The most basic form of fermentation is called homolactic fermentation, in which the only end product is lactic acid. It starts with glycolysis and its end product, pyruvate, but goes to make lactic acid in a single redox reaction. This is one of the only respiratory pathways that doesn’t produce a gaseous byproduct. It happens in mammalian muscle cells and in lactobacilli and some fungi. The sour taste of yogurt comes from the lactic acid made by lactobacilli. There is also heterolactic fermentation, in which lactic acid isn’t the only end product. Some goes on to make carbon dioxide and ethanol (using phosphoketolase). Some also goes on to make acetate or other waste products. The reasoning behind the need to do this at all is because lactic acid is too acidic to drive certain biological processes. This is why food is fermented in the first place—it drives out other bacteria and keeps them from taking hold, extending the shelf life of food. Another reason to have heterolactic fermentation is because, if things like ethanol and CO2 are produced as end products, they are volatile, leaving the situation so that the rate of forward reactions is kept up. Too high a concentration of an end product like lactic acid will drive the concentration backward, slowing the growth of the organism itself. Things like propionic acid and butyric acid become better end products because they aren’t as acidic and won’t interfere with cellular growth. Several organisms will produce hydrogen gas as part of the fermentation process, including those that make butanol, butyric acid, caproate, and glyoxylate. The hydrogen gas will help to regenerate NAD from NADH. Remember that this is how NAD is recycled in many fermentation processes. The hydrogen gas is used by methanogens and organisms that are sulfate reducers as well and, in high concentrations, can be given off as a gaseous substance.
PHOTOSYNTHESIS Photosynthesis is the process by which light energy becomes chemical energy— something common to plants, most algae, and cyanobacteria. The chemical energy is stored in carbohydrate molecules (sugars) that are essentially made by CO2 and water. In the majority of cases, the waste product of this reaction process is O2 gas. It is these
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