Membrane proteins

Membrane proteins are proteins associated to their lipid bayer in some way, to provide that membrane with its characteristic functional properties. The amount and types of proteins in a membrane are highly variable, for example in a typical plasma membrane about 50% mass is membrane proteins, whereas in plasma membranes involved in ATP production there is around 75% mass. 

Membrane proteins can be organised into two main categories- integral (intrinsic) proteins and peripheral (extrinsic). They are organised based on their membrane-proteins interactions. Integral membrane proteins have portions embedded in the phospholipid bilayer. An example of intrinsic proteins are transmembrane and lipid linked proteins. Peripheral membrane proteins are bound to the membrane either indirectly via interactions with integral membrane proteins or directly via interactions with lipid polar head groups. Protein attached proteins are examples of extrinsic proteins.

There are many ways that the membrane proteins can be associated to the membrance. Transmembrane proteins are directly placed in the lipid bilayer and contain a hydrophobic region within the phospholipids and hydrophillic regions in the cytosol and outside the cell. However some membrane proteins are non covalently bonded to other proteins embedded in the lipid bilayer. These proteins are known as peripheral membrane proteins, these proteins do not extend into the hydrophobic interior of the bilayer. This type of protein can be released from the bilayer by simple procedures, which affect the protein-protein interactions. Whereas transmembrane proteins also known as integral membrane proteins, cannot be released by simple procedures.