Energy and Metabolism

ID #2199

I was wondering if someone could explain why these are exergonic/endergonic? I know they're both reduced. But how does that imply exer/endergonic? NAD+ --> NADH FAD --> FADH2

You could figure this out in two ways. 

First, a hydrogen (H) atom has both a proton (positive charge) AND an electron (negative charge). Electrons are very energy rich. By accepting a H atom, a cofactor is reduced and contains more energy in the form of the electron than a cofactor without the hydrogen when it's oxidized. 

Or, in the second line of thinking which you learned during exam 1 material covering biosynthetic/catabolic or anabolic reactions, you can consider whether or not you are breaking or building a bond. Building a larger molecule by forming a bond requires energy input (endergonic) and breaking a bond releases energy (exergonic). 

Using either line of reasoning I believe would lead you to the conclusion that the reduction reactions (NAD+ + 2H-->NADH + H+ or FAD + 2 H --> FADH2) are both endergonic and the oxidation reactions (NADH + H+--> NAD+ + 2H or FADH2--> FAD + 2H) are both exergonic.

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