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Carbohydrate metabolism denotes the various biochemical processes responsible for the formation, breakdown and interconversion of carbohydrates in living organisms. The most important carbohydrate is glucose, a simple sugar (monosaccharide) that is metabolized by nearly all known organisms. Glucose and other carbohydrates are part of a wide variety of metabolic pathways across species: plants synthesize carbohydrates from carbon dioxide and water by photosynthesis storing the absorbed energy internally, often in the form of starch or lipids. Plant components are consumed by animals and fungi, and used as fuel for cellular respiration. Oxidation of one gram of carbohydrate yields approximately 4 kcal of energy, while the oxidation of one gram of lipids yields about 9 kcal. Energy obtained from metabolism (e.g., oxidation of glucose) is usually stored temporarily within cells in the form of ATP.〔(Energetics of Cellular Respiration (Glucose Metabolism) )〕 Organisms capable of aerobic respiration metabolize glucose and oxygen to release energy with carbon dioxide and water as byproducts. Carbohydrates can be chemically divided into complex and simple. Simple carbohydrates consist of single or double sugar units (monosaccharides and disaccharides, respectively). Sucrose or table sugar (a disaccharide) is a common example of a simple carbohydrate. Complex carbohydrates contain three or more sugar units linked in a chain, with most containing hundreds to thousands of sugar units. They are digested by enzymes to release the simple sugars. Starch, for example, is a polymer of glucose units and is typically broken down to glucose. Cellulose is also a polymer of glucose but it cannot be digested by most organisms. Bacteria that produce enzymes to digest cellulose live inside the gut of some mammals, such as cows, and when these mammals eat plants, the cellulose is broken down by the bacteria and some of it is released into the gut. Doctors and scientists once believed that eating complex carbohydrates instead of sugars would help maintain lower blood glucose. Numerous studies suggest, however, that both sugars and starches produce an unpredictable range of glycemic and insulinemic responses. While some studies support a more rapid absorption of sugars relative to starches〔Wolever, Thomas M. S. (2006), ''(The Glycaemic Index: A Physiological Classification of Dietary Carbohydrate )'', CABI, pg. 65, ISBN 9781845930516. “Indeed, blood glucose responses elicited by pure sugars and fruits suggest rapid absorption because the blood glucose concentration rises more quickly and falls more rapidly than after bread (Wolever ''et al''., 1993; Lee and Wolever, 1998). Further evidence that sugars are rapidly absorbed is provided by recent studies indicating that the switch from oxidation of fat to carbohydrate occurs more rapidly after a high-sucrose meal than a high-starch meal, with the increase in carbohydrate oxidation being sustained for longer after the starch than the sucrose meal (Daly ''et al''., 2000).”〕 other studies reveal that many complex carbohydrates such as those found in bread, rice, and potatoes have glycemic indices similar to or higher than simple carbohydrates such as sucrose. Sucrose, for example, has a glycemic index lower than expected because the sucrose molecule is half fructose, which has little effect on blood glucose.〔Wolever, Thomas M. S. (2006), ''(The Glycaemic Index: A Physiological Classification of Dietary Carbohydrate )'', CABI, pg. 65, ISBN 9781845930516. "Sucrose has a low GI because only half of the molecule is glucose and the other half is fructose. So sugars with a low GI are considered to have a low GI primarily because they contain less glucose than starch rather than because they are slowly absorbed."〕 The value of classifying carbohydrates as simple or complex is questionable. The glycemic index is a better predictor of a carbohydrate's effect on blood glucose. Carbohydrates are a superior short-term fuel for organisms because they are simpler to metabolize than fats or those amino acids (components of proteins) that can be used for fuel. In animals, the most important carbohydrate is glucose. The concentration of glucose in the blood is used as the main control for the central metabolic hormone, insulin. Starch, and cellulose in a few organisms (e.g., some animals (such as termites) and some microorganisms (such as protists and bacteria)), both being glucose polymers, are disassembled during digestion and absorbed as glucose. Some simple carbohydrates have their own enzymatic oxidation pathways, as do only a few of the more complex carbohydrates. The disaccharide lactose, for instance, requires the enzyme lactase to be broken into its monosaccharide components; many animals lack this enzyme in adulthood. Carbohydrates are typically stored as long polymers of glucose molecules with glycosidic bonds for structural support (e.g. chitin, cellulose) or for energy storage (e.g. glycogen, starch). However, the strong affinity of most carbohydrates for water makes storage of large quantities of carbohydrates inefficient due to the large molecular weight of the solvated water-carbohydrate complex. In most organisms, excess carbohydrates are regularly catabolised to form acetyl-CoA, which is a feed stock for the fatty acid synthesis pathway; fatty acids, triglycerides, and other lipids are commonly used for long-term energy storage. The hydrophobic character of lipids makes them a much more compact form of energy storage than hydrophilic carbohydrates. However, animals, including humans, lack the necessary enzymatic machinery and so do not synthesize glucose from lipids, though glycerol can be converted to glucose.〔G Cooper, The Cell, American Society of Microbiology, p 72〕 All carbohydrates share a general formula of approximately CnH2nOn; glucose is C6H12O6. Monosaccharides may be chemically bonded together to form disaccharides such as sucrose and longer polysaccharides such as starch and cellulose. ==Catabolism== (詳細はglycoside hydrolases. The monosaccharide units then enter monosaccharide catabolism. Organisms vary in the range of monosaccharides they can absorb and use and they can also vary in the range of more complex carbohydrates they are capable of disassembling. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「carbohydrate metabolism」の詳細全文を読む スポンサード リンク
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