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The citric acid cycle is a series of chemical reactions that occurs during cellular respiration, the process by which cells in organisms produce energy. It is also referred to as the Krebs cycle or the tricarboxylic acid cycle. In the cycle, a series of energy-generating chemical reactions are catalyzed, or sped up, by various enzymes. Typically, students first study the citric acid cycle in advanced high school biology classes or in introductory college biology classes. It is then approached in greater detail in biochemistry classes, as it is an inherently chemical process.
There are many different complex chemical reactions in the cycle, most of which involve molecules composed of carbon, oxygen, and hydrogen. There are roughly ten steps to the cycle, and each step is catalyzed by a different enzyme. Immediately before the citric acid cycle, a process called glycolysis produces pyruvate, an energy-containing molecule that is further broken down into usable energy as the cellular respiration process proceeds. The citric acid cycle breaks pyruvate molecules down into NADH (Nicotinamide adenine dinucleotide) and ATP (Adenosine triphosphate) molecules, both of which contain a significant amount of energy that can be quickly accessed by the body. Most of the energetic molecules produced during cellular respiration are, through a series of metabolic processes, derived from glucose (sugar) molecules.
Cellular respiration is usually an aerobic process, meaning that it uses oxygen. If oxygen is present, the pyruvate produced through glycolosis will proceed as normal to the citric acid cycle to be broken down into NADH and ATP (Adenosine triphosphate). If no oxygen is present, however, the pyruvate begins a process called fermentation. Fermentation does not use oxygen, meaning that it is an anaerobic process. Fermentation is a far less efficient energy producing process than the citric acid cycle; it does not produce enough energy to sustain an organism for a prolonged period of time.
In most complex, multicellular organisms, the citric acid cycle occurs in the mitochondria, an energy-producing organelle that exists inside of cells. Mitochondrion are the main producers of ATP, which cells are able to utilize for energy very efficiently. Some processes in cellular respiration occur outside of the mitochondria, and others occur as molecules are transported across the mitochondria's membrane. The citric acid cycle occurs in the matric of the mitochondria, which contains the many different enzymes that catalyze many of the chemical reactions involved in the cycle and in the rest of cellular respiration.