Water-Soluble Ligands
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A water-soluble ligand is polar, and either cannot cross the plasma membrane at all or cannot cross it unaided, so it binds a cell-surface receptor's extracellular domain instead.
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Peptide hormones and small water-soluble signaling proteins bind exclusively at the cell surface, never entering the target cell.
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Polarity, not size alone, keeps a water-soluble ligand out of the membrane's hydrophobic core.