The citric acid cycle, also known as the TCA (Tri-carboxylic acid) cycle or the Krebs cycle, is the name given to the cycle of oxidation-reduction reactions in the mitochondria of the cells of aerobic organisms that produce energy or ATP molecules by breaking down organic molecules like fatty acids, glucose, and amino acids in the presence of oxygen.
The citric acid cycle begins with the reaction of the oxaloacetic acid or oxaloacetate (a four-carbon molecule) with acetyl-coenzyme A or acetyl CoA (a two-carbon molecule) to form citric acid or citrate (a six-carbon molecule). Note that Acetyl CoA is derived from a three-carbon compound called pyruvic acid (after glycolysis in cytosol). In a series of chemical reactions that take place in the presence of enzymes, the isomer of citrate undergoes oxidation and decarboxylation to form α-ketoglutarate (a five-carbon compound) that again undergoes oxidative decarboxylation to generate succinate (a four-carbon compound). Oxaloacetate is formed from succinate at the end of the cycle, along with the production of carbon dioxide and ATP molecules.
Hence, two carbon molecules (in the form of acetyl CoA) enter the system and two leave it (in the form of two carbon dioxide molecules). And in the process, high-energy electrons in the form of NADH and FADH2 are released, along with the generation of energy. Note that oxaloacetate, the four-carbon molecule that initiates the first step of the reaction, gets catalytically formed or regenerated in the end again to take part in another round of the citric acid cycle. One such molecule of oxaloacetate is capable of participating in many cycles.