The Krebs cycle, also called the citric acid cycle, is a central pathway in cellular respiration. It completes the breakdown of fuel molecules by processing acetyl-CoA, a two-carbon molecule made from carbohydrates, fats, and some amino acids. This cycle matters because it captures high-energy electrons that cells later use to make most of their ATP.
In eukaryotic cells, it occurs in the mitochondrial matrix, the fluid-filled space inside the inner mitochondrial membrane.
Each turn of the cycle adds acetyl-CoA to a four-carbon molecule, releases two carbon atoms as CO2, and regenerates the starting molecule so the cycle can continue. The main energy products are NADH and FADH2, which carry electrons to the electron transport chain in the inner mitochondrial membrane. A small amount of ATP or GTP is made directly during the cycle.
Because one glucose molecule produces two acetyl-CoA molecules, the Krebs cycle turns twice for each glucose molecule.
Key Facts
- Location in eukaryotes: the Krebs cycle occurs in the mitochondrial matrix.
- Main input per turn: acetyl-CoA + 3 NAD+ + FAD + ADP or GDP + Pi + 2 H2O.
- Main output per turn: 2 CO2 + 3 NADH + 1 FADH2 + 1 ATP or GTP + CoA.
- Per glucose: the cycle turns 2 times because glycolysis produces 2 pyruvate, which form 2 acetyl-CoA.
- Per glucose from the Krebs cycle: 4 CO2 + 6 NADH + 2 FADH2 + 2 ATP or GTP.
- Overall purpose: oxidize acetyl-CoA and load electron carriers for oxidative phosphorylation.
Vocabulary
- Krebs cycle
- A cyclic metabolic pathway that oxidizes acetyl-CoA to CO2 while producing NADH, FADH2, and ATP or GTP.
- Citric acid cycle
- Another name for the Krebs cycle, named because citrate is the first six-carbon molecule formed in the pathway.
- Mitochondrial matrix
- The fluid-filled compartment inside the inner mitochondrial membrane where the Krebs cycle occurs in eukaryotic cells.
- Acetyl-CoA
- A two-carbon molecule that carries an acetyl group into the Krebs cycle for oxidation.
- Electron carrier
- A molecule such as NADH or FADH2 that transports high-energy electrons to another process, such as the electron transport chain.
Common Mistakes to Avoid
- Saying the Krebs cycle happens in the cytoplasm. This is wrong for eukaryotic cells because the cycle occurs in the mitochondrial matrix.
- Counting one turn of the cycle per glucose. This is wrong because one glucose forms two acetyl-CoA molecules, so the cycle turns twice per glucose.
- Thinking the Krebs cycle makes most of the cell's ATP directly. This is wrong because it makes only a small amount directly, while most ATP comes later from oxidative phosphorylation using NADH and FADH2.
- Forgetting that CO2 is released in the Krebs cycle. This is wrong because each turn releases two CO2 molecules as acetyl-CoA is oxidized.
Practice Questions
- 1 One turn of the Krebs cycle produces 3 NADH, 1 FADH2, 1 ATP, and 2 CO2. How many of each are produced from 4 turns of the cycle?
- 2 If a cell breaks down 3 glucose molecules completely through the Krebs cycle, how many total turns of the cycle occur, and how many CO2 molecules are released by the cycle?
- 3 Explain why the Krebs cycle depends on the electron transport chain even though they are separate parts of cellular respiration.