Cell-Surface Receptors: Three Domains
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A cell-surface receptor spans the membrane with an extracellular domain that binds ligand, a transmembrane domain, and an intracellular domain that passes the signal onward.
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The extracellular domain binds ligand outside the cell, so the ligand never has to cross the membrane.
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The receptor converts a signal at the membrane — the ligand's job ends at binding, not delivery inward.