Encyclopædia Britannica, Inc.

Cellular respiration is the process by which organisms utilise oxygen to break down food molecules to get chemical energy for jail cell functions. Cellular respiration takes place in the cells of animals, plants, and fungi, and as well in algae and other protists. It is often called aerobic respiration because the process requires oxygen (the root aer comes from the Greek give-and-take for "air"). In the absence of oxygen, cells can get energy past breaking down nutrient through the process of fermentation, or anaerobic respiration. Of the two processes, cellular respiration is more efficient, yielding considerably more free energy than that released through fermentation.

Cellular respiration is a chemical reaction in which glucose is broken down in the presence of oxygen, releasing chemical free energy and producing carbon dioxide and water equally waste products:

glucose + oxygen → chemical free energy + carbon dioxide + water

The free energy released is captured in molecules of adenosine triphosphate, or ATP, which then supply it to fuel other cellular processes (see biochemistry).

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All cells need energy to function. Just as a car must burn fuel to get the free energy it needs to run, the cell must burn fuel—for example, food—to get energy to bear out the tasks of life. Glucose, a simple sugar, provides the fuel the cell needs. Although free energy is also stored in larger molecules, such equally circuitous carbohydrates and fats, they must exist broken down into molecules of glucose before the cell tin can use their free energy.

Most of cellular respiration takes place in sausage-shaped organelles called mitochondria. Although mitochondria play a key role in other cellular processes, their main function is to produce large amounts of free energy through cellular respiration. The number of mitochondria per cell varies; liver and muscle cells, which require big amounts of energy to office, may have thousands. (See also cell.)

Cellular respiration begins in the cell's cytoplasm. There, glucose is cleaved down through a series of chemic reactions to produce small molecules of a substance called pyruvate. This part of the procedure is chosen glycolysis; information technology does not require oxygen and releases a small amount of energy, which is captured by a few ATPs. The pyruvate molecules then enter the mitochondria, where they undergo a serial of chemical reactions with oxygen. So much energy is released in these reactions that it takes many molecules of ATP to capture it all. The reactions also release hydrogen, which combines with oxygen to produce water; and carbon, which combines with oxygen to produce carbon dioxide. The water and carbon dioxide are released as waste products; the ATPs leave the mitochondria and deliver their captured energy to places in the prison cell where it is needed to power cellular activities.