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What effect do varying oxygen levels have on Saccharomyces cerevisiae's (baker's yeast) production of ethanol from glucose?
Question Date: 2002-10-28
Answer 1:

Your question is so specific, it sounds like a science fair project! While I study microorganisms, I don't study yeast. I'll will try my best to answer your question, however.

First, a little background on fermentation: Every organism on the planet needs a source of energy for life and growth. Organisms that don't photosynthesize get this energy by oxidizing chemicals (e.g. carbon dioxide, glucose) with the help of an added substance that accepts electrons, known as an electron acceptor. (Oxidation simply means a loss of electrons.) The electron acceptor can be oxygen, iron, pyruvate, sulfate, carbon dioxide or other compounds.

Most of the time, animals get their energy via a complex metabolic process known as the citric acid (or TCA) cycle, wherein oxygen is the electron acceptor. This particular process generates the most amount of energy per unit organic matter oxidized. Many other processes, which employ different electron acceptors, can also generate energy. Bakers' yeast is part of a group of microorganisms that gain energy through fermentation. In fermentation, organic matter serves as both the energy source and the electron acceptor, and the energy yield is 18 times lower. There are different fermentation pathways, and fermentation products can include acids (such as the lactic acid that builds up in your muscles) or carbon dioxide plus ethanol (as with Saccharomyces cerevisiae). The alcohol in beer, wine, whisky, etc are all produced through fermentation, and foods such as yogurt, cheese and most types of bread depend upon fermentation as well. Yeasts are very commercially important organisms.

To answer your question (finally!): The relationship between oxygen availability and ethanol production in yeast is complex. The production of ethanol by Saccharomyces cerevisiae is done commercially in large quantities, both for biofuel and for alcoholic beverages, and so manufacturers are interested in finding ways to maximize ethanol production. For this reason, the effect of oxygen on ethanol production has been fairly well studied. The presence of oxygen at normal atmospheric concentrations will inhibit any fermentation process. At very low concentrations, however, oxygen can actually increase the yield of ethanol. This is sometimes referred to as the microaerobic effect. As the level of oxygen is increased beyond this point, byproducts such as glycerol and acetic acid (vinegar) are produced by the yeast in addition to ethanol, and the yield and purity of the ethanol are reduced. As I mentioned before, fermentation will stop altogether once oxygen concentrations become too high. This is sometimes referred to as the Pasteur effect.

Answer 2:

Both yeasts and people get energy for our life processes and activities from breaking down molecules like glucose. The first step is called glycolysis (which, conveniently, means "sugar breakdown"). The end product of this process is called pyruvic acid. If there's oxygen around, this product is broken down even further to carbon dioxide and water to release a lot more energy. This is called "aerobic respiration".

In animals like us, when there's not enough oxygen around we turn the pyruvic acid into lactic acid until we have enough oxygen to get back to normal. Lactic acid is one of the reasons your muscles get sore when you exercise hard enough that you can't keep up with your body's oxygen demand ("anerobic respiration"). In yeasts, the pyruvic acid is turned into ethanol and carbon dioxide when there's a lack of oxygen. Ethanol is a poison, though, so they will produce it until the solution around them is about 12% alcohol, then they die from ethanol poisoning. So yeasts do aerobic respiration when they can because they get a lot more energy from their food and don't produce the poisonous ethanol. When they can't do aerobic respiration, they switch to anearobic respiration or fermentation. What gas do you think is in the holes in bread?

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