Two of the main ways in which diffused substances are transported across cells are osmosisand diffusion. Diffusion is the movement of a fluid across a concentration gradient useful to the cells; osmosis is the movement of wateracross a semi-permeable membrane. However, sometimes substances need to be transported against a concentration gradient or membrane, which is when active transporttakes place.
By active transport, cells are able to move substances from an area of low concentration to an area of high concentration. This is what is meant by moving against the gradient. Because the substances are being transported against a gradient, energy is required for an active transport systemto carry a molecule across the membrane and then return to its original position:
The energy required for active…