Cellular Respiration: Krebs Cycle: Videos & Practice Problems

The Krebs cycle, also known as the citric acid cycle or TCA cycle, is a crucial component of aerobic cellular respiration, representing its third stage. Understanding the terminology is essential, as different sources may refer to this cycle using any of these names interchangeably. In this cycle, Acetyl CoA is oxidized, leading to the production of energy primarily in the form of NADH and FADH₂, along with a small amount of ATP.

Prior to the Krebs cycle, glycolysis occurs outside the mitochondria, resulting in the formation of two pyruvate molecules. Following this, pyruvate oxidation converts these pyruvates into Acetyl CoA, which serves as the starting point for the Krebs cycle. As the course progresses, a deeper exploration of the Krebs cycle will reveal its significance in energy production and its role in the broader context of cellular respiration.

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, 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.

The final phase, Oxaloacetate Regeneration, is critical for the cycle's continuity. It regenerates oxaloacetate, allowing the cycle to begin anew. This phase produces one NADH and one FADH₂ molecule, completing the cycle. The regeneration of oxaloacetate ensures that the Krebs cycle can continue to process additional Acetyl CoA molecules.

For every glucose molecule that enters the cell, two rounds of the Krebs cycle occur, as each 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 Krebs cycle includes two FADH₂ molecules, two ATP molecules, six NADH molecules, and four CO₂ molecules. The CO₂ produced is exhaled as a waste product.

A helpful mnemonic to remember the outputs of the Krebs cycle is "Krebs Fan Company," where each letter corresponds to a product: F 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, representing the total output from two cycles.

Understanding the phases of the Krebs cycle is essential, as the NADH and FADH₂ produced will be utilized in the next stage of aerobic respiration, the electron transport chain (ETC), where they play a vital role in ATP production. This foundational knowledge sets the stage for further exploration of cellular respiration processes.

To determine how many turns of the Krebs cycle are required to completely break down one molecule of glucose, it's essential to understand the metabolic pathway involved. Initially, one molecule of glucose undergoes glycolysis, a process that breaks it down into two molecules of pyruvate. Following glycolysis, each pyruvate molecule is converted into Acetyl CoA through a process known as pyruvate oxidation. This results in the formation of two Acetyl CoA molecules from one glucose molecule.

Each Acetyl CoA then enters the Krebs cycle, also known as the citric acid cycle, where it undergoes a series of reactions that produce energy carriers such as NADH and FADH₂, as well as ATP. Since there are two Acetyl CoA molecules produced from one glucose molecule, the Krebs cycle must turn twice to fully process the glucose. Therefore, the answer to the question is that two turns of the Krebs cycle are needed to completely break down one molecule of glucose.