Fermentation Lesson 2
In last month’s lesson we explored what yeast are and we took a superficial look at what some of its contributions are in today’s economy.
Q: Why would nature evolve an organism that wants to use its food source to produce alcohol, rather than applying all that potential energy into reproduction (biomass generation) and growth?
Fermentation is highly inefficient at releasing energy because it produces only 2 molecules of ATP from one molecule of glucose (ATP is the essential energy source of living cells). In contrast, most other organisms generate 36 molecules of ATP from one glucose molecule! They do so by importing the intermediate break-down product pyruvate into their mitochondria, where it is metabolized via the Krebs cycle and oxidative phosphorylation.
Not all scientists agree, but the leading theory explaining this metabolic inefficiency suggests that fermenting yeast’s ability and/or preference to produce alcohol allowed it to “sanitize” or sterilize the surrounding area, killing competing bacteria and fungus, guaranteeing that the food source would be available only to its kin.
Q: How does fermentation occur within the yeast?
The transformation starts with glycolysis: all organisms have glycolysis occurring in their cytoplasm. The process works on glucose (6-C), splitting it into two 3-C compounds.
Glycolysis.jpg
The end of the glycolysis process yields two pyruvic acid (3-C) molecules, and a net gain of 2 ATP and two NADH per glucose molecule.
Under anaerobic conditions, the pyruvic acid is then routed by the yeast into the alcohol fermentation pathway, as illustrated below:
Glycolysis 2
Q: Why release CO2? Is this also a part of the yeast’s survival strategy?
The release of carbon dioxide (CO2) is not a deliberate action by the yeast, it is rather the result of the glycolysis transformation and the fact that CO2 is a co-product of that reaction. CO2, however, is a heavier gas than oxygen alone, thus it creates a buffer between the surface of the fermentation vessel (tank) and the ambient air, further guaranteeing an anaerobic environment, conducive to the continuous production of alcohol.
To summarize: yeast appear to have evolved an energy-inefficient way to metabolize fermentable sugars into alcohol, but this strategy has given it an evolutionary advantage, by using the resulting alcohol to kill competing organisms. Us humans are benefiting immensely from this ability, seldomly thinking about its unexpected origins or its future!
Join us again next month, as we continue our deep-dive into this fascinating world!