Cell-to-Cell Communication and Signal Transduction

Vocabulary practice flashcards reviewing cellular communication mechanisms, chemical messenger classes, synthesis, transport, and signal transduction pathways.

Gap Junction

A membrane protein structure composed of connexons that directly links the cytosol of two adjacent cells, allowing ions and small molecules to pass between them; commonly found in smooth and cardiac muscle.

Ligand

A chemical messenger produced by a source cell that travels to a target cell and binds specifically to target cell receptors to trigger a response.

Paracrine Messenger

A functional class of chemical messenger that signals nearby cells by diffusing through the interstitial fluid over short distances (e.g., histamine during an inflammation response).

Autocrine Messenger

A specialized subclass of paracrines where the chemical signals the exact same cell that secreted it, making the source and target cells identical.

Neurotransmitter

A chemical messenger produced by neurons and released by exocytosis into the extracellular fluid of the synaptic cleft to act on a postsynaptic target cell (e.g., acetylcholine, glycine, serotonin).

Hormone

A chemical messenger produced by endocrine cells that is secreted into the blood via interstitial fluid to travel long distances to target cells (e.g., insulin, estrogen, thyroxin).

Neurohormone

A special class of hormone produced by neurons and secreted into the blood via interstitial fluid for long-distance transport (e.g., antidiuretic hormone [ADH], oxytocin).

Lipophobic Ligand

A water-soluble messenger that does not readily cross the lipid bilayer of the cell membrane, requiring binding to cell-surface membrane receptors to trigger enzyme activation or permeability changes.

Lipophilic Ligand

A lipid-soluble messenger that easily crosses the cell membrane to bind to intracellular receptors located in the cytosol or nucleus, typically mediating responses via gene activation.

Amino Acid Messengers

A chemical class of four lipophobic neurotransmitters—glutamate, aspartate, glycine, and GABA—synthesized within neurons, stored in vesicles, and released via exocytosis.

Catecholamines

A group of amine chemical messengers derived from the amino acid tyrosine, including dopamine, norepinephrine (noradrenalin), and epinephrine (adrenalin).

Peptide and Protein Messengers

The most abundant chemical class of ligands; lipophobic chains of amino acids categorized as peptide ligands ($<50$ amino acids) or protein ligands ($>50$ amino acids).

Steroid Ligands

Lipophilic hormones derived from cholesterol that are synthesized on demand, diffuse freely across membranes, and bind to intracellular target receptors.

Eicosanoids

A class of lipophilic messengers synthesized on demand from arachidonic acid (a cell membrane phospholipid) via the cyclooxygenase or lipoxygenase pathways.

Cyclooxygenase Pathway

The enzymatic pathway that converts arachidonic acid into prostaglandins, prostacyclins, and thromboxanes.

Lipoxygenase Pathway

The enzymatic pathway that converts arachidonic acid into leukotrienes.

Blood-Borne Hormone Transport

The transport mechanism in which lipophobic ligands dissolve directly in plasma, whereas hydrophobic messengers travel predominantly bound to carrier proteins ($>99\%$ bound, $<1\%$ free hormone).

Messenger Half-Life

The time required for a chemical messenger's concentration in blood or interstitial fluid to decrease by half; messengers dissolved in plasma exhibit short half-lives ($<10\text{ min}$ for insulin), whereas carrier-bound messengers exhibit long half-lives ($90\text{ min}$ for cortisol).

Receptor Affinity

The binding strength between a chemical messenger and its receptor, which together with messenger concentration and receptor number determines target response magnitude.

Down-Regulation

A reduction in the number of target cell receptors that can result from chronically excess messenger, decreasing cellular sensitivity and producing tolerance.

Up-Regulation

An increase in target cell receptor density that may occur in response to prolonged deficiency of messenger, leading to heightened cellular sensitivity.

Agonist

A chemical that binds to a receptor and mimics the normal cellular response of the endogenous ligand (e.g., morphine acting on mu opiate receptors).

Antagonist

A chemical that binds to a receptor without generating a biological response, competing with the native ligand to block its effects (e.g., naloxone blocking mu opiate receptors).

Hormone Response Element (HRE)

A specific sequence of DNA in the nucleus where an activated lipophilic hormone-receptor complex binds to initiate or modulate mRNA transcription.

Channel-Linked Receptor

A fast ligand-gated channel where the receptor and ion channel form the same protein; direct binding of the ligand opens or closes the channel to alter cell electrical properties.

Enzyme-Linked Receptor

A membrane protein functioning simultaneously as a receptor and an enzyme (such as a tyrosine kinase), directly activating intracellular catalytic activity upon ligand binding.

G Protein-Linked Receptor

A membrane receptor coupled to an intracellular heterotrimeric regulatory protein ($\alpha$, $\beta$, and $\gamma$ subunits) that exchanges GDP for GTP upon activation to indirectly regulate ion channels or amplifier enzymes.

Adenylate Cyclase

A membrane-bound amplifier enzyme activated by $G_s$ proteins that synthesizes the second messenger cyclic adenosine monophosphate (cAMP) from ATP.

Protein Kinase A (PKA)

An intracellular enzyme activated by cAMP that phosphorylates target proteins using ATP, executing the specific physiological response within the cell.

Phospholipase C

A membrane amplifier enzyme activated by G proteins that hydrolyzes inositol-4,5-biphosphate ($\text{PIP}_2$) into diacylglycerol (DAG) and inositol triphosphate ($\text{IP}_3$).

Inositol Triphosphate ($\text{IP}_3$)

A water-soluble second messenger generated by phospholipase C that binds to receptors on the endoplasmic reticulum to trigger the release of stored calcium ions into the cytosol.

Diacylglycerol (DAG)

A lipid-bound second messenger generated by the cleavage of $\text{PIP}_2$ that remains in the plasma membrane and activates protein kinase C (PKC).

Calmodulin

A cytosolic calcium-binding protein that binds $\text{Ca}^{2+}$ released into the cytosol, forming a complex that activates specific protein kinases to regulate contraction, metabolism, and transport.

Signal Amplification

The cascade phenomenon in second messenger systems where a single messenger molecule leads to the progressive activation of thousands of downstream enzymes and the phosphorylation of millions ($2,500,000$) of proteins.

Nervous vs. Endocrine Long-Distance Communication

The nervous system transmits signals via action potentials and synapses with immediate, brief effects on specific cells; the endocrine system secretes hormones into the bloodstream with delayed, long-lasting effects on widespread target cells.