What is the Krebs Cycle?
The Krebs cycle (citric acid cycle) is the second major stage of cellular respiration, taking place in the mitochondrial matrix. Each turn oxidizes an acetyl-CoA molecule, releasing carbon dioxide and capturing high-energy electrons in NADH and FADH2 for the electron transport chain.
The Krebs cycle is a series of eight enzyme-catalyzed reactions in the mitochondrial matrix that oxidize acetyl-CoA, producing 3 NADH, 1 FADH2, 1 GTP (ATP), and 2 CO2 per turn.
- 1.Acetyl-CoA + Oxaloacetate — Acetyl-CoA joins oxaloacetate to form citrate.
- 2.Citrate → Isocitrate — Citrate is rearranged into its isomer, isocitrate.
- 3.Isocitrate → α-Ketoglutarate — Oxidative decarboxylation releases CO₂ and forms NADH.
- 4.α-Ketoglutarate → Succinyl-CoA — A second CO₂ is released and another NADH is made.
- 5.Succinyl-CoA → Succinate — Substrate-level phosphorylation produces GTP (ATP).
- 6.Succinate → Fumarate — Oxidation forms FADH₂.
- 7.Fumarate → Malate — Water is added across the double bond.
- 8.Malate → Oxaloacetate — Final oxidation regenerates oxaloacetate and forms NADH.
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Step-by-step worked examples
One acetyl-CoA molecule enters the Krebs cycle for a single turn. How many NADH, FADH2, GTP, and CO2 are produced?
Per turn: 3 NADH 1 FADH₂ 1 GTP 2 CO₂
A cell fully oxidizes one glucose molecule. Glycolysis yields 2 pyruvate, which become 2 acetyl-CoA, so the Krebs cycle turns twice. Find the total NADH, FADH2, and GTP produced by the Krebs cycle.
2 turns × 3 NADH = 6 NADH 2 turns × 1 FADH₂ = 2 FADH₂ 2 turns × 1 GTP = 2 GTP
Using ATP-equivalent conversion factors (NADH ≈ 2.5 ATP, FADH2 ≈ 1.5 ATP, GTP = 1 ATP), calculate the total ATP yield from 2 Krebs cycle turns.
NADH: 6 × 2.5 = 15 ATP FADH₂: 2 × 1.5 = 3 ATP GTP: 2 × 1 = 2 ATP Total = 15 + 3 + 2 = 20 ATP
Flashcards
Quick quiz
Q1.Where in the cell does the Krebs cycle occur?
Q2.What molecule combines with acetyl-CoA to start the cycle?
Q3.How many NADH molecules are produced per turn of the Krebs cycle?
Q4.How many total turns of the Krebs cycle occur per glucose molecule?
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Common mistakes
The Krebs cycle directly produces most of a cell's ATP. — Correct: Most ATP comes later from the electron transport chain, using the NADH/FADH₂ made by the Krebs cycle.
The Krebs cycle only happens once per glucose molecule. — Correct: It turns twice per glucose, since glycolysis yields two acetyl-CoA molecules.
The Krebs cycle occurs in the cytoplasm. — Correct: It occurs in the mitochondrial matrix.
Oxygen is directly consumed in the Krebs cycle reactions. — Correct: Oxygen is used later, as the final electron acceptor in the electron transport chain.
FAQ
What is the Krebs cycle?
The Krebs cycle (citric acid cycle) is a set of eight reactions in the mitochondrial matrix that oxidize acetyl-CoA to capture energy in NADH, FADH2, and GTP while releasing CO2.
What is the Krebs cycle's summary formula?
One turn: Acetyl-CoA + 3 NAD⁺ + FAD + GDP + Pi + 2 H₂O → 2 CO₂ + 3 NADH + FADH₂ + GTP + CoA.
What are examples of the Krebs cycle's products?
One turn yields 3 NADH, 1 FADH₂, 1 GTP, and 2 CO₂; a full glucose molecule drives two turns, doubling those totals.
How do you calculate ATP yield from the Krebs cycle?
Multiply NADH by ~2.5 ATP and FADH2 by ~1.5 ATP, add the GTP (1 ATP each) — e.g. 2 turns give about 20 ATP-equivalents.