With regard to the action of hormones and neurotransmitters on cellular receptors, which of these describes "amplification"?
A. A single first-messenger molecule activates multiple second-messenger molecules, each of which activate thousands of enzymes.
B. Only hydrophilic first-messenger molecules can activate second-messenger molecules within the cell cytosol.
C. When the extracellular concentration of a chemical messenger reaches a very high level, it overwhelms transporter molecules and the chemical floods into the cell.
D. Some cellular receptors have such low affinity for chemical ligands that it can require a million or more molecules to activate them.
Answer: A
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