The role of the cell membrane in cellular transport mechanisms.

Diffusion

Diffusion is the net movement of molecules or ions from a region of high concentration to low concentration which can be through a partially permeable membrane.

The cell membrane will make itself selectively permeable to allow products which are needed e.g. oxygen and water to enter or leave across the cell. Diffusion relies on the free movement of molecules and the rate is affected by the concentration gradient, area over which diffusion takes place and thickness of exchange surface.

 Facilitated diffusion

Facilitated diffusion is similar to diffusion however because the molecules being moved are of a larger size, the permeable membrane cannot diffuse them therefore carrier proteins which are embedded across the whole of the cell membrane change shape to transport the molecules from one side to another.

Particles e.g. glucose bind to the carrier protein from outside of the cell where the concentration gradient is most likely to be higher and as the carrier protein changes shape so the molecule can be released on the other side of the cell membrane. Due to the concentration gradient difference this does not require any energy.

Active transport

Active transport is much like facilitated diffusion as it uses a protein which acts as a sodium pump however the difference is that this protein requires energy in the form of ATP in order to carry out its process.

The sodium/potassium pump is embedded in the phospholipids bi layer and works by ATP giving it energy which causes the carrier protein to change shape so the sodium ions can bind to the protein. The binding of the sodium ions makes the protein change shape to release the sodium ions on the other side. Furthermore by releasing the sodium ions on the other side it causes the protein to change shape again to make the potassium ions bind to it and once binded it releases them on the other side. This procedure requires 1 ATP molecule and to repeat itself it will need another.