Membrane Transport After binding to a receptor on a membrane, the major effect of some hormones is to activate carrier molecules in or near the membrane to increase the movement of substrates or ions from outside to inside the cell. For example, insulin binds to receptors on the surface of the cell and mobilizes glucose transport to the membrane of the cell. The transporters link up with glucose on the outside of the cell membrane where the concentration of glucose is high, and the glucose diffuses to the inside of the membrane for use in the cell (69). lf an individual does not have adequate insulin, as exists in uncontrolled
diabetes, glucose accumulates in the plasma because the glucose transporters in the membrane are not activated. In contrast, insulin does not use second messenger mechanisms to bring about its effects on the cell. Insulin binds to a tyrosine kinase receptor's alpha (o) subunits, which reside outside the cell (see Fig. 5.5). his binding causes the beta (B) subunits located inside the cell to phosphorylate themselves. The activated ty rosine kinase phosphorylates insulin-response proteins that lead to the movement of glucose transporters (called GLUT4) from the cytoplasm to the membrane so glucose can enter the cell. The insulin-response pro- tein also activates the enzyme glycogen synthase to form glycogen from the glucose molecules brought into the cell, as shown in Fig. 5.5 (107).