Glycolysis
Figure %: The Glycolytic Pathway.
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. In this section we will look at glycolysis, the metabolism of glucose, a digestive product of carbohydrates found in many food products that we ingest.
Taking place in the cell cytoplasm, glycolysis actually comprises a series of nine steps involving a number of intermediate structures and specific enzymes that help catalyze each reaction. In this section, we will go through each of these reactions, learning the roles of the associated intermediates and enzymes.
Over the course of glycolysis' nine 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. 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 aneorobic respiration in a process called fermentation.