The plasma membrane on the outer surface of the protoplast regulates what enters and leaves the cell. Other membranes within the cell compartmentalize the protoplast, separating the interior into units of differing chemical composition, each with their own differentially (selectively) permeable membranes—membranes, like the plasma membrane, that selectively permit the passage of some materials while inhibiting the passage of others. (A freely permeable membrane that allowed everything to pass would be useless for the cell as would an impermeable membrane that allowed nothing to pass.) The cell is thus a collection of “factories” that import, manufacture, and export metabolic substances and that are separated from one another by membranes.
The membranes are channels of communication within and between cells and carry the directions supplied by the genes in the nucleus. Signals transmitted via specific molecules that institute reactions in their receptors organize the individual cells into an integrated multicellular organism. All of the cellular membranes are essentially alike in structure, but differ in their constituent proteins. Consequently, they vary in the types of materials they recognize and in the reactions they initiate and conduct.
The currently acceptable version of membrane structure is the fluid‐mosaic model, which visualizes the membrane as a double layer of fluid lipids in which proteins float laterally, forming a changing mosaic pattern (refer to Figure ).
The lipid bilayer is formed primarily of phospholipids, which, in a watery solution, orient with their hydrophilic (water-loving) heads toward the outside and theirhydrophobic (water-hating) tails to the inside. Other lipids, the sterols, are present in small numbers. (In animals, the representative sterol is cholesterol; in plants,stigmasterol).
Embedded in this bilayer are proteins that function in three cellular activities: transport, reception, and communication. These proteins are structured with two different configurations: Some extend through the lipid layer, with their hydrophobic portions amongst the lipid tails and their hydrophilic portions protruding on either side of the lipid layer. These are the transmembrane proteins. The second group, the peripheral proteins, are mostly glycoproteins (proteins with short carbohydrate side chains) attached to the projecting ends of the transmembrane proteins. Some of the proteins are not attached and appear to float; others, the integral proteins, are firmly embedded and don't move.