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Diffusion = the spread of particles from a region of higher concentration to a region of lower concentration, until the particles are evenly spread out.

Diffusion takes place when you use a spray in a room, for example. The particles in the spray move randomly, knocking each other, which results in them spreading throughout the room gradually, from high concentration to low concentration. Therefore, diffusion acts down (or along) a concentration gradient.

It is important to know what affects the rate of diffusion. These are:

1. Surface area - the greater the surface area, the faster diffusion will occur
2. Difference in concentration - the higher the difference (the steeper the gradient), the faster diffusion will take place
3. The thickness of the exchange surface - the thicker the exchange surface, the slower the rate of diffusion.

Of course there are other factors such as temperature (increased kinetic energy results in faster diffusion) and the diffusion pathway (distance). The latter is a side effect of (3.) The thickness of the exchange surface, in some respects.

In some cases, diffusion is aided by certain proteins. This is called facilitated diffusion. The responsible proteins speed up diffusion of substances which would otherwise take longer to pass through the plasma membrane. The key points about facilitated diffusion which differentiate it from active transport (which also uses proteins):

-it occurs down a concentration gradient
-it uses no metabolic energy

Two kinds of protein achieve facilitated diffusion: carrier proteins and ion channels. Carrier proteins transport substances from one side of the membrane to the other, usually by co-transport. For example, glucose is transported along with an Na+ ion.

Ion channels are proteins with gates that can be open or closed to allow or stop certain ions from entering, e.g. Na+ (sodium) and K+ (potassium)  ions.