[fc3stud]

Can anyone help an ailing chem teacher?  I always tell my students that bond breaking is endothermic, energy always being needed to break a chemical bond.  One of my students, jo, rebutted me. "Sorry Sir, you're wrong.  our Biology teacher taught us that when ATP converts to ATP, a phosphate bond is broken, and energy is released. ???DOH!  PLease help.  Is it anything to do with free energy?  I've had a look in the books, and she{Jo} seems to be right.  Thanks.

[Mitch]

Firstly, you are right, bond breaking is always an endothermic process. But, it doesn't mean a reaction that involves bond breaking is endogonic (Delta G).

delta H of reaction = Bonds broken in reactant - bonds formed in product

If the product releases more energy from the bonds formed than those broken in the reactant than the overal process is actually exothermic. This is of course a highly simplified description on how it works.

[Sandra]

This is a great Q! But I hope "Jo" is not killing you slowly.

Mitch is right. The energy released for the hydrolysis of ATP is the gibbs free energy (delta G). This is different from the energy released
when breaking a bond (bond enthalphy).
 

rxn 1: ATP + H2O = ADP + Pi

The gibbs free energy of the reaction 1, is described by: delta G_rxn = delta H_rxn -T delta S_rxn

Using MItch's equation for delta H_rxn, we see that in oder for the hydrolysis of ATp to "release energy" we need the enthalpic term to be a negative value.  

From Donald Voet's biochem book (3rd ed, pg 568), we see that the rxn is enthalpicly driven. One reason that the hydrolysis of ATP "releases energy" is due to the decrease in electrostatic repulsions btwn
the charged phosphate grps in ATP vs. ADP. Another reason is the smaller solvation energy of the ATP molecule relative to the ADP + Pi products.  

I hope this helps. Good luck!

sandra

 

[fc3stud]

Many thanks to Mitch and Sandra, now i can go back to my students and put them in the knowledge!