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Why is energy needed for active transport? Active transport moves molecules up a concentration gradient, generating potential energy across a membrane. That energy must come from somewhere. Active transport converts energy stored in chemical bonds into energy stored in a concentration gradient. When ATP reacts with the transport protein the energy stored in a phosphate-phosphate bond is released. The reaction of the phosphate with the transport protein changes the shape of the transport protein, moving the molecule across the membrane. The energy is thus transferred from the ATP to the transport protein and then to the concentration gradient.