What is the main energy currency of the cell?

The main energy currency of the cell is adenosine triphosphate, or ATP. ATP serves as a fundamental energy transfer molecule in biological systems. When cells require energy for various processes, such as muscle contraction, active transport, or biosynthesis, they hydrolyze ATP to release energy. The hydrolysis of ATP to adenosine diphosphate (ADP) and an inorganic phosphate (Pi) is highly exergonic, providing the necessary energy for cellular activities.

In contrast, while FADH2 and NADH are important electron carriers that play vital roles in cellular respiration, particularly in the electron transport chain, they are not regarded as the primary energy currency. Instead, they function to shuttle electrons and protons, ultimately leading to the production of ATP. ADP, while involved in the ATP cycle as a precursor to ATP, does not serve as an energy currency itself; rather, it is the product formed when ATP is utilized.

Overall, ATP's critical role in energy transfer and its ability to quickly regenerate makes it the central energy currency of the cell.