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What happens during anaerobic respiration?

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Anaerobic respiration refers to the enzymatic breakdown of sugar for energy in the absence of oxygen. Most cells in the human body can perform anaerobic respiration, at least for short periods of time. One molecule of glucose metabolized anaerobically to pyruvate yields two molecules of adenosine triphosphate (ATP); this is a low yield compared to aerobic respiration, in which 36 ATP molecules are generated for each glucose molecule metabolized.

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Unlike aerobic respiration, which uses special organelles called mitochondria to synthesize ATP, anaerobic respiration relies exclusively on substrate phosphorylation in the cytoplasm. The basic strategy is to transfer an energy-rich phosphate group from a three-carbon derivative of glucose to adenosine diphosphate (ADP), resulting in the formation of ATP.

There are two specific phosphorylation reactions in anaerobic respiration responsible for ATP production. First, Phosphoglycerate kinase converts 1,3 bisphosphoglycerate into 3-phosphoglycerate, yielding an ATP. Second, Pyruvate kinase removes a phosphate group from phosphoenol pyruvate, converting it to pyruvate and simultaneously converting ADP to ATP.

In the absence of oxygen, pyruvate can be converted into lactic acid, acetic acid (vinegar), or ethanol. The first two products are metabolic dead ends in human cells. The last pathway is found in brewer's yeast. Also known as fermentation, it is the basis of wine and beer production throughout the world.

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