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ATP is the energy currency of physiological processes. It stands for adenosine triphosphate, and it's the product for which both aerobic and anaerobic respiration evolved! The removal of a phosphate group from ATP results in an adenosine diphosphate (ADP) molecule, the inorganic phosphate (Pi) and energy being released.

This reaction is reversible, so an ATP molecule can be formed back from an ADP molecule + Pi, but this time energy is being used instead of released.

The "cellular work" ATP is involved in can mean anything from maintaining electrochemical gradients through active transport needed for neuron function, to muscle contraction and immune activity. It's much of what being alive is made of!