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Aerobic and anaerobic are terms used to describe the presence or absence of oxygen. (Anaerobic means "without air.") All living things require energy, and when oxygen is used to metabolize (convert or break down)


German-British biochemist Hans Krebs (1900–1981) was the first to explain how cells release energy during respiration (the chemical process by which food is broken down to release energy). His discovery of a very complicated chain of reactions, which came to be called the Krebs cycle, explained how cells break down glucose (a common sugar) and obtain needed energy. For this discovery he received the 1953 Nobel Prize in Physiology and Medicine and was knighted by Queen Elizabeth in 1958.

Hans Krebs was born in Hildesheim, Germany and received his medical degree from the University of Hamburg in 1925. His father was a doctor, and Krebs took up his father's specialty as an ear, nose, and throat specialist. It was not long, however, before he realized that he preferred doing research to working with patients, and in 1926 he became an assistant to the noted biochemist, Otto Heinrich Warburg (1883–1970) in Berlin. Warburg studied respiration and would himself win a Nobel Prize in 1931 for his work on that subject. By 1932, Krebs was making a name for himself with his own work on amino acids (the building blocks of protein), but in 1933 the German dictator, Adolf Hitler (1889–1945), was appointed chancellor. This political change in Germany meant that all people of Jewish origin, including Krebs, would be persecuted and eventually sent to concentration camps to be worked to death. That year however, Krebs was able to leave Germany and move to England. There he had the good fortune to work with another Nobel Prize winner, the English biochemist, Frederick Gowland Hopkins (1861–1947), at Sheffield University. It was there that Krebs would discover the process for which he is best known, the citric acid cycle, which is also called the Krebs cycle.

The Krebs cycle is an important step in the process used by cells when they convert food, such as carbohydrates and fats, into usable energy. During this energy-producing process called respiration, one molecule of glucose combines with six molecules of oxygen to produce six molecules of carbon dioxide, six molecules of water, and a considerable amount of energy. Krebs discovered that this does not happen all at once, but that a complicated chain of reactions occurs during which a little of the original energy is released each time. His work revealed that this series of reactions was actually a chain, or a cycle, of events. His explanation of this highly complex cycle proved to be a major breakthrough in biochemistry and in understanding how an animal's metabolism really works. Metabolism is all of the chemical processes (all the building up and breaking down) that takes place in an organism to stay alive and grow. The Krebs cycle focuses specifically on the breaking-down aspects of an animal's metabolism by which energy is released. It explains how, through a series of six chemical reactions that take place inside an animal cell in a recurring loop, food is combined with oxygen to produce the energy needed for life.

organic materials to release energy, it is described as being an aerobic process. When no oxygen is needed to metabolize materials, the process is referred to as anaerobic. The terms aerobic and anaerobic are used mostly to describe types of respiration and types of bacteria.


All living cells need a constant supply of energy to power the chemical activities they conduct to support life. The living cells in both plants and animals use glucose, the most common form of sugar, as their energy source or fuel. However, glucose must be broken down during a process called respiration before it will release usable energy. Finally, energy is produced by the cell in one of two ways: aerobic respiration or anaerobic respiration. Aerobic cellular respiration is a process that necessarily involves the use of oxygen. During this process, glucose and oxygen are chemically combined, which is known as oxidation, in the cell's mitochondria (part of the cell that produces energy) to yield energy and to release carbon dioxide and water as waste. During aerobic respiration, one molecule of glucose is combined with six molecules of oxygen to produce six molecules of carbon dioxide and six molecules of water. Aerobic respiration also releases a large amount of energy in the form of energy-carrying molecules called adenosine diphosphate (ADP) and adenosine triphosphate (ATP). The series of events or reactions that occur during aerobic respiration are known as the Krebs cycle. This cycle is named after the German biochemist Hans Krebs (1900–1981), who discovered that glucose is broken down in a chain of reactions. Aerobic respiration, therefore, results in the release of a large amount of energy, but only if oxygen is present. It is in this way in which animals and plants obtain energy.


Anaerobic respiration is the opposite of aerobic, since it involves a type of respiration that does not involve oxygen. Also called glycolysis (which literally means the splitting of carbohydrates), this process takes place in the cell and is very slow compared the to oxygen-rich process of aerobic respiration. Anaerobic respiration does not result in the production of a great deal of energy since glucose is only partly broken down. Instead, most of the glucose forms new organic compounds such as acid, methane gas, and alcohol. The most common anaerobic reactions take place during the process known as alcoholic fermentation. During this process, microorganisms like bacteria, molds, or yeast change sugar into carbon dioxide and alcohol. In the best known examples of making bread, beer and wine, yeast is used to bring about this conversion without the use of oxygen. Although the anaerobic process produces little immediate energy, its by-products like methane and alcohol contain a significant amount of potential energy and can be used as fuels.


Bacteria can also be described as being aerobic or anaerobic. These single-celled microorganisms are among the most abundant living things on Earth and live and feed in many different ways. Bacteria that need oxygen in order to grow are called aerobic bacteria. An example are the bacteria that cause the lung disease tuberculosis. The moist, warm lungs have a steady supply of oxygen and provide an ideal breeding ground for this potentially fatal bacterium. However, many bacteria can only grow in the absence of oxygen and are called anaerobic bacteria. The bacteria that live in soil and those that inhabit the intestinal tracts of mammals carry out anaerobic respiration.


Another type of anaerobic respiration occurs in the stressed muscles of an animal when is uses oxygen faster than the blood can supply it. The exhausted muscle quickly switches from aerobic respiration to anaerobic respiration and begins to break down glucose without oxygen. This type of anaerobic respiration results in little energy and produces muscles that ache and eventually shut down. Anaerobic respiration in the muscles produces an acid by-product known as lactic acid. It is the buildup of lactic acid that makes an athlete's muscles "burn" and then stop working, which is why some runners collapse and can barely use their muscles immediately after a race.

[See alsoBacteria; Respiration ]