BB ch 11/12

Biological membrane

A dynamic, lipid-bilayer structure that encloses cells and organelles, controlling passage of substances and information.

Transport

Movement of molecules or ions across biological membranes by passive or active mechanisms.

Biosignaling

Cellular communication process in which external or internal signals are detected and converted into specific biochemical responses.

Lipid composition

The particular mixture of lipids in a membrane that determines its physical properties and functions.

Cholesterol

A sterol abundant in plasma membranes that modulates fluidity and forms part of lipid rafts.

Plasma membrane

The outer membrane of a cell, rich in cholesterol and glycolipids, separating cytosol from the environment.

Mitochondrial membrane

Membrane surrounding mitochondria, low in cholesterol and specialized for energy transduction.

Glycerophospholipid

A major phospholipid class containing glycerol, two fatty acids, and a phospho-head group.

Sphingolipid

Lipid with a sphingosine backbone, often enriched in lipid rafts and outer leaflets.

Sodium dodecyl sulfate (SDS)

Anionic detergent that forms micelles and can disrupt membranes and denature proteins.

Free fatty acid

Unesterified fatty acid that can self-associate or integrate into membranes.

Lipid bilayer

Two leaflets of amphipathic lipids forming the basic membrane barrier.

Bilayer thickness

Approximate 30–50 Å distance across the hydrophobic core of a membrane.

Liposome

Artificial spherical vesicle with a lipid bilayer enclosing an aqueous compartment.

Fluid mosaic model

Concept describing membranes as fluid lipid bilayers with freely diffusing proteins.

Integral membrane protein

Protein embedded within the bilayer, often spanning it with hydrophobic helices.

Lateral diffusion

Side-to-side movement of lipids or proteins within one leaflet of a membrane.

Transverse diffusion

Flip-flop movement of lipids from one leaflet to the other, normally slow without enzymes.

Carbohydrate moiety

Sugar group covalently attached to membrane proteins or lipids, exposed extracellularly.

Membrane asymmetry

Unequal distribution of specific lipids and proteins between the two bilayer leaflets.

Phosphatidylserine (PS)

Negatively charged phospholipid normally on inner leaflet; externalization signals clotting or apoptosis.

Blood clotting signal

Exposure of PS on platelet outer surface, triggering coagulation cascade.

Programmed cell death (apoptosis) signal

Cell-surface PS presentation marking cells for phagocytic removal.

Peripheral membrane protein

Protein loosely attached to membrane surfaces via electrostatic or hydrogen bonds.

Amphitropic protein

Protein that reversibly associates with membranes in response to regulatory signals.

Detergent extraction

Use of surfactants to solubilize and isolate integral membrane proteins.

Calcium-dependent membrane association

Binding of certain proteins to membranes triggered by Ca²⁺ ions.

Chelating agent

Molecule such as EDTA that removes divalent cations, releasing some peripheral proteins.

Urea treatment

Disrupts hydrogen bonding to detach peripheral proteins without destroying bilayer.

Integral protein hydrophobic domain

Transmembrane segment rich in non-polar residues anchoring the protein in lipid.

Glycosylphosphatidylinositol (GPI) anchor

Glycolipid that tethers proteins to the extracellular face of membranes.

Lipid-linked protein

Protein covalently attached to one or more lipid groups for membrane anchoring.

Glycophorin

Single-pass integral protein of erythrocytes containing sialic acid–rich carbohydrates.

Transmembrane helix

α-Helical segment that spans the lipid bilayer once.

Positive-inside rule

Tendency for basic residues to occur on cytosolic side of transmembrane proteins.

Fe²⁺ uptake channel

Membrane transporter facilitating ferrous iron import, obeying positive-inside rule.

Phospholipase A

Enzyme that cleaves fatty acids from phospholipids, sometimes membrane-bound.

Dimeric transporter

Transport protein functioning as two associated subunits.

Trimeric transporter

Membrane transporter composed of three subunits forming a pore or carrier.

Bacteriorhodopsin

Light-driven proton pump with seven transmembrane helices in archaea.

Liquid-ordered state

Membrane phase with ordered acyl chains but lateral lipid mobility.

Liquid-disordered state

More fluid membrane phase with less acyl chain order.

Lipid raft

Cholesterol- and sphingolipid-rich microdomain that organizes signaling proteins.

Cholesterol enrichment

High concentration of cholesterol in particular membrane areas, promoting order.

Sphingolipid enrichment

Accumulation of sphingolipids that helps create lipid rafts.

GPI-anchored protein clustering

Gathering of GPI-linked proteins within lipid rafts.

