Editing ATP hydrolysis (section)

==Amount of energy produced==

Hydrolysis of the terminal phosphoanhydridic bond is a highly exergonic process. The amount of released energy depends on the conditions in a particular cell. Specifically, the energy released is dependent on concentrations of ATP, ADP and P<sub>i</sub>.  As the concentrations of these molecules deviate from values at equilibrium, the value of [[Gibbs free energy]] change (Δ''G'') will be increasingly different. In standard conditions (ATP, ADP and P<sub>i</sub>  concentrations are equal to 1M, water concentration is equal to 55 M) the value of Δ''G'' is between -28 and -34 kJ/mol.<ref>{{Cite web|url=http://bionumbers.hms.harvard.edu/bionumber.aspx?s=y&id=101989&ver=2|title=Standard Gibbs free energy of ATP hydrolysis - Generic - BNID 101989|website=bionumbers.hms.harvard.edu|access-date=2018-01-25}}</ref><ref name=":1">{{Cite web|url=http://book.bionumbers.org/how-much-energy-is-released-in-atp-hydrolysis/|title=» How much energy is released in ATP hydrolysis?|last=Philips|first=Ron Milo & Ron|website=book.bionumbers.org|language=en|access-date=2018-01-25}}</ref>

The range of the Δ''G'' value exists because this reaction is dependent on the concentration of Mg<sup>2+</sup> cations, which stabilize the ATP molecule. The cellular environment also contributes to differences in the Δ''G'' value since ATP hydrolysis is dependent not only on the studied cell, but also on the surrounding tissue and even the compartment within the cell. Variability in the Δ''G'' values is therefore to be expected.<ref name=":1" />

The relationship between the standard Gibbs free energy change Δ<sub>r</sub>''G''<sup>o</sup> and chemical equilibrium is revealing. This relationship is defined by the equation Δ<sub>r</sub>''G''<sup>o</sup> = -''RT'' ln(''K''), where ''K'' is the [[equilibrium constant]], which is equal to the [[reaction quotient]] ''Q'' in equilibrium. The standard value of Δ''G'' for this reaction is, as mentioned, between -28 and -34 kJ/mol; however, experimentally determined concentrations of the involved molecules reveal that the reaction is not at equilibrium.<ref name=":1" /> Given this fact, a comparison between the equilibrium constant, ''K'', and the reaction quotient, ''Q'', provides insight. ''K'' takes into consideration reactions taking place in standard conditions, but in the cellular environment the concentrations of the involved molecules (namely, ATP, ADP, and P<sub>i</sub>) are far from the standard 1 M. In fact, the concentrations are more appropriately measured in mM, which is smaller than M by three orders of magnitude.<ref name=":1" /> Using these nonstandard concentrations, the calculated value of ''Q'' is much less than one. By relating ''Q'' to Δ''G'' using the equation Δ''G'' = Δ<sub>r</sub>''G''<sup>o</sup> + ''RT'' ln(''Q''), where Δ<sub>r</sub>''G''<sup>o</sup> is the standard change in Gibbs free energy for the hydrolysis of ATP, it is found that the magnitude of Δ''G'' is much greater than the standard value. The nonstandard conditions of the cell actually result in a more favorable reaction.<ref name=":2">{{Cite web|url=https://cnx.org/contents/GFy_h8cu@11.1:mcCadNdV@8/ATP-Adenosine-Triphosphate|title=ATP: Adenosine Triphosphate|website=cnx.org|date=21 October 2016 |access-date=2018-05-16}}</ref>

In one particular study, to determine Δ''G'' in vivo in humans, the concentration of ATP, ADP, and P<sub>i</sub> was measured using nuclear magnetic resonance.<ref name=":1" /> In human muscle cells at rest, the concentration of ATP was found to be around 4 mM and the concentration of ADP was around 9 μM. Inputing these values into the above equations yields Δ''G'' = -64 kJ/mol. After [[ischemia]], when the muscle is recovering from exercise, the concentration of ATP is as low as 1 mM and the concentration of ADP is around 7 μM. Therefore, the absolute Δ''G'' would be as high as -69 kJ/mol.<ref>{{Cite journal|last1=Wackerhage|first1=H.|last2=Hoffmann|first2=U.|last3=Essfeld|first3=D.|last4=Leyk|first4=D.|last5=Mueller|first5=K.|last6=Zange|first6=J.|date=December 1998|title=Recovery of free ADP, Pi, and free energy of ATP hydrolysis in human skeletal muscle|journal=Journal of Applied Physiology|volume=85|issue=6|pages=2140–2145|doi=10.1152/jappl.1998.85.6.2140|issn=8750-7587|pmid=9843537|s2cid=2265397 }}</ref>

By comparing the standard value of Δ''G'' and the experimental value of Δ''G'', one can see that the energy released from the hydrolysis of ATP, as measured in humans, is almost twice as much as the energy produced under standard conditions.<ref name=":1" /><ref name=":2" />