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A carbohydrate is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms. Usually, carbon, hydrogen, and oxygen atoms are in a ratio of 1:2:1 hence, the empirical formula for (CH2O) n. Chemically, carbohydrates are polyhydroxy aldehydes (an aldehyde, is a functional group in which a carbon atom shares a double bond with an oxygen atom, a single bond with a hydrogen atom) or ketones (it is a functional group having carbonyl group =C=O bonded to two hydrocarbon groups). It can also be said that carbohydrates are the compounds that yield aldehydes or ketones upon hydrolysis.
Carbohydrates are the most abundant molecules on earth. Carbohydrates perform numerous roles in living organisms like:
a) Polysaccharides (like starch and glycogen) serve as the storage of energy and polysaccharides like cellulose in plants and chitin in arthropods serve as structural components.
b) Ribose (a 5-carbon monosaccharide) is an important component of coenzymes (ATP, FAD and NAD) and that of the genetic molecule, RNA while deoxyribose is a component of DNA.
c) Saccharides and their derivatives include many other important biomolecules that play key roles in the immune system, fertilization, preventing pathogenesis, blood clotting, and development.
d) They are found in a wide variety of natural and processed foods. Starch is a polysaccharide, which is abundantly present in cereals (wheat, maize, rice), potatoes etc. Sugars like (sucrose, extracted from sugarcane or sugar beets), lactose (abundant in milk), glucose and fructose, both of which occur naturally in honey, many fruits, and some vegetables. All form of sugar is consumed by humans.
e) Cellulose is a polysaccharide found in the cell walls of all plants and is one of the main components of insoluble dietary fibre. Although it is not digestible, it is an insoluble but an important dietary fibre (roughage), that helps to maintain a healthy digestive system by easing defecation.
Read other chemistry notes for class 12 Band Theory of Solids
There are three major classes of carbohydrates:
The word “saccharide” is derived from the Greek word “sakkharon” which means sugar.
They are also known as simple sugars as they cannot be further hydrolysed to simpler chemical compounds. Monosaccharides consist of single polyhydroxy aldehyde or ketone. They are usually colourless, water-soluble, and crystalline solids that are readily soluble in water. Most of them have a sweet taste. Polymerization of monosaccharides forms the oligosaccharides and polysaccharides. The most abundant monosaccharide occurring in nature is Glucose. The backbone of monosaccharide is an unbranched carbon chain in which all the carbon atoms are linked by single bonds. One of the carbons is double bonded to oxygen to form a carbonyl group, the other carbon atoms form a single bond with one hydroxyl group.
Based on the position of the carbonyl group, the monosaccharides are classified into two classes-
If the carbonyl group is at an end of the carbon chain, the monosaccharide is an aldehyde and it is known as Aldose. Given below are the example of aldoses and their further classification as aldo- tetrose, aldo-pentose, aldo-hexose and so on.
Just like aldoses, ketoses are also divided into keto-tetrose, keto-pentose, keto- hexose and so on.
a) All monosaccharides, except dihydroxyacetone (see the chart of ketoses above), contain one or more asymmetric carbons also known as chiral carbon, hence, monosaccharides have optically active isomeric forms (the two or more forms of a compound which have the same structure but they are mirror images of each other and typically differ in optical activity). Optical isomers are also known as enantiomers/stereoisomers
A chiral carbon is a carbon centre that contains four nonidentical substituents. Sometimes the chiral centre is also known as a stereogenic centre or anomeric carbon. In general, a molecule with n chiral centres will have 2n. The two stereoisomers of glyceraldehyde having one chiral carbon are shown below:
b) Simple monosaccharides are reducing agents. They can be oxidized by mild oxidizing agents like; ferric ions (Fe3+) or cupric ions (Cu2+). This property is useful in the laboratory analysis of sugars; it is the basis of Fehling’s Reaction. Glucose and other sugars that are capable of reducing ferric or cupric ions are called reducing sugars.
c) Monosaccharides with five or more carbon atoms in the backbone usually occur in aqueous solution as cyclic or ring structure. Such cyclic structures are known as pyranose because they resemble six-membered ring structure pyran. In the cyclic structure, the carbonyl group forms a covalent bond with the oxygen of the hydroxyl group along the chain.
They are short chains of monosaccharide units that are joined together by glycosidic linkages. A glycosidic bond is formed when the hydroxyl group on one sugar reacts with the anomeric carbon of the other. When the anomeric carbon is involved in a glycosidic bond, it cannot be oxidized by cupric or ferric ion. The most common oligosaccharides are the disaccharides, which contain two monosaccharide units. For example, maltose is a disaccharide which is made up of two glucose units.
All common monosaccharides and disaccharides have their names ending in the suffix as “-ose”. Most oligosaccharides that have three or more monosaccharide units do not occur as free entities but are joined to non-sugar molecules like lipids or proteins in hybrid structures. They are known as glycoconjugates.
The polysaccharides consist of long chains having hundreds or thousands of monosaccharide units. They are polymers of high molecular weight. Some polysaccharides like cellulose occur as linear chains (having many glucose units), whereas some like glycogen have branched chain.
Depending upon the monosaccharide units, the polysaccharides are divided into two classes:
Homopolysaccharides: These polysaccharides contain only a single type of monosaccharide units. These include starch, glycogen, cellulose, chitin etc. Starch and glycogen are storage form of carbohydrates and are used as fuel. Cellulose and chitin are the structural elements in the plant cell wall and exoskeleton of animals respectively.
Homopolysaccharides: They contain two or more different kinds of monomeric units. These carbohydrates provide extracellular support for all living organisms. The bacterial cell wall is a heteropolysaccharide built from two alternating monosaccharide units. In animal tissues, the extracellular space is occupied by several types of heteropolysaccharides built from alternating monosaccharide units. This forms the matrix that holds the individual cells and provides protection, support and shape of the cells, tissues and organs.
Read another note PREPARATION OF DIOXYGEN of Class 12 Chemistry
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