What Is the Meaning of Cellular Respiration?
Cellular respiration is the name for an essential biological process that extracts energy from foods in a usable, chemical form known as adensosine triphosphate, or ATP. Cellular respiration occurs in the mitochondria of cells and its main pathways can occur with or without the presence of oxygen. The process can be broken down into three different parts: glycolysis, the Krebs cycle and electron transport. When oxygen is not present, cellular respiration can occur by the process of fermentation.
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ATP
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ATP, or adenosine triphosphate, is a molecule that yields the energy used to drive most of the essential chemical reactions that occur within the body. ATP is a coenzyme: it's a non-protein compound that binds to proteins and makes them able to perform their needed biological role. The whole purpose of cellular respiration is to produce ATP molecules that can then be used by the body to trigger crucial cellular processes and to activate required enzymes at the appropriate times.
Glycolysis
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Glycolysis is the first step in cellular respiration. In glycolysis, glucose that has been obtained by the body from food is chemically altered into the compound pyruvate. When this chemical change occurs, energy is released from the glucose compound in the form of two NADH molecules and 2 ATP molecules. These chemical reactions take place in the cytosol of cells--the watery liquid contained within the cell membrane.
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The Krebs cycle
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The Krebs cycle--also known as the citric acid cycle--is the second phase of cellular respiration. After the production of pyruvate from glycolysis in the cell's cytosol, the pyruvate molecules are transported into the cell's mitochondria. The mitochondria are organelles located within all cells; their main function is the production of ATP in order to provide the cell with energy. Within the mitochondria, the pyruvate molecules are converted to acetyl-CoA. They are then converted to carbon dioxide in the metabolic pathway known as the Krebs cycle. Energy is released from the Krebs cycle in the form of ATP, NADH and FADH2.
Electron transport
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The final step of cellular respiration involves the conversion of NADH and FADH2 molecules produced in the earlier stages of respiration into reduced forms of each molecule through an electron transport chain. The reduction process releases energy from both in the form of ATP: the reduction of an NADH molecule produces three ATP molecules, and the reduction of an FADH2 molecule produces two ATP molecules.
Fermentation
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Even if oxygen is not present within a cell, the process of fermentation allows the cell to continue producing ATP. Fermentation occurs when the NADH molecules produced by glycolysis within the cytosol are oxidized, resulting in more molecules to be used in glycolysis. By recycling the NADH, fermentation allows glycolysis to continue producing ATP for the cell to use.
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