Proteins make up about 50 percent of the plasma membrane’s mass. The proteins in the membrane can occur in two forms – integral (intrinsic) proteins and peripheral (extrinsic) proteins.
Channel proteins are integral proteins that allow specific molecules and ions to pass through the membrane by creating a pore.
Carrier proteins bind to specific molecules or ions to temporarily allow them to cross the membrane. The shape of the carrier protein is changed by this process and may require ATP as an energy source. An example of this is a sodium-potassium pump.
Cell recognition proteins are usually glycoproteins with the carbohydrate portion projecting out of the cell and into the extracellular space. These give the cell a biochemical personality and are important in cell-to-cell recognition. For example the ABO blood group is based on the presence or absence of antigens on the red blood cell membranes. Cell recognition proteins are also important in the correct association of cells during development.
Receptor proteins on membranes recognise and respond to different extracellular stimulating molecules enabling specific responses to be generated within the cell. This is called signal transduction.
Some receptor proteins are enzymes, in which the cytoplasmic portion of the protein catalyses a reaction in response to the binding of a ligand.
Membrane proteins for cell adhesion bind to other proteins like those found in the extracellular matrix that surrounds cells or to membrane proteins of adjacent cells to form intercellular junctions. Cell adhesion is important in maintaining the structure of tissues and organs.
Membrane proteins for structural support are linked to the cytoskeleton and help to maintain the shape of the cell.
