- The Citric Acid Cycle (also called the Krebs Cycle or TCA Cycle) is a series of chemical reactions that take place in the mitochondria of cells.
- Its job is to release energy from food molecules (mainly glucose, fats, and proteins).
- It is the second stage of cellular respiration, after glycolysis and before oxidative phosphorylation.
Main Purpose
- Break down a small molecule called acetyl-CoA (made from glucose, fats, proteins).
- Produce:
- NADH and FADH₂ (energy carriers for the next step)
- ATP (or GTP) (direct energy)
- CO₂ (waste gas we breathe out)
Steps of the Citric Acid Cycle (Simplified)
- Acetyl-CoA + Oxaloacetate → Citrate
- Acetyl-CoA (2 carbons) combines with oxaloacetate (4 carbons) to form citrate (6 carbons).
- Citrate → Isocitrate
- Molecule rearranges slightly.
- Isocitrate → α-Ketoglutarate + CO₂ + NADH
- One CO₂ is released, and NADH is made.
- α-Ketoglutarate → Succinyl-CoA + CO₂ + NADH
- Another CO₂ is released, more NADH formed.
- Succinyl-CoA → Succinate + ATP (or GTP)
- Direct energy (ATP/GTP) is produced.
- Succinate → Fumarate + FADH₂
- FADH₂ is made.
- Fumarate → Malate
- Rearrangement continues.
- Malate → Oxaloacetate + NADH
- Cycle regenerates oxaloacetate, ready to start again.
Energy Yield (per 1 Acetyl-CoA)
- 3 NADH
- 1 FADH₂
- 1 ATP (or GTP)
- 2 CO₂ (waste)
Since each glucose makes 2 Acetyl-CoA, the cycle runs twice per glucose, doubling the yield.
Why is it Important?
- Provides most of the high-energy electron carriers (NADH, FADH₂) for oxidative phosphorylation.
- Releases CO₂ (what we exhale).
- Central hub of metabolism → connects carbohydrates, fats, and proteins.
In short:
The Citric Acid Cycle is a cycle of reactions in the mitochondria that breaks down acetyl-CoA, produces NADH, FADH₂, ATP, and releases CO₂. It is the energy center of the cell’s metabolism.