Krebs Cycle: Videos & Practice Problems

The Krebs Cycle, also known as the citric acid cycle or TCA cycle, is the third stage of aerobic cellular respiration. It begins with Acetyl CoA entering the cycle, combining with oxaloacetate to form citrate. The cycle consists of three phases: Acetyl CoA Entry, citrate oxidation, and oxaloacetate regeneration. Each complete cycle produces 2 CO₂, 1 ATP, 3 NADH, and 1 FADH₂. For every glucose molecule, two rounds of the cycle occur, yielding a total of 4 CO₂, 2 ATP, 6 NADH, and 2 FADH₂.

Krebs Cycle

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 is the starting point for the Krebs Cycle. As the course progresses, further details about the Krebs Cycle will be explored, emphasizing its role in energy production and its significance in cellular respiration.

Phases of The Krebs Cycle

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.

Krebs Cycle Example 1

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, corresponding to the two acetyl CoA molecules generated during the earlier stages of cellular respiration.

Which product of the Krebs cycle is also used as a reactant in the Krebs cycle?

• A. Citrate.
• B. ATP.
• C. Acetyl-CoA.
• D. Oxaloacetate.

Answer: (D)

Taking one molecule of glucose through glycolysis, pyruvate oxidation, and the Krebs cycle generates:

• A. 6 CO₂, 8 NADH, 2 FADH₂ and 4 ATP.
• B. 6 CO₂, 8 NADH, 1 FADH₂ and 2 ATP.
• C. 6 CO₂, 10 NADH, 2 FADH₂ and 4 ATP.
• D. 6 CO₂, 10 NADH, 2 FADH₂ and 2 ATP.

Answer: (C)

Here’s what students ask on this topic:

What is the Krebs Cycle and why is it important in cellular respiration?

The Krebs Cycle, also known as the citric acid cycle or TCA cycle, is the third stage of aerobic cellular respiration. It plays a crucial role in energy production by oxidizing Acetyl CoA to produce ATP, NADH, and FADH₂. These molecules are essential for the electron transport chain, which generates the majority of ATP in cellular respiration. The cycle also produces CO₂ as a byproduct, which is exhaled. Understanding the Krebs Cycle is vital for grasping how cells convert nutrients into usable energy.

What are the main phases of the Krebs Cycle?

The Krebs Cycle consists of three main phases: Acetyl CoA Entry, Citrate Oxidation, and Oxaloacetate Regeneration. In the first phase, Acetyl CoA combines with oxaloacetate to form citrate. The second phase involves the oxidation of citrate, producing ATP, NADH, and CO₂. The final phase regenerates oxaloacetate, producing additional NADH and FADH₂. These phases ensure the continuous production of energy carriers and the regeneration of cycle intermediates.

What are the key outputs of the Krebs Cycle per glucose molecule?

For each glucose molecule, the Krebs Cycle produces 2 ATP, 6 NADH, 2 FADH₂, and 4 CO₂. These outputs are crucial for the electron transport chain, where NADH and FADH₂ are used to generate a significant amount of ATP. The CO₂ produced is expelled from the body through exhalation. These outputs highlight the Krebs Cycle's role in cellular energy metabolism.

Why is the Krebs Cycle also called the citric acid cycle?

The Krebs Cycle is also known as the citric acid cycle because the first molecule produced in the cycle is citrate, or citric acid. When Acetyl CoA enters the cycle, it combines with oxaloacetate to form citrate. This naming highlights the importance of citrate as a key intermediate in the cycle's series of reactions.

How does the Krebs Cycle contribute to the electron transport chain?

The Krebs Cycle contributes to the electron transport chain by producing NADH and FADH₂, which are electron carriers. These molecules transport high-energy electrons to the electron transport chain, where they are used to generate a large amount of ATP through oxidative phosphorylation. This connection underscores the Krebs Cycle's role in the overall process of cellular respiration and energy production.