Rucete ✏ Campbell Biology In a Nutshell
Unit 2 THE CELL — Concept 9.3 The Citric Acid Cycle Completes the Energy-Yielding Oxidation of Organic Molecules
The citric acid cycle, also known as the Krebs cycle, is a crucial metabolic pathway that completes the oxidation of organic molecules, leading to the production of ATP, NADH, and FADH₂. This cycle plays a vital role in cellular respiration and energy production.
1. Overview of the Citric Acid Cycle
- Location: Mitochondrial matrix in eukaryotic cells.
- Function: Oxidizes acetyl-CoA into carbon dioxide and generates NADH and FADH₂.
2. Key Steps and Products
- Acetyl-CoA Formation: Pyruvate is converted to acetyl-CoA, releasing CO₂ and producing NADH.
- Citrate Formation: Acetyl-CoA combines with oxaloacetate to form citrate.
- Isomerization: Citrate is rearranged into isocitrate.
- First Oxidation: Isocitrate is oxidized to α-ketoglutarate, releasing CO₂ and forming NADH.
- Second Oxidation: α-Ketoglutarate is converted to succinyl-CoA, releasing CO₂ and generating NADH.
- ATP (or GTP) Production: Succinyl-CoA is converted to succinate, producing ATP (or GTP).
- Third Oxidation: Succinate is oxidized to fumarate, producing FADH₂.
- Hydration: Fumarate is hydrated to form malate.
- Fourth Oxidation: Malate is oxidized to oxaloacetate, producing NADH and completing the cycle.
3. Energy Yield per Acetyl-CoA Molecule
- NADH: 3 molecules
- FADH₂: 1 molecule
- ATP (or GTP): 1 molecule
- CO₂: 2 molecules
4. Significance in Metabolism
- Electron Carriers: NADH and FADH₂ feed into the electron transport chain for ATP production.
- Metabolic Integration: Provides intermediates for biosynthesis and links major nutrient pathways.
In a Nutshell
- The citric acid cycle is a key energy-producing process that generates electron carriers and CO₂ from acetyl-CoA.
- It connects carbohydrate, fat, and protein metabolism and fuels the electron transport chain.