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 SparkNote 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. (Note: specific knowledge of the nine steps of glycolysis is not necessary for the AP Biology test. In regard to that test, this summary presents all information necessary about glycolysis and the first two sections of this SparkNote can be skipped. The third section on fermentation, however, will be covered on the AP test).
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, covered in the third section of this SparkNote on glycolysis.
In the first two sections of this SparkNote, we will look at glycolysis in two major stages. The first involves the phosphorylation of the glucose ring in preparation for an eventual breakdown into two 3-carbon molecules. In the second stage, the two 3-carbon molecules are converted into pyruvate.