Of the constituents of the ordinary mixed food of man the carbohydrates are usually the most abundant and the most economical sources of energy. They are also considered to be the first of the three great groups of foodstuffs to be formed by synthesis from simple inorganic substances in plants; "in the long run, all the energy of living matter comes from them." The synthesis of carbohydrates in nature is therefore a logical starting point for the study of the organic foodstuffs.

In the chlorophyll cells of the leaves of green plants the energy of the sun's rays brings about reaction between carbon dioxide and water which results in the liberation of oxygen and the formation of organic compounds. There is still doubt as to the exact mechanism of the process and no certainty that it is the same in all cases. It has, however, been quite generally found that the volume of oxygen liberated is equal to that of carbon dioxide consumed. The simplest possible representation of the reaction would be CO2 + H20 → CH20 + O2 according to which the first product of the synthesis would be formaldehyde. There is considerable (though not conclusive) evidence that formaldehyde is thus formed and that it is rapidly built up into less reactive compounds. Whatever the steps in the process* there is normally an early production of carbohydrate. Usually the first product which can be demonstrated as accumulating in the plant as the result of the photosynthesis is a sugar (glucose or sucrose) or starch. Assuming glucose as a typical product and neglecting the intermediate stages, the photosynthesis of carbohydrate may be represented thus:

6 CO2 + 6 H20 -> C6H12O6 + 6 O2

Glucose is the most familiar representative of a group of simple sugars (monosaccharides or monosaccharoses) which are in composition direct polymers of formaldehyde (CH20) and which are classified, according to the number of carbon atoms in the monosaccharide molecule, as trioses, pentoses, hexoses, etc.

Classification

Definitions of the term "simple sugar" vary somewhat, depending chiefly upon the views of different authors as to how simple a compound may properly be called a sugar.

According to Browne, a simple sugar or monosaccharide may be defined as an aldehyde alcohol or ketone alcohol of the aliphatic series, the molecule of which contains one carbonyl and one or more alcohol groups, one of the latter being always adjacent to the carbonyl group. All simple sugars contain, therefore, as a characteristic group upon the presence of which the chief chemical properties of the sugars depend.

* For concise discussion of the synthesis of carbohydrates in plants the reader may be referred to Armstrong's The Simple Carbohydrates and the Glucosides, pages 92-96; Browne's Handbook of Sugar Analysis, pages 532-534; and Mathews' Physiological Chemistry, pages 44-49. A somewhat fuller account will be found in Jost's Pflanzenphysiologie and Euler's PJlanzenchemie, and a very detailed treatment of the subject in Czapek's Biochemie der Pflanzen. For discussion from a more physiological standpoint, see Pfeffer's Plant Physiology and summary of recent work by Jorgenson and Stiles in The New Phytologist.

The simplest possible sugar according to this definition is glycolaldehyde, CH2OH-CHO, which (in analogy with the nomenclature of the familiar sugars) may also be called glyco-lose. The structural formulae of glucose and fructose, the most familiar representatives of the aldehyde-alcohol (aldose) and ketone-alcohol (ketose) sugars, respectively, are as follows:

Glucose

Fructose

Since glucose gives aldehyde reactions but not so readily as the above structural formula would lead one to expect, it is believed that in ordinary solutions of glucose the substance exists partly in the condition indicated by the aldehyde formula and partly in a tautomeric form represented by the lactone or "oxygen bridge" formula.

Following are the aldehyde and lactone formulae written without reference to the spatial relationships of the hydrogen and hydroxyl groups:

Aldehyde form:

CH2OH-CHOH-CHOH-CHOH-CHOH-CHO

Lactone form:

The name monosaccharide ("single sugar") implies that the monosaccharide molecule contains only one sugar radicle - that it cannot be split by hydrolysis into sugars of lower molecular weight. A substance like cane sugar which on hydrolysis splits to two molecules of simple sugar is called a disaccharide or disaccha-rose ("double sugar"). Trisaccharides and tetrasaccharides are also known. Substances which like starch are of high molecular weight and on complete hydrolysis yield many molecules of simple sugar are called polysaccharides * or polysaccharoses. The term "carbohydrates" covers all the simple sugars and all substances which can be converted into simple sugars by hydrolysis. The term "glucosides" is applied to substances which consist of combinations of carbohydrate radicles with radicles of other kinds and which therefore yield on hydrolysis both a simple sugar and one or more products of other than carbohydrate nature.