All carbohydrates contain carbon, hydrogen and oxygen of these hydrogen and oxygen occur in the same proportion as they do in water i.e. H2O. When these substances are heated the water escapes and carbon is left belind as a black mass. Some carbohydrates are soluble in water e.g. sugar and inulin while other are insoluble e.g. starch and glycogen.
(1) SUGAR:
These are various kinds of sugars formed in plants of these grape sugar or glucose is chiefly found in grapes and cane sugars or sucrose in sugarcane and beets. Grape sugar is the simplest of all carbohydrates and is formed in the leaf by chloroplasts in the presence of sunlight. Other forms of carbohydrates are derived from it. Commonly all the glucose formed in the leaf becomes converted into starch, an insoluble carbohydrate. At night this starch is reconverted into sugar which then travels to the storage organs where it is again converted into starch by leucoplasts. The chemical formula of gape sugar is C6H12O6 and that of cane sugar is C12H22O11. Glucose contents of gapes are 12.15% or more sucrose contents of sugar canes 10.15% and of beet roots 10.20%.
(2) INULIN
Inulin is a soluble carbohydrate and occurs in solution in the cell sap. When required for nutrition it is converted into a form of sugar (fruit sugar or fructose). Inulin is present in the tuberous roots of Dahlia and some other plants. When pieces of Dahlia roots are kept in alcohol or glycerine for 6 to 7 days preferably more, inulin becomes precipitated in the form of spherical crystals. Under the microscope fully formed inulin crystals are seen to be star of wheel shaped and half formed ones more or less fan shaped. These crystals are deposited mostly across the cell walls and occasionally only in the cell cavity. Sometimes these crystals are so large that they extend through many cells. Inulin has same chemical composition as starch viz (C6H10O5).
(3) STARCH
This is an insoluble carbohydrate and occurs in the form of minute grains starch grains occur as a reserve food in all green plants in their storage organs. Rice, wheat, maize and millets which constitute the staple food of mankind are specially rich in starch. Starch grains may be oval, spherical, rounded and flat or polygonal. They also vary very much in size the largest known being about 100 microns in length as in rhizome and canna and the smallest about 5 microns in length as in rice.
In potato they are of varying sizes. Starch is always derived from sugar either in the leaf by the chloroplasts or in the storage organ by the leucoplasts. When required for nutrition starch is converted into sugar. In the starch grain a dark roundish or elongated spot may be seen; this is the point of origin of the grain and is known as lilum. Around the lilum a variable number of strata (layers) of different dristities are alternately deposited. In some grains as in those of Potato the layers are laid down on one side of the lilum; such starch grains are said to be concentric. In others as in those of pea the layers are deposited concentrically round the lilum, such starch grains are said to be concentric. Commonly as in Potato and pea starch grains occur singly with lilum when they are said to be simple.
Sometimes however two or more grains occur together in a solid group with as many hila as there are grains in it. This group then is said to form a compound grain as in rice and oat. A few compound grains are also sometimes formed in Potato. Starch has the same chemical composition as cellulose and inluin (C6H12O5). It is insoluble in water and alcohol.
Rice contains 70-80% of starch, wheat about 70%, maize about 68%, barley 60-65% and potato 20%.
(4) GLYCOGEN
This is very common form of carbohydrates occurring in Fungi. In yeast, a unicellular fungus, it occurs to the extent of about 30% of the dry weight of the plant. It is not found in higher plants but is widely distributed among animals and is therefore sometimes called animal starch. It occurs in the form of granules in the cytoplasm of the cell. Glycogen dissolves in hot water. It is coloured reddish brown with iodine solution. Its chemical formula is (C6H10O5) n.
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