ATP bond energy

[Astra]

I just wanna make sure I have this down correct.

Breaking of a bond is endergonic ( requires energy input) +G

Formation of a bond is exergonic ( energy is released) -G

So, when ATP hydrolysis releases energy, it is due to the following.

Bonds in ATP are broken and this process is endergonic,

New bonds are formed in ADP and this is exergonic.

The bonds formed release more energy than the energy used to break the bonds.

Therefore, the ATP hydrolysis is -G and a spontaneous process.

The overall free energy generated from this process is then coupled with endergonic processes in the body to allow them to proceed spontaneously.

Correct?

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[yeasureyoubetcha]

So, when ATP hydrolysis releases energy, it is due to the following.

Bonds in ATP are broken and this process is endergonic,

It may require a small amount of activation energy.

New bonds are formed in ADP and this is exergonic.

There are no new bonds formed. Its just the loss/transfer of the gamma phosphate. The process as a whole is exergonic.

The bonds formed release more energy than the energy used to break the bonds.

See above. I believe the release of free energy comes from spreading out of the cluster of negative charges from the phosphates upon the loss of gamma. Its really energetically unfavorable to tack on 3 negative charges right next to each other. To counter the clustering, i think the delocalization helps stabilize ATP. This is where the Ea may come into play in breaking the bond.

Therefore, the ATP hydrolysis is -G and a spontaneous process.

Yes

The overall free energy generated from this process is then coupled with endergonic processes in the body to allow them to proceed spontaneously.
Correct?

yes

 

[aldol16]

It may require a small amount of activation energy.

@Astra118 is talking about a thermodynamic ground state effect when he/she talks about "endergonic" vs. "exergonic" so activation energy isn't relevant to that process since it is purely a kinetic quantity - that's not to say it doesn't matter though and you were right to bring it up. But just make sure to keep the two concepts separate because many students tend to confuse them.

 

[yeasureyoubetcha]

@Astra118 is talking about a thermodynamic ground state effect when he/she talks about "endergonic" vs. "exergonic" so activation energy isn't relevant to that process since it is purely a kinetic quantity - that's not to say it doesn't matter though and you were right to bring it up. But just make sure to keep the two concepts separate because many students tend to confuse them.

I was just picturing it like a rxn coordinate diagram. You're right though. It's easy to confuse the 2


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[aldol16]

I was just picturing it like a rxn coordinate diagram

Yes, but it has nothing to do with the energy from bond forming/breaking. Just to make the point clear.

Also, there is bond-forming in this reaction. It's ATP hydrolysis.

 

[Astra]

Yes, but it has nothing to do with the energy from bond forming/breaking. Just to make the point clear.

Regarding my post, do you see any mistakes?

 

[aldol16]

Regarding my post, do you see any mistakes?

Only some minor points.

Bond energies only factor into the energy calculation as one factor. In other words, BDEs don't tell you whether a reaction is spontaneous or not. Only BDFEs will tell you that. So can you tell me why?

 

[Astra]

Only some minor points.

Bond energies only factor into the energy calculation as one factor. In other words, BDEs don't tell you whether a reaction is spontaneous or not. Only BDFEs will tell you that. So can you tell me why?

A reaction is spontaneous if the products are more stable than the reactants. The stability of the product is determined by the energy of formation for the bonds of the product?

disclaimer, I do not know what BDFE is. Took an educated guess

 

[aldol16]

Close. BDE is bond dissociation energy and BDFE is bond dissociation free energy. Bond making/breaking is talked about in terms of BDE. That's the well-known "delta H" of the Gibbs equation. But that doesn't determine spontaneity completely - you also have the entropic consideration. So you should always consider that as well when you want to know if a reaction is spontaneous or not.

 

[Astra]

Close. BDE is bond dissociation energy and BDFE is bond dissociation free energy. Bond making/breaking is talked about in terms of BDE. That's the well-known "delta H" of the Gibbs equation. But that doesn't determine spontaneity completely - you also have the entropic consideration. So you should always consider that as well when you want to know if a reaction is spontaneous or not.

And this goes back to the equation G = H - TS correct?

Your point is that you cannot assume a reaction is spontaneous because it has a -H value ( heat is released) because the S value might be positive enough to overpower the fact that heat is released.

 

[aldol16]

And this goes back to the equation G = H - TS correct?

Your point is that you cannot assume a reaction is spontaneous because it has a -H value ( heat is released) because the S value might be positive enough to overpower the fact that heat is released.

Yes, exactly. So bond energies is only one part of the equation. You still have to consider whether ATP + H20 to ADP + Pi is entropically favored or disfavored. It won't be skewed to one extreme or the other in this case because you go from 2 molecules to 2 molecules, but delta S still matters. And so does temperature.