[joelhall]

having some trouble understanding the correlation here...

now according to the organic chemistry books:

energy is absorbed when bonds are broken
energy is lost when bonds are formed

according to biochemistry in the a&p books:

energy is lost when bonds are broken
energy is absorbed when bonds are formed.

can anyone explain why this is so in biology?  i cant quite see what im missing here

thanks guys,

joel

[Yggdrasil]

It all depends on your perspective.  In order to break chemical bonds, energy must be transferred from the surroundings to the molecule whose bond will be broken.  You could say that in order to break a bond, a molecule absorbs energy from the surroundings, but it is equally correct that the surroundings lose energy due to the breakage of a chemical bond.

Similarly, when molecules form a bond, they transfer some of their chemical energy to the surroundings.   Thus, in the formation of a bond, the molecule loses energy to the surroundings, which is equivalent to saying that the surroundings absorb the chemical energy released by the molecules forming a bond.

Personally, I like the organic chemists' way of looking at things from the point of view of the molecule.  When in doubt, I would trust organic chemists' explanations over biologists' explanations.  For example, most biochemistry text books do not have a good explanation of why ATP hydrolysis releases energy.  Most biochemistry text books say something to the effect that the phosphodiester bonds in ATP are high energy bonds, so breaking the bond releases a lot of energy.  A more correct explanation is that, because the phosphodiester bond of ATP is so high energy, you don't have to put a lot of energy into the system to break the bond (creating a higher energy state than ATP).  Thus, when you form a new bond with water, much more of the energy released by the formation of the bond is available to be used to perform chemical work.  As an analogy, it's like increasing your net profit by keeping your income the same but decreasing your expenditures.


(note: in reality the process of bond breakage and bond formation occurs simultaneously.  I treat them as separate steps here in order to make the picture a little clearer.  This is justified because the thermodynamics of a process is independent of the path taken.  Thus, the sittuation where you sequentially break a bond then form another bond has the same thermodynamics as the case where the bonds are broken and formed simultaneously).

[joelhall]

i agree it makes far more sense the way its explained in chemistry - bonds are formed for stability.  to say that atp bonds act as stores for energy seems a departure.

like you said, in my molecular cell biology book it explains that simultaneous breaking/formation occurs, one feeding the other, but the physiology texts still seem... well wrong to put it bluntly if bonds are so 'high' in energy surely one would expect molecules to gain this energy and degrade pretty much instantaneously?

thanks for the reply though, much appreciated.

joel