The Plasma Membrane (or cell membrane)
The generation (development) of the plasma membrane was a crucial step in the generation of the earliest forms of life without it cellular life is impossible. The plasma membrane that encloses every cell defines the cell's extent and maintains the essential differences between contents and the environment. This membrane is also a highly selective filter that maintains the unequal concentration of ion on either side and allows nutrients to enter and waste products to leave the cell.
All biological membranes, including the plasma membrane and the internal membranes of eukaryotic cells, have a common overall structure:
They are assemblies of lipid and protein molecules held together by "non covalent interactions"
The lipid molecules are arranged as a continuous "bilayer" (Double layer) approximately 4 to 5 mm thick.
The bilayer provides the basic structure of the membrane and serves as a relatively impermeable barrier to the flow of water soluble molecules. The protein is "dissolved" in the lipid bilayer and mediates the various functions of the membrane:
1) Transport specific molecules into or out of the cell.
2) Enzymes that caralyze membrane associated reactions.
3) Serve as receptors for receiving and transducing chemical signals from the cell environment.
4) Structural links between the cells ?? to skeleton and extra cellular matrix.
NB
All cell membranes are dynamic, fluid structures: most of their lipids and proteins are able to move about rapidly in the plane of the membrane.
This has lead to the term "Fluid mosaic model" as a description of the structure of cell membranes.
Many membrane proteins are held in the Bilayer by "Hydrophobic" interactions with lipid molecules. Membrane proteins may be associated with the lipid bilayer in one of three ways.
1) Some proteins are known to extend across the bilayer and
2) Some to be held by non-covalent interactions with other membrane proteins.
3) Recently, it has been shown that some membrane proteins have one or more covalently attached fatty acid chains that help anchor the protein in one monolayer or the other (These may or may not be transmembrane proteins).