What is Diffusion?
The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, until equilibrium is reached.
Diffusion is one of the most important ways that substances move in and out of cells — and it requires no energy from the cell. This makes it a passive process.
Particles in gases and liquids are in constant, random motion. Because of this random movement, particles naturally spread out from where they are crowded together (high concentration) to where there are fewer of them (low concentration).
Imagine spraying perfume in one corner of a room. At first, the scent particles are all bunched together near the spray. Over time, they spread out in all directions until the scent is even throughout the room. That spreading is diffusion in action!
Key things to remember:
- Diffusion is a passive process — no energy (ATP) is needed
- Movement is net — particles move in both directions, but more move from high to low concentration
- Diffusion continues until equilibrium is reached (equal concentration everywhere)
- It works in gases and liquids (not solids)
Concentration Gradient
The difference in concentration of a substance between two regions. The steeper the gradient, the greater the difference in concentration, and the faster diffusion occurs.
The amount of a substance present in a given volume, often measured in mol/dm³ or simply described as high or low.
Think of a concentration gradient like a slope. Just as a ball rolls faster down a steep hill than a gentle slope, particles diffuse faster when the concentration gradient is steep (large difference in concentration).
When we say particles move "down" the concentration gradient, we mean from high to low concentration — the same direction as rolling downhill. This direction is automatic and requires no energy input.
The gradient eventually disappears when equilibrium is reached — the concentration is equal on both sides. At equilibrium, particles still move randomly, but there is no longer a net movement in any direction.
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