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We use creatine phosphate as an energy storage to resupply ADP with a phosphate group as our muscle cells only contain about 2-5 mM ATP. But why doesn't the muscle cells just keep 20-30 mM ATP instead of 20-30 mM creatine phosphate?

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Because all available ATP will likely be quickly spent by various cellular machinery. If there was 20-30mM of ATP, what would stop muscle from contraction and how could you keep ATP level constant at rest, be ready for rapid movement? By blocking all myosin? This creates enormous regulatory problems for the cell.

Phosphocreatine buffers energy for a period of time when it will be actually needed. Having buffer allows for more robust control of energy spending, for example via regulation creatine kinases and other enzymes. Just by activating single enzyme you then start rapid conversion of ADT into ATP via phosphocreatine.

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