Functions:

The phospholipid bilayer provides the basic structure of membranes. It also restricts entry and exit of polar molecules and ions.

• Channel proteins and carrier proteins are involved in the selective transport of polar molecules and ions across the membrane.
• Enzymes on epithelial cells lining some parts of the gut contain digestive enzymes in their cell surface membrane.
• Receptor molecules for chemical signaling between cells. e.g. hormones are chemical massengers which circulate in the blood but only bind to specific target cells which have the correct receptor sites.
• Antigens act as cell indentity markers or ‘name tags’. They are glycoproteins, i.e. proteins with branching carbohydrate side chains like ‘antennae’. There is an enormous number of possible shapes to these side chains, so each type of cell can have its own specific markers.