Carbohydrate

Carbohydrates are the polyhydroxy aldehyde or ketone compounds. Carbohydrates are also called as “saccharides” and this word is derived from Greek word “saccharo” which means “sugar”. Another definition of carbohydrate is that they are polyhydroxylated compounds with at least 3 Carbon atoms with potentially active carbonyl group which may be either aldehyde or ketone. Carbohydrates also include those substances that yield such compounds upon hydrolysis. Some carbohydrates also contain N, P and S.

The name “carbohydrate” is derived from the fact that the first compounds of this group studied had an empirical formula C_x(H₂O)_y (hydrates of carbon). Now it is known that many carbohydrates do not fit into this formula e.g. deoxyribose which is C₅H₁₀O₄. However, when carbohydrates are heated then carbon and water are obtained.

Carbohydrates are the most abundant biomolecules on the earth. They form only 2% of human body mass. They are source of about 80% of human caloric intake. Each year, photosynthesis converts more than 100 billion metric tons of CO₂ and H₂O into cellulose and other plant products. These carbohydrate products have important structural and metabolic roles. In plants, glucose is synthesized from carbon dioxide and water by photosynthesis and stored as starch or used to synthesize the cellulose of the plant cell walls. Animals can synthesize carbohydrates from amino acids, but in most animal carbohydrate is derived ultimately from plants source. Mostly, dietary carbohydrate is absorbed into the bloodstream as glucose while other sugars that are absorbed are converted to glucose in the liver.

Glucose is most important among all carbohydrates. Glucose is the precursor for synthesis of all the other carbohydrates in the body including glycogen for storage; ribose and deoxyribose for nucleic acids; galactose for lactose of milk, in glycolipids, and in glycoproteins. Diseases associated with carbohydrate metabolism include diabetes mellitus, galactosemia, glycogen storage diseases, and lactose intolerance.

Blood glucose is serving as direct food for brain. Mammary glands take the glucose from the blood convert it into the galactose. Then, mammary glands combine this galactose molecule with another glucose molecule to form the lactose. Glucose is oxidized preferably by all tissues of the body to provide energy. Orally more than half of the energy of the body is provided by oxidation of glucose. Excess of glucose is readily converted into fats and stored in the fat depots.

Classification

There is no single satisfactory classification of carbohydrates. Commonly used classification is given below

1.   Monosaccharides: These are simple sugars which cannot be further hydrolyzed and have the empirical formula (CH₂O)_n where n = 3 or greater. They may be classified as trioses, tetroses, pentoses, hexoses, or heptoses, depending upon the number of carbon atoms. They are either aldoses (contain aldehyde group) or ketoses (contain ketone group).

No. of Carbons	Generic Name	Examples
		Aldoses	Ketoses
3	Trioses	Glyceraldehyde	Dihydroxyacetone
4	Tetroses	Erythrose	Erythrulose
5	Pentoses	Ribose	Ribulose
6	Hexoses	Glucose	Fructose
7	Heptoses	Glucoheptose	Seduheptulose

In nomenclature of carbohydrates, the type of carbonyl group (aldehyde or ketone) and the generic name are combined to name the monosaccharides. For example, glucose is an aldose as well as a hexose; it is therefore, an aldohexose while fructose is a ketohexose. Monosaccharides are white crystalline solids, freely soluble in water and most have a sweet taste.

2.   Disaccharides: Disaccharides are the condensation product of two monosaccharide units linked together by a covalent bond e.g., maltose (2 glucose molecules), sucrose (1 glucose + 1 fructose), lactose (1 glucose + 1 galactose). Monosaccharides are joined together by glycosidic linkage.

3.   Oligosaccharides: The word oligosaccharide is derived from the Greek word “oligo” which means “few”. These are the condensation product of 3 to 10 monosaccharide units. Sometime disaccharides are also included in this group. The monosaccharides are joined together through glycosidic linkages. Examples include maltotriose (3 glucose molecules), α-dextrin (8 glucose molecules).

           

4.   Polysaccharides: Polysaccharides are the condensation product of more than ten monosaccharides units. Monosaccharides units may be either in straight or in branched chains (e.g. cellulose, glycogen, starch, and dextrin). There are two types of polysaccharides.

a.    Homopolysaccharides: These are polysaccharides that are formed from combination of only one type of monosaccharide units. They are also called homoglycans. Examples include starch, glycogen, cellulose and dextrins. All of which yield glucose on hydrolysis.

b.   Heteropolysaccharides: These are the polysaccharides that formed from combination of two or more types of monosaccharides units. They are also called as heteroglycans. These are divided into following three groups.

 i. Mucopolysaccharides: these polysaccharides in addition to containing carbohydrate group also contain acid group e.g. hyaluronic acid, heparin, blood groups polysaccharides, serum mucoids.

ii. Mucilages: These polysaccharides occur in plants e.g. agar, vegetable gums and pectin

iii. Hemicellulose: These polysaccharides occur in plants e.g. hemicellulose.

5.   Derived Carbohydrates: These carbohydrates are derived from carbohydrates by various chemical reactions. These may be:

a.   Oxidation products: These sugar acids derived from glucose on its oxidation e.g., gluconic acid, glucuronic acid, glucaric acid, Ascorbic acid (vitamin C).

b.   Reduction products: These are the reduced products of carbohydrates. These are polyhydroxy alcohols e.g., glycerol (derived from glyceraldehydes), ribitol (derived from ribose).

c.    Amino sugars: These have –NH₂ group at carbon No.2 e.g., glucosamine, galactosamine, mannosamine.

d.   Deoxy sugars: These have less number of oxygen atoms than other sugars e.g., deoxyribose (present in DNA and is one oxygen atom lesser than ribose).

Carbohydrates also present in combination with lipids (glycolipids) and proteins (glycoproteins).