Section 4.2 Diffusion.

Diffusion is explained as 'passive transport'. This comes from the energy coming from natural , inbuilt motion of the products, rather than a external souce of ATP.

The net movemnt of molecuels or ions from a region where they are more highly concentrated to one where their concentration is lower until they are evenly distrobuted.

Facilitated diffusion is a passive progress. It relies only on the inbuilt motion ( kenetic energy ) of diffusion molecuels. It occurs down a concentration gradient, but occurs a specific points on the plasma membrane where special protein molecuels and two types- protein channel and carrier proteins.

Protein channels- These proteins form water-filled hydrophobic channelsacross the membrane. They allow water soluable ions to pass through. If a particualr ion isnt present the channel remains closed. 

Carrier proteins- Involves proteins which span the entire length of the plasma membrane. When a specific molecuel is present, for example gluecose, it causes it to change shape in such a way the plasma membrane allows the molecuel to be released inside of the membrane.