Carbohydrates: Carbohydrates occur abundantly in living organisms. They are found in all organisms and in almost 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 carbohydrates. They play both structural and functional roles. Simple form of them are the main source of energy in cells. Some of them are the main constituents of cell wall in plants and microorganisms. The word carbohydrate literally means hydrated carbons. They are composed of carbon, hydrogen and oxygen and the ratio of hydrogen and oxygen is the same as in water. Chemically, carbohydrates are defined as polyhydroxy aldehydes or ketones or complex substances which on hydrolysis yield polyhydroxy aldehyde or ketones subunits. hydrolysis involves the break down of large molecules into smaller once utilizing water molecules.
Sources of carbohydrates are green plants. These are primary products of photosynthesis. Other compounds of plants are produced from them by various chemical changes. Carbohydrates in cell combine with proteins and lipids and the resultant compounds are called glycoproteins and glycolipids, respectively. Glycoproteins and glycolipids have structural role in extracellular matrix of animals and bacterial cell wall. Both these conjugated molecules are components of biological membranes.
Classification of carbohydrates: They are also called saccharides(derived from Greek word sakcharon meaning sugar) and are classified into three groups: monosaccharides, oligosaccharides and polysaccharides.
Monosaccharides: These are simple sugars. They are sweet in taste, are easily soluble in water and cannot be hydrolysed into simpler sugars. Chemically they are either polyhydroxy aldehydes or ketones. All carbon atoms in a monosaccharide except one have a hydroxyl group. The remaining carbon atom is either a part of an aldehyde group or a keto group. The sugar with aldehyde group is called aldo-sugar and with keto group as keto-sugar. In nature monosaccharides with 3 to 7 carbon atoms are found. They are called trioses, tetroses, pentoses, hexoses and heptoses. Two trioses are intermediate in respiration and photosynthesis. Tetroses are rare in nature and occur in some bacteria. Pentoses and hexoses are most common. From the biological point of view the most important hexose is glucose and it is an aldose sugar. Most of the monosaccharides form ring structure when in solution. For example ribose will form five concerned ring known as ribofuranose, whereas glucose will form six concerned ring known as glucopyranose. In free state glucose is present in all fruits being abundant in grapes, figs and dates. Our blood normally contains 0.08% glucose. In combined form it is found in many disaccharides and polysaccharides. Starch, cellulose and glycogen yield glucose on complete hydrolysis.
Oligosaccharides: These are comparatively less sweet in taste and less soluble in water. On hydrolysis oligosaccharides yield from two to ten monosaccharides. The ones yielding two monosaccharides are known as disaccharides those yielding three are known as trisaccharides and so on. The covalent bond between two monosaccharides is called glycosidic bond. Physiologically important disaccharides are maltose, sucrose and lactose. Most familiar disaccharide is sucrose which on hydrolysis yields glucose and fructose both of which are reducing sugars.
Polysaccharides: Polysaccharides are most complex and most abundant carbohydrates in nature. They are usually branched and tasteless. They are formed by several monosaccharide units linked by glycosidic bonds. Polysaccharides have high molecular weights and are only sparingly soluble in water. Some biologically important polysaccharides are starch, glycogen, cellulose, dextrin, agar, pectin and chitin.
Starch: It is found in fruits, grains, seeds and tubers and it is main source of carbohydrates for animals. On hydrolysis it yields glucose molecules. Starches are of two types amylose and amylopectin. Amylose starches have unbranched chains of glucose and are soluble in hot water. Amylopectin starches have branched chains and are insoluble in hot or cold water. Starches give blue color with iodine.
Glycogen: It is also called animal starch. It is chief form of carbohydrates stored in animal body. It is found abundantly in liver and muscles though found in all animal cells. It is insoluble in water and gives red color with iodine. It also yields glucose on hydrolysis.
Cellulose: It is most abundant carbohydrate in nature. Cotton is pure form of cellulose. It is main constituent of cell wall of plants and is highly insoluble in water. On hydrolysis it yields glucose molecules. It is not digested in human digestive tract. In herbivores it is digested because of microorganisms in their digestive tract. These microorganisms secrete an enzyme called cellulase for its digestions. Cellulose gives no color with iodine.
0 Comments