Membrane transport

Membrane transport is the movement of ions and molecules across biological membranes through passive processes or energy-dependent mechanisms.

Simple diffusion: The net movement of particles from higher to lower concentration without transport proteins or energy input. Small nonpolar molecules cross lipid bilayers this way, following their concentration gradients.
Cell membrane: A selectively permeable boundary surrounding a cell, built mainly from a lipid bilayer and proteins. Its structure creates the barrier that membrane transport mechanisms cross.
Sodium-potassium pump: An ATP-powered membrane protein that exports three sodium ions and imports two potassium ions per cycle. It maintains ion gradients used by cells for electrical and transport functions.
Passive transport: The movement of substances down their electrochemical gradients without direct cellular energy input. It contrasts with transport that spends energy to move substances uphill.
Facilitated diffusion: Passive movement down a concentration or electrochemical gradient through membrane channels or carriers. It lets polar solutes cross membranes without direct energy expenditure.
Phospholipid bilayer: A double layer of amphipathic lipids whose hydrophobic interior restricts passage of many polar substances. Its hydrophobic core makes unassisted passage depend strongly on a solute's properties.
Glucose transporter: A membrane carrier that moves glucose across a membrane by facilitated diffusion or coupled transport. Glucose uptake illustrates how cells move a polar nutrient across lipid bilayers.
Primary active transport: Transport that uses energy directly, often from ATP hydrolysis, to move substances against gradients. It is the direct-energy form of active transport.
Active transport: The movement of substances across membranes against a gradient using energy directly or indirectly. It moves solutes uphill when passive transport cannot establish the needed distribution.
Concentration gradient: A spatial difference in the concentration of a substance. Passive transport follows this difference, while active transport can oppose it.

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