CARBOHYDRATES

                                             CARBOHYDRATES:

More than half of all the organic carbon on planet Earth in stored in just two carbohydrates molecules-Starch and cellulose. Both are the polymers of the sugar monomer, glucose.

The only different between them is the manner in which the glucose units are joined together.

When the word ‘’carbohydrate’’ was coined, it originally referred to compounds of the general formula Can (H2O2)n, However, only the simple sugars, or mono-saccharides, fit this formula exactly.

ü Monosaccharaides: Structure and Stereochemistry:-

A Monosaccharaides can be a poly hydroxyl aldehyde (aldose) or a poly hydroxyl ketone (ketoses). The simplest Monosaccharaides contain three carbon atoms are called trioses (tri meaning three). Glyceraldehyde is the aldose with three carbon (an aldose triose), and di hydroxyl acetone is the ketose with three carbon atoms (a keto triose).

Six-carbon sugars are the most abundant in nature, but to five-carbon sugars, ribose and

deoxyribose occur in the structures of RNA and DNA, respectively. Four-carbon and seven-carbon sugars play roles in photosynthesis and other metabolic pathways.

Ø Cyclic Structures: Anomers

Sugars, especially those with five and six carbons atoms, normally exist as cyclic molecules rather than as the open-chain forms. They cy-citation takes place as a result of interaction between for functional groups on distant carbons such as C-1 and C-5 to form a cyclic hemiacetal (in aldohexoses). Another possibility in interaction beyween C-2 and C-5 to form a cyclic hemiketal (in ketohexoses). In either case, the carbonyl carbon becomes a new chiral center called the enomeric carbon. The cyclic sugar can take either of tow different forms, designated a and B, and are called anomers of each other.

ü Oligosaccharides:

Oligomers of sugars frequently occur as disaccharides, form by linking tow monosaccharide’s units by glycosides’ bonds. They are sucrose, lactose and maltose.

Sucrose is the common table sugar extracted from sugarcane and sugar beets. The Monosaccharaides units that make up sucrose are a-D glucose and B-D-fructose. Glucose (an aldohexoses) is a pyranose, and fructose (a ketohexoses) is a furanose. The a C-1 carbon of the glucose is linked to the B C-2 carbon of the fructose in a glycosidic linkage that has the notation Ab (1-2). Sucrose is not a reducing sugar because both anomeric groups.

Lactose is disaccharides made up of B-D-galactose and D-glucose, Galactose is the C-4 epimer of glucose. In other words, the only difference between glucose and galactose is inversion of configuration at C-4.

ü Polysaccharides:

Polysaccharides that occur in organisms are usually composed of a very few types monosaccharide components. A polymer that consists of only one type of monosaccharide is a homopolysaccharide. Glucose is the most common monomer. Cellulose and chitin are polysaccharides with B-glycosidic linkages and are structural materials. Starch and glycogen, also polysaccharides, have a-glycosidic linkages, and serve as carbohydrates storage polymers in plants and animals, respectively.

·        Chitin:

Polysaccharides that is similar to cellulose in both structure and function is chitin, which is also a linear homopolysaccharides with all the residues linked in B (1-4) glycosides’ bonds. Chitin differs from cellulose in the nature of the monosaccharide unit; in cellulose the monomer is B-D-glucose, and in chitin the monomer is N-acetyl-B-D-glucosamine.

·        Glycoproteins:

Glycoproteins contain carbohydrates residues in addition on the polypeptide chain, some of the most important example of glycoproteins are involved in the immune response; for example, antibodies.