Diffusion, Osmosis and Active Transport
Learn the difference between diffusion, osmosis and active transport, including movement direction, membranes, energy and clear GCSE Biology examples.
Diffusion moves particles from a higher concentration to a lower concentration. Osmosis is the movement of water from a dilute solution to a concentrated solution through a partially permeable membrane. Active transport moves substances from a more dilute solution to a more concentrated solution using energy from respiration.
This AQA GCSE Biology content applies to both Foundation and Higher tiers. The explanation assumes separate GCSE Biology, although this topic is also shared with Combined Science.
Key differences
| Process | What moves? | Direction | Membrane needed? | Energy needed? |
|---|---|---|---|---|
| Diffusion | Particles of a substance | Higher to lower concentration | Not always | No |
| Osmosis | Water only | Dilute to concentrated solution | Yes: partially permeable | No |
| Active transport | Substances such as mineral ions or sugar | Lower to higher concentration | Across a cell membrane | Yes, from respiration |
A concentration gradient is the difference in concentration between two areas. Moving down the concentration gradient means moving from higher to lower concentration. Moving against it means moving from lower to higher concentration.
A partially permeable membrane allows some small molecules, such as water, to pass through but prevents some other substances from crossing.
Worked biological example
A root hair cell can take in substances in two different ways:
- Water enters the cell by osmosis through its partially permeable cell membrane.
- Mineral ions can enter by active transport when their concentration is lower in the soil than inside the root hair cell. This requires energy from respiration.
Oxygen can move from the blood into body cells by diffusion because its concentration is higher in the blood than in the cells.
Common exam mistake
Do not describe osmosis as the movement of “particles” or “solution”. Osmosis is specifically the movement of water. Also, do not say active transport moves substances from high to low concentration: it moves them against the concentration gradient and requires energy.
