cell signalling

Meaning: Cell signalling is how a cell detects an extracellular signal (a ligand) and converts it into a response: reception, transduction, response. Polar or large ligands, such as peptide hormones, typically cannot cross the membrane and bind surface receptors: ligand-gated ion channels, G protein-coupled receptors, or receptor tyrosine kinases. These relay the message through second messengers such as cAMP, IP₃, DAG, and Ca²⁺, and enzyme cascades amplify it. Lipid-soluble ligands such as steroid hormones typically cross the membrane and bind intracellular receptors that alter transcription. Range varies: endocrine (blood), paracrine (neighbors), autocrine (same cell), or juxtacrine (direct contact).

On the MCAT: Passages give a pathway plus a change, such as an inhibitor, a mutant receptor, or a toxin that locks a G protein on. You predict whether cAMP, kinase activity, or gene expression rises or falls. Questions also match receptor location to ligand solubility and ask why a response is fast (channels, phosphorylation) or slow (new transcription).

Common confusion: The ligand is the first messenger and need not enter the cell; a second messenger carries the signal inside. Receptor location follows ligand solubility, not the hormone's importance.