Lesson overview
Diffusion and active transport: 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.
Suggested sequence
- Lesson 1 · Diffusion and cell size. Diffusion, surface area to volume ratio and the factors that change the rate of diffusion. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Diffusion in living things. Diffusion through cell membranes: gases in the lungs and leaves, and dissolved substances. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- Lesson 3 · Active transport. Active transport against a concentration gradient, with protein carriers and energy from respiration. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- End of module · final check. 4 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 5 questions, 15 min, lesson closed.
Learning goals
Part 1: Diffusion and cell size
- Describe diffusion as the net movement of particles down a concentration gradient as a result of their random movement, with energy from the kinetic energy of the particles (3.1.1, 3.1.2)
- Explain the effect of surface area to volume ratio and distance on diffusion, using agar cubes (3.1.5)
- Investigate the factors that affect the rate of diffusion: surface area, temperature, concentration gradient and distance (3.1.5)
Part 2: Diffusion in living things
- State that substances move into and out of cells by diffusion through the cell membrane (3.1.3)
- Describe the importance of diffusion of gases and solutes in living organisms (3.1.4)
Part 3: Active transport
- Describe active transport as the movement of particles through a cell membrane against a concentration gradient using energy from respiration (3.1, 3.3.1)
- Explain the importance of active transport, including ion uptake by root hairs (3.3.2)
- State that protein carriers move molecules or ions across a membrane during active transport (3.3.3)
Misconceptions to listen for
- “Particles diffuse because they want to spread out.” — Particles have no wants. They move at random; more happen to move away from where they are crowded than move back.
- “Diffusion needs energy from the cell.” — The energy is the kinetic energy of the moving particles themselves; the cell supplies none.
- “The surface area is 9 cm².” — That is one face. A cube has 6 faces: multiply by 6.
- “The acid soaked into the small cube faster.” — The border was the same width on every cube: the acid moved at the same rate. The small cube cleared because there was less distance to go.
- “A smaller cube makes the acid particles move faster.” — The particles move just as fast; in a small cube they have less distance to go to reach the centre.
- “A bigger cube clears sooner because it has more surface.” — It has more surface in total but much more volume, and its centre is further away.
- “Diffusion has stopped, so the particles have stopped.” — The particles never stop moving; only the net movement stops.
- “A mouse has more surface, so it gets enough oxygen.” — It has more surface in total, but much less for each cm³ of body, and its inner cells are far from the skin.
- “Diffusion goes from low to high concentration.” — That is active transport. Diffusion goes down the gradient, from high to low.
- “Q clears first because it has more surface.” — Q has more surface in total, but less for each cm³, and its centre is four times further away.
Scope
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.