We've seen how substances can move passively down a concentration gradient through diffusion and osmosis. But sometimes, a cell needs to move substances in the opposite direction. This process is called active transport.

Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration. Because this is like pushing something uphill, it moves particles against the concentration gradient.

Moving substances against a concentration gradient requires energy. The cell provides this energy through the process of respiration. Without energy from respiration, active transport cannot happen.