Edexcel GCSE Biology
Transport in and out of cells — Edexcel GCSE Biology revision
Free revision notes, key terms, common exam traps and 5 practice questions with answers. About 4 minutes to read.
Three transport routes
- Diffusion: particles spread from high to low concentration. Passive — no energy needed.
- Osmosis: water moves through a partially permeable membrane from a dilute to a more concentrated solution.
- Active transport: moves substances against the concentration gradient, so it needs energy from respiration.
Rate factors
- Bigger concentration difference → faster diffusion.
- Higher temperature → particles move faster → faster diffusion.
- Larger surface area → faster exchange (villi, alveoli, gills).
Key terms
- Concentration gradient
- The difference in concentration between two areas.
- Partially permeable
- A membrane that lets some particles through but not others.
- Active transport
- Movement against the gradient using energy from respiration.
- Surface area : volume ratio
- Surface area divided by volume — smaller organisms have a bigger ratio.
- Diffusion
- Net movement of particles from high to low concentration, needing no energy.
- Osmosis
- Net movement of water from dilute to concentrated solution across a partially permeable membrane.
Common exam traps
- Saying osmosis moves 'particles' — osmosis is specifically water.
- Forgetting to say active transport requires energy from respiration.
- Describing water moving 'to where there is more water' — examiners want dilute → concentrated solution wording.
Practice questions with answers
1. Which process requires energy from respiration?
- • Diffusion
- • Osmosis
- • Active transport
- • Evaporation
Answer: Active transport
It works against the concentration gradient, so energy is needed.
2. Define osmosis.
Answer: The movement of water through a partially permeable membrane from a dilute solution to a more concentrated solution.
Three marking points: water, partially permeable membrane, dilute → concentrated.
3. Potato cylinders placed in strong sugar solution lose mass. Why?
- • Water leaves the cells by osmosis
- • Sugar enters by diffusion and dissolves the cells
- • Cells respire faster
- • Water enters the cells by osmosis
Answer: Water leaves the cells by osmosis
The solution outside is more concentrated than the cell contents.
4. Give two ways an exchange surface can be adapted to increase the rate of diffusion.
Answer: Large surface area and a thin membrane (short diffusion path); good blood supply or ventilation also count.
Any two valid adaptations score.
5. Which process needs energy from respiration?
- • Diffusion
- • Osmosis
- • Active transport
- • Evaporation
Answer: Active transport
Active transport moves particles against the gradient, so it needs energy.
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