I have a very basic question. Energy is absorbed to break bonds. Bond-breaking is an endothermic process. Energy is released when new bonds form. Bond-making is an exothermic process.

But ATP breaks down to ADP and releases energy right? Is there anything wrong in my understanding?


Bonds are formed between water and phosphate. Here is a good link:

Bonds are both broken and made in chemical reactions but many biology teachers and textbooks state that "Breaking ATP bonds releases energy." In reactions bonds are broken and made. If the strength of the bonds formed is exceeded by the strength of the bonds broken the reaction is exothermic.

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    $\begingroup$ I was confused about this for a long time as a student, and so were many of my classmates. Goes to show how slightly imprecise language can be problematic. Once I realized the $\rm{P-O-P}$ bonds weren't broken (bond homolysis or heterolysis) so much as hydrolyzed, it all made sense. Hydrolysis reactions can release quite a bit of energy. $\endgroup$ – Nicolau Saker Neto Sep 18 '14 at 0:02
  • $\begingroup$ @brinnb : "If the strength of the broken bonds is exceeded by the strength of the bonds formed the reaction is exothermic." That is a great point. $\endgroup$ – dexterdev Sep 18 '14 at 5:23
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    $\begingroup$ Shouldn't it be If the strength of the bonds formed is exceeded by the strength of the broken bonds the reaction is exothermic? $\endgroup$ – Bøbby Leung Feb 24 '19 at 5:05

The previous answers are not very good, so I feel compelled to clarify: ATP is not broken in a vacuum, but hydrolyzed by water, which is disrespected and not shown in the reaction pathway, but is hugely important. The overall free energy of delta G remember is not just the enthalpy of the reaction (heat given off or absorbed) but also composed of an entropy which is the ordering or disordering of the surrounding solvent water molecules. It takes far more water molecules to be ordered around the ATP gamma phosphate once it is released in the ADP + P form than in the ATP form when still attached. Energy is always required to break bonds and always is released when bonds are formed, even with ATP, but the part that is omitted is that more molecules of water are bound and ordered after the ATP hydrolysis than before and so more ordering plus many more highly energetic hydrogen bonds formed than broken equals a large overall release of energy for the overall process. Don't forget the water!


So yes, energy is absorbed to break ATP --> ADP... ATP, yes requires energy to break it up into ADP & AMP.. BUT the energy is very little compared to the amount of energy it releases when one of the phosphate leaves. That is why when one phosphate is lost (ATP--> ADP), ALOT of energy is gained in comparison to the small amount that was absorbed to break the bond. Its kind of like investing in something so that it will pay back later. You give some energy, and in return you get a lot more back. Hope this helps!

  • $\begingroup$ Welcome to chemistry.se! If you have questions about how to beautify your posts, have a look at the help center. Do you want to know more about this site, please take the tour. In it's current state I do not understand how your statement answers the question. Where does the phosphate leave to and how does that "generate" energy? $\endgroup$ – Martin - マーチン Jan 27 '15 at 4:13

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