Why reduced atoms are more energetic?

[godingly]

For example, in glycolysis, NAD+ is reduced to NADH and then it's more energetic. Why adding electrons to an atom gives it more energy?

 

[Borek]

It is not just about atoms, it is about whole molecules (or ions), and it is not just about adding electrons. Sometimes it is a reduced molecule that has more energy than the starting form, sometimes it is oxidized molecule. Which one and why - that's what the chemistry is about, there is no short, universal answer.

 

[Quantum Defect]

For example, in glycolysis, NAD+ is reduced to NADH and then it's more energetic. Why adding electrons to an atom gives it more energy?

This is not quite right. "More energetic" is probably not the best term to use, more reactive is better -- relative reactivity usually refers to the thermodynamic stability of products compared with reactants. This can change with the context.

You can have strong oxidizing agents, in which the reduced form is more thermodynamically stable. You can also have strong reducing agents, where the oxidized form is the more stable from. In these cases, it is also understood that you are reacting them with something.

For example sodium metal can reduce water to hydrogen gas, leaving sodium hydorxide in solution. [in the example, sodium metal is a stronger reducing agent that hydrogen gas].

In the NAD+, NADH case, with NADH, you have effectively added a hydride (H-) to NAD+. Hydrides are typically reducing agents. In its reactions, NAD+ is more stable than NADH.

 

[epenguin]

.

For example, in glycolysis, NAD+ is reduced to NADH and then it's more energetic. Why adding electrons to an atom gives it more energy?

Is this something you have been told? And is it a systematic theme in a textbook you have? If so can you give some quotations?

And if it matters at all does it matter in glycolysis where the NADH is reoxidised further along the chain so there is no net oxidation?
I do not find it a helpful way of looking at anything. You can hardly say NADH is more or less energetic than NAD⁺ because you cannot convert between them. Unless you have another pair of substances for them to oxidise/reduce. In this case one of them is phohoglyceraldehyde.

What I'd rather say is that you may have seen in chemistry that aldehydes are rather easily oxidised - by mild oxidising agents. It is not surprising that in oxidation by NAD⁺ the equilibrium lies strongly over towards the products NADH and phosphoglycerate. (But then of course there are plenty of other things that can oxidize the NADH). However the cell's enzymes make something cleverer than phosphoglycerate. They use the energy, let's say, implicit in that far-over equilibrium to couple in an enzyme mechanism to a reaction that would normally be energetically unfavorable, the condensation of two acids, phosphoglycerate and inorganic phosphate to form an anhydride. (Again I appeal to the chemistry lab. If you've ever opened a bottle of acetic anhydride what does it smell like? Acetic acid! Because by the time it gets into your nose that's what it is! That's what anhydrides want to do - hydrolyse. IOW it is energetically unfavourable to condense acids. So in the PGA dehydrogenase is one mechanism Nature has found for coupling this energetically unfavourable reaction to a favorable one (that above) and the result is the overall reaction only slightly unfavourable and even at equilibrium substantial amounts of diphosphoglycerate are formed.

The diphosphoglycerate can then via transphosphorylations phosphorylate ADP to form ATP. My old textbook says the discovery of what was called "substrate phosphorylation" (as distinct from the later-discovered "oxidative phosphorylation" ) "is considered one of the most important achievements in modern biology since it was the first demonstration of an enzymatic and chemical mechanism by which energy yielded by the oxidation of an organic molecule could be conserved in the form of ATP."

You could say that, and they must have felt they'd discovered the Secret of Life.. Or you could say it held the subject back for several decades because it reinforced the chemist's professional deformation, the conviction that the secret of everything was focussed into one or a few well localized and identifiable chemical bonds with stoichiometries and stuff, whereas the solution to the more important oxidative phosphorylation was a biophysicists' more abstract picture where it was not loocalised but diffuse in just a spatial or compartmental pH gradient.