Unit 3 Deck 3: Membrane Transport Mechanisms

Simple Diffusion

The passive, non-protein-mediated net movement of small, non-polar molecules (such as O2, CO2, ethanol, and steroids) down their concentration gradient until dynamic equilibrium is reached.

Osmosis

The passive net diffusion of water molecules across a selectively permeable membrane down its own concentration gradient, moving from an area of lower solute concentration to higher solute concentration.

Aquaporins

Transmembrane channel proteins that facilitate the rapid transport of water molecules into and out of cells across the hydrophobic core of the bilayer.

Tonicity

The capability of an extracellular solution to drive water movement into or out of a cell by osmosis, classified as isotonic, hypertonic, or hypotonic based on osmolarity differences.

Facilitated Diffusion

Passive transmembrane movement of polar molecules and ions down their concentration gradient with the assistance of carrier proteins or channel proteins, requiring no input of metabolic energy.

Carrier Proteins

Transmembrane transport proteins that selectively bind a specific solute and undergo conformational shape changes to shuttle that solute across the membrane.

Channel Proteins

Transmembrane proteins that form open hydrophilic pores through the membrane, allowing rapid passive diffusion of specific ions or small molecules.

Gated Ion Channels

Channel proteins that open or close their pores in response to specific stimuli—categorized as ligand-gated, voltage-gated, mechanically gated, or light-gated.

Active Transport

The cellular movement of substances across a membrane against their concentration or electrochemical gradient via transmembrane protein pumps, directly driven by the hydrolysis of ATP

Sodium-Potassium Pump (Na⁺/K⁺ Pump)

An electrogenic primary active transport protein that hydrolyzes one molecule of ATP to export three Na+ ions and import two K+ ions against their concentration gradients.

Proton Pump

An ancient electrogenic active transport protein conserved across Bacteria, Archaea, and Eukarya that hydrolyzes ATP to move hydrogen ions (H+) across biological membranes, creating a proton or pH gradient.

Chemiosmosis

A cellular process in which the potential energy stored in an electrochemical proton (H+) gradient across a membrane is harnessed to perform biological work.

Secondary Active Transport

A transport mechanism where an electrochemical gradient initially established by ATP-driven primary active transport (such as a proton gradient) is coupled to drive the simultaneous transport of a second solute against its concentration gradient.

Pinocytosis

A form of bulk endocytosis in which a cell invaginates its plasma membrane to engulf droplets of extracellular fluid and non-specific dissolved solutes into internal vesicles

Endocytosis and Exocytosis

Bulk vesicular transport mechanisms: endocytosis brings extracellular materials into the cell inside membrane-bound vesicles, while exocytosis fuses intracellular vesicles with the plasma membrane to secrete substances.