Carbohydrate literary means hydrated carbon. Carbohydrates are composed of carbon, hydrogen and oxygen and the ratio of hydrogen and oxygen is the same as in water. Carbohydrates polyhydroxy aldehydes or ketones or complex substances that on hydrolysis yield polyhdroxy aldehydes or ketones subunits. Hydrolysis involve break down of large molecules into smaller ones utilizing water molecules. Carbohydrate occurs abundantly in living organisms. They are found in all parts of the cell, cellulose of wood, cotton and paper starches present in cereals, root tubers, cane sugar and milk sugar are all examples of carbohydrate. The sources of carbohydrates are green plants. These are primary products of photosynthesis. Other compounds of plants are produced from carbohydrates by various chemical changes. Carbohydrates are animal’s major source of energy. Most animal cells have chemical machinery to breakdown the energy rich carbon hydrogen (C – H) bonds in sugars and starches. Carbohydrates in cell combine with proteins and lipids and the resultant compounds are called glycoproteins and glycolipids. Glycoproteins and glycolipids have structural role in extracellular matrix of animals and bacterial cell wall. Carbohydrates play structural and functional roles. Simple carbohydrates are main constituents of cell walls in plants and micro organisms.
Classification of carbohydrates:
They are also called saccharides which means sugar. They have three groups.
(1) Monosaccharides
(2) Disaccharides
(3) Polysaccharides
(1) Monosacchrides:
They are single sugars and are simple sugars. They have following properties.
They are sweet in taste, they are easily soluble in water and they cannot be hydrolyzed into simple sugars. Chemically they are either polyhdroxy aldehydes or ketones. All carbon atoms in a monosaccharaide except one have a hydroxyl group. Remaining carbon atom is either a part of an aldehyde group or a keto group. The sugar with aldehyde group is called also sugar and with keto group as keto sugar e.g. Glucose, Fructose.
(2) Disaccharides:
It is formed by removing a molecule of water from two monosaccharides. Disaccharides have the same molecular formula C12H22O11 e.g. Sucrose, lactose and maltose. A molecule of glucose combines with a molecule of fructose through 1 – 2 glycosidic bond to form sucrose, the table sugar. If a glucose molecule bonds to another monosaccharide galactose through 1 – 4 glycosidic bonds, the disaccharides formed is lactose, the milk sugar, when two glucose submits join together by 1 – 4 glycosidic bonds they form maltose that is present in seeds, especially it gives barley seeds a sweet taste. Beer brewers ferment barley into alcohol. Organic compounds having same molecular formula but different structural formulae are called isomers and each isomer has unique properties.
(3) Polysaccharides:
They are most complex and abundant carbohydrates in nature. They are usually branched and tasteless. Several monosaccharide units linked by glycosidic bonds form polysaccharides. They are tasteless and insoluble in water and thus are responsible for making structural part of cell and organelles. On hydrolysis they produce large number of monosaccharides. Monosaccharides are bonded together with glycosidic bond. Polysaccharides may be in straight chain formed through 1 – 4 glycosidic bonds or have branched chain formed by 1 – 4 and 1 – 6 glycosidic bonds e.g. starch, glycogen, cellulose, dextrin, agar, pectin and chitin. Starch is main source of carbohydrate, for animals and on hydrolysis provides glucose molecules for animal. It is found in fruits, grains, seeds and tubers.
Glycogen is generally known as animal starch. It is stored in liver and muscles of animals. It is insoluble in water and converted into glucose on hydrolysis. Cellulose is most abundant carbohydrate in nature. Cotton is the pure form of cellulose. It is main constituent of cell walls of plants and is highly insoluble in water. On hydrolysis it also yields glucose molecules. In herbivores it is digested because of presence of micro organisms like bacteria, yeasts and protozoa in the digestive tract. It is not digested in human digestive tract.
Chitin is major component of exoskeleton of insects and crustaceans. The monomer of chitin is amino sugar comprising of glucose with a nitrogen containing appendage.
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