The Citric Acid Cycle is a series of eight chemical reactions utilized by all aerobic organisms to generate energy. The pathway uses
acetyl CoA and oxaloacetate to produce carbon dioxide, ATP, NADH, and FADH2. Clicking on each of the
thumbnail images will bring up a larger, labeled version of the
see the Flash movie for the following sequence of images,
The citric acid cycle is one of the metabolic pathways involved in aerobic respiration, an important process that produces energy for cells.
The citric acid cycle takes place in the matrix, or inner fluid of the mitochondrion.
The citric acid cycle involves eight chemical reactions that produce carbon dioxide, ATP, NADH and FADH2. The NADH and FADH2 are electron carriers that can be used by the electron transport chain (ETC).
In the first step of the citric acid cycle, acetyl CoA (a two-carbon molecule) and oxaloacetate (a four-carbon molecule) are combined to form citrate (a six-carbon molecule).
The citrate molecule then undergoes multiple biochemical changes during which two molecules of carbon dioxide are created before it is converted back to oxaloacetate.
However, before the citric acid cycle can begin, acetyl CoA must be produced. Acetyl CoA is created from pyruvate (the end product of glycolysis) during pyruvate oxidation.
Pyruvate oxidation results in one molecule of acetyl CoA, one molecule of carbon dioxide, and one molecule of NADH.
During the first step of the citric acid cycle, the acetyl group from acetyl CoA is transfered to oxaloacetate to form citrate.
Citrate then undergoes four more reactions to form succinate, along with producing two molecules of carbon dioxide, two molecules of NADH, and one ATP.
Succinate goes through three more reactions before finally being converted back to oxaloacetate. These reactions also produce one FADH2 molecule and one NADH.