Molecular Organization of Cell Membranes, Receptors, and Transport Mechanisms

Vocabulary practice flashcards covering cell membrane ultrastructure, glycocalyx, cell signaling receptors, passive and active transport mechanisms, ion pumps, macromolecular translocation, and vesicular export.

Fluid Mosaic Model

The molecular model describing cellular membrane ultrastructure as a dynamic phospholipid bilayer with intercalated integral and peripheral proteins possessing mobility within the lipid matrix.

P-face (Protoplasmic Face)

The interior fracture face of the inner leaflet of the cell membrane that remains facing the cytoplasm during freeze-fracture preparation.

E-face (Extracellular Face)

The interior fracture face of the outer leaflet of the split cell membrane that borders the extracellular space in freeze-fracture preparations.

Lateral Diffusion

The rapid two-dimensional movement of membrane lipids and proteins within the plane of the same bilayer leaflet, demonstrated experimentally by the intermixing of surface proteins following human-mouse cell fusion.

Flip-Flop (Transverse Diffusion)

The extremely slow and thermodynamically unfavorable movement of a phospholipid molecule from one membrane leaflet across the hydrophobic core to the opposing leaflet.

Glycocalyx

A carbohydrate-rich peripheral zone on the external surface of the plasmalemma formed by oligosaccharide side chains covalently linked to membrane glycoproteins and glycolipids.

Cellular-Type Antigens

Terminal glucidic (carbohydrate) sequences located on membrane glycoproteins and glycolipids that establish immunological cellular identity.

Paracrine Mediators

Local chemical messengers secreted into the extracellular fluid that diffuse short distances to act selectively upon adjacent target cells.

Endocrine Mediators

Hormones produced by specialized endocrine glands that are released directly into the bloodstream to reach distant target tissues throughout the body.

Autocrine Mediators

Locally acting signaling molecules that are secreted into the interstitial space and bind to specific surface receptors on the very cell that produced them.

Robert Leftkowitz & Brian Kobika

Researchers awarded the 2012 Nobel Prize in Chemistry for their seminal work and discoveries concerning the structure and function of G-protein-coupled receptors (GPCRs).

Calmodulin

An $\alpha$-helix-loop-helix protein equipped with four EF-hand motifs that undergoes a conformational change upon binding four $Ca^{2+}$ ions, enabling it to bind and activate target regulatory proteins.

Cholera Toxin

An enterotoxin produced by Vibrio cholerae that binds cell surface gangliosides and persistently activates adenylate cyclase, resulting in excessive cAMP production, chloride and sodium secretion, and severe dehydrating watery diarrhea.

Uniport

A carrier-mediated transport mechanism that translocates a single substance across the membrane down its concentration gradient without coupling to another solute.

Symport

A form of coupled transport in which a carrier protein moves two distinct solutes across the cell membrane simultaneously in the exact same direction.

Antiport

A coupled membrane transport process in which the movement of one solute across the membrane in one direction is linked to the simultaneous movement of another solute in the opposite direction.

Ionophores

Lipophilic carrier molecules or channel formers—such as valinomycin for $K^+$ or gramycidin for $Na^+$—that increase the permeability of lipid bilayers to specific inorganic ions.

Band 3 Protein

An integral erythrocyte membrane transport protein consisting of membrane-spanning segments and a $40\,\text{kDa}$ cytosolic domain that mediates electroneutral $HCO_3^- / Cl^-$ anion exchange.

Aquaporin 1 (AQP 1)

A specialized membrane channel protein that facilitates the rapid, selective passage of water molecules across cellular membranes while blocking the translocation of ions and protons.

Voltage-Gated Ion Channel

An ion-selective transmembrane pore that transitions between closed, open, and inactivated states in direct response to alterations in the electrical potential across the membrane.

Bacteriorhodopsin

A light-driven proton pump containing seven transmembrane helices and a retinal cofactor that generates an electrochemical proton gradient across a $3\,\text{nm}$ lipid bilayer upon light absorption.

P-Type Pump

A class of ATP-driven ion pumps that undergo reversible autophosphorylation at a conserved aspartate residue during the active transport cycle, exemplified by the $Na^+/K^+$ pump and $Ca^{2+}$-ATPase.

ABC Transporters

A family of transport proteins characterized by two transmembrane domains and two cytosolic nucleotide-binding domains (NBDs) that hydrolyze ATP to translocate small molecules across biological membranes.

V-Type Proton Pump

An ATP-dependent vacuolar proton pump that acidifies intracellular compartments such as lysosomes and endosomes, as well as the extracellular space in osteoclasts and specific kidney tubule cells.

F-Type ATP Synthase

A rotary ATP-driven machine in mitochondria and chloroplast thylakoids that normally operates in reverse mode, harnessing proton flow across the membrane to synthesize ATP from ADP and inorganic phosphate.

$Na^+/K^+$ Pump ($Na^+/K^+$-ATPase)

An electrogenic P-type ATPase that utilizes the hydrolysis of one ATP molecule to export $3\,Na^+$ ions and import $2\,K^+$ ions against their concentration gradients, regulating cell volume and maintaining resting membrane potential between $-20\,\text{mV}$ and $-200\,\text{mV}$.

Ouabain

A specific cardiac glycoside that binds competitively to the extracellular potassium-binding site of the $Na^+/K^+$ pump, directly inhibiting its enzymatic activity.

SERCA2

A sarcoplasmic/endoplasmic reticulum $Ca^{2+}$-ATPase that pumps two $Ca^{2+}$ ions out of the cytoplasm and into the sarcoplasmic reticulum lumen per ATP hydrolyzed, regulated by phospholamban (PLN) to induce muscle relaxation.

CFTR (Cystic Fibrosis Transmembrane Conductance Regulator)

An ABC-family ATP-gated chloride channel containing two transmembrane domains, two nucleotide-binding domains (NBD1, NBD2), and a regulatory domain; mutations such as the deletion of phenylalanine ($\Delta F508$) cause cystic fibrosis.

Secondary Active Transport (Coupled Transport)

The active translocation of a solute (such as glucose) against its concentration gradient driven by the dissipation of an electrochemical ion gradient (such as $Na^+$) previously established by primary ATP pumps.

TOM and TIM Complexes

Multisubunit protein translocases located respectively in the outer membrane (TOM) and inner membrane (TIM) of mitochondria that coordinate the translocation and sorting of nuclear-encoded polypeptides into the mitochondrial matrix.

Bacterial Conjugation

A direct mechanism of macromolecular transfer in bacteria wherein donor cells transfer plasmid DNA to recipient acceptor cells across a specialized conjugation channel or bridge.

Bacteriophage Tail Sheath Contraction

The mechanical shortening of a bacteriophage tail sheath from 24 rings to 12 rings upon receptor binding, driving its core tube through the bacterial cell envelope to inject viral DNA directly into the host cytoplasm.

Exocytosis

A vesicular transport process wherein intracellular secretory granules or transport vesicles fuse with the plasma membrane to release their luminal contents into the extracellular environment.

Cellular Defecation

The exocytic elimination of undigested macromolecular residues and waste materials retained within secondary lysosomes (residual bodies) to the exterior of the cell.