Carbohydrate

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A carbohydrate is an organic compound that is made up of carbon, hydrogen and oxygen and is considered as one of the major class of biomolecules.The general formula of carbohydrate is Cn(H2O)n. The more common carbohydrates are made up of between three to six carbon atoms. A carbohydrate with three carbon atoms is referred to as a triose, with four a tetraose, with five a pentose and six a hexose. Glucose, mannose, galactose are hexoses and ribose, xylose and arabinose are pentoses.

Monosaccharides exist as open chains or ring forms although only the ring forms exist as isomers. And sugars with 5 or more carbons often form rings because both the carbonyl and alcohol groups can react.

One of the most commonly known carbohydrate would be glucose which is one of the main energy source for living organisms.
Monosaccharides are the simplest carbohydrates such as glucose and galactose. They are generally aldehydes or ketones that have multiple hydroxyl groups attached. The smallest monosaccharides are dihydroxyacetone and D- and L-glyceraldehyde.  Dihydroxyacetone is a ketose because it contains a keto group, whereas glyceraldehyde is an aldose because it contains an aldehyde group [1] (there are two stereoisomers of glyceraldehyde due to a single asymmetric carbon atom).

When simple monosaccharides (such as maltose) are linked together through 1,4-glycosidic bonds to other monosaccharides, a wide variety of information rich oligosaccharides are formed that can be used in complex processes such as cell signalling [2].

Polysaccharides are extremely information rich due to the complexity that can be created through the bond variety possible. This is possible due the multiple hydroxyl groups which means individual monosaccharide’s can join through a glycosidic bond in several different ways - this is also results in branching of some polysaccharides like glycogen and starch.

In eukaryotes both lipids and proteins in the outer layer of the plasma membrane can have carbohydrates attached to them and are therefore called glycolipids and glycoproteins respectively. The glycolipids and glycoproteins also share a common name a glycocalyx. There are several functions of a glycocalyx namely in cell recognition, adhesion and protection [3].

References

  1. Berg J., Tymoczko J and Stryer L. (2007) Biochemistry, 6th edition, New York: WH Freeman. pg 304
  2. Berg J., Tymoczko J and Stryer L. (2001) Biochemistry fifth edition, New York:WH Freeman. pg 301
  3. Berg J., Tymoczko J., Stryer L. (2001) Biochemistry fifth edition, New York: WH Freemen page 307


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