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Now that we have a general understanding of the broad topics of metabolism and respiration, we will turn our discussion to more specific metabolic pathways that lead to the derivation of ATP. The first is glycolysis, the metabolism of glucose, a digestive product of carbohydrates found in many food products that we ingest. Glucose is a six-membered ring molecule found in the blood and is usually a result of the breakdown of complex carbohydrates into simple sugars. It enters cells through specific transporter proteins that move it from outside the cell into the cell's cytosol.
Taking place in the cell cytoplasm, glycolysis comprises a series of ten steps involving a number of intermediate structures and specific enzymes that help catalyze each reaction. These distinct reactions convert glucose into pyruvate. Over the course of glycolysis' ten steps, the 6-carbon molecule glucose is broken down to two 3-carbon pyruvate molecules. The reaction does not occur spontaneously: 2 ATP molecules must be broken down to drive the splitting of glucose into the 2 pyruvates. However, in the course of the breakdown of glucose, the glycolysis reaction produces four ATP, resulting in a net gain of two ATP for the entire process. Glycolysis also results in the production of 2 NADH molecules, which eventually play an important role in the production of additional ATP in the electron transport chain. Glycolysis itself is an anaerobic process. If we go back and take count of our ATP usage and generation, we find that we have consumed two molecules of ATP and generate four to leave a net gain of two ATP molecules from the glycolytic pathway. We have gone from our starting product, glucose, to our final product, pyruvate. After a cell has completed glycolysis, and depending on the circumstances in which the cell finds itself, that cell can either move into the process of aerobic respiration and commence the citric acid cycle or continue with less efficient anaerobic respiration in a process called fermentation.
