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 transporters located in the membrane of the cell. The transporters link up with glucose on the out-side of the cell membrane where the concentration of glucose is high, and the glucose transporter diffuses to the inside of the membrane to release glucose for use in the cell (77). If an individual does not have ade-quate insulin, as exists in uncontrolled diabetes, glu-cose accumulates in the plasma because the glucose transporters in the membrane are not activated.

Altering Activity of DNA in the Nucleus



Due to their lipid-like nature, steroid hormones diffuse easily through cell membranes, where they become bound to a protein receptor in the cytoplasm of the cell. Figure shows that the steroid-receptor complex enters the nucleus and binds to a specific protein linked to DNA, which contains the instruction codes for protein synthesis. This activates (or, in a few cases, suppresses) genes that lead to the synthesis of a specific messenger RNA (mRNA) that carries the codes from the nucleus to the cytoplasm where the specific protein is synthesized. While thyroid hor-mones are not steroid hormones, they act in a similar manner. These processes the activation of DNA and the synthesis of specific protein take time to turn on (making the hormones involved “slow-acting” hor-mones), but their effects are longer lasting than those generated by “second messengers” .

Second Messengers


Many hormones, because of their size or highly charged structure, cannot easily cross cell membranes. These hormones exert their effects by binding to a receptor on the membrane sur-face and activating a G protein located in the mem-brane of the cell. The G protein is the link between the hormone-receptor interaction on the surface of the membrane and the subsequent events inside the cell. The G protein may open an ion channel to allow Ca’ to enter the cell, or it may activate an enzyme in the membrane. If the G protein activates adenylate cyclase, then cyclic AMP (cyclic 3′,5′-adenosine mono-phosphate) is formed from ATP . In turn, the cyclic AMP concentration increases in the cell and activates proteins, which directly alter cellular activity. For example, this mechanism is used to break down glycogen to glucose (by activating phosphorylase) and break down triglyceride molecules to free fatty acids (by activating hormone sensitive lipase IHSL1). The cyclic AMP is inactivated by phosphodiesterase, an enzyme that converts cyclic AMP to 5’AMP Factors that inter-fere with phosphodiesterase activity, such as caffeine.



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