BIOLOGY Volume 1 - A Guide to General Biology - 2004

5. CELLS

5.9. Cell membranes

5.9.7. Functions of membranes

As already noted, the phospholipid bilayer forms the structural basis of the membrane. It also restricts The entry of polar molecules and ions into The Cell and their exit from it. A number of other functions are performed by other membrane components.

1. Channel and carrier Proteins facilitate the selective transport of polar molecules and ions across the membrane (for Active Transport and Facilitated Diffusion, see Section 5.9.8).

2. Enzymes. Proteins frequently function as enzymes. A classic example is found in the microvilli of the epithelium lining certain PARTS OF THE intestine. The Plasma Membranes of these epithelial Cells contain digestive enzymes.

3. Receptor molecules. All protein molecules possess a highly specific conformation, as discussed in Chapters 3 and 4. This makes them ideal receptors, i.e., molecules used to transmit specific signals from cell to cell. For example, Hormones, which act as chemical messengers, circulate in the Blood but bind exclusively to specific target cells possessing the corresponding receptors. Neurotransmitters—the chemicals responsible for Nerve Impulse Conduction—likewise bind to specific receptor proteins on Nerve Cells.

4. Antigens act as markers, a sort of cellular "name tag" that allows a cell to be recognized. These are Glycoproteins—proteins with attached branched oligosaccharide side chains that function as "antennas." There is an infinite variety of possible configurations for these side chains, enabling each cell type to have its own unique marker. These markers allow cells to recognize one another and coordinate their activities, such as during The formation of Tissues and Organs in Multicellular Organisms. This same property enables The Immune System to recognize and attack foreign antigens.

5. Glycolipids also feature branched oligosaccharide side chains and likewise assist in cell-to-Cell Recognition. Furthermore, glycolipids can serve as receptors for chemical signals. Together with glycoproteins, glycolipids ensure proper Cell Adhesion when cells aggregate to form tissues.

6. Energy Transduction. During Photosynthesis and Respiration, energy-transducing systems operate within the membranes of METABOLISM/14.html">Chloroplasts and Mitochondria, respectively, and these systems also involve proteins.

7. Cholesterol acts as an additional "barrier," restricting the movement of polar molecules across the membrane in both directions—into and out of the cell.