[NewtoAtoms]

Hello Chemists,

acetyl phosphate + ADP <-----> acetate + ATP         ∆G=-2.8 kcal/mol

Q:  Is the free energy difference per molecule of acetyl phosphate, relative to that of acetate, greater than, lesser than, or equal to that of ATP relative to its dephosphorylated coutnerpart?

A:  I am new to chemistry, and I am being taught chemistry during summer school. Unfortunately I have a teacher who speaks terrible English so I am having to revert to posting on here for some help.

My teacher told me that this is a biochemical question in regards to the body, and I am to answer it.
I know that the equation is exothermic, due to the - 2.9 kcal/mol
I also know that during a chemical reaction, each part of the reaction is proportional to the other, therefore
1 mol acetyl phosphate:1 mol ADP: 1 mol acetate: 1 mol ATP.

Therefore is it safe to say that the free energy difference  per molecule of acetyl phosphate equal to that of ATP realative to it's dephosphorlyated counterpart? 

Unfortunately I don't really understand the question, and I have read it a million times, as well as the section in my text, so I am trying to answer it according to what I think I am understanding.

can anyone clarify this gumble for me and tell me if I am at all in the right field?

Thank you for your time

Newtobiology

[Yggdrasil]

Wow, that question is phrased very awkwardly.  Here's what I think it is asking:

Consider the reactions:
1) acetate + phosphate --> acetyl phosphate
2) ADP + phosphate --> ATP

Is the change in free energy for reaction (1) greater than, equal to, or  less than the free energy change for reaction (2).


Also, a slight correction to your statements:

I know that the equation is exothermic, due to the - 2.9 kcal/mol

A reaction is exothermic if ΔH is negative.  The correct term for a reaction where ΔG is negative is exergonic.  While an exergonic reaction can be exothermic, this is not always the case.

[NewtoAtoms]

Thank you Yggdrasil!

The question is indeed very awkward!  But I think I now understand it due to the way that you have organized it. 

However does the -2.9 kcal/mol represent the free energy of the #1 reaction or the entire reaction?
I have researched and this is what I have found:

1.  acetyl phosphate ---> acetate + phosphate ∆G = -2.9 kcallmol (CAN I ASSUME THIS)
2.  ATP + H20 ---> ADP + Pi    ∆G = -7.3 kcal/mol

If I can assume that #1 is indeed -2.9 kcal/mol, then the free energy difference per molecule of acetyl phosphate relative to that of acetate is less than that of aTP relative to its dephosphorylated counterpart.

Thank you so much for your time and assistance!

[Yggdrasil]

1.  acetyl phosphate ---> acetate + phosphate ∆G = -2.9 kcallmol (CAN I ASSUME THIS)

This is incorrect.  The ΔG = -2.9 kcal/mol refers to the entire coupled reaction of acetyl phosphate + ADP --> acetate + ATP.

[NewtoAtoms]

Hello again Yssdrasil,

The original reaction is: acetyl phosphate + ADP <---> Acetate + ATP     ∆G -2.8 kcal/mol

I also know that ATP + H20 <--> ADP + Pi    ∆G -7.3 kcal/mol

However it I write it like this:

Acetyl phosphate <--> acetate + phospate      ∆G = ?
ADP + Pi             <--> ATP + H20                   ∆G = 7.3 kcal/mol ( I HAVE TO FLIP IT AND THE '-' of ∆G - CORRECT?)
----------------------------------------------------------------
Acetyl phosphate + ADP <--> Acetate + ATP    ∆G = -2.8 kcal/mol

Therefore acetyl phosphate <--> acetate + phosphate ∆G = 4.5 kcal/mol

Am I correct now?

Thank you for your help.

[Yggdrasil]

Yes, you have the sign of the ΔG for ADP + Pi --> ATP + H2O correct.  However, I'm not quite sure you've calculated the ΔG for acetyl phosphate + H2O --> acetate + phosphate correctly  (I accidentally omitted the water in the reaction previously).   You would add the ΔG values for both "halves" of the reaction to obtain the overall ΔG of the transformation, so I don't see how you can add two positive numbers (7.3 + 4.5) to obtain a negative value.

However, you are definitely on the right track and very close to the correct answer.