Cell membranes and transport
The cell membrane is a boundary which seperates the living cell from its non-living surroundings. It controls which substances can enter or leave the cell.
The structure of the membrane is described by Singer and Nicholson as a fluid mosaic:
- Fluid because the indiviudal phospholipids can move about within the layer
- Mosaic because the proteins appear dotted through the layer like a mosaic
The cell membrane is almost entirely made up of phospholipids and proteins. Phospholipids are made by replacing one of the fatty acids of a trigylceride with a phosphate group. A phospholipid is a polar molecule, with the phosphate head being hydrophillic and liphobic; the fatty acid tails are hydrophobic and lipophillic. Phospholipids therefore create a bilyar.
Singer and Nicholson put forward a model for the structure of the cell/plasma membrane and it states that:
- There's a bimolecular phospholipid layer
- Associated with the bilayer are a variety of proteins
- Some of these proteins occur on just one layer of the membrane and are therefore called extrinsic proteins
- Some proteins span both layers of the membrane and are called intrinsic proteins
Roles of various parts of the plasma membrane
- These create the bilayer and act as a barrier to most water-soluble substances
- As their tails are non-polar it's also difficult for molecules or ions to pass through…