Membranes: Structure, Function, and Transport

Vocabulary practice flashcards covering membrane structure, lipid composition, protein classes, diffusion kinetics, tonicity, active transport systems, and bulk transport mechanisms.

Fluid Mosaic Model

The structural model proposed in 1972 describing biological membranes as a mosaic of proteins discontinuously embedded within or attached to a fluid lipid bilayer.

Plasma Membrane Thickness

The standard lipid bilayer thickness of the mammalian plasma membrane, measuring between $7\text{ nm}$ and $10\text{ nm}$.

Exoplasmic Face

The membrane surface that faces outward toward the extracellular environment in the plasma membrane, inside the lumen of the ER, Golgi, and lysosomes, or toward the intermembrane space in mitochondria, chloroplasts, and the nucleus.

Cytoplasmic Face

The membrane surface that is oriented directly toward the cytoplasm across all cellular and organellar membranes.

Phosphoglycerides

Glycerol-based membrane phospholipids, including phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylthreonine, phosphatidylinositol, phosphatidylglycerol, and diphosphatidylglycerol (cardiolipin).

Sphingomyelin

A major phospholipid and sphingolipid composed of a sphingosine backbone linked to a fatty acid, a phosphate group, and a choline polar head group.

Glycolipids

Membrane lipids featuring carbohydrate groups (such as glucose or galactose) covalently attached to a sphingosine-containing ceramide unit, including cerebrosides and gangliosides.

Sterols

Four-ring planar membrane lipids (cholesterol in animals, phytosterols such as campesterol, sitosterol, and stigmasterol in plants) that regulate membrane fluidity; absent in prokaryotes and inner organellar membranes.

Transition Temperature ($T_m$)

The melting point temperature at which a lipid bilayer shifts between a solid-like gel state and a fluid state; longer fatty acid chains and saturation increase $T_m$, while double bonds lower it.

Cholesterol (Membrane Function)

A sterol that hydrogen-bonds to fatty acid chains near lipid head groups, moderating the membrane melting temperature and decreasing bilayer permeability to ions and small polar molecules.

Integral Membrane Proteins

Proteins embedded directly within the lipid bilayer, typically containing one or more hydrophobic transmembrane domains that interact with the core lipids.

Peripheral Membrane Proteins

Proteins bound to the outer or inner membrane surface through weak electrostatic forces and hydrogen bonding to lipid head groups or integral membrane proteins.

Lipid-Anchored Membrane Proteins

Proteins positioned on the membrane surface that are covalently tethered to lipid molecules embedded in the bilayer, such as fatty acids or prenyl groups (cytoplasmic) or GPI anchors (exoplasmic).

GPI Anchor

Glycosylphosphatidylinositol; a glycolipid structure that covalently tethers specific lipid-anchored proteins to the exoplasmic leaflet of the plasma membrane.

Porin

A rigid, barrel-shaped transmembrane channel composed of anti-parallel $\beta$-sheets found in bacteria and mitochondria that does not open and close.

Glycocalyx

A carbohydrate coat formed on the exoplasmic leaflet of the plasma membrane by the oligosaccharide chains of membrane glycoproteins and glycolipids.

Simple Diffusion

The direct, unassisted movement of small, nonpolar solutes (such as $O_2$ and $CO_2$) down their concentration gradient, displaying a transport rate that increases linearly with solute concentration.

Facilitated Diffusion

Protein-mediated passive transport of polar or charged solutes down their concentration gradient, characterized by a hyperbolic rate curve that exhibits saturation kinetics.

Carrier Protein

A transport protein that alternates between conformations by binding specific solute molecules on one face of the membrane and releasing them on the opposite face.

Channel Protein

A transport protein that creates a hydrophilic pore across the membrane, allowing rapid passive movement of ions or water down their electrochemical gradient.

Uniport

A transport protein mechanism that moves a single specific solute across the lipid bilayer in one direction.

Symport

A coupled transport mechanism in which two distinct solutes are transported simultaneously across the membrane in the same direction.

Antiport

A coupled transport mechanism in which two distinct solutes are translocated simultaneously across the membrane in opposite directions.

Osmosis

The net diffusion of water across a selectively permeable membrane toward the side with higher solute concentration (lower free energy).

Hypertonic Solution

An external environment with a higher solute concentration than the intracellular fluid, causing net water loss, cell shriveling (crenation) in animal cells, and plasmolysis in plant cells.

Hypotonic Solution

An external environment with a lower solute concentration than the intracellular fluid, driving water entry that results in cell swelling/lysis in animal cells and normal turgor pressure in plant cells.

Direct Active Transport

The movement of a solute against an electrochemical gradient coupled directly to an exergonic chemical reaction, primarily ATP hydrolysis.

Indirect Active Transport

Active transport powered by the simultaneous downhill movement of a co-transported ion (such as $Na^+$) driving another solute uphill against its concentration gradient.

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

A primary active transport protein that couples ATP hydrolysis to the export of $3\,Na^+$ ions and the import of $2\,K^+$ ions against their concentration gradients.

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

An indirect active transport carrier that binds two external $Na^+$ ions to facilitate the binding and transport of one glucose molecule into the cell against its gradient.

Phagocytosis

A form of bulk endocytosis ('cellular eating') in which the plasma membrane extends pseudopods to engulf large particles or microorganisms into an internal vacuole.

Pinocytosis

A nonspecific form of bulk endocytosis ('cellular drinking') where the membrane invaginates to take up droplets of extracellular fluid and dissolved solutes within small vesicles.

Receptor-Mediated Endocytosis

Targeted bulk uptake in which specific macromolecules bind to cell-surface receptors, inducing internalization through clathrin-coated pits into coated vesicles.

Exocytosis

The bulk export process in which intracellular secretory vesicles migrate to and fuse with the plasma membrane, releasing their contents into the extracellular space.