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Ethanol and Lactic Acid Fermentation
In the process of glycolysis, NAD+ coenzymes are used up and none are actually regenerated by the end. That means that without some other type of process that regenerates these coenzymes, the limited amount of NAD+ in the cell would eventually run out. In order to regenerate these NAD+ molecules and keep the process going, the cell metabolizes the pyruvate molecules. Under aerobic conditions, the pyruvate molecules move into the mitochondria to undergo pyruvate decarboxylation and the citric acid cycle to regenerate the NAD+ and produce even more ATP. But under anaerobic conditions, the cell uses a process known as fermentation to regenerate the NAD+ molecules. Although there are many fermentation processes, the two most common ones are ethanol fermentation and lactic acid fermentation. Yeast cells and several other organisms carry out ethanol fermentation, also known as alcoholic fermentation. In this two step process, the pyruvate first undergoes a decarboxylation process that is catalyzed by the enzyme pyruvate decarboxylase. In this process, thiamine pyrophosphate is used as a coenzyme. This produces a carbon dioxide molecule in the gas state and an acetylaldehyde. The acetylaldehyde then undergoes a second process in which alcohol dehydrogenase transfers a hydride group onto the substrate from an NADH molecule that was produced during glycolysis to form an ethanol molecule and regenerate the NAD+. Since two pyruvate molecule enter fermentation, two NAD+ molecules are regenerated. Many other organisms such as bacterial cells and eukaryotic cells (including our own cells) utilize a second mode of fermentation called lactic acid fermentation. This is a one step process catalyzed by lactate dehydrogenase. In this process the NADH is oxidized and the hydride ion is transferred onto the pyruvate to form the lactate molecule. This step occurs twice and so two NAD+ are regenerated. In our body, skeletal muscle cells utilize this process when there is an inadequate supply of oxygen, such as during rigorous exercise. Bacterial cells such as clostridium tetani, clostridium botulinum and clostridium perfringens all use this process. These are obligate anaerobes and depend purely on fermentation to make ATP energy molecules.
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