AP Biology Unit 4 Notes: How Cells Use Signals to Coordinate Life

Cell communication

The process by which cells detect and respond to signals from their environment or from other cells to coordinate activities like growth, division, secretion, contraction, or programmed cell death.

Signal

Any detectable factor that triggers a cellular response; can be chemical (e.g., ligands) or non-chemical (e.g., light, mechanical pressure).

Ligand

A signaling molecule (such as a hormone or growth factor) that binds specifically to a receptor protein to initiate a response.

Receptor

A (usually) protein that binds a specific signal/ligand and undergoes a conformational change that initiates intracellular signaling; receptor presence helps determine whether a cell responds.

Contact-dependent (juxtacrine) signaling

Signaling that requires physical contact between cells, where a membrane-bound ligand on one cell binds a receptor on an adjacent cell.

Paracrine signaling

Local signaling in which a cell releases molecules that diffuse through extracellular fluid to nearby target cells; effects remain local due to limited diffusion and/or rapid breakdown.

Synaptic signaling

A specialized local signaling used by neurons in which neurotransmitters are released into a synaptic cleft and act rapidly and specifically on a nearby target cell.

Neurotransmitter

A chemical signal released by a neuron into the synaptic cleft that binds receptors on a target cell to transmit a fast, specific signal.

Endocrine signaling

Long-distance signaling in which endocrine cells release hormones into the bloodstream (circulatory system) to reach distant target cells.

Hormone

A signaling molecule carried through the bloodstream in endocrine signaling; only cells with the appropriate receptor respond.

Autocrine signaling

Signaling in which a cell releases a signal that binds to receptors on the same cell (or same cell type), often reinforcing the response.

Signal transduction

The process by which a cell converts an external signal into a specific internal response, typically via a chain of molecular events rather than the ligand doing the work directly.

Reception

The stage of signaling in which a ligand (signal) binds to its receptor.

Transduction

The stage of signaling in which receptor activation triggers a series of intracellular steps (often a cascade) that relay and process the signal.

Response

The final outcome of signaling, such as changes in gene expression, enzyme activity, cytoskeleton behavior, secretion, or other cellular actions.

Cell-surface receptor

A plasma-membrane receptor used for ligands that are large and/or polar and cannot cross the hydrophobic lipid bilayer easily.

Intracellular receptor

A receptor in the cytoplasm or nucleus that binds small, nonpolar (often hydrophobic) ligands that can diffuse across the membrane; the complex often affects gene expression.

G protein-coupled receptor (GPCR)

A membrane receptor that, after ligand binding, activates a G protein on the cytoplasmic side to trigger downstream signaling (often including second messengers and kinase cascades).

Receptor tyrosine kinase (RTK)

A membrane receptor with an intracellular kinase domain that typically dimerizes upon ligand binding and becomes phosphorylated, creating docking sites for relay proteins.

Ligand-gated ion channel

A membrane protein that opens or closes in response to ligand binding, allowing specific ions to cross the membrane and rapidly change membrane potential or ion concentration.

Protein kinase

An enzyme that transfers a phosphate group from ATP to a protein (phosphorylation), often propagating signals through phosphorylation cascades.

Protein phosphatase

An enzyme that removes phosphate groups from proteins, helping terminate/reset signaling pathways by reversing kinase effects.

Second messenger

A small, non-protein molecule or ion that spreads a signal inside the cell; can diffuse quickly and be produced in large amounts to extend/amplify signaling.

Amplification (in signaling)

A feature of many pathways where activation of one receptor leads to activation/production of many downstream molecules (e.g., via enzymes or second messenger generation), producing a large response.

Desensitization

A reduction in cellular responsiveness after prolonged stimulation, often via receptor internalization or receptor modification that decreases activation.