Energy and Metabolism

ID #2214

I was just going back through my notes, and something Mr. Mehrten's said kind of confused me. Before he starts describing step 4 of glycolysis, he says, "now every step from here down will be exergonic" (or something along those lines,) and then when he moves on to step 6 he clearly says "this is the first exergonic step." Is step 6 the first exergonic step because it is the first one coupled with an endergonic reaction, or are steps 4-10 all considered exergonic, even those not coupled to an endergonic reaction?

I think you are recalling Prof. Mehrtens describing the investment phase of glycolysis (where a cell "spends money to make money") and the payoff phase (when ATP is made and energy is transferred to NAD+). He also uses the terms endergonic and exergonic to describe portions of glycolysis, even if some of the steps are rearrangements and do not require or release energy. Step 4 (where fructose 1,6-bisphsophate is broken down into 2 3 carbon molecules) begins the overall "payoff phase" of glycolysis. Step 6 (2 glyceraldehyde-3-phosphate --> 2 1,3-bisphosphoglycerate) is the first exergonic reaction in glycolysis that is coupled to the reduction of NAD+ to NADH.  

There are two endergonic steps and 3 exergonic steps of glycolysis, but the "investment phase" (steps 1-3) is mostly endergonic and the "payoff phase" (steps 4-10) is mostly exergonic. Please refer to your textbook to check which of the reactions are endergonic and exergonic.

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