When considered from the standpoint of efficiency, the sugars are among the most valuable of all the carbohydrates, although in quantity they are less important than the starches, at least in raw food materials.

Unlike starch, they are found in solution in the sap of growing plants. It is probable that these are the forms in which carbohydrate material is transferred from one part of the plant to another. It is easy to see that some such medium of exchange is necessary. The actual production of new vegetable substance takes place in the leaves. When, therefore, cell-walls and starch grains are to be constructed in the stem and fruit, the building material must be carried from the leaves to these parts in forms which will readily pass through intervening membranes. Excepting certain soluble compounds, closely related to starch, the sugars appear to be the only available bodies fitted for this office.

It is very seldom that a plant contains only a single sugar. Generally two or more sugars are found together. This is especially the case in the corn plant, sorghum, and the fruits; and the proportions of each depend somewhat upon the stage of growth of the plant.

67. Classification Of Sugars According To Structure

The structure of certain. sugars is such that their molecules cannot be divided into simpler compounds that retain the carbohydrate character, and these are known as monosaccharides. To this class belong glucose (grape sugar) and fructose (fruit sugar). On the other hand, there are a large number of carbohydrates, one molecule of which by treatment in certain ways may be converted into two or more molecules of a mono- (simple) sugar. For instance, one molecule of starch, when submitted to the action of an acid or of certain ferments, breaks up into several molecules of glucose, and we call starch a poly-saccharide; and to this class belong sucrose (cane sugar), maltose (malt sugar), lactose (milk sugar), cellulose, the starches and gums, all of which maybe split up into mono- or simple sugars. The poly-sugars are subdivided into di-, tri-, and so on, according as they break up into two, three, or more molecules of a simple sugar.

There are subdivisions of the mono-sugars also, on the basis of the number of carbon, atoms in their molecules, and thus we have the names diose, triose, tetrose, pentose, hexose, heptose, etc., for sugars having two, three, four, five, six, seven, or more carbon atoms in the molecule. It may be remarked here that it is among the hexose (six carbon) sugars or their multiples that we find the carbohydrates most important to human nutrition.