Generally, particles move randomly and end up evenly spaced. The movement of materials into and out of cells depends on this fact.

### What is diffusion?

The process of diffusion occurs when particles of one substance spread out through the particles of another substance. The ability of smells to spread out through the air and the way concentrated liquids travel through water is the result of diffusion.

The diffusion of particles occurs when they spread out from a high concentration area, where there are many particles, to a low concentration area, where there are fewer particles.

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The diffusion of particles occurs when they spread out from a high concentration area, where there are many particles, to a low concentration area, where there are fewer particles.

### What are cell membranes?

A cell's membrane holds the cell together, but it also allows stuff into and out of the cell. Diffusion allows dissolved substances to enter and leave cells.

It is only possible for very small molecules to diffuse through cell membranes through things like oxygen, glucose, amino acids and water. Proteins and starches are too large to pass through cell membranes.

As is the case with diffusion in air, particles move through the cell membrane from where there is a greater concentration (more particles) to where there is a lower concentration (fewer particles). The particles move randomly, so they go both ways, but if there are a lot more particles on one side of the membrane, then there is more overall movement.

### What are the rates of diffusion?

There can be a variation in the rate of diffusion. This is caused by a number of factors.

• Concentration gradients, or the difference in particle concentration, increase diffusion speed, the larger the gradient. This occurs because there is a greater net movement from one side than the other.
• A higher temperature results in a faster diffusion rate. This is due to the particles having more energy, so they diffuse faster.
• If a cell membrane, for example, has a large surface area, diffusion will be faster. The reason for this is that a larger number of particles can pass through simultaneously.

## Revision Quiz for Diffusion

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