Citric Acid Cycle
Definition and meaning of Citric Acid Cycle in chemistry.
The citric acid cycle is a closed biological loop of eight enzyme-catalyzed chemical reactions. This cycle oxidizes acetyl-CoA into carbon dioxide gas while capturing the newly released energy. The process captures this chemical energy by forming complex molecules of NADH, FADH2, and GTP. It serves as the central hub of aerobic respiration inside the mitochondrial matrix of eukaryotic cells.
In more detail
Each turn of the cycle begins with a two-carbon acetyl group from an acetyl-CoA molecule. This small group joins a four-carbon oxaloacetate molecule to form a six-carbon molecule called citrate. Citrate is the specific tricarboxylic acid that gives this metabolic cycle its primary name.
A series of chemical oxidation steps then removes two carbon atoms to create CO2 gas. These chemical steps release energy and successfully regenerate the starting oxaloacetate molecule for the next turn. This crucial regeneration allows the biological cycle to turn again and repeat the process continuously.
The cycle produces high-energy NADH and FADH2 molecules that carry excited, active electrons. These carrier molecules transport their electrons directly to the cell's nearby electron transport chain. The electron transport chain uses these active electrons to manufacture most of the cell's ATP energy.
ATP is the primary energy currency that powers almost all cellular functions in the body. Scientists also call this pathway the Krebs cycle, named after the pioneering biochemist Hans Krebs. Another common biological name for this important metabolic process is the tricarboxylic acid (TCA) cycle.
Key facts
| Also known as | Krebs cycle, TCA cycle |
|---|---|
| Location | Mitochondrial matrix |
| Yield per turn | 3 NADH, 1 FADH2, 1 GTP, 2 CO2 |
| Starting molecules | Acetyl-CoA and oxaloacetate |
One complete turn of the cycle oxidizes a single acetyl group to yield usable chemical energy. This single metabolic turn produces three NADH, one FADH2, one GTP, and two CO2 molecules. Breaking down one single glucose molecule naturally produces two separate acetyl-CoA starting molecules. Therefore, a single glucose molecule provides enough chemical fuel to drive two full turns of the cycle.
Frequently asked questions
Why is it called the citric acid cycle?
The first molecule formed in each turn is citrate. This compound is the ion of citric acid, which gives the cycle its common name.
What does the citric acid cycle produce?
Each turn produces two molecules of CO2, three NADH, one FADH2, and one GTP. The NADH and FADH2 molecules help generate most of the cell's ATP.