CARBOHYDRATE, in chemistry, a com pound consisting of carbon, hydrogen and oxygen, and having the general formula C..11.70o. As will be seen, the number of car bon atoms in a carbohydrate is always divisible by six, and the oxygen and hydrogen are present in the same proportion in which they occur in water. It is not implied, however, that the compound contains water as such, but only that the oxygen and hydrogen atoms are present in the proportion of two atoms of the latter to one of the former. It will also be ob served that a carbohydrate and a hydrocarbon are two essentially different things, inasmuch as a carbohydrate contains oxygen, while a hydrocarbon is a compound containing no ele ment but carbon and hydrogen.
The carbohydrates constitute a large and very important class of substances, embracing the starches, sugars, glucoses and gums, as well as cellulose. Their chemical relations are in tricate, and are far from being thoroughly understood. Several schemes have been pro posed for their classification, but owing to the present imperfection of our knowledge none is entirely satisfactory. The classification pro posed by O'Sullivan is convenient, however, and will be adopted here.
Class 1.— SAccRARANz: Amorphous substances, having the general formula nCellieGs, soluble in water but insoluble in alcohol, and further characterized by the fact that when they are treated with acids they yield substances of the type nealuOt, directly. and without the formation of intermediate compounds. Dextran, kevulan, the amylans and the galactans are examples. (These bodies are gums).
Class 2.— SAccmulgrts: Substances possessing a certain amount of structure, having the general formula nC411100i, insoluble in either water or alcohol, and transformed by the action of acids and certain ferments first into nCisHnOil, and finally.- by the action of acids, into nC441106. Cellulose, starch, inulin and tunicin are examples; the first two falling under in the less elaborate classification which divides the carbohy drates merely into amylose, saccharose and glucose. • Class 3.— SAcctuaubrs: Amorphous substances, having the general formula nCsHio0i, soluble in water, but insoluble in alcohol; converted by acids first into nCizilaGii, and finally into nCslits0a; and by certain ferments into nC1,14,20u• Glycogen, dextrin and malto-dextrin are examples.
Class 4.— SACCHAROSES (sugars).
Group (a).— SACCHARONS: Sweet. crystallizable bodies, soluble in water and in moderately strong alcohol.
. having the general formula nCi2H2r0a, and convert ible by acids and sometimes by ferments into nesliirOi. Sucrose (cane sugar), lactose (milk sugar), maltose and raffinose are examples.
Group (b).— GLUCOSES: Substances crystallizing, though not so readily as the members of the preced ing group; having the general formula nCalu0.; soluble in both water and alcohol; and converted by the prolonged action of acids into substances that are no longer carbohydrates. Some of these, such as dextrose, bevulose and galactose, are fermentable by yeast; while others, such as sorbinose, are not fermentable.
Group (c)•— Certain substances, such as inosite and scyllit, which probably belong in the aromatic series and bear no special resemblance to the other mem bers of the carbohydrate family.
O'Sullivan has also a fifth class, including those substances which, though they may not be carbohydrates in the strict sense, are neverthe less closely allied to the carbohydrates and are easily converted into them when hydrolysed. In this class he places the glucosides and cer tain of the gums, mucilages and pectins.
The carbohydrates are exceedingly import ant elements in the world's food supply, and may indeed be said to be essential to the mainte nance of life. They are practically all of vege table origin, and are derived ultimately from certain simple fundamental substances that are formed in the green leaves of plants. Under the influence of sunlight the chlorophyll con tained in the leaves is competent to split up the carbon dioxide of the air, retaining the carbon and setting the oxygen free. The carbon that is abstracted in this way is caused to combine with the water that the leaves contain, with the production of carbohydrates; but the identity of the carbohydrate that is first formed in this way, and which serves as the starting-point for the others, is not yet established. According to the views of Sachs the °first obvious product° is starch, the formation of which he explained by the equation: Carbon Water Starch Free Dioxide Oxygen