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Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Citric Acid Cycle

Citric Acid Cycle, series of chemical reactions occurring within the cell, responsible for the final breakdown of food molecules to form carbon dioxide, water, and energy. This process, which is carried out by seven enzymes, is also known as the tricarboxylic acid cycle, or the Krebs cycle. The citric acid cycle is active in all animals and higher plants and in most bacteria. In organisms that have cells with nuclei, the cycle is contained within a membrane-bound organelle called the mitochondrion, a structure often referred to as the power plant of the cell. Discovery of the citric acid cycle is credited to Sir Hans Adolf Krebs, a British biochemist who outlined its essential steps in 1937.

Before a food can enter the citric acid cycle, it must be broken down into small units called acetyl groups. Each acetyl group (CH3CO) contains only two atoms of carbon, along with hydrogen and oxygen. At the start of the cycle, an acetyl group combines with a four-carbon molecule called oxaloacetate to yield a six-carbon compound, citric acid. In the remaining steps of the cycle, the citric acid molecule is gradually rearranged and stripped of two of its carbon atoms, which are given off in the form of carbon dioxide. Four electrons also are released. These travel along a nearby series of carrier molecules in the cell, the electron transport chain, where they produce energy in the form of an energy-rich molecule called adenosine triphosphate, or ATP, before combining with oxygen to form water. An additional product of the cycle is another energy-containing molecule, called guanosinetriphosphate, or GTP. These fuel molecules, ATP and GTP, can be used by the cell for many energy-requiring processes. Another fuel molecule, creatine phosphate, may also be used to provide extra energy to brain and muscle cells. At the end of the cycle, the original molecule of oxaloacetate is regenerated. This molecule can then combine with another acetyl group to start the cycle over again. At each turn of the cycle, more energy is produced.

The citric acid cycle is an economical way of turning the food components in a cell into usable energy. Only the acetyl groups are destroyed in the cycle; the seven enzymes that carry out the various reactions, and the intermediate compounds that these enzymes act on, can be used again and again. Many of the intermediate compounds produced in the citric acid cycle also are of value as starting materials for the synthesis of amino acids, carbohydrates, and other cellular products. See Metabolism; Sugar Metabolism.

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