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LESSON B2 · EXPLAINER
Diffusion, Osmosis and Active Transport
Sprinkle salt on a slug and it shrivels. Put a raisin in water and it swells. Same process — why?
Narration uses your device’s built-in voice. No sound? Switch to “Captions only” — the captions advance on their own.
Explore in 3D
Diffusion and osmosis in 3D
Watch particles spread, and see what happens to cells in different solutions.
Step 2 · Read
Key notes
Copy these into your notebook in your own words — that is how they stick.
Diffusion
Net movement of particles from high to low concentration down a gradient; passive.
Osmosis
Movement of water from a dilute to a more concentrated solution across a partially permeable membrane; passive.
Active transport
Movement from low to high concentration using energy from respiration and carrier proteins.
Plant cells
In water → turgid. In strong solution → flaccid, plasmolysed (wilting).
Animal cells
In water → swell and burst (haemolysis). In strong solution → crenate/shrink.
Potato well
Salt in well draws water from dish through living cells. Boiled potato: membranes destroyed → no osmosis.
Importance
Gas exchange, water uptake, absorption of food, turgidity/support, mineral uptake, kidney reabsorption.
EXAM TIPS & COMMON MISTAKES
Osmosis definitions MUST mention water, partially permeable membrane and direction (dilute → concentrated).
For “which process?” tables: water movement = osmosis; gases/dissolved food down a gradient = diffusion; minerals up a gradient = active transport.
To make water leave a potato into the dish, make the DISH solution concentrated (add salt to the dish).
Step 2 · Learn from examples
Worked examples & study resources
The kind of support a teacher gives after the lesson: worked examples to attempt first, a board illustration, key terms, common misconceptions and memory aids.
Worked example 1
Potato strips in sugar solutions
Potato strips, each 50 mm long, were left in sucrose solutions for 2 hours and measured again.
Sucrose (mol dm⁻³)
0.0
0.2
0.4
0.6
Final length (mm)
54
52
49
47
(a) Calculate the percentage change in length in 0.6 mol dm⁻³. (b) Explain the results in water and in 0.6 mol dm⁻³. (c) Estimate the concentration at which the length would not change.
Try it in your notebook first, then reveal the solution one step at a time.
In water (0.0) the solution is more dilute than the cell sap, so water enters the cells by osmosis through the partially permeable membranes. The cells become turgid and the strip gets longer.
In 0.6 mol dm⁻³ the solution is more concentrated than the cell sap, so water leaves the cells by osmosis. The cells become flaccid and the strip shortens.
No change happens where the solution has the same concentration as the cell sap. The change goes from +2 mm (0.2) to −1 mm (0.4), so zero is about two-thirds of the way: about 0.33 mol dm⁻³.
Answer. (a) −6%. (b) Water entered by osmosis in water (strip longer); water left by osmosis in 0.6 mol dm⁻³ (strip shorter). (c) About 0.3–0.35 mol dm⁻³, where the solution matches the cell sap.
Examiner’s tip. Use percentage change, not just change in length, so that strips of different starting lengths can be compared fairly. Always say water moves “by osmosis through a partially permeable membrane”.
Worked example 2
Why salting preserves fish
Saltfish has been preserved in the Caribbean for centuries by packing fish in salt. Explain how salt stops the fish from going bad.
Try it in your notebook first, then reveal the solution one step at a time.
Fish goes bad because bacteria and fungi grow on it.
Salt dissolves in the moisture on the fish, making a very concentrated solution.
Water leaves the bacterial and fungal cells by osmosis, from their dilute cytoplasm to the concentrated salt solution outside.
Without enough water the microorganisms cannot carry out their reactions or reproduce, so they cannot decompose the fish. Salting also draws water out of the fish itself, drying it.
Answer. The salt makes a concentrated solution around the fish, so water leaves any bacteria or fungi by osmosis. Dehydrated microorganisms cannot grow or reproduce, so the fish does not decompose.
Examiner’s tip. The same idea explains preserving with sugar (jams, guava cheese). Questions often ask you to apply osmosis to everyday situations — always say which way the water moves and why.
On the board · Three ways substances cross membranes
Diffusion
Osmosis
Active transport
What moves
Any particles (gases, solutes)
Water only
Particles such as mineral ions, glucose
Direction
High → low concentration
Dilute solution → concentrated solution
Low → high concentration (against the gradient)
Energy from respiration?
No (passive)
No (passive)
Yes
Membrane needed?
No
Yes, partially permeable
Yes, with carrier proteins
Example
O₂ into blood in the alveoli
Water into root hair cells
Mineral ions into root hairs
Ask two questions: what is moving, and is energy needed?
Key terms
Diffusion
Net movement of particles from a high to a low concentration.
Osmosis
Movement of water from a dilute to a more concentrated solution through a partially permeable membrane.
Active transport
Movement against a concentration gradient using energy from respiration.
Concentration gradient
A difference in concentration between two areas.
Turgid
A plant cell swollen and firm with water.
Flaccid / plasmolysed
A plant cell that has lost water; in plasmolysis the membrane pulls away from the wall.
Crenated
An animal cell shrivelled after losing water.
Common misconceptions
✗ In osmosis, the salt or sugar moves.✓ In osmosis only the water moves. The solute particles are too large to pass through the membrane.
✗ Plant cells burst in pure water.✓ The cell wall stops them bursting — they become turgid. Animal cells, with no wall, can burst.
✗ Diffusion needs energy.✓ Diffusion and osmosis are passive. Only active transport uses energy from respiration.
Memory aid
Osmosis = water Only. Active transport needs ATP and goes Against the gradient.
Real-life link
Oral rehydration solution (clean water with the right amounts of salt and sugar) saves lives in cases of severe diarrhoea: the salt and sugar are absorbed in the gut, and water follows them into the blood by osmosis.
Try this
Put one carrot stick in tap water and another in very salty water overnight. Compare how bendy they are next morning, and explain the difference using the word “osmosis”.
Play & practise
Games
3 quick games on this lesson. Earn XP and badges as you play — your best score for each game is saved. Games are for practice and do not change your exam readiness.
Step 3 · Practise
Paper 1 practice — multiple choice
8 items modelled on real CSEC Paper 1 questions. Choose one answer for each, then check. Aim for 80% or better.
Step 4 · Apply
Paper 2 practice — structured questions
Write full answers, then reveal the mark scheme and tick the points you earned (18 marks in total). Your answers are saved on this device.
Based on CSEC P2 May/June 2024, Question 1(a)–(d)
Question 1 [11 marks]
A well was cut in a raw potato and salt crystals were placed in it. The potato stood in a Petri dish of water for two hours.
(a) Describe what would be observed after two hours, and explain it. [3]
MARK SCHEME — (a) (max 3)
(b) Suggest a suitable hypothesis for the experiment. [2]
MARK SCHEME — (b) (max 2)
(c) Suggest TWO ways the observations would differ if a boiled potato were used. [2]
MARK SCHEME — (c) (max 2)
(d) For EACH observation name the process and give a reason: (i) slugs shrivel when salted; (ii) sea-water fish die in fresh water. [4]
MARK SCHEME — (d) (max 4)
Based on CSEC P2 January 2026 (hybrid), Question 5(c)
Question 2 [7 marks]
Diffusion and active transport are essential in living organisms.
(a) Explain THREE ways in which diffusion is important to processes in living organisms. [6]
MARK SCHEME — (a) (max 6)
(b) Outline ONE way in which active transport is important. [1]
MARK SCHEME — (b) (max 1)
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Sources
Where this topic appears in CSEC papers
Find these in your own past-paper booklets for extra timed practice. The practice above is adapted from them.