Membrane fusion

Coalescence of two lipid bilayers into one continuous membrane.

Caveolin

Membrane protein binding cholesterol to form caveolae invaginations.

Caveolae

Flask-shaped plasma-membrane invaginations rich in caveolin and cholesterol.

Membrane curvature

Bending of bilayers, often induced by caveolin or protein scaffolds.

Electrochemical potential

Combined effect of concentration gradient and membrane voltage on ion movement.

Membrane potential (Vm)

Electrical voltage difference across a membrane, typically −50 to −70 mV in cells.

Concentration gradient

Difference in solute concentration across a membrane.

Electrochemical gradient

Spatial variation in both charge and concentration that stores potential energy.

Passive transport

Solute movement down its electrochemical gradient without energy input.

Transporter protein

Membrane protein that facilitates solute passage by specific binding and conformational change.

Dehydration of solute

Removal of water shell when a transporter binds an ion or molecule.

Hydrophilic transmembrane pathway

Polar channel provided by transporters to allow solutes through hydrophobic core.

GLUT1

Erythrocyte glucose uniporter mediating facilitated diffusion of D-glucose.

Facilitated diffusion

Carrier-mediated passive transport of solutes across membranes.

Chloride–bicarbonate exchanger

Erythrocyte antiporter swapping HCO₃⁻ and Cl⁻ without changing Vm.

Anion exchange protein

Alternative name for chloride-bicarbonate exchanger.

CO₂ transport

Movement of carbon dioxide in blood facilitated by conversion to HCO₃⁻ and Cl⁻ exchange.

Antiporter

Transport system moving two solutes in opposite directions.

Symporter

Transporter that moves two substrates in the same direction.

Uniporter

Transport protein that carries a single type of molecule across the membrane.

Primary active transport

Energy-driven solute movement directly coupled to ATP hydrolysis or light.

Secondary active transport

Use of an existing ion gradient to drive uphill movement of another solute.

Sodium pump

Na,K-ATPase that expels Na⁺ and imports K⁺ using ATP.

Na,K-ATPase

P-type ATPase maintaining high intracellular K⁺ and low Na⁺ levels.

Membrane potential maintenance

Role of Na,K-ATPase in establishing negative cytosolic voltage.

ATP hydrolysis

Exergonic reaction supplying energy for active transport.

Ouabain

Plant-derived steroid inhibitor of Na,K-ATPase linked to hypertension.

Hypertension

Chronic high blood pressure, partly from sodium pump inhibition in vessels.

Ca²⁺ accumulation

Rise of intracellular Ca²⁺ when Na,K-ATPase is blocked, promoting vasoconstriction.

Proton gradient

Difference in H⁺ concentration across a membrane, powering ATP synthesis.

FoF₁ ATPase

Complex that uses proton motive force to synthesize or hydrolyze ATP.

ATP synthase

Enzyme converting ADP + Pi to ATP using energy of H⁺ flow.

Aquaporin

Tetrameric water channel permitting rapid, selective water transport.

NPA motif

Asn-Pro-Ala sequence repeated in aquaporins forming constriction sites.

Signal-transducing system

Molecular machinery converting a signal into a cellular response.

Specificity in signaling

Selective binding of a signal to its matching receptor.

Amplification

Cascade process in which one receptor event activates many downstream molecules.

Desensitization

Decreased receptor responsiveness upon continuous stimulation.

Adaptation

Cell’s adjustment restoring sensitivity after prolonged exposure to a signal.

Integration of signals

Combination of multiple signaling inputs to set a balanced cellular response.

Second messenger

Small intracellular molecule relaying signals from receptors to targets.

Antigen

Foreign molecule recognized by the immune system as a signal.

Extracellular matrix component

Structural molecule outside cells that can act via integrin receptors.

Growth factor

Protein hormone that stimulates cell proliferation or differentiation.

Hormone

Chemical messenger secreted into blood to regulate distant cells.

Light as signal

Photons acting on photoreceptors to trigger biological responses.

Mechanical stimulus

Physical touch or pressure sensed by mechanoreceptors.

Neurotransmitter

Chemical released by neurons to signal across synapses.

Nutrient signal

Metabolite level changes that modulate cellular pathways.

Odorant

Volatile molecule detected by olfactory receptors.

Pheromone

Chemical secreted by an organism to influence another of the same species.

Tastant

Compound that elicits a taste sensation via taste receptors.

G protein–coupled receptor (GPCR)

Seven-helix receptor that activates heterotrimeric G proteins upon ligand binding.

Heterotrimeric G protein

GTP-binding protein with α, β, γ subunits regulating effectors downstream of GPCRs.