Cell Communication and Homeostasis

Population

A group of individuals of the same species living in a defined area at the same time

Signaling molecule

A chemical messenger used to convey information between cells or organisms

Target cell

A cell that has specific receptors for a signaling molecule and responds to that signal.

Ligand

A molecule that binds specifically to a receptor to initiate a cellular response.

Quorum sensing

A system by which bacteria find their population density through signal molecules to coordinate behaviors.

Hormone

A signaling molecule produced by an endocrine gland that travels through the bloodstream to act on distant cells.

Plasmodesmata

Microscopic holes through cell walls in plants that connect adjacent cells and allow transport and communication.

Sensor

A protein or cell component that detects a signal and initiates a response.

Set point

The normal or desired value for a regulated variable around which feedback mechanisms act.

Negative feedback

A control mechanism that reduces the deviation from a set point, restoring stability.

Positive feedback

A control mechanism that reinforces a process, pushing the system further from the set point.

Amplification

The process by which a single signaling event leads to a larger downstream response.

G protein-coupled receptor protein

A large family of cell-surface receptors that activate G proteins upon ligand binding, triggering intracellular signaling.

Second messenger

A small, non-protein molecule or ion that relays signals inside the cell after receptor activation.

Cyclic adenosine monophosphate (cAMP)

A second messenger produced from ATP by adenylyl cyclase that activates protein kinases and other targets.

Kinase

An enzyme that adds a phosphate group to a substrate, thereby altering its activity.

Signaling cascade

A series of sequential biochemical events in which each step activates the next, amplifying the initial signal.

Receptor-protein kinase

A receptor that itself has intrinsic kinase activity, phosphorylating target proteins upon ligand binding.

Ligand-gated channel

A membrane channel that opens or closes in response to a ligand binding, altering ion flow and membrane potential.

Gene expression

The process by which information in a gene is used to synthesize a functional gene product, often regulated by signaling pathways.

Mutation

A heritable change in DNA sequence that can alter gene function or regulation.

Agonist

A molecule that binds a receptor and activates it to produce a biological response.

Antagonist

A molecule that binds a receptor but does not activate it, blocking or dampening the effect of an agonist.

Blood sugar regulation

(negative feedback) After a meal, blood glucose rises; in response, insulin is released to lower glucose levels, returning them toward the set point. If glucose drops too low, glucagon is released to raise glucose, again restoring the set point.

Labor/birth

(positive feedback) Uterine contractions trigger the release of oxytocin, which intensifies contractions, leading to more oxytocin release and increasingly strong contractions until birth.