2.5—Membrane transport mechanisms

Simple diffusion moves small suitable molecules through the bilayer down a concentration gradient. Facilitated diffusion uses a channel or carrier but still moves down the gradient; active transport uses energy to move a substance against its electrochemical gradient.

Channels provide a hydrophilic route and can be gated. Carrier proteins bind a solute and change shape. Pumps use ATP directly or couple movement to another gradient.

Glucose may enter a cell through a carrier down its gradient, while a sodium–potassium pump uses ATP to maintain ion gradients that support nerve and epithelial transport.

The mechanism predicts sensitivity: diffusion depends on gradient and permeability; active transport depends on ATP supply, pump density and the gradient being maintained.

A membrane protein does not automatically imply active transport. State the direction, energy source and coupling before assigning the mechanism.