tutor FAQ

Why does concentration run opposite to water potential

Solute occupies space and binds water, so more solute means a lower potential.

Why does a plant cell not burst in pure water

The wall generates pressure potential that rises until it balances the solute potential.

Why does the rate graph flatten

Carriers are finite, so once saturated the rate cannot rise with concentration.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.