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I am currently in AP Bio, and our textbook has a diagram of a sodium-potassium pump:

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If I am understanding correctly, every time this cycle occurs, an ATP is hydrolyzed in step 2 to produce a new phosphate group. Why can't the pump reuse the phosphate group it released in step 4 from the previous cycle?

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The phosphate group must be detached from the protein to cause a change in conformation. The lost phosphate group is drifting in the cytoplasm and cannot be reused without reattachment to ADP (creating ATP), since attachment of phosphate to the protein requires energy.

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