Two main ways in which different substances are transported across cells are osmosis and diffusion.
Diffusion: It is the movement of fluids across a concentration gradient, and moves from a high concentration to a low concentration, this is useful to cells.
Osmosis: Is the movement of water across a partially permeable membrane. It also moves from a high concentration to a low concentration.
However, sometimes substances need to be transported against concentration gradient or membrane. This is when active transport takes place.
Active transport: Cells are able to move substances from an area of low concentration to an area of high concentration. This is what it is meant by moving against the concentration gradient. As the substances are moving against the concentration gradient, energy is required for an active transport system to carry molecules across the membrane and then return to its original position.
The energy required for active transport comes from cellular respiration. The rate of active transport and rate of respiration in cells are closely linked. Cells involved in active transport have a lot fo mitochondria to provide the energy needed from respiration.