Citric Acid Cycle
Definition and meaning of Citric Acid Cycle in chemistry.
The citric acid cycle is a closed loop of eight enzyme-catalyzed reactions that oxidizes acetyl-CoA to carbon dioxide, capturing the released energy as NADH, FADH2, and GTP. It is the central hub of aerobic respiration and runs in the mitochondrial matrix of eukaryotic cells.
In more detail
Each turn begins when a two-carbon acetyl group from acetyl-CoA joins the four-carbon oxaloacetate to form six-carbon citrate, the tricarboxylic acid the cycle is named for. A series of oxidation and decarboxylation steps then releases two carbons as CO2 and regenerates oxaloacetate, so the cycle can turn again. The NADH and FADH2 it produces carry electrons to the electron transport chain, where most of the cell's ATP is made. The pathway is also called the Krebs cycle, after Hans Krebs, and the tricarboxylic acid (TCA) cycle.
Key facts
| Field | Biochemistry |
|---|---|
| Also known as | Krebs cycle, tricarboxylic acid (TCA) cycle |
| Location | Mitochondrial matrix in eukaryotes; cytoplasm in prokaryotes |
| Yield per turn | 3 NADH, 1 FADH2, 1 GTP, 2 CO2 |
| Starting molecules | Acetyl-CoA and oxaloacetate |
One turn of the cycle oxidizes a single acetyl group and yields 3 NADH, 1 FADH2, 1 GTP, and 2 CO2. Because breaking down one glucose molecule produces two acetyl-CoA, a single glucose drives two turns of the cycle.
Frequently asked questions
Why is it called the citric acid cycle?
The first molecule formed in each turn is citrate, the ion of citric acid, which is a tricarboxylic acid. The cycle is named for this compound, and its other name, the tricarboxylic acid (TCA) cycle, refers to the same thing.
What does the citric acid cycle produce?
Per turn it produces 2 molecules of CO2, 3 NADH, 1 FADH2, and 1 GTP (which is equivalent to ATP). The NADH and FADH2 then feed the electron transport chain, which generates most of the cell's ATP.
Where does the citric acid cycle take place?
In eukaryotic cells it occurs in the mitochondrial matrix. In prokaryotes, which have no mitochondria, it takes place in the cytoplasm.