Carbohydrate metabolism

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 atmospheric gases by photosynthesis storing the absorbed energy internally, often in the form of starch or lipids. Plant components are eaten by animals and fungi, and used as fuel for cellular respiration. Oxidation of one gram of carbohydrate yields approximately 4 kcal of energy and from lipids about 9 kcal. Energy obtained from metabolism (eg, oxidation of glucose) is usually stored temporarily within cells in the form of ATP. Organisms capable of aerobic respiration metabolize glucose and oxygen to release energy with carbon dioxide and water as byproducts.
Carbohydrates are a superior short-term fuel for organisms because they are simpler to metabolize than fats or those amino acid portions of proteins that are used for fuel. In animals, the most important carbohydrate is glucose; so much so, that the level of glucose is used as the main control for the central metabolic hormone, insulin. Starch, and cellulose in a few organisms (eg, termites, ruminants, and some bacteria), being both 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 monosaccharides 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. <ref, 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
Main article: Carbohydrate catabolism
Oligo/polysaccharides are typically cleaved into smaller monosaccharides by enzymes called Glycoside hydrolases. The monosaccharide units then enter monosaccharide catabolism. Organisms vary in the range of monosaccharides they can absorb and use, and also in the range of more complex carbohydrates they are capable of disassembling.


Metabolic pathways
Carbon fixation, or photosynthesis, in which CO2 is reduced to carbohydrate.
Glycolysis - the oxidation metabolism of glucose molecules to obtain ATP and Pyruvate
Pyruvate from glycolysis enters the Krebs cycle, also known as the Citric Acid Cycle, in aerobic organisms after moving through pyruvate dehydrogenase complex.
The Pentose phosphate pathway, which acts in the conversion of hexoses into pentoses and in NADPH regeneration.
Glycogenesis - the conversion of excess glucose into glycogen as a cellular storage mechanism; this prevents excessive osmotic pressure buildup inside the cell
Glycogenolysis - the breakdown of glycogen into glucose, which provides a glucose supply for glucose-dependent tissues.
Gluconeogenesis - de novo synthesis of glucose molecules from simple organic compounds. an example in humans is the conversion of a few amino acids in cellular protein to glucose.


Glucoregulation
Glucoregulation is the maintenance of steady levels of glucose in the body; it is part of homeostasis, and so keeps a constant internal environment around cells in the body.
The hormone insulin is the primary regulatory signal in animals, suggesting that the basic mechanism is very old and very central to animal life. When present, it causes many tissue cells to take up glucose from the circulation, causes some cells to store glucose internally in the form of glycogen, causes some cells to take in and hold lipids, and in many cases controls cellular electrolyte balances and amino acid uptake as well. Its absence turns off glucose uptake into cells, reverses electrolyte adjustments, begins glycogen breakdown and glucose release into the circulation by some cells, begins lipid release from lipid storage cells, etc. Circulatory glucose levels are the most important signal to the insulin producing cells, and as they are largely due to dietary carbohydrate intake, diet controls major aspects of metabolism via insulin. In humans, insulin is made by beta cells in the pancreas, fat is stored in adipose tissue cells, and glycogen is both stored and released as needed by liver cells. Regardless of insulin levels, no glucose is released to the blood from internal glycogen stores from muscle cells.
The hormone glucagon, on the other hand, acts in the opposite direction to insulin, forcing the conversion of glycogen in liver cells to glucose which is then put into the blood, though not from muscle cells, as they lack the ability to export glucose into the blood. The release of glucagon is controlled by low levels of blood glucose.


Human diseases of carbohydrate metabolism
Diabetes mellitus
Lactose intolerance
Fructose intolerance
Galactosemia
Glycogen storage disease