Molecular Organization and Transport Mechanisms of Cell Membranes

Comprehensive vocabulary flashcards covering the molecular ultrastructure of cell membranes, glycocalyx, cell surface receptors, signaling cascades, passive/active transport systems, and vesicular trafficking.

Fluid Mosaic Model

The structural model of cellular membranes describing a dynamic phospholipid bilayer with a hydrophobic core and hydrophilic polar heads, embedded with mobile integral and peripheral proteins, glycoproteins, and glycolipids.

P-face (Protoplasmic Face)

The interior fracture half of the split lipid bilayer in freeze-fracture electron microscopy that faces the protoplasm (cytosol).

E-face (Extracellular Face)

The fracture half of the split lipid bilayer produced by freeze-fracture electron microscopy that faces the extracellular space or exterior environment.

Basculement (Flip-Flop)

The slow, transverse movement of membrane lipid molecules from one monolayer leaflet across the bilayer to the opposite leaflet.

Lateral Diffusion (Difuziune Laterală)

The rapid, spontaneous sideways translation of lipid molecules and intrinsic proteins within the plane of the same monolayer leaflet.

Glycocalyx

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

Sialic Acid

A terminal negatively charged sugar component of the glycocalyx responsible for binding and storing cations such as $Ca^{2+}$.

Cellular-Type Antigens

Terminal glucides (carbohydrates) of membrane glycoproteins and glycolipids that establish the individual identity and immunological specificity of a cell.

Autocrine Mediators (Médiateurs Autocrines)

Signal molecules released by a cell that bind to specific receptors situated on the surface of the very same cell to elicit a response.

Paracrine Mediators (Médiateurs Paracrines)

Local chemical regulators secreted into the extracellular interstitial fluid that diffuse locally to act on neighbouring target cells.

Endocrine Mediators (Hormones)

Signaling molecules synthesized by endocrine cells and secreted into the bloodstream to travel to distant target tissues throughout the organism.

Fluoxetine (Prozac)

A therapeutic agent that acts at the synaptic cleft by blocking the reabsorption (reuptake) of the neurotransmitter serotonin by the presynaptic neuron.

Calmodulin

An intracellular $\alpha$-helix-loop-helix regulatory protein containing four EF-hand motifs; binding of four $Ca^{2+}$ ions induces conformational activation to bind and regulate target proteins.

G-Protein-Coupled Receptors (GPCRs)

A major class of cell surface receptors consisting of seven membrane-spanning $\alpha$-helical domains that transduce signals by activating intracellular heterotrimeric G proteins.

Robert Lefkowitz and Brian Kobilka

Researchers awarded the 2012 Nobel Prize in Chemistry for their seminal work elucidating the structure, function, and molecular mechanisms of G-protein-coupled receptors.

Cholera Toxin

An enterotoxin produced by Vibrio cholerae that binds cell-surface gangliosides and permanently activates adenylyl cyclase, sharply increasing cAMP and causing massive efflux of $Cl^-$, $Na^+$, and $H_2O$, resulting in life-threatening dehydration.

Symport

A form of coupled secondary active transport in which a carrier protein moves two distinct solute species simultaneously across the membrane in the same direction.

Antiport

A form of coupled transport across a membrane in which the translocation of one solute species in one direction drives the movement of another solute in the opposite direction.

Gramicidin

A channel-forming ionophore that embeds within the lipid bilayer to create a transmembrane pore allowing the selective passive passage of $Na^+$ ions.

Valinomycin

A mobile carrier (shuttle / navette) ionophore that encapsulates a dehydrated $K^+$ ion within a hydrophobic ring to diffuse it across the lipid bilayer.

Band 3

An erythrocyte integral membrane glycoprotein carrier that mediates the electroneutral facilitated exchange of $HCO_3^-$ and $Cl^-$ across the red blood cell membrane.

Aquaporin 1 (AQP1)

A specialized tetrameric channel-forming protein that facilitates the rapid, selective passage of water molecules across biological membranes while preventing the movement of ions.

Bacteriorhodopsin

A light-driven, seven-transmembrane-helix proton pump containing retinal that spans a $3\,\text{nm}$ lipid bilayer hydrophobic core and uses light energy to transport $H^+$ out of the cytoplasm.

P-type Pump

An ATP-driven ion transporter family that autophosphorylates a conserved aspartate residue during the reaction cycle to transport ions such as $H^+$, $K^+$, $Na^+$, or $Ca^{2+}$.

SERCA2 ($Ca^{2+}$-ATPase)

A P-type ATP-driven pump located in the sarcoplasmic reticulum membrane of muscle cells that pumps two $Ca^{2+}$ ions from the cytoplasm into the SR lumen per ATP hydrolyzed, inducing muscle relaxation.

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

An electrogenic P-type ATPase that utilizes one ATP to export $3\,Na^+$ and import $2\,K^+$, controlling cellular volume and establishing a negative membrane potential between $-20\,\text{mV}$ and $-200\,\text{mV}$.

Ouabain

A cardiotonic glycoside that selectively binds to the extracellular potassium-binding site of the $Na^+/K^+$ ATPase, inhibiting its active ion transport activity.

ABC Transporters

A transport superfamily possessing transmembrane domains (TMDs) and nucleotide-binding domains (NBDs) that hydrolyze ATP to transport various small molecules or ions across biological membranes.

CFTR (Cystic Fibrosis Transmembrane Conductance Regulator)

An ABC-family member functioning as a regulated chloride ($Cl^-$) channel whose gene defect most commonly involves a specific deletion of a phenylalanine residue.

$Na^+/\text{Glucose}$ Symporter

A secondary active symporter that drives the uptake of glucose against its gradient coupled to the downhill inward translocation of $Na^+$ along an electrochemical gradient ($[Na^+]_{\text{out}} = 0.14\,\text{M}$ vs. $[Na^+]_{\text{in}} = 0.014\,\text{M}$).

F-Type ATP Synthase

A turbine-like proton-driven multi-subunit complex that normally operates in reverse mode in mitochondria and chloroplasts to generate ATP on the exoplasmic (matrix) face driven by an electrochemical proton gradient.

V-Type Proton Pump

An ATP-dependent proton transporter found in vacuolar and lysosomal membranes that hydrolyzes ATP to pump $H^+$ into lumens and maintain organelle acidification.

TOM and TIM Complexes

Translocases of the outer and inner mitochondrial membranes that orchestrate the direct translocation of nuclear-encoded precursor proteins across the mitochondrial envelope.

Importin

A nuclear transport receptor protein that binds nuclear localization signals (NLS) on target proteins in the cytosol to chaperone them through the nuclear pore complex into the nucleus.

Bacterial Conjugation

The direct transfer of genetic material (DNA) between two bacterial cells via direct cytoplasmic connection through a conjugation pilus.

SNARE Complex

A superfamily of membrane-bridging proteins that assemble into trans-complexes between vesicular and target membranes to drive membrane apposition and fusion pore formation.

Cellular Defecation

The exocytic evacuation of undigested, insoluble residual materials from residual bodies (secondary lysosomes) out into the extracellular space.