The citric acid cycle is part of the metabolic process called aerobic respiration. Aerobic respiration occurs in the cells of all living organisms, from plants to humans. In aerobic respiration, pyruvate molecules, derived from glucose, produce energy for the cell within the citric acid cycle. During the period pyruvate spends in the citric acid cycle, its carbon molecules are expelled as waste.
Aerobic respiration is also known as aerobic metabolism, oxidative metabolism or cellular respiration. It is a metabolic process where chemical bonds are broken to create energy in the form of adenosine triphosphate, or ATP. The process is termed aerobic, because it only occurs when oxygen is readily available for the cell to use. The carbohydrate glucose and oxygen produce energy for the living cell.
The first process in cellular respiration is glycolysis, which occurs in the cytoplasm of the plant or animal cell. ATP breaks down a single glucose molecule into two molecules of three-carbon pyruvate, or CH3COCOO−. Glycolysis releases four molecules of ATP by releasing some of the energy stored in the chemical bonds of glucose. The remaining energy from the glucose is in the two molecules of pyruvate produced in the glycolysis reaction. In the presence of oxygen, the pyruvate molecules undergo active transport into the mitochondria where they will enter the citric acid cycle.
Citric Acid Cycle
n the mitochondrial matrix, the pyruvate dehydrogenase complex removes a single carbon and two oxygen molecules from the pyruvate’s carboxyl group. This produces a single molecule of carbon dioxide, or CO2. A two-carbon fragment of the pyruvate remains, called an acetyl group (C2H3O). The pyruvate dehydrogenase complex catalyzes the attachment of this acetyl group to coenzyme A, forming acetyl-CoA. For every glucose molecule, two molecules of acetyl-CoA enter the citric acid cycle. The citric acid cycle is also known as the Krebs cycle or the tricarboxylic acid cycle. Acetyl-CoA is very reactive, and transfers the acetyl group to oxaloacetate producing citrate -- the first intermediate in the citric acid cycle.
The two carbons in acetyl-CoA react with the four carbons in oxaloacetate to produce citrate, a six-carbon molecule. In subsequent reactions within the citric acid cycle, ATP is produced and the two original pyruvate carbons are lost as two molecules of waste CO2. At the end of the cycle, only the four-carbon oxaloacetate remains. During aerobic respiration, the three carbon atoms from the pyruvate molecules are exhaled from the organism as waste carbon dioxide.