Carbohydrate

Carbohydrates'[note 1] or saccharides are the most abundant of the four major classes of biomolecules. They fill numerous roles in living things, such as the storage and transport of energy (e.g., starch, glycogen) and structural components (e.g., cellulose in lants and chitin in arthropods). In addition, carbohydrates and their derivatives play major roles in the working process of the immune system, fertilization, pathogenesis,blood clotting]], and development.

Carbohydrates are simple organic compounds that are aldehydes or ketones with many hydroxyl groups added, usually one on each carbon atom that is not part of the aldehyde or ketone functional group. The basic carbohydrate units are called monosaccharides; examples are glucose, galactose, and fructose. The general stoichiometric formula of an unmodified monosaccharide is (C·H2O)n, where n is any number of three or greater; however, not all carbohydrates conform to this precise stoichiometric definition (e.g., uronic acids, deoxy-sugars such as fucose), nor are all chemicals that do conform to this definition automatically classified as carbohydrate.

Monosaccharides

Monosaccharide are the simplest carbohydrates in that they cannot be hydrolyzed to smaller carbohydrates. They are aldehydes or ketones with two or more hydroxyl groups. The general chemical formula of an unmodified monosaccharide is (C•H2O)n, literally a "carbon hydrate." Monosaccharides are important fuel molecules as well as building blocks for nucleic acids. The smallest monosaccharides, for which n = 3, are dihydroxyacetone and D- and L-glyceraldehyde.

Disaccharides

Two joined monosaccharides are called a disaccharide and these are the simplest polysaccharides. Examples include sucrose and lactose. They are composed of two monosaccharide units bound together by a covalent bond known as a glycosidic linkage formed via a dehydration reaction, resulting in the loss of a hydrogen atom from one monosaccharide and a hydroxyl group from the other. The formula of unmodified disaccharides is C12H22O11. Although there are numerous kinds of disaccharides, a handful of disaccharides are particularly notable.

Trisaccharide

Trisaccharides are oligosaccharides composed of three monosaccharides with two glycosidic bonds connecting them. Similar to the disaccharides, each glycosidic bond can be formed between any hydroxyl group on the component monosaccharides. Even if all three component sugars are the same (e.g., glucose), different bond combinations (regiochemistry) and stereochemistry (alpha- or beta-) result in triaccharides that are diastereoisomers with different chemical and physical properties.

Glycosidicbond

A glycosidic bond is formed between the hemiacetal group of a saccharide (or a molecule derived from a saccharide) and the hydroxyl group of some organic compound such as an alcohol. If the group attached to the carbohydrate residue is not another saccharide it is referred to as an aglycone. If it is another saccharide, the resulting units can be termed as being at the reducing end or the terminal end of the structure. This is a relative nomenclature where the reducing end of the di- or polysaccharide is towards the last anomeric carbon of the structure, and the terminal end is in the opposite direction.

In the literature, the bond between an amino group or other nitrogen-containing group and the sugar is often referred to as a glycosidic bond (although IUPAC seems to suggest that the term is a misnomer). For example, the sugar-base bond in a nucleoside may be referred to as a glycosidic bond.[1] A substance containing a glycosidic bond is a glycoside.

Polysaccharides

A glycosidic bond can join two monosaccharide molecules to form a disaccharide, as, for instance, in the linkage of glucose and fructose to create sucrose. More complicated polysaccharides such as starch (an important nutrient), glycogen, cellulose (cell walls of plants) or chitin (found in fungi) consist of numerous monosaccharide units joined by glycosidic bonds.

While the cyclic structures of monosaccharide units are fairly rigid, the glycosidic bonds confer flexibility to polysaccharide molecules.

Main article: Carbohydrate conformation

Glycosidic bonds join monosaccharides to form polysaccharides, just like peptide bonds join amino acids to form proteins. The conformation of the torsional angles about the glycosidic bond are the most flexible point of a polysaccharide. The orientations of these torsions typically fall within an expected range of values.

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Done by student: baraa derar al-ahmaarclass / 10/2

Under the supervision of Professor: Mr. Ayman Elsangary