Energy charge is an index used to measure the energy status of biological cells. It is related to ATP, ADP and AMP concentrations. Energy charge was first defined by Atkinson and Walton who found that it was necessary to take into account the concentration of all three nucleotides, rather than just ATP and ADP, to account for the energy status in metabolism.
Many reactions in metabolism are controlled by the energy status of the cell. One index of the energy status is the energy charge, which is proportional to the mole fraction of ATP plus half the mole fraction of ADP, given that ATP contains two anhydride bonds whereas ADP contains one. Hence the energy charge is defined as: