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I know that when ATP is hydrolyzed into ADP it gives 7.3 kcal/mol. The question is, does this value vary among different types of cells?

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  • $\begingroup$ NADH and FADH are not hydrolyzed, but they can be oxidized, and the resulting energy used to drive ATP synthesis in mitochondria. Can you give a source for your numbers? 3.7 calories (kcal / mol) for ATP hydrolysis sounds way too low. $\endgroup$ – Roland Oct 26 '17 at 17:23
  • $\begingroup$ thanks for your comment. yes, you are right it oxidized and it gives 7.3, I wrote it by mistake. I will edit the question. thanks again. $\endgroup$ – Reem M.Al Haj Oct 27 '17 at 5:49
  • $\begingroup$ I see, you are simply asking about the energy of ATP hydrolysis. The way you question was worded, it sounds like you were asking about the total energy from NADH and FADH2 oxidation. And you got an answer on that. I will edit to clarify. $\endgroup$ – Roland Oct 27 '17 at 5:59
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Yes, the energy from ATP hydrolysis can vary between different cell types and condition, because it depends on several factors. Hence, you will find different values in various textbooks.

First, like any chemical reaction, the free energy $\Delta G$ of ATP hydrolysis depends on the concentrations of the reactants. You can explore different values of $\Delta G$ with this calculation tool. Typical values are [ATP] = 10 mM, [ADP] = 1 mM and [P$_i$] = 20 mM, which gives $\Delta G$ = 42 kJ/mol (10 kcal/mol). But all these concentrations can vary between cells, and there is not much data available on intracellular concentrations in specific cell types. Many textbooks give free energy values denoted $\Delta G^{\circ}{'}$ which are calculated at concentrations of 1M, which is of course completely unphysiological.

In addition, the $\Delta G$ is strongly affected by the concentration of Mg$^{2+}$, because this ion associates with phosphate groups and alters their free energy. It also depends on pH, since the phosphate groups can take up/release protons. If you are interested, an in depth analysis is found here.

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  • $\begingroup$ The point you make is valid, that the actual free energy of hydrolysis of ATP can differ from the standard free energy. However the OP asks about variation in “different types of cells”, by which I presume he means e.g. muscle v. liver. Surely the answer to this is either “no” or “we don‘t know”. $\endgroup$ – David Oct 28 '17 at 13:30
  • $\begingroup$ I don't think it's just a matter of getting the textbook values right. Because the free energy depends quite a bit on concentrations of reactants, it can differ between cell types or conditions. For example, the reaction quotient ATP/(ADP * Pi) can drop as much as 50-fold in exercising muscle (jap.physiology.org/content/73/2/728) You're right though that we don't know much about these parameters between various cell types, as intracellular concentrations are hard to measure accurately. $\endgroup$ – Roland Oct 28 '17 at 14:37

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