Bond Formation/Breakage, ATP, and ∆G

[ScrubswithnoSleeves]

Hey Everyone!

So I have always taken for fact and not tried to understand in depth that bond breakage is endergonic and bond formation is exergonic. That has gotten me pretty far in life, but now that I'm studying for the MCAT, I'd like to understand that better.

This has always confused me in regards to molecules like ATP, as well as processes like Catabolism and Anabolism. Then, watching a Kaplan video, they said that ATP formation (ADP + Pi > ATP) is an endergonic reaction (HUH????). Also, why is it that ATP hydrolysis releases energy while bonds are being broken?

My idea (have no clue if this even on the right track) is that although the breakage of the ATP into ADP + Pi does require an initial endergonic addition of energy, the overall process is exergonic because of how high Energy/unstable the bonds are.

Can someone please clarify this for me?

---

Members don't see this ad.

 

[pretty_positron]

ATP wouldn't be a source of energy if breaking it down did not release energy.

Don't follow the hard and fast rule of bond breakage being endergonic and bond formation being exergonic. This is not always the case. Recall what you learned in General Chemistry about Gibbs Free Energy.

For instance, enthalpy of bond formation = sum of bonds broken - sum of bonds formed.

 

[NextStepTutor_1]

Hi @ScrubswithnoSleeves -

Yes, there are a few reasons why the bonds in ATP are relatively high-energy/unstable. A major one is that the triphosphate unit of ATP carries four negative charges at physiological pH that are in close proximity to each other. The repulsion among these charges can be minimized by hydrolysis. Also, the free phosphate group (Pi, sometimes referred to as orthophosphate) has resonance stabilization, which improves the energetic favorability of ADP + Pi compared to ATP. Additionally, ADP and Pi are more stabilized by hydration than is ATP, because additional negatively charged groups are made available to interact with and bind water following hydrolysis. Finally, ATP hydrolysis is entropically favorable.

In terms of how to square this fact with the general principle that bond formation is exergonic and bond breakage is endergonic, a key insight is that this principle is used to compare bonded to non-bonded atoms, not to compare various bonds. Basically, the idea is that bond formation in general is exergonic, but not all bonds are equally exergonic (or equally energetically favorable). Physical elevation might be a good analogy. All mountains are "up" from the reference point of someone at sea level, but some mountains are more "up" than others. Therefore, going from a 10,000-foot mountain to a 5,000-foot mountain is going "down", even if both of those mountains are "up" from the point of view of sea level. In this analogy, "up" or "down" from sea level is like "endergonic"/"exergonic" in the context of bond formation—that's a baseline comparison with the corresponding unbonded atoms. ADP + Pi is like the 10,000-foot mountain, while ATP is like the 5,000-foot mountain.

Hope this helps clarify the issue !