Chemical Signaling (AHL) – Lecture Vocabulary

A comprehensive set of vocabulary flashcards covering key terms from the lecture on cellular chemical signaling, receptor types, signaling molecules, and intracellular pathways.

Ligand

A signaling molecule that selectively binds to a specific site on a receptor protein.

Receptor

A protein that contains a binding site for a ligand and, upon binding, undergoes a conformational change that initiates a cellular response.

Quorum sensing

Bacterial communication mechanism in which accumulation of signaling molecules enables population-density–dependent coordinated behavior.

Autoinducer

Small signaling molecule secreted by bacteria that diffuses between cells and triggers quorum-sensing responses once a threshold concentration is reached.

Vibrio fischeri

Marine bacterium whose quorum sensing controls bioluminescence, forming a mutualistic light organ association with the Hawaiian bobtail squid.

LuxR

Cytoplasmic receptor protein in Vibrio fischeri that binds autoinducer and activates transcription of bioluminescence genes.

Luciferase

Enzyme produced in response to LuxR activation that catalyzes an oxidation reaction emitting greenish-blue light.

Hormone

Chemical messenger produced by endocrine glands, transported via bloodstream to distant target cells to regulate physiology.

Neurotransmitter

Chemical messenger released from a presynaptic neuron that diffuses across a 20–40 nm synaptic gap to stimulate a postsynaptic cell.

Cytokine

Small protein released, often by immune cells, that binds membrane receptors on nearby cells to coordinate inflammation, defense, or development.

Calcium signaling

Use of Ca²⁺ ion concentration changes inside cells (e.g., muscle fibers, neurons) to transmit and regulate signals.

Steroid hormone

Lipid-soluble hormone derived from cholesterol (e.g., testosterone, progesterone) that diffuses through membranes to bind intracellular receptors.

Amine hormone

Hormone derived from amino acids, often water-soluble (e.g., epinephrine), binding to membrane receptors.

Peptide hormone

Hormone composed of short chains of amino acids (e.g., insulin) that is hydrophilic and binds transmembrane receptors.

Local signaling

Communication in which a cell releases molecules that affect nearby target cells, such as paracrine growth factors.

Synaptic signaling

Localized signaling where neurotransmitters are released into the synaptic gap between neurons or between neuron and target cell.

Distant signaling

Endocrine communication in which hormones travel through the circulatory system to far-away target tissues.

Transmembrane receptor

Integral membrane protein whose ligand-binding domain is extracellular and whose cytoplasmic portion transmits signals inside the cell.

Intracellular receptor

Soluble cytoplasmic or nuclear protein that binds hydrophobic ligands and directly influences gene expression.

Signal transduction pathway

Series of molecular events triggered by receptor activation that convert an extracellular signal into a specific intracellular response.

Ligand-gated ion channel

Transmembrane receptor that opens an ion channel upon ligand binding, altering membrane potential.

Membrane depolarization

Reduction in electrical potential difference across the plasma membrane, often initiated by ion influx.

Acetylcholine receptor

Ligand-gated ion channel in muscle and neuronal membranes that opens to allow Na⁺ influx when acetylcholine binds.

G protein

Membrane-associated protein that binds GDP/GTP and acts as a molecular switch to relay signals from activated receptors.

G protein–coupled receptor (GPCR)

Large family of seven-transmembrane receptors that activate heterotrimeric G proteins upon ligand binding.

Second messenger

Small intracellular molecule (e.g., cAMP, Ca²⁺) that propagates and amplifies signals inside the cell following receptor activation.

Cyclic AMP (cAMP)

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

Adenylyl cyclase

Enzyme activated by Gα-GTP that converts ATP to many molecules of cAMP.

Adrenergic receptor

GPCR that binds epinephrine or norepinephrine and triggers fight-or-flight cellular responses.

Epinephrine (Adrenaline)

Amine hormone and neurotransmitter released from adrenal medulla that prepares the body for vigorous activity via adrenergic receptors.

Tyrosine kinase

Enzyme that transfers a phosphate group from ATP to the amino acid tyrosine on target proteins.

Receptor tyrosine kinase (RTK)

Transmembrane receptor with intrinsic tyrosine kinase domains that dimerize and autophosphorylate upon ligand binding.

Insulin receptor

An RTK that, after insulin binding and autophosphorylation, initiates pathways leading to glucose transporter insertion into the plasma membrane.

Phosphorylation

Addition of a phosphate group to a molecule, often regulating protein activity.

Dimerization

Process where two receptor subunits associate to form a functional dimer, commonly initiating RTK activation.

Gene transcription factor

Protein that binds specific DNA sequences to regulate transcription; often activated by intracellular steroid hormone receptors.

Positive feedback

Regulatory mechanism in which the end product of a pathway amplifies the initial signal, increasing pathway output.

Negative feedback

Regulatory mechanism where the end product inhibits an earlier step, reducing pathway output to maintain homeostasis.

Inositol triphosphate (IP3)

Second messenger that binds ER receptors to release Ca²⁺, frequently engaging positive feedback loops.

Ethylene

Plant hormone whose production during fruit ripening promotes further ethylene synthesis in nearby fruits—an example of positive feedback.

GLUT-4 transporter

Glucose carrier protein inserted into cell membranes in response to insulin signaling, facilitating glucose uptake.

Glycogen phosphorylase

Enzyme activated by the epinephrine-cAMP pathway that breaks down glycogen to release glucose.

Hydrophilic ligand

Water-soluble signaling molecule that cannot cross the lipid bilayer and therefore binds extracellular domains of receptors.

Hydrophobic ligand

Non-polar signaling molecule that can diffuse through the lipid bilayer to bind intracellular receptors.

Signal amplification

Increase in strength or number of signaling molecules as a message is relayed, allowing a small ligand concentration to elicit a large response.