Carbohydrate metabolism
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'''Carbohydrate metabolism''' denotes the various [[biochemistry|biochemical]] processes responsible for the [[anabolism|formation]], [[catabolism|breakdown]] and interconversion of [[carbohydrates]] in [[life|living]] [[organism]]s.
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 atmospheric gases by [[photosynthesis]], which can then be consumed by other organisms and used as fuel for [[cellular respiration]]. Oxidation of one gram of carbohydrate yields approximately 4 kcal of [[Chemical energy|energy]]. Energy obtained from carbohydrate metabolism is usually stored in the form of [[Adenosine triphosphate|ATP]]. Organisms capable of [[aerobic respiration]] metabolize glucose and [[oxygen]] to release energy with [[carbon dioxide]] and [[water]] as byproducts.
All carbohydrates share a general formula of approximately C<sub>n</sub>H<sub>2n</sub>O<sub>n</sub>; glucose is C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>. Monosaccharides may be chemically bonded together to form [[disaccharides]] such as [[sucrose]] and longer [[polysaccharides]] such as [[starch]] and [[cellulose]].
Carbohydrates are a superior short-term energy reserve for organisms, because they are much simpler to metabolize than [[fat]]s or [[proteins]]. In animals, all dietary carbohydrates are delivered to cells in the form of glucose. Carbohydrates are typically stored as long polymers of glucose molecules with [[Glycosidic bond]]s for structural support (e.g. [[chitin]], [[cellulose]]) or energy storage (e.g. [[glycogen]], [[starch]]). However, the strong affinity of carbohydrates for water makes storage of large quantities of carbohydrates inefficient due to the large molecular weight of the solvated water-carbohydrate complex. In some organisms, excess carbohydrates are catabolised to form [[Acetyl-CoA]], where they enter 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.
==Catabolism==
{{Main|Carbohydrate catabolism}}
Oligo/polysaccharides are cleaved first to smaller monosaccharides by enzymes called [[Glycoside hydrolase]]s. The monosaccharide units can then enter into monosaccharide catabolism
==Metabolic pathways==
* [[Carbon fixation]], whereby CO<sub>2</sub> is reduced to carbohydrate.
* [[Glycolysis]] - the breakdown of the [[glucose]] molecule in order to obtain [[adenosine triphosphate|ATP]] and [[Pyruvate]]
** Pyruvate from glycolysis enters the [[Krebs cycle]] in [[aerobic]] organisms.
* The [[Pentose phosphate pathway]], which acts in the conversion of [[hexose]]s into [[pentose]]s and in [[NADPH]] regeneration.
* [[Glycogenesis]] - the conversion of excess glucose into [[glycogen]] in order to prevent excessive [[osmotic pressure]] buildup inside the cell
* [[Glycogenolysis]] - the breakdown of glycogen into glucose, in order to provide a steady level of glucose supply for glucose-dependent tissues.
* [[Gluconeogenesis]] - ''de novo'' synthesis of glucose molecules from simple [[organic chemistry|organic]] compounds
==Glucoregulation==
Glucoregulation is the maintenance of steady levels of [[glucose]] in the body; it is part of [[homeostasis]], and keeps a constant internal environment around cells in the body.
The hormone [[insulin]] makes the body convert glucose into glycogen and puts it into the liver; the insulin is made in the pancreas, and is secreted when the [[blood sugar]] is too high. Insulin also promotes the use of glucose by the muscles.
The hormone [[glucagon]], on the other hand, acts in the opposite direction and promotes the conversion of glycogen to glucose in response to low blood sugar.
==Human diseases of carbohydrate metabolism==
* [[Diabetes mellitus]]
* [[Lactose intolerance]]
* [[Fructose intolerance]]
* [[Galactosemia]]
* [[Glycogen storage disease]]
==External links==
* {{MeshName|Carbohydrate+metabolism}}
*[http://diabetescorner.blogspot.com/2007/09/glucose-metabolism.html Glucose Metabolism and Diabetes]
*[http://www.bbc.co.uk/schools/gcsebitesize/biology/humansasorganisms/6homeostasisrev4.shtml BBC - GCSE Bitesize - Biology | Humans | Glucoregulation]
{{Carbohydrate metabolism}}
{{Glycogenolysis}}
{{Gluconeogenesis}}
{{Fructose and galactose metabolism}}
{{Glycosaminoglycan metabolism enzymes}}
{{Glycoprotein metabolism enzymes}}
[[Category:Metabolism]]
[[de:Glucose-Stoffwechsel]]
[[fr:Métabolisme des glucides]]
[[ja:炭水化物代謝]]
[[th:การเผาผลาญคาร์โบไฮเดรต]]