Curriculum fit: GCSE Biology and Combined Science (transport in cells). US: high school life science (structure and function of cell membranes).
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What Is It?
Cells need to take in substances such as oxygen and glucose and get rid of waste. There are three main ways this happens: diffusion, osmosis and active transport.
Diffusion and osmosis are passive: they need no energy from the cell. Active transport uses energy from respiration to move substances against a concentration gradient.
Key Formulas and Ideas
- Diffusion: net movement of particles from a higher to a lower concentration (down a concentration gradient).
- Osmosis: movement of water across a partially permeable membrane from a dilute solution to a more concentrated solution.
- Active transport: movement from a lower to a higher concentration, using energy.
- Percentage change in mass = (final mass − initial mass) ÷ initial mass × 100.
Worked Examples
Diffusion
Explain why oxygen moves from the air in the lungs into the blood.
- The concentration of oxygen is higher in the alveoli than in the blood.
- Oxygen diffuses down the concentration gradient, across the thin alveolar wall.
✓ Answer Oxygen diffuses from high to low concentration
Osmosis Practical
A potato cylinder has mass 5.0 g and, after soaking in a solution, has mass 5.6 g. Find the percentage change.
- Change = 5.6 − 5.0 = 0.6 g.
- 0.6 ÷ 5.0 × 100 = 12%.
✓ Answer +12% (the potato gained water)
Active Transport
Why do root hair cells use active transport to take up mineral ions?
- Mineral ion concentration is lower in the soil than in the cell.
- Diffusion would move ions the wrong way, so the cell uses energy to pull them in.
✓ Answer Ions are absorbed against the concentration gradient
Common Mistakes to Avoid
- Saying osmosis is the movement of any particles. It is specific to water.
- Describing diffusion without mentioning a concentration gradient.
- Writing that active transport does not need energy.
- Getting the direction of water movement the wrong way round.
Practice Questions
Frequently Asked Questions
What affects the rate of diffusion?
A steeper concentration gradient, a higher temperature and a larger surface area all increase the rate.
What does 'partially permeable' mean?
Only some molecules, such as water, can pass through the membrane.
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Reviewed by StudyMorrow editorial review on 5 Nov 2026.