In the absence of O2, fermentation pathways use NADH formed by glycolysis to reduce pyruvate and regenerate NAD+. The energy yield of glycolysis coupled to fermentation is low because glucose is only partially oxidized. When O2 is present, the electron carriers of cellular respiration allow for the full oxidation of glucose, so the energy yield from glucose is much higher.
learning outcomes
You should be able to:
Describe the process of fermentation.
Why is replenishing NAD+ required to continue glycolysis in the absence of O2?
NAD+ is an electron acceptor for reactions in glycolysis and pyruvate oxidation. If it is not present in its oxidized form, the oxidation of these substrates in the two reactions does not occur, and the pathways stop.
How does fermentation replenish NAD+?
Fermentation replenishes NAD+ by oxidizing NADH during chemical oxidations forming lactate or ethanol from pyruvate.
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Now that you’ve seen how cells harvest energy, let’s see how that energy moves through other metabolic pathways in the cell.