Passive transport

Passive transport is the movement of substances across a cell membrane down a concentration or electrochemical gradient without direct energy input from the cell.

Simple diffusion: Net movement of particles from higher to lower concentration without crossing a membrane protein. Small nonpolar molecules can pass directly through the lipid bilayer this way.
Concentration gradient: A spatial difference in the concentration of a substance. A concentration difference drives net diffusion toward the region of lower concentration.
Active transport: Membrane transport that uses energy to move substances against their electrochemical gradients. Unlike passive transport, it can move substances uphill by coupling movement to an energy source.
Aquaporin: A membrane channel protein that selectively facilitates water movement across cell membranes. Aquaporins increase the rate of passive water transport in many tissues.
Osmotic pressure: The pressure required to stop net solvent movement across a selectively permeable membrane. It quantifies the force associated with osmotic water movement across cell membranes.
Facilitated diffusion: Down-gradient membrane transport through channels or carrier proteins without direct energy input. Channels and carriers let ions and polar solutes cross membranes that lipids exclude.
Cell membrane: A selectively permeable lipid bilayer with embedded proteins that separates a cell from its surroundings. Its lipid core and proteins determine which substances can cross passively.
Primary active transport: Transport against a gradient powered directly by an energy source such as ATP hydrolysis. ATP-driven pumps establish gradients that passive transport later dissipates.
Resting membrane potential: The voltage difference across a cell membrane when the cell is not actively signaling. Passive ion movement through open channels helps establish and maintain this voltage.
Tonicity: The effect of a solution's nonpenetrating solutes on a cell's volume. Water transport changes cell volume according to the effective solute differences across the membrane.

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