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 smaller 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 is 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 a four-carbon molecule called oxaloacetate, resulting in the formation of 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 step 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, leading to 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. The regeneration of oxaloacetate ensures that the cycle can start anew with the next Acetyl CoA molecule.

For every glucose molecule that enters the cell, two Acetyl CoA molecules are produced, necessitating two complete revolutions of the Krebs Cycle. Therefore, the total output from two cycles includes two ATP, six NADH, two FADH₂, and four CO₂ molecules. A helpful mnemonic to remember these outputs is "Krebs Fan Company," where each letter corresponds to the products: 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.

Ultimately, the NADH and FADH₂ produced in the Krebs Cycle will be utilized in the next stage of aerobic cellular respiration, the electron transport chain (ETC), where they play a vital role in generating additional ATP. Understanding the phases and outputs of the Krebs Cycle is essential for grasping the overall process of energy production in cells.

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 enzymatic reactions. Since there are two acetyl CoA molecules produced from one glucose molecule, the Krebs Cycle will turn twice to fully process the glucose. Therefore, the total number of turns of the Krebs Cycle needed to completely break down one molecule of glucose is two.

In summary, for every molecule of glucose, two turns of the Krebs Cycle are necessary, confirming that the correct answer is two turns.