CARBOHYDRATES
These substances comprise an exceedingly important group of compounds, the members of which constitute the major proportion of the dry matter of plants. The name "carbohydrate" indicates the fact that these compounds contain only carbon, hydrogen, and oxygen, the last two elements usually being present in the same proportions as in water. As a rule, natural carbohydrates contain six, or some multiple of six, carbon atoms and the same number of oxygen atoms less one for each additional group of six carbons above the first one; e.g., C6H12O6, C12H22O11, C18H32O16, etc.
Carbohydrates are classed as open-chain compounds, that is, they may be regarded as derivatives of the aliphatic hydrocarbons. From the standpoint of the characteristic groups which they contain, they are aldehyde-alcohols. In common with many other polyatomic open-chain alcohols, they generally possess a characteristic sweet, or mildly sweetish, taste. In the case of the more complex and less soluble forms, this sweetish taste is scarcely noticeable and these compounds are commonly called the "starches," as contrasted with the more soluble and sweeter forms, known as "sugars."
The characteristic ending ose is added to the names of the members of this group. As systematic names, the Latin numeral indicating the number of carbon atoms in the molecule is combined with this ending; e.g., C5H10O5, pentose, C6H12O6, hexose, etc.
In recent years, as a matter of scientific interest, many sugarlike substances which contain from two to nine carbon atoms combined with the proper number of hydrogen and oxygen atoms to be equivalent to the same number of molecules of water in each case, have been artificially prepared in the laboratory and designated as dioses, trioses, tetroses, pentoses, hexoses, heptoses, octoses, and nonoses, respectively. Substances corresponding in composition and properties with the artificial tetroses and one or two derivatives of heptoses are occasionally found in plant tissues, and a considerable number of pentoses and their condensation products are common constituents of plant gums, etc.; but the great majority of the natural carbohydrates are hexoses and their derivatives.
GROUPS OF CARBOHYDRATES
Since the simpler carbohydrates are sugars, i.e., they possess the characteristic sweet taste, the name "saccharide" is used as a basis for the classification of the entire group. The simplest natural sugars, the hexoses, C6H12O6, are known as mono-saccharides. The group of next greater complexity, those which have the formula C12H22O11 and may be regarded as derived from the combination of two molecules of a hexose with the dropping out of one molecule of water at the point of union, are known as di-saccharides. Compounds having the formula C18H32O16 (i.e., three molecules of C6H12O6 minus two molecules of H2O) are tri-saccharides; and the still more complex groups, having the general formula (C6H10O5)n, are called the poly-saccharides. The mono-, di-, and tri-saccharides are generally easily soluble in water, have a more or less pronouncedly sweet taste, and are known as the sugars; while the polysaccharides are generally insoluble in water and of a neutral taste, and are called starches. As will be seen later, there are many natural plant carbohydrates belonging to each of these groups.
In addition to these saccharide groups, there are other types, or groups, of compounds which resemble the true carbohydrates in their chemical composition and properties and are often considered as a part of this general group. These are the pentoses, C5H10O5, and their condensation products, the pentosans (C5H8O4)n, and their methyl derivatives, C6H12O5; certain polyhydric alcohols having the formula C6H8(OH)6; pectose and its derivatives, pectin and pectic acid; and lignose substances of complex composition. It is doubtful whether these compounds are actual products of photosynthesis in plants, or have the same physiological uses as the carbohydrates and it has seemed wise to consider them in a separate and later chapter.
ISOMERIC FORMS OF MONOSACCHARIDES
Four sugars having the formula C6H12O6, namely, glucose, fructose, mannose, and galactose, occur very commonly and widely distributed in plants. In addition to these, thirteen others having the same percentage composition have been artificially prepared, while seven additional forms are theoretically possible. In other words, twenty-four different compounds, all having the same empirical formula and similar sugar-like properties are theoretically possible. In order to arrive at a conception of this multiplicity of isomeric forms, it is necessary to understand the two types of isomerism which are involved. One of these is structural isomerism, and the other is space- or stereo-isomerism.