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Cell-surface receptors span the plasma membrane and bind ligands outside the cell. What is true of the ligand in this arrangement?
AThe ligand does not have to enter the cell that it affects
BThe ligand must cross into the cytoplasm to trigger a response
CThe ligand must bind directly to nuclear DNA to work
DThe ligand is degraded inside the nucleus before use
Answer & Solution
Correct answer: A. The ligand does not have to enter the cell that it affects
1. Cell-surface receptors are transmembrane proteins that bind external ligand molecules.
2. Ligands that interact with these receptors do not have to enter the cell that they affect.
3. This differs from internal receptors, where the ligand crosses the membrane and enters the cytoplasm.
4. Binding directly to nuclear DNA describes the endpoint of internal receptor signaling, not the ligand-receptor interaction at the cell surface.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 9 "Cell Communication", section 9.1 Signaling Molecules and Cellular Receptors_
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