The Krebs cycle, also called the citric acid cycle, is the central pathway that finishes the breakdown of fuel molecules after glycolysis and pyruvate oxidation. This cheat sheet helps students track where carbon atoms, electrons, and energy carriers go during aerobic cellular respiration. It is especially useful because the cycle has many similar-sounding intermediates, enzymes, and products.
A clear reference makes it easier to connect the cycle to ATP production in the electron transport chain.
The cycle begins when acetyl-CoA combines with oxaloacetate to form citrate in the mitochondrial matrix. Each turn uses one acetyl-CoA and produces 3 NADH, 1 FADH2, 1 ATP or GTP, and 2 CO2. Because one glucose forms two acetyl-CoA molecules, the cycle turns twice per glucose.
The most important idea is that the Krebs cycle captures high-energy electrons in NADH and FADH2, which then fuel oxidative phosphorylation.
Key Facts
- The Krebs cycle occurs in the mitochondrial matrix in eukaryotic cells and in the cytoplasm of prokaryotic cells.
- Before the cycle, pyruvate oxidation converts pyruvate into acetyl-CoA, producing 1 NADH and 1 CO2 per pyruvate.
- One turn of the Krebs cycle begins when acetyl-CoA plus oxaloacetate forms citrate.
- The net yield per acetyl-CoA is 3 NADH, 1 FADH2, 1 ATP or GTP, and 2 CO2.
- The net yield per glucose from the Krebs cycle is 6 NADH, 2 FADH2, 2 ATP or GTP, and 4 CO2.
- The two carbon atoms from acetyl-CoA are released as CO2 during oxidation steps of the cycle.
- NAD+ is reduced to NADH, and FAD is reduced to FADH2 when they accept high-energy electrons and hydrogen.
- Oxaloacetate is regenerated at the end of the cycle, allowing the pathway to continue as long as acetyl-CoA and oxidized electron carriers are available.
Vocabulary
- Krebs cycle
- A cyclic pathway in cellular respiration that oxidizes acetyl-CoA and produces NADH, FADH2, ATP or GTP, and CO2.
- Citric acid cycle
- Another name for the Krebs cycle, named because citrate is the first stable molecule formed.
- Acetyl-CoA
- A two-carbon molecule that delivers an acetyl group into the Krebs cycle after pyruvate is oxidized.
- Oxaloacetate
- A four-carbon molecule that combines with acetyl-CoA to start the cycle and is regenerated at the end.
- NADH
- A reduced electron carrier that transports high-energy electrons from the Krebs cycle to the electron transport chain.
- FADH2
- A reduced electron carrier made in the Krebs cycle that donates electrons to the electron transport chain.
Common Mistakes to Avoid
- Counting one turn per glucose is wrong because one glucose produces two acetyl-CoA molecules, so the Krebs cycle turns twice per glucose.
- Saying the Krebs cycle directly makes most ATP is wrong because it mainly produces NADH and FADH2, which power ATP production in the electron transport chain.
- Placing the Krebs cycle in the cytoplasm of eukaryotic cells is wrong because it occurs in the mitochondrial matrix.
- Forgetting that oxaloacetate is regenerated is wrong because the cycle depends on oxaloacetate being available to combine with the next acetyl-CoA.
- Mixing up NADH and NAD+ is wrong because NAD+ is the empty electron carrier and NADH is the reduced, energy-rich form.
Practice Questions
- 1 If 1 acetyl-CoA enters the Krebs cycle, how many NADH, FADH2, ATP or GTP, and CO2 are produced?
- 2 If 1 glucose molecule produces 2 acetyl-CoA molecules, what is the total Krebs cycle yield of NADH and FADH2 per glucose?
- 3 A cell runs the Krebs cycle 6 times. How many CO2 molecules are released during those 6 turns?
- 4 Explain why the Krebs cycle would slow down if the electron transport chain stopped working.