Fermentation: why?

Your teacher has advised you to open your textbook on the day before the lab and read pages 170-173. If you did this, you found a story about how cells can get energy from sugar when there is no oxygen around. It's called fermentation, and it is nicely summarized in Figure 9.17 of the textbook.

Recall that you can harvest a little bit of energy out of glycolysis: even before you send your pyruvate to be oxidized into carbon dioxide in the Krebs cycle, you've made a net profit of 2 ATP per glucose. Not much, but if there's no oxygen to accept electrons from NADH and FADH2, then the Krebs cycle stops, and glycolysis is the best you can do.

The real problem is that step 6 of glycolysis consumes NAD+. If you keep on doing glycolysis, you will quickly run out of NAD+ (it all gets reduced to NADH, which is useless without oxygen) and glycolysis will stop. How to turn that built-up NADH back into NAD+? Fermentation.

What you need is an oxidizing agent to take electrons away from NADH. Happily, the pyruvate that you get out of glycolysis can do this job. In the cells of animals such as ourselves, this happens directly: with the help of an enzyme, pyruvate oxidizes NADH to NAD+, and is converted (reduced) to lactate - which gets shipped out of the cell and detoxified by your liver.


Other critters, like yeast, take a short detour and first convert pyruvate to 2-acetaldehyde. This is used as an oxidizing agent to regenerate NAD+. In this reaction, 2-acetaldehyde is reduced to ethanol.


Regardless of how it's done, it's an elegant way to keep glycolysis going in the absence of oxygen. In one fell swoop, you do two essential jobs: regenerate NAD+ and funnel away pyruvate. Both of these jobs are important. Why? Because if the product of any reaction builds up to a high concentration, the reaction will slow down - and can even run backwards.

(Why is this so?)



The products of fermentation can be experienced directly: when you sprint up a long hill, your legs hurt as a result of lactic acid flooding out of the muscle cells. When you consume fruit that has been heavily colonized by yeast, ethanol enters your brain and your thinking becomes sloppy. If this happens, do not drive or operate machinery.

Here is another question: is all this writing helpful? If not, I won't do it.