However, if a cell relied solely on diffusion, it would eventually reach a state of equilibrium where the concentration of nutrients inside the cell matched the environment outside. For a living organism, equilibrium is dangerous; it implies a lack of potential energy and a scarcity of resources. To thrive, cells must hoard nutrients and expel waste, often forcing molecules to move "up the hill"—from areas of low concentration to areas of high concentration. This uphill battle is the definition of active transport.