What is the difference between diffusion and osmosis?
Short answer
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. Osmosis is a special case: the diffusion of water molecules across a partially permeable membrane, from a dilute solution to a more concentrated solution. Both are passive and need no energy from the cell.
Step by step
- Diffusion can happen with any particles, in liquids or gases, with or without a membrane.
- Osmosis only involves water and always needs a partially permeable membrane.
- In osmosis, water moves from where there is more water (dilute) to less water (concentrated).
- Active transport is different: it moves substances against the concentration gradient and uses energy from respiration.
Worked example
Potato cylinders in sugar solutions
- In pure water, the potato gains water by osmosis and gets heavier
- In concentrated sugar solution, it loses water and gets lighter
- Where the mass does not change, the solution matches the concentration inside the cells
Common mistakes
- Saying osmosis is the movement of solute. Only water moves.
- Describing movement from low to high concentration of water.
- Saying animal cells become turgid. Plant cells become turgid; animal cells can burst because they have no cell wall.
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