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【多选题】
Explain the three main classes of c ell-surface receptors.A.
All cell-surface receptor proteins bind to an extracellular signal molecule and transduce its message into one or more intracellular signaling molecules that alter the cell’s behavior. Most of these receptors belong to one of three large classes, which differ in the transduction mechanism they use.
B.
1. Ion-channel-coupled receptors change the permeability of the plasma membrane to selected ions, thereby altering the membrane potential and, if the conditions are right, producing an electrical current.
C.
2. G-protein-coupled receptors activate membrane-bound, trimeric GTP-binding proteins (G proteins), which then activate (or inhibit) an enzyme or an ion channel in the plasma membrane, initiating an intracellular signaling cascade.
D.
3. Enzyme-coupled receptors either act as enzymes or associate with enzymes inside the cell; when stimulated, the enzymes can activate a wide variety of intracellular signaling pathways. Many enzyme-coupled receptors have their own enzyme activity, while others rely on an enzyme that becomes associated with the activated receptor.
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A.
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B.
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C.
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D.
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A.
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B.
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C.
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D.
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