GCSE Biology Diffusion, Osmosis & Active Transport Notes
Substances move in and out of cells by three main mechanisms — diffusion, osmosis and active transport. Each depends on concentration and, for active transport, on energy from respiration.
Key concepts
- Diffusion
- Net movement of particles from a high to low concentration down a concentration gradient. Passive — no energy needed.
- Osmosis
- Diffusion of water across a partially permeable membrane from a dilute (high water) to a concentrated (low water) solution.
- Active transport
- Movement of substances against a concentration gradient using energy from respiration (ATP).
- Rate of diffusion increases with a steeper concentration gradient, higher temperature and a larger surface area.
- Plant cells placed in pure water become turgid; in concentrated solutions they become plasmolysed.
- Root hair cells use active transport to absorb mineral ions from soil where concentration is lower than in the cell.
Required practicals
- Investigate the effect of sucrose concentration on the mass of plant tissue (osmosis).
Worked example
Explain why potato cylinders lose mass when placed in a strong sugar solution.
- 1The sugar solution has a lower water concentration than the cell sap.
- 2Water moves out of the potato cells by osmosis down the water concentration gradient.
- 3The cells lose water, so the cylinders lose mass.
Answer: Water leaves the cells by osmosis, so the cylinders lose mass.
Common mistakes
✗ Saying osmosis moves 'water and sugar'.
✓ Osmosis is water only — the membrane blocks solutes.
✗ Describing diffusion as needing energy.
✓ Diffusion is passive; only active transport uses energy from respiration.
✗ Confusing 'high water' with 'high solute'.
✓ A dilute solution has HIGH water concentration; a concentrated solution has LOW water concentration.
Quick check
Try these, then reveal the answers.
Which process moves particles against a concentration gradient?Reveal answer
Active transport.
Give two factors that increase the rate of diffusion.Reveal answer
Steeper gradient and higher temperature (also larger surface area).
In osmosis, what moves and through what?Reveal answer
Water, across a partially permeable membrane.
Exam tips
- Always use the phrase 'down/against the concentration gradient' when describing transport.
- For osmosis, mention 'partially permeable membrane' explicitly.
- Link active transport to 'energy from respiration' (ATP) for the mark.
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Frequently asked questions
What is the difference between diffusion and osmosis?+
Diffusion is the movement of any particles down a concentration gradient; osmosis is specifically the movement of water across a partially permeable membrane.
Why do root hair cells use active transport?+
Mineral ions are more concentrated inside the cell than in the surrounding soil, so absorbing them requires energy from respiration.
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