What Is a Citric Acid Cycle?


The citric acid cycle is the first stage of a two tier process called cellular respiration. It is also known as the tricarboxylic acid cycle (TCA) and, more commonly, as the Krebs Cycle. Cellular respiration is the process by which glucose is transformed into ATP (adenosine triphosphate) or a useful form of chemical energy.

The Facts

Cellular respiration is a complicated biological and chemical process that is typically divided into two stages. The process starts with glucose and oxygen and forms carbon dioxide, water, and free energy. The free energy is then captured and stored as ATP, according to teachersdomain.org.


Glucose is a simple sugar molecule that consists of six atoms of oxygen, six atoms of carbon and 12 atoms of hydrogen. Living organisms consume food (complex carbohydrates) that can be broken down into glucose by the liver. The liver converts molecules into glucose, according to teachersdomain.org. Cells can then obtain glucose from the blood (through capillary walls). When glucose reaches the inside of a cell, it is absorbed by mitochondria (the organelles in which the citric acid cycle takes place).


The citric acid cycle is a series of enzyme catalyzed chemical reactions that takes place in the mitochondria. In 1937, this first stage of cellular respiration was first identified by Albert Szent-Gyorgyi and Hans Krebs, hence the name Krebs Cycle (officially referred to as the Svent-Gyorgyi-Krebs Cycle). Szent-Gyorgyi was awarded the Nobel Prize in 1937 for his discoveries of a biological combustion process and Krebs was awarded the Nobel Prize in 1953 for the discovery of the citric acid cycle.


The citric acid cycle takes certain compounds that donate protons and electrons to the electron transport chain, according to "Citric Acid (Krebs) Cycle" at ccbcmd.edu. The electron transport chain then generates ATP through the process of oxidative phosphorylation. Furthermore, Krebs cycle also produces 2 ATP through the process of substrate phosphorylation. Through the supply of precursor metabolites, the citric acid cycle also plays an important role in the flow of carbon through the cell. In general, it is made up of eight distinct steps (each of which is catalyzed by a unique enzyme).


The citric acid cycle begins when coenzyme A transfers its 2-carbon acetyl group to the 4-carbon compound oxaloacetate. This step results in a 6-carbon molecule citrate. In step 2, the citrate is rearranged to form isocitrate (an isometric form of the molecule), according to ccbcmd.edu. In step 3, the isocitrate is oxidized and a carbon dioxide molecule is removed. The removal produces a 5-carbon molecule called alpha-ketoglutarate. In step 4, alpha-ketoglutarate is oxidized, the carbon dioxide molecules is removed and coenzyme A is added to form succinyl-CoA (a 4-carbon compound). In step 5 of the citric acid cycle, CoA is removed from succinyl-CoA to produce succinate. Energy is released and is used to make GTP (guanosine triphosphate) and, in turn, to make ATP. In step 6, succinate is oxidized to create furmarate and, in step 7, water is added to furmarate to form malate. In step 8, malate is oxidized to produce oxaloacetate (the beginning compound of the citric acid cycle).

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