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KREBS CYCLE MADE EASY.
The Krebs cycle (named after Hans Krebs) is a part of cellular respiration. Its other names are the citric acid cycle, and the tricarboxylic acid cycle (TCA cycle). The Krebs cycle comes after the link reaction and provides the hydrogen and electrons needed for the electron transport chain.
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Glucose is the molecule that is ultimately metabolized by living things to derive energy, in the form of adenosine triphosphate, or ATP. Glucose can be stored in the body in numerous forms; glycogen is little more than a long chain of glucose molecules that is stored in muscle and liver cells, while dietary carbohydrates, proteins and fats have components that can be metabolized to glucose as well. When a molecule of glucose enters a cell, it is broken down in the cytoplasm into pyruvate.
What happens next depends on whether the pyruvate enters the aerobic respiration path (the usual result) or the lactate fermentation path (used in bouts of high-intensity exercise or oxygen deprivation) before it ultimately allows for ATP production and the release of carbon dioxide (CO2) and water (H2O) as by-products.
The Krebs cycle - also called the citric acid cycle or the tricarboxylic acid (TCA) cycle - is the first step in the aerobic pathway, and it operates to continually synthesize enough of a substance called oxaloacetate to keep the cycle going, although, as you'll see, this is not really the cycle's "mission." The Krebs cycle supplies other benefits as well. Because it includes some eight reactions (and, correspondingly, nine enzymes) involving nine distinct molecules, it is helpful to develop tools to keep the important points of the cycle straight in your mind.
As noted previously, the fate of pyruvate depends on the metabolic demands and the environment of the organism in question. In prokaryotes, glycolysis plus fermentation provides almost all of the single cell's energy needs, although some of these organisms have evolved electron transport chains that allow them to make use of oxygen to liberate ATP from metabolites (products) of glycolysis. In prokaryotes as well as in all eukaryotes but yeast, if there is no oxygen available or if the cell's energy needs cannot be fully met through aerobic respiration, pyruvate is converted to lactic acid via fermentation under the influence of the enzyme lactate dehydrogenase, or LDH.