Cell Communication and Signaling Pathways

Comprehensive practice flashcards defining core vocabulary, signaling mechanisms, receptor classes, second messengers, and pathways in cellular communication.

Endocrine signaling

A form of long-distance cell communication where signaling molecules, such as hormones like estrogen and testosterone, travel through the circulatory system to reach distant target cells.

Paracrine signaling

A form of short-distance cell communication where a cell secretes local regulator molecules, such as growth factors, that stimulate nearby target cells to grow and divide.

Synaptic signaling

A specialized, very short-distance type of paracrine signaling occurring in the animal nervous system where an electrical signal triggers the release of neurotransmitters that diffuse across a synapse to a target cell.

Autocrine signaling

A mode of cell signaling in which both the signaling cell and the responding cell are the same cell; important during embryonic development.

Contact-dependent signaling

Direct intercellular communication that occurs when a transmembrane protein on one cell functions as the signal and binds to a receptor protein on an adjacent cell.

Gap junctions

Specialized cell junctions in animal cells that directly connect adjacent plasma membranes, allowing direct chemical and signaling molecule communication between cells.

Plasmodesmata

Microscopic channels traversing the cell walls of plant cells that provide direct cytoplasmic connections and communication between adjacent plant cells.

Reception

The first stage of cell signaling in which a target cell detects a signaling molecule that binds to a specific receptor protein located on the cell surface or within the cell.

Signal transduction

The second stage of cell signaling where the binding of a signaling molecule alters the receptor and initiates a pathway of molecular steps to transmit the signal into the interior of the cell.

Cellular response

The final stage of cell signaling (also termed the output response) where the transduced signal triggers a specific cellular activity, such as activating an enzyme or regulating gene transcription.

Intracellular receptor

A receptor protein located in the cytoplasm or nucleus that binds small, nonpolar signaling molecules (such as steroid hormones) that can freely cross the plasma membrane.

Cell-surface receptor

A transmembrane protein receptor that binds polar signaling molecules unable to cross the cell membrane, undergoing a conformational change to transmit the signal inward.

G protein-coupled receptor (GPCR)

The largest family of cell-surface transmembrane receptors that span the plasma membrane and interact with an intracellular G protein to relay external signals.

G protein

An intracellular membrane-associated protein with $\alpha$, $\beta$, and $\gamma$ subunits that binds GDP when inactive and associates with GTP upon activation by an activated GPCR.

Receptor tyrosine kinase (RTK)

A membrane receptor kinase that transfers phosphate groups from ATP to another protein, playing a central role in signaling pathways such as cell growth and pigment production.

Dimerization

The pairing together of two receptor kinase subunits upon ligand binding, enabling each receptor member to attach phosphate groups to the other member to create intracellular binding sites.

Ligand-gated ion channel

A membrane receptor that functions as a gate, opening or closing to regulate ion flow across the plasma membrane when bound by a specific chemical signaling molecule.

Protein kinase

An enzyme that catalyzes phosphorylation by transferring a phosphate group from ATP to a target protein, typically converting an inactive protein into an active state.

Protein phosphatase

An enzyme that catalyzes dephosphorylation by removing a phosphate group ($P_i$) from an active protein, thereby returning it to an inactive state.

Kit

A receptor kinase responsible for pigment production in skin, feathers, scales, and hair, which leads to patchy pigmentation loss across species when mutated.

Adenylyl cyclase

A plasma membrane enzyme activated by the active $\alpha$ subunit of a G protein that converts ATP into cyclic AMP (cAMP) and pyrophosphate.

Cyclic AMP (cAMP)

A common water-soluble second messenger synthesized from ATP by adenylyl cyclase that activates protein kinase A (PKA) to propagate cellular signaling.

Phosphodiesterase

A cytosolic enzyme that hydrolyzes cyclic AMP (cAMP) into AMP in the presence of $H_2O$, serving as an off-switch that terminates cAMP-mediated signaling.

Phospholipase C

A membrane enzyme activated by a G protein that cleaves the membrane phospholipid $PIP_2$ to generate two second messengers: diacylglycerol (DAG) and inositol triphosphate ($IP_3$).

Inositol triphosphate ($IP_3$)

A water-soluble second messenger produced by phospholipase C that diffuses through the cytosol and binds to open $IP_3$-gated calcium channels on the endoplasmic reticulum.

Diacylglycerol (DAG)

A lipid second messenger produced alongside $IP_3$ from the cleavage of $PIP_2$ by phospholipase C that remains embedded in the plasma membrane to activate downstream target proteins.

Scaffolding protein

A large relay protein to which multiple other relay proteins, such as several different protein kinases, are simultaneously attached to enhance the speed and efficiency of signaling pathways.

Adrenocorticotropic hormone (ACTH)

A hormone secreted by the pituitary gland during the fight-or-flight response following signal processing in the amygdala and hypothalamus, leading to the downstream release of cortisol and adrenaline.