What is the Krebs Cycle?
The Krebs cycle (citric acid cycle) is the second stage of aerobic respiration, taking place in the mitochondrial matrix. It fully oxidizes acetyl-CoA from pyruvate, releasing carbon dioxide and capturing energy in electron carriers that fuel the electron transport chain.
The Krebs cycle oxidizes acetyl-CoA in the mitochondrial matrix, producing 3 NADH, 1 FADH2, 1 ATP (GTP), and 2 CO2 per turn — and it turns twice for every glucose molecule.
- 1.Acetyl-CoA + Oxaloacetate → Citrate — Acetyl-CoA combines with oxaloacetate to form the 6-carbon citrate, starting the cycle.
- 2.Citrate → Isocitrate → α-Ketoglutarate — Citrate is rearranged and oxidized, releasing CO2 and producing 1 NADH.
- 3.α-Ketoglutarate → Succinyl-CoA — A second CO2 is released and a second NADH is produced.
- 4.Succinyl-CoA → Succinate — Substrate-level phosphorylation directly generates 1 ATP (as GTP).
- 5.Succinate → Fumarate → Malate — Succinate is oxidized, producing 1 FADH2.
- 6.Malate → Oxaloacetate — Malate is oxidized to regenerate oxaloacetate, producing a third NADH — the cycle is ready to repeat.
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Step-by-step worked examples
One acetyl-CoA molecule enters the Krebs cycle for one turn. How many NADH, FADH2, and ATP are produced?
Each turn produces 3 NADH Each turn produces 1 FADH2 Each turn produces 1 ATP (GTP) directly
One glucose molecule yields 2 acetyl-CoA, so the Krebs cycle turns twice. Total NADH produced?
NADH per turn = 3 Turns per glucose = 2 Total NADH = 2 × 3 = 6 NADH
If a cell has 4 acetyl-CoA molecules available, how many total ATP (GTP) come directly from the Krebs cycle?
ATP per turn = 1 Turns = 4 (one per acetyl-CoA) Total ATP = 4 × 1 = 4 ATP
Flashcards
Quick quiz
Q1.Where does the Krebs cycle take place?
Q2.How many NADH are produced per turn of the Krebs cycle?
Q3.How many times does the Krebs cycle turn for one glucose molecule?
Q4.What molecule enters the Krebs cycle to start it?
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Common mistakes
The Krebs cycle uses glucose directly. — Correct: Glucose is broken down in glycolysis first; the Krebs cycle uses acetyl-CoA.
The Krebs cycle only turns once per glucose. — Correct: It turns twice per glucose, because glycolysis produces two acetyl-CoA molecules.
The Krebs cycle happens in the cytoplasm. — Correct: It takes place in the mitochondrial matrix.
The Krebs cycle produces most of a cell's ATP directly. — Correct: It produces only 1 ATP (GTP) per turn directly — most ATP comes later from the electron transport chain using its NADH and FADH2.
FAQ
What is the Krebs cycle?
The Krebs cycle (citric acid cycle) is the stage of aerobic respiration that oxidizes acetyl-CoA in the mitochondrial matrix, producing NADH, FADH2, ATP, and CO2.
What is the formula for NADH output in the Krebs cycle?
NADH = turns × 3, where turns is the number of acetyl-CoA molecules entering (2 per glucose).
What are examples of Krebs cycle calculations?
For 2 turns (one glucose), total NADH = 2 × 3 = 6 NADH, plus 2 FADH2 and 2 ATP.
How is the Krebs cycle related to the electron transport chain?
The NADH and FADH2 produced by the Krebs cycle carry electrons to the electron transport chain, which uses them to generate most of the cell's ATP.