Final answer:
ATP synthase typically synthesizes ATP during chemiosmosis as protons flow down their gradient. If it pumps H+ against the gradient, it consumes ATP to move protons from low to high concentration, opposite of its usual function.
Step-by-step explanation:
When ATP synthase pumps hydrogen ions (H+) across a membrane against the electrochemical proton gradient, this process goes against the typical function of ATP synthase during chemiosmosis. In normal cellular respiration, ATP synthase utilizes the potential energy stored in the proton gradient to facilitate the synthesis of ATP from ADP and inorganic phosphate. This is achieved as protons flow down their electrochemical gradient through the ATP synthase, turning it like a turbine. However, if the ATP synthase were to pump protons against the gradient, it would be consuming ATP rather than synthesizing it, as it temporarily functions in reverse, moving protons from an area of low concentration to an area of high concentration, which requires energy input.