3.3 Transport Across the Cell Membrane

Vocabulary flashcards covering essential terms and definitions related to membrane transport, diffusion, membrane proteins, and electrochemical gradients.

Simple diffusion

Spontaneous movement of molecules directly through the lipid bilayer from an area of high concentration to low concentration without energy or proteins.

Facilitated diffusion

Passive movement of molecules down a concentration gradient with the help of selective integral membrane proteins (channels or carriers).

Primary active transport

Transport process that moves substances against their concentration gradient using energy released directly from ATP hydrolysis.

Hydrophilic (polar) head

Phosphate-containing portion of a phospholipid that faces the aqueous exterior or interior of the cell membrane.

Hydrophobic (non-polar) tail

Fatty-acid chains of a phospholipid that orient toward the interior of the lipid bilayer and avoid water.

Integral membrane protein

Protein that spans the entire lipid bilayer; often functions as a channel, transporter, or receptor.

Peripheral membrane protein

Protein loosely attached to one surface of the membrane via non-covalent interactions; frequently involved in signaling or enzymatic activity.

Lipid-anchored protein

Protein covalently bound to a lipid molecule, anchoring it to the membrane for signaling, enzymatic, or protective roles.

Aquaporin

Selective channel protein that greatly accelerates water movement across a semipermeable membrane.

Osmosis

Diffusion of water through a semipermeable membrane toward the side with higher solute concentration.

Membrane fluidity

Measure of how easily lipid molecules move within the bilayer; influenced by temperature and fatty-acid saturation.

Saturated fatty acid

Fatty acid with no double bonds; straight chains pack tightly, decreasing membrane fluidity and increasing rigidity.

Monounsaturated fatty acid

Fatty acid containing one double bond; introduces a kink that prevents tight packing and increases membrane fluidity.

Phosphatidylethanolamine (PE)

Major phospholipid in fish liver whose fatty-acid saturation changes with temperature to regulate membrane fluidity.

Passive transport

General term for membrane transport processes that require no metabolic energy (e.g., diffusion, osmosis, facilitated diffusion).

Permeability hierarchy

Relative ease of membrane crossing: small non-polar molecules > small uncharged polar molecules > large uncharged polar molecules > ions.

Sodium-potassium pump (Na⁺/K⁺-ATPase)

Primary active transporter that expels 3 Na⁺ from and imports 2 K⁺ into the cell per ATP, helping establish membrane potential.

Membrane potential (Vm)

Electrical potential difference across the cell membrane, typically ~−70 mV, with the cell interior more negative than exterior.

Electrochemical gradient

Combined influence of a solute’s concentration gradient and the electrical potential across the membrane that drives ion movement.

ΔG_concentration

Free-energy change caused by a concentration gradient; calculated as ΔG = RT ln(Cout/Cin).

ΔG_charge

Free-energy change due to the electrical gradient; calculated as ΔG = −zFVm (where z is ion charge).

Secondary active transport

Uphill movement of one solute powered by the downhill movement of another (e.g., Na⁺-driven glucose uptake).

Driving force (for ions)

Direction and energy associated with ion movement, determined by the sum of ΔGconcentration and ΔGcharge.

Microvilli

Membrane protrusions that enlarge surface area, increasing the rate of diffusion or osmotically driven swelling.

Hydraulic conductivity

Property of a membrane that describes how readily water can pass through it, influencing the speed of cell swelling or shrinking.