Carbohydrates (F.Sc-Biology-Chapter-2.4): 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. Glucose is naturally produced in green plants which take carbon dioxide from the air and water from the soil to synthesize glucose.
As indicated n the equation, energy is consumed in this process which is provided by sunlight. This is why the process is called photosynthesis. It is noteworthy that for the synthesis of 10g of glucose 717.6 kcal of solar energy is used. This energy is stored in the glucose molecules as chemical energy and becomes available in all organisms when it is oxidized in the body.
Oligosaccharides:
These are comparatively less sweet in taste, and less soluble in water. On hydrolysis oligosaccharides yield from two to ten monosaccharide’s. The ones yielding two monosaccharide’s are known as disaccharides, those yielding three are known as trisaccarides and so on. The covalent bond between two monosaccharide’s is called glycoside bond.
Physiologically important disaccharides are maltose, sucrose, and lactose (see fig. 2.5). most familiar disaccharide is sucrose (cane sugar) which on hydrolysis yields glucose and fructose, both of which are reducing sugars. Its molecular formula is C12H22 O11. Its structural formula is given in fig. 2.5.
Fig. 2.5 A disaccharide. Not carefully the glycoside linkage between the two monosaccharide’s.
Polysaccharides:
Polysaccharides are the most complex and the most abundant carbohydrates in nature. They are usually branched and tasteless. They are formed by several monosaccharide units linked by glycoside bonds (fig. 2.6) polysaccharides have high molecular weights and are only sparingly soluble in water. Some biologically important polysaccharides are starch, glycogen, cellulose, dextrin’s, agar, pectin, and chitin.
Starch:
It is found in fruits, grains, seeds, and tubers. It is the main source of carbohydrates for animals. On hydrolysis, it yields glucose molecules. Starches are of two types, amylase and amylopectin. Amylase 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 colour with iodine.
Fig. 2.6 : polysaccharides are polymers of monosaccharide’s.
Glycogen:
It is also called animal starch. It is the chief from of carbohydrate 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 colour with iodine. It also yields glucose on hydrolysis.
Cellulose:
It is the most abundant carbohydrate in nature. Cotton is the pure form of cellulose. It is the main constituent of cell walls of plants and is highly insoluble in water. On hydrolysis it also yields glucose molecules. It is not digested in the human digestive tract. In the herbivores, it is digested because of micro-organisms (bacteria, yeasts, protozoa) in their digestive tract. These micro-organisms secrete an enzyme called cellulose for its digestion. Cellulose gives no colour with iodine.