Cell Signaling and Homeostasis

Vocabulary practice flashcards covering cell signaling pathways, reception, transduction cascades, apoptosis, and homeostatic negative and positive feedback mechanisms.

Signal Transduction Pathway

The multi-step process and sequence of biochemical reactions by which an extracellular signal bound to a receiving cell is converted into a specific cellular response.

Mating Factors

Chemical signaling molecules (such as $a$ factor and $\alpha$ factor) secreted by unicellular yeast cells of different mating types to coordinate reciprocal binding and sexual reproduction.

Cell-Cell Recognition

A form of direct contact communication in animal cells where membrane-bound surface molecules, such as glycoproteins on immune cells, interact directly with receptors on adjacent cells.

Local Regulator

A secreted signaling molecule that influences target cells in the immediate vicinity by diffusing across short distances through the extracellular fluid.

Paracrine Signaling

A type of local signaling in which a secreting cell discharges local regulator molecules (such as growth factors) into the extracellular fluid to act on neighboring target cells.

Synaptic Signaling

A specialized form of local communication in the nervous system where an electrical impulse travels along an axon, triggering the release of neurotransmitters across a synapse to stimulate a target cell.

Hormone

A chemical messenger used for long-distance signaling, released by endocrine cells into body fluids (such as blood) in animals, or moving through vessels/air in plants, to reach distant target cells.

Ligand

A signaling molecule that specifically binds to another, usually larger molecule (such as a receptor protein), typically inducing a conformational change in that receptor.

G Protein-Coupled Receptor (GPCR)

A plasma membrane receptor that undergoes a conformational change upon ligand binding, activating an associated G protein via GTP binding, which then diffuses along the membrane to activate an enzyme.

Ligand-Gated Ion Channel

A membrane receptor with a gate region that opens or closes upon the binding of a specific ligand, allowing or blocking the diffusion of ions down their concentration gradient.

Intracellular Receptor

A receptor protein located inside the cytoplasm or nucleus of target cells that binds to small, hydrophobic signaling molecules (such as testosterone and thyroid hormones) capable of traversing the plasma membrane.

Protein Kinase

An enzyme that transfers a phosphate group from ATP to an inactive target protein (phosphorylating it) to change its conformation and activate it.

Protein Phosphatase

An enzyme that removes a phosphate group from an active phosphorylated protein (dephosphorylating it), thereby inactivating the protein and turning off the signal transduction pathway.

Phosphorylation Cascade

A sequence of relay interactions where a series of protein kinases are successively phosphorylated and activated, amplifying the original signal and enabling fine regulation of cellular activities.

Second Messenger

A small, nonprotein, water-soluble molecule or ion (such as cyclic AMP or $Ca^{2+}$) that rapidly spreads throughout the cytosol via diffusion to relay a signal received by cell-surface receptors.

Cyclic AMP (cAMP)

A common intracellular second messenger converted from ATP by the enzyme adenylyl cyclase that activates protein kinase A to initiate downstream responses.

Adenylyl Cyclase

A membrane-bound enzyme activated by a G protein that catalyzes the conversion of ATP into cyclic AMP (cAMP) and inorganic pyrophosphate ($PP_i$), rapidly amplifying intracellular cAMP levels up to $20\times$.

Apoptosis

Programmed cell death triggered by death ligands, nuclear DNA damage, or ER protein misfolding, involving organelle fragmentation, DNA chopping, membrane blebbing, and clearance by scavenger cells.

Blebbing

The morphological formation of outward, lobed protrusions from the plasma membrane of a cell undergoing apoptosis prior to its packaging into vesicles.

Homeostasis

The maintenance of a relatively stable internal physiological environment in an organism through coordinated control systems and feedback mechanisms.

Negative Feedback

A homeostatic mechanism in which a physiological perturbation initiates effector responses that counteract the initial stimulus, returning the variable back to its normal set point.

Positive Feedback

A physiological control process that amplifies an initiating stimulus, driving the system progressively farther away from its set point until a specific climatic endpoint is reached.

Thermoregulation

A negative feedback mechanism that stabilizes body temperature by triggering vasodilation and sweating when temperatures rise, or vasoconstriction and shivering when temperatures fall.

Antagonistic Hormones

A pair of hormones whose physiological actions directly counter one another to precisely regulate a variable, such as insulin and glucagon regulating blood glucose levels.

Glucagon

A hormone released by pancreatic alpha cells when blood glucose is low that directs the liver to break down glycogen and release glucose into the bloodstream.

Insulin

A hormone secreted by pancreatic beta cells in response to elevated blood glucose that promotes glucose uptake by body cells and glycogen storage in the liver.

GLUT4 Transporter

A glucose transport protein stored in intracellular vesicles that translocates to the plasma membrane via exocytosis following insulin receptor activation to allow glucose entry into cells.

Oxytocin

A hormone produced by the hypothalamus and released from the posterior pituitary during labor that stimulates uterine smooth muscle contractions in a positive feedback loop.

Ethylene

A gaseous plant hormone that promotes fruit ripening and participates in positive feedback, where ripening fruit releases gas that stimulates neighboring fruit to ripen and produce more hormone.

Prolactin

A hormone that stimulates milk production after childbirth, maintained through a positive feedback loop where infant suckling stimulates ongoing hormone release.