What’s the Citric Acid Cycle?

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The citric acid cycle is a series of chemical reactions that occur during cellular respiration, the process by which the cells of organisms produce energy. It is also called the Krebs cycle or the tricarboxylic acid cycle. In the cycle, a series of energy-generating chemical reactions are catalyzed, or accelerated, by various enzymes. Typically, students study the citric acid cycle first in advanced high school biology classes or introductory college biology classes. It is therefore covered in more detail in biochemistry lessons, as it is an inherently chemical process.

There are many different complex chemical reactions in the cycle, most of which involve molecules made up of carbon, oxygen and hydrogen. There are about ten steps in the cycle and each step is catalyzed by a different enzyme. Immediately prior to 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 down pyruvate molecules into NADH (nicotinamide adenine dinucleotide) and ATP (adenosine triphosphate) molecules, both of which contain a significant amount of energy that the body can access quickly. Most of the energy molecules produced during cellular respiration are, through a series of metabolic processes, derived from glucose (sugar) molecules.

Cellular respiration is usually an aerobic process, which means it uses oxygen. If oxygen is present, the pyruvate produced through glycolosis will normally proceed 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, which means it is an anaerobic process. Fermentation is a much less efficient energy-producing process than the citric acid cycle; it does not produce enough energy to sustain an organism for an extended period of time.

In most complex multicellular organisms, the citric acid cycle occurs in the mitochondria, an energy-producing organelle that exists within cells. Mitochondria are the main producers of ATP, which cells are able to use for energy very efficiently. Some processes in cellular respiration occur outside the mitochondria and others occur when molecules are transported across the mitochondrial membrane. The citric acid cycle occurs in the mitochondrial matrix, which contains the many different enzymes that catalyze many of the chemical reactions involved in the cycle and the rest of cellular respiration.

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