Diffusion
Diffusion is the net movement of particles from a region of their higher concentration to a region of their lower concentration, down a concentration gradient, as a result of their random motion. It is a passive process – no energy from respiration is needed. Gases and dissolved substances diffuse; for example oxygen diffuses into cells and carbon dioxide diffuses out.
Key idea
The rate of diffusion increases with a steeper concentration gradient, a higher temperature (faster-moving particles), a larger surface area and a shorter distance to travel.
Osmosis
Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane. It is a special case of diffusion involving only water.
Worked example
A piece of potato is placed in pure water. The cell contents have a lower water potential than the surrounding water, so water enters the cells by osmosis. The cells become turgid and the potato gains mass. In a strong sugar solution the reverse happens – water leaves the cells, they become flaccid (and may plasmolyse), and the potato loses mass.
Remember
- Osmosis is only about water moving across a partially permeable membrane.
- Both diffusion and osmosis are passive – they need no energy.
- Active transport, in contrast, moves substances against the gradient and needs energy.