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 derived from carbohydrates, fats, and proteins into carbon dioxide, while producing high-energy electron carriers.
2. Key Steps and Products
Acetyl-CoA Formation: Pyruvate from glycolysis is converted into 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, producing NADH and releasing CO₂, forming α-ketoglutarate.
Second Oxidation: α-Ketoglutarate is further oxidized, yielding NADH, releasing CO₂, and producing succinyl-CoA.
ATP (or GTP) Production: Succinyl-CoA is converted to succinate, generating 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 produced donate electrons to the electron transport chain, driving further ATP synthesis.
Metabolic Integration: Provides intermediates for biosynthetic pathways, linking carbohydrate, fat, and protein metabolism.
In a Nutshell
The citric acid cycle is essential for energy production, oxidizing acetyl-CoA to CO₂ and generating high-energy electron carriers.
It integrates various metabolic pathways, underscoring its central role in cellular metabolism.