Cell signaling

Cell signaling is the process by which cells detect signals and convert them into responses, coordinating behavior within and between organisms.

Receptor (biochemistry): A molecule, often a protein, that binds a specific signal and initiates or alters a cellular response. Receptors detect extracellular signals and start many signaling pathways.
Ligand (biochemistry): A molecule that binds selectively to another molecule, such as a receptor. Many cell signals act as ligands that bind receptors.
Hormone: A signaling molecule produced by cells and transported to affect target cells, often at a distance. Hormones coordinate responses among distant tissues through signaling.
Endocrine signaling: Long-distance communication in which hormones travel through circulation to affect target cells. It contrasts with locally restricted forms of cell communication.
Signal transduction: The conversion of a signal received by a cell into a sequence of intracellular molecular events. It describes the intracellular relay connecting signal detection to a response.
Phosphorylation: The addition of a phosphate group to a molecule, often regulating protein function. Phosphorylation serves as a reversible switch in many signaling pathways.
Synaptic transmission: The transfer of information from a neuron to another cell across a synapse. Neurons use rapid chemical signaling to communicate with target cells.
Paracrine signaling: Local communication in which a cell releases signals that affect nearby cells. Its short-range action differs from hormone delivery across the body.
Second messenger: A small intracellular molecule or ion that relays signals from activated receptors to cellular targets. Second messengers spread and amplify signals inside the cell.
G protein: A guanine-nucleotide-binding protein that switches between active and inactive states to regulate cellular processes. G proteins relay signals from many cell-surface receptors to intracellular effectors.

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