AP Biology — Cell Signaling & Gene Expression

Cell communication is one of the highest-yield AP Biology topics. Signaling occurs in three stages. Reception: a signaling molecule (ligand) binds to a specific receptor protein on the target cell. This binding is specific—only the correct ligand fits the receptor (like a lock and key). Receptors can be on the cell surface (for hydrophilic/polar ligands like peptide hormones and neurotransmitters) or inside the cell (for hydrophobic ligands like steroid hormones that cross the membrane freely). Transduction: binding of the ligand changes the shape of the receptor, triggering a series of molecular changes—the signal transduction cascade. This cascade often involves second messengers (like cAMP or Ca²⁺) and relay proteins that pass the signal through the cell. Response: the cascade ultimately triggers a cellular response—which may be a change in gene expression (activation of transcription factors), a change in enzyme activity, or a change in cell behavior (like contraction or secretion). Crucially, cells can respond differently to the same signal depending on what receptor proteins and relay molecules they possess.