Carbohydrates are most abundant biomolecules in nature. They are primarily composed of carbon, hydrogen and oxygen. Carbohydrates generally possess the empirical formula (CH2O)n. However, there are several non-carbohydrate compounds which also appear as hydrates of carbon e.g. acetic acid (C2H4O2), lactic acid (C3H6O3) etc. Some of genuine carbohydrates e.g. Rhamnohexose (C6H12O5), deoxyribose (C5H10O4), etc. do not satisfy the general formula. Hence, carbohydrates cannot always be considered as hydrates of carbon. Therefore, carbohydrates can be defined as polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis.
Carbohydrates are partially oxidized organic compounds widely used in biological systems as structural constituents, for metabolism to generate energy and biosynthetic intermediates. Carbohydrates are aldehyde or ketone compounds with multiple hydroxyl groups. These groups are reactive in carbohydrates of five or more carbons: carbonyl carbon tends to undergo a non-enzymatic attack by one of hydroxyl groups, resulting in a hemiacetal or hemiketal ring structure.
Most carbohydrates are chiral compounds. Most of commonly used carbohydrates have D configuration, although some carbohydrates used for post-translational modification and some other non-metabolic purposes have L configuration. Unmodified carbohydrates are present in aqueous solution as a mixture of ring and linear forms. The free carbonyl carbon of linear form may react with other biological compounds to produce potentially deleterious covalent adducts.
Carbohydrates may form chains involving ether linkages between monosaccharide units. At least one of carbons involved in these linkages is anomeric carbon (Carbon derived from Carbonyl Carbon of open chain form). Oligosaccharide units are used in glycoproteins. Polysaccharides act as structural molecules (such as cellulose), and as energy storage molecules (sucrose, starch, and glycogen).
Some major functions of carbohydrates are:
- They are most abundant dietary source of energy (4cal/gm) for all organisms.
- Precursors for many organic compounds (fats, amino acids).
- They have cellular functions such as cell growth, adhesion and fertilization.
- Used in synthesis of Nucleic acids and generation of energy.
- Carbohydrate polymers (also called glycans) serve as structural and protective elements in the cell walls of bacteria and plants and in the connective tissues of animals.
- Serve as storage form of energy (glycogen) to meet immediate energy demands of body.
Carbohydrates are often referred to as saccharides (Greek: sakcharon-sugar). On the basis of number of sugar units, there are three major classes of carbohydrates:
Mono and oligosaccharides are water soluble, sweet to taste and crystalline in character. Hence, they are commonly known as sugars.
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