The cell membrane (also called plasma membrane) is a biological membrane that separates the interior of the cell (the cytosol) from the exterior of the cell (though, in certain organisms, such as plants, an additional layer exterior to this, the cell wall also exists as a boundary). It has a thickness of about 7.5nm. The cell membrane selectively allows substances in and out of the cell, regulates the environment of the cell and ensures conditions are right for reactions within the cytosol.
Proteins in the cell membrane
The membrane is a sort of 'fluid mosaic' (Singer and Nicholson model) - a fluid structure with a mosaic of proteins in it. The proteins are attached to or embedded into a membrane (extrinsic); and some span the entire level (intrinsic). A number serve as transporters for protons (H+), ions (e.g., Cl-) and molecules (e.g., glucose) across the membrane. The proteins are allowed to move about, which accounts for the ease with which membranes are able to join on to each other.
Glycoproteins are proteins with attached carbohydrates. This type of modification is found on the outside part of transmembrane proteins.
Lipids in the cell membrane
Lipids have a hydrophillic (attracted to water) head, and two hydrophobic (repelled) fatty acid chains. The head contains phosphate which is attracted to water. The bilayer originates from the heads of the lipids wanting to be in contact with water so a layer surrounds the cell with the heads directed towards the watery outside. The other layer of lipids have their heads pointed inwards towards the watery cytosol.
There are three types of lipids present:
- Phospholipids - phospholipids these are lipids which contain phosphates, making up the bulk of the lipids.
- Glycolipids - these are lipids with a carbohydrate (polysaccharide) 'stalk' sticking from them. Glycolipds always occur in the outer of the lipid bilayer. The carbohydrate portions are called glycocalyx.
- Cholesterol - cholesterol ensure that the membrane does not freeze at low temperatures. These also have polar regions, and line up with the lipids in the outer layer.
Most of the transport is controlled by regulatory mechanisms, and some transports require energy to take place (active transport). Some of the membrane proteins are used as relays between the interior and the exterior of the cell (e.g., the insulin receptor). Incoming signals are further processed by signal transduction.
Within eukaryotes, the periphery of various organelles are also defined by the extent of a membrane. The interior structure of some organelles is also composed of membrane folded back upon itself (e.g., in mitochondria) to increase the total membrane surface. Vesicles and vacuoles are also surrounded by a membrane.
Transport across the cell membrane
This can occur by 4 ways:
- Endocytosis and exocytosis - endocytosis and exocytosis is respectively the uptake of materials in to cells and the way materials are removed. This forms the main transport of substances in to cells.
- Diffusion - diffusion is the random moving of particles between a membrane which results in substances moving from a high concentration to a low concentration.
- Osmosis - osmosis is the movement of water through membranes to try and even the concentrations of substances.
- Active transport - active transport is the movement of molecules across a membrane up a concentration gradient; to do this requires energy,