Diffusion

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.

Particles diffuse down a concentration gradient and eventually reach equilibrium in which all particles are evenly distributed (but still moving).

Diffusion does not require energy and so is described as a passive process.

You need to know about two types of diffusion:

1. Simple diffusion
2. Facilitated diffusion

Simple diffusion

Some molecules can diffuse directly across cell membranes in a process known as simple diffusion.

Facilitated diffusion

Large or polar molecules can cross cell membranes in a process known as facilitated diffusion.

Facilitated diffusion involves two types of proteins:

1. Carrier proteins
2. Channel proteins

Each carrier or channel protein is highly specific, allowing only one or two types of molecule to pass through.

Channel proteins

Channel proteins mainly transport ions across the cell membrane.

These proteins form pores in the cell membrane, which ions can travel through.

Factors affecting the rate of diffusion

1. Temperature - At higher temperatures, particles have more kinetic energy and diffuse faster.
2. Concentration gradient - The steeper the concentration gradient, the faster the rate of diffusion.
3. Thickness of membrane - The particles travel shorter distances through thin exchange surfaces, so diffuse faster.
4. Surface area - Larger surface areas mean more particles can cross the membrane at once, making diffusion faster.
5. Number of carrier or channel proteins - The more of these proteins, the faster the rate of facilitated diffusion.