Active transport

Active transport moves substances across a membrane against their concentration or electrochemical gradients by coupling movement to an energy source.

Primary active transport: Membrane transport powered directly by an energy source such as ATP hydrolysis or light. It uses energy directly to move substances uphill, as in ATP-driven pumps.
Cell membrane: A selectively permeable lipid bilayer with embedded proteins that separates a cell from its surroundings. Active transport occurs through proteins embedded in this barrier.
Glucose-sodium symporter: A transporter that uses sodium-ion movement to carry glucose into cells across a membrane. It absorbs glucose by coupling its uphill entry to sodium moving downhill.
Facilitated diffusion: Passive movement down a concentration or electrochemical gradient through a membrane channel or carrier protein. It uses transport proteins but does not move substances uphill.
Secondary active transport: Membrane transport powered by the downhill movement of another substance along its electrochemical gradient. It couples a substance’s uphill movement to another substance moving downhill.
Concentration gradient: A difference in the concentration of a substance between two regions. Moving against this difference requires an energy source.
Sodium-glucose cotransporter 2: A renal transporter that reabsorbs filtered glucose together with sodium in the proximal tubule. Its activity illustrates secondary transport in kidney glucose reabsorption.
Simple diffusion: Net movement of particles down a concentration gradient without a transport protein or direct energy input. Unlike active transport, it proceeds spontaneously down a gradient.
Sodium-potassium pump: An ATP-powered membrane pump that moves three sodium ions out of a cell for every two potassium ions moved in. This primary pump establishes ion gradients used by many cells.
Membrane potential: The voltage difference across a membrane, produced by unequal distributions of electrical charge. Charged substances can move uphill electrically even when their concentrations favor movement.

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