 These values can be used to calculate the change in energy under physiological conditions and the cellular ATP / ADP ratio . However , a more representative value ( which takes AMP into consideration ) called the Energy charge is increasingly being employed . The values given for the Gibbs free energy for this reaction are dependent on a number of factors , including overall ionic strength and the presence of alkaline earth metal ions such as Mg2 + and Ca2 + . Under typical cellular conditions , ΔG is approximately − 57 kJ / mol ( − 14 kcal / mol ) . 
