Membrane Transport

Diffusion

The passive movement of molecules or ions from an area of high concentration to an area of low concentration.

Concentration Gradient

The difference in the concentration of a substance between two areas; diffusion moves substances down this gradient.

Passive Transport

The movement of substances across a cell membrane without the use of energy, moving down their concentration gradient.

Simple Diffusion

The direct, passive diffusion of small, nonpolar molecules (like O2 and CO2) through the phospholipid bilayer.

Facilitated Diffusion

The passive transport of ions or polar molecules across the membrane through specific transport proteins.

Osmosis

The passive diffusion of water across a selectively permeable membrane from an area of low solute concentration to an area of high solute concentration.

Active Transport

The energy-requiring (ATP) movement of substances across a membrane against their concentration gradient, usually via protein pumps.

Electrochemical Gradient

The combined influence of a solute's concentration gradient and the electrical charge (membrane potential) across the membrane.

Membrane Potential

The voltage (electrical potential difference) across a cell's plasma membrane.