The Krebs Cycle, also known as the citric acid cycle, is a crucial part of aerobic cellular respiration, consisting of a series of reactions that can be divided into three distinct phases: Acetyl CoA Entry, Citrate Oxidation, and Oxaloacetate Regeneration.
In the first phase, Acetyl CoA Entry, the two carbon atoms from Acetyl CoA combine with oxaloacetate, a four-carbon molecule present in the mitochondria, to form citrate (or citric acid), a six-carbon compound. It is important to note that the CoA portion of Acetyl CoA does not enter the cycle; it is recycled for future reactions. This initial reaction is significant as it marks the beginning of the Krebs Cycle.
The second phase, Citrate Oxidation, involves the rearrangement and oxidation of citrate. During this phase, citrate loses electrons, resulting in the production of one ATP molecule through substrate-level phosphorylation, two NADH molecules, and two carbon dioxide (CO2) molecules. This oxidation process is essential for energy production and the release of CO2 as a waste product.
In the final phase, Oxaloacetate Regeneration, the cycle must regenerate oxaloacetate to continue the process. This phase also involves further oxidation, producing one NADH and one FADH2 molecule, thus completing the cycle and allowing it to start anew with another Acetyl CoA molecule.
For every glucose molecule that enters the cell, two rounds of the Krebs Cycle occur, as one glucose is split into two pyruvate molecules, which are then converted into two Acetyl CoA molecules. Therefore, the total output from two revolutions of the cycle includes two FADH2, two ATP, six NADH, and four CO2 molecules. This can be remembered using the mnemonic "Krebs Fan Company," where 'F' stands for FADH2, 'A' for ATP, 'N' for NADH, and 'C' for CO2. The numbers associated with these products are 2, 2, 6, and 4, respectively.
Understanding these phases and their outputs is essential, as the NADH and FADH2 produced will be utilized in the next stage of aerobic cellular respiration, the electron transport chain (ETC), while the CO2 is exhaled as a waste product. This foundational knowledge of the Krebs Cycle sets the stage for further exploration of cellular respiration processes.