BIOL 3030 – Membrane Transport - LEC 5- Stevens

Vocabulary flashcards summarizing key terms, proteins, and concepts related to membrane transport mechanisms from BIOL 3030 lectures.

Membrane Transport Protein

Any integral membrane protein that facilitates the movement of specific solutes across a biological membrane.

Passive Transport

Movement of molecules down their electrochemical gradient without an external energy source (e.g., simple diffusion, facilitated diffusion).

Active Transport

Energy-requiring movement of molecules against their electrochemical gradient, typically driven by ATP hydrolysis or ion gradients.

Electrochemical Gradient

Combined influence of a solute’s concentration gradient and the membrane potential; a form of stored energy.

Simple Diffusion

Unaided passage of small, non-polar molecules directly through the lipid bilayer down their gradient.

Facilitated Diffusion

Passive, protein-mediated transport of polar molecules or ions down their gradient via channels or carriers (uniporters).

Uniporter

A carrier that transports a single type of molecule down its concentration gradient (e.g., GLUT1).

Symporter

Cotransporter that moves two different solutes in the same direction; typically one down its gradient to drive the other up (e.g., Na⁺/glucose symporter).

Antiporter

Cotransporter that moves two solutes in opposite directions; one down its gradient powers the uphill transport of the other (e.g., Cl⁻/HCO₃⁻ exchanger).

GLUT1

High-affinity glucose uniporter expressed in most cells; exhibits low Km and high Vmax at physiological glucose levels.

GLUT2

Low-affinity, high-capacity glucose uniporter found in liver and pancreas; functions when blood glucose is high (high Km, low Vmax at low glucose).

Km (Transporter)

Substrate concentration at which a transporter operates at half its maximal velocity; reflects binding affinity (low Km = high affinity).

Vmax (Transporter)

Maximum rate of solute transport when all transporter binding sites are saturated.

Osmosis

Net diffusion of water across a semipermeable membrane from high to low water concentration (or low to high solute concentration).

Aquaporin

Tetrameric water channel protein that greatly increases membrane water permeability; discovered by frog oocyte experiments.

Osmotic Pressure

Hydrostatic pressure required to stop the net flow of water across a membrane due to osmotic differences.

Primary Active Transport

Transport in which ATP hydrolysis directly drives the uphill movement of solutes (e.g., Na⁺/K⁺ pump).

Secondary Active Transport

Uphill transport powered indirectly by an ion gradient established by primary active transport (e.g., Na⁺/glucose symport).

P-class Pump

ATP-powered pump that becomes transiently phosphorylated during transport, such as the Na⁺/K⁺-ATPase or Ca²⁺-ATPase.

Na⁺/K⁺-ATPase

P-class pump that exports 3 Na⁺ and imports 2 K⁺ per ATP hydrolyzed, maintaining ion gradients and membrane potential.

V-class H⁺-ATPase

Multisubunit proton pump that acidifies intracellular organelles (lysosomes, endosomes) or extracellular spaces (stomach lumen).

ABC Transporter

ATP-Binding Cassette family pump with two ATP-binding domains; exports diverse substrates including lipids and drugs.

Flippase (ABCB4)

An ABC transporter that translocates phospholipids from the outer to inner leaflet of a membrane, demonstrated by fluorescent lipid experiments.

CFTR

Cystic fibrosis transmembrane conductance regulator; an ABC family protein that functions as a Cl⁻ channel rather than a pump.

Resting Membrane Potential

Steady-state voltage across the plasma membrane (≈ –60 mV) produced mainly by K⁺ leak channels and ion gradients.

K⁺ Leak Channel

Nongated channel selective for K⁺; its activity is central to establishing the resting membrane potential.

Selectivity Filter

Narrow region in ion channels whose carbonyl oxygens coordinate dehydrated K⁺ ions, excluding smaller Na⁺ ions.

AE1 (Band 3)

Cl⁻/HCO₃⁻ antiporter in erythrocytes; crucial for CO₂ transport between tissues and lungs.

Transcellular Transport

Net movement of a solute across an epithelial cell, entering at one domain and exiting at the opposite (e.g., intestinal glucose uptake).

Na⁺/Glucose Symporter (SGLT)

Secondary active transporter in intestinal and kidney epithelia that imports glucose with 2 Na⁺ ions.

H⁺/K⁺-ATPase

P-class pump in gastric parietal cells that secretes protons into the stomach lumen in exchange for K⁺, lowering pH to ~1.

Ion Channel Gating

Process by which channels open or close in response to stimuli (voltage, ligand, mechanical force).

Hydrophobicity

Measure of a molecule’s tendency to avoid water; high hydrophobicity increases simple diffusion through lipid bilayers.

Michaelis–Menten Kinetics

Mathematical description of the rate of enzyme or transporter activity as a function of substrate concentration.

Fluorescent Lipid Quenching Assay

Technique used to show phospholipid flipping by monitoring fluorescence loss when external labels are quenched.

Ion Concentration Gradient

Difference in ion concentration across a membrane; provides the driving force for many transport processes.

Tight Junction

Seal between epithelial cells that prevents paracellular diffusion, ensuring solutes cross via transcellular pathways.