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Is an external source of energy always needed to drive active transport? In some cases it is possible to couple the transport of two different molecules across a membrane so that the energetic effects cancel one another out. One molecule is moving down a concentration gradient (thus reducing potential energy). This molecule reacts with the transport protein to change its conformation and transport the second molecule across the membrane against its concentration gradient. Thus the energy is transferred from one concentration gradient to the transport protein and then to a second concentration gradient. In this case the ultimate source of energy is the force causing the initial concentration gradient.