Diffusion and active transport · 3 parts · syllabus 3.1, 3.3

Why are cells so small?

Cut pink agar cubes of three sizes, put them in acid and time how long each takes to turn colourless. Cut a fresh set in half, compare surface area and volume, then test warmer and stronger acid.

Start learning Explore the experiment

A printed full stop seen close up; a magnifier circle shows human cheek cells, five or six across the dot.

Follow the idea

Diffusion and cell size
Diffusion, surface area to volume ratio and the factors that change the rate of diffusion
Not started
Diffusion in living things
Diffusion through cell membranes: gases in the lungs and leaves, and dissolved substances
Not started
Active transport
Active transport against a concentration gradient, with protein carriers and energy from respiration
Not started

Three agar cubes on mesh platforms in beakers of acid: the large cube has a big pink core, the middle one a small core, the small one is colourless; each has a clear layer of the same thickness.

The experiment · Agar cubes in acid · time them, cut them, change one factor

Run it yourself, or project it for the class

Use the apparatus step by step and look closely at what happens. The whole experiment fits one screen, so it works on a projector too.

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Final check

Explain it on your own across all three parts. Hints and keys stay closed until you submit.

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Return and recall

Come back after one to two weeks and answer with the lesson closed.

Recall questions

For teachers

Printable worksheets with their answer keys, the final check and the return questions for this module.

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Scope and sources

Cambridge IGCSE Biology 0610 · 2026–2028 v3 · Original learning materials.

Diffusion as the net movement of particles down a concentration gradient, its energy and the four factors (3.1.1, 3.1.2, 3.1.5) through the agar-cube practical; diffusion through cell membranes and its importance for gases and solutes (3.1.3, 3.1.4); active transport (3.3.1) with root hairs and protein carriers (Supplement 3.3.2–3.3.3). Osmosis is in the next module. Times in the experiment come from a teaching model.

Apparatus, amounts and times in the experiment are illustrative. Real practical work follows a teacher-approved safety method.

Official Cambridge syllabus

Why are cells so small?

Follow the idea

Diffusion and cell size

Diffusion in living things

Active transport

Run it yourself, or project it for the class