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UPSC MainsZOOLOGY-PAPER-II201915 Marks
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Q25.

Make a diagrammatic presentation of Krebs' cycle.

How to Approach

This question requires a detailed, diagrammatic representation of the Krebs cycle (also known as the citric acid cycle). The answer should focus on accurately depicting the cycle's steps, including the input molecules, intermediate compounds, enzymes involved (mentioning key regulatory enzymes is beneficial), and the output molecules (NADH, FADH2, ATP/GTP, CO2). A clear, labelled diagram is crucial. The answer should also briefly explain the significance of the cycle in cellular respiration. Focus on clarity and accuracy of the diagram.

Model Answer

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Introduction

The Krebs cycle, a series of chemical reactions crucial to all aerobic organisms, is the central metabolic pathway in cellular respiration. Discovered by Hans Krebs in 1937, it occurs in the mitochondrial matrix and plays a pivotal role in oxidizing acetyl-CoA, derived from carbohydrates, fats, and proteins, to generate energy carriers (NADH and FADH2) and precursor metabolites for biosynthesis. The cycle doesn’t directly produce a large amount of ATP, but its primary function is to generate high-energy electron carriers that fuel the electron transport chain, ultimately leading to substantial ATP production. A diagrammatic representation is essential to understand its complex interplay of reactions.

The Krebs Cycle: A Diagrammatic Presentation

The Krebs cycle begins with the condensation of acetyl-CoA (a two-carbon molecule) with oxaloacetate (a four-carbon molecule) to form citrate (a six-carbon molecule). The cycle then proceeds through a series of enzymatic reactions, regenerating oxaloacetate and releasing two molecules of carbon dioxide (CO2) for each molecule of acetyl-CoA processed. Crucially, the cycle generates three molecules of NADH, one molecule of FADH2, and one molecule of GTP (which is readily converted to ATP) per acetyl-CoA molecule.

Krebs Cycle Diagram

Key Enzymes and Regulatory Steps: Several enzymes within the Krebs cycle are subject to regulation, controlling the cycle's rate based on cellular energy needs. Citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase are key regulatory enzymes. These enzymes are allosterically regulated by molecules like ATP, ADP, NADH, and succinyl-CoA.

Detailed Steps of the Krebs Cycle:

  • Step 1: Acetyl-CoA + Oxaloacetate → Citrate (Catalyzed by Citrate Synthase)
  • Step 2: Citrate → Isocitrate (Catalyzed by Aconitase)
  • Step 3: Isocitrate → α-Ketoglutarate + CO2 + NADH (Catalyzed by Isocitrate Dehydrogenase – a key regulatory step)
  • Step 4: α-Ketoglutarate + CoA → Succinyl-CoA + CO2 + NADH (Catalyzed by α-Ketoglutarate Dehydrogenase Complex)
  • Step 5: Succinyl-CoA → Succinate + CoA + GTP (Catalyzed by Succinyl-CoA Synthetase)
  • Step 6: Succinate → Fumarate + FADH2 (Catalyzed by Succinate Dehydrogenase)
  • Step 7: Fumarate → Malate (Catalyzed by Fumarase)
  • Step 8: Malate → Oxaloacetate + NADH (Catalyzed by Malate Dehydrogenase)

Output per Acetyl-CoA molecule:

  • 2 molecules of CO2
  • 3 molecules of NADH
  • 1 molecule of FADH2
  • 1 molecule of GTP (converted to ATP)

The NADH and FADH2 produced are then utilized in the electron transport chain to generate a proton gradient, which drives ATP synthesis via oxidative phosphorylation.

Conclusion

The Krebs cycle is a fundamental metabolic pathway that links glycolysis and fatty acid oxidation to the electron transport chain, enabling efficient energy production in aerobic organisms. Its intricate series of reactions, regulated by key enzymes, ensures that energy generation is responsive to cellular demands. Understanding the Krebs cycle is crucial for comprehending cellular metabolism and its implications for various physiological processes and diseases. Further research continues to refine our understanding of its regulation and its role in metabolic flexibility.

Answer Length

This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.

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Additional Resources

Key Definitions

Acetyl-CoA
Acetyl-CoA is a molecule that carries chemical energy in the form of a thioester bond to coenzyme A. It is a central metabolic intermediate, derived from carbohydrates, fats, and proteins, and serves as the primary fuel for the Krebs cycle.
Oxidative Phosphorylation
Oxidative phosphorylation is the metabolic pathway in which cells use enzymes to oxidize nutrients, thereby releasing energy which is used to produce adenosine triphosphate (ATP).

Key Statistics

Approximately 20 ATP molecules are produced per glucose molecule through oxidative phosphorylation, fueled by the NADH and FADH2 generated during glycolysis, pyruvate oxidation, and the Krebs cycle.

Source: Textbook of Biochemistry with Clinical Correlations (Devlin, 2011)

The mitochondrial matrix, where the Krebs cycle occurs, occupies approximately 25% of the volume of a typical mammalian cell.

Source: Alberts et al., Molecular Biology of the Cell (2015)

Examples

Pyruvate Dehydrogenase Complex Deficiency

A deficiency in the pyruvate dehydrogenase complex (PDC), which converts pyruvate to acetyl-CoA, disrupts the entry of carbohydrates into the Krebs cycle, leading to lactic acidosis and neurological problems. This illustrates the critical importance of the Krebs cycle for energy metabolism.

Frequently Asked Questions

▶What happens to the CO2 produced during the Krebs cycle?

The CO2 produced during the Krebs cycle is transported via the bloodstream to the lungs and exhaled during respiration. It is a waste product of cellular respiration.

Topics Covered

BiologyBiochemistryKrebs CycleCellular RespirationMetabolic Pathways
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