Releaseing energy???

Question

(there might be one or more answers)

Why is it better to release energy in many small packets?

A. It matches the energy required for various reactions.

B. It lets you combine energy from many glucose molecules.

C. It avoids the requirement for oxygen.

D. It involves many pathways.

E. Glucose has too much energy to release all at once.

Answer 1

Your teacher/book probably wants you to answer A, D and E. So, if all you wanted is to answer it, stop reading. But I disagree with E. It's a common misunderstanding in high school level that the hydrolisis of ATP happens simultaneously with another reaction and that the energy released from the hydrolisis somehow finds its way into the other reaction and 'supplies it with energy'. That is not what truly happens.

The phosphoric ester bond that binds the second and third phosphate groups in ATP is easily interchangeable between molecules, so that a reaction that is usually thermodinamically impossible (Delta G greater than 0) that  occurs with ATP as a reagent may become thermodinamically possible (Delta G < 0). One classic and easy example is the conversion of glutamate to glutamine: with the help of an enzyme, ATP loses its phosphate group, that replaces the hydroxil part of glutamate's carboxyl part of theside chain. The phosphate group is then replaced by an amino group, creating glutamine. So, as an end result, you have glutamine, Pi and ADP.

So, I said all of that just to say that the reason glucose is not used as a whole molecule is simply that it does not have easily interchangeable, small functional groups that can make a generally impossible reaction become possible. It's not because it 'releases too much energy'. In fact, Gibbs energy that is released free into the cytosol (or in any other means really) will be simply lost, unable to be used in any reaction.

Hope that was at least interesting =).

Answer 2

A. It matches the energy required for various reactions.

D. It involves many pathways - thus allowing many control points.

E. Glucose has too much energy to release all at once.