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Carbohydrates are a basic class of chemical compounds in biochemistry. They are a primary biological means of storing or consuming energy; other forms being fat and protein. In autotrophs, such as plants, sugars are converted into starch for storage. In heterotrophs, such as animals, they are used as metabolic fuel.

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Typically, carbohydrates are classified into the sweet sugars (monosaccharides and disaccharides) and the unsweet, starchy, polysaccharides. Monosaccharides are simple, crystalline sugars. Disaccharides are composed of two monosaccharides joined together (hence di-saccharides). Polysaccharides are very large molecules such as starch or glycogen, which are formed from many monosaccharides joined together (poly-saccharides).


Pure carbohydrates consist of just three elements: carbon, hydrogen, and oxygen. The ratio varies, but not by much. Usually, the molar amount of carbon is the same as or slightly greater than the amount of oxygen, and the amount of hydrogen is twice (or slightly greater) the amount of oxygen. The traditional general structure of carbohydrates is: CxH2xOx, but many important carbohydrates, like deoxyribose C5O4H10 have more hydrogen.


Three sugars, glucose, galactose and fructose share the same molecular formula: C6H12O6. However, the arrangement of atoms is different in each of these three sugars : they are called isomers.


Two monosaccharides can be linked together to form a disaccharide. The common disaccharides are sucrose (cane or beet sugar - made from one glucose and one fructose), lactose (Milk sugar - made from one glucose and one galactose) and maltose (made of two glucoses).
The formula of these disaccharides is C12H22O11. The binding between the two sugars results in the loss of a hydrogen atom H from one molecule and a hydroxyl group from the other.



Starches are are polymers of glucose. Amylose[?] consists of a linear chain of several hundred of glucose molecules. Amylopectine[?] is a branched molecules made of several thousand of glucose units.
Starches are insoluble in water. They can be digested by hydrolysis done by amylases[?]. Potato, rice, wheat, and maize are major sources of starch in the human diet.


Glycogen is the storage form of excess glucose in animals. It is essentially a glucose polysaccharide with frequent convalent bonds between adjacent layers. The bond is between the methanol branch and the opposite hydroxyl branch of the carbon ring. Glycogen can be broken down to form glucose for respiration, through the process of glycogenolysis[?]. This involves the breaking of the bonds between the glucose molecules (C-O-C) by the phosphate group of the ATP molecule. Normal cellular metabolism can then gain energy from the respiration of the glucose once the phosphate group has been detached.


The structural components of plants are primarily cellulose. Wood is largely cellulose and lignin, while paper and cotton are nearly pure cellulose. Cellulose is a polymer made with repeated glucose units.

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