H-C-OH H-C-OH H-C-OH
| | | |
CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH
Glucose Mannose Fructose Enolic Form

The preceding technical discussion of the chemical constitution and
reactions of the hexoses has been presented, not because it has any direct
connection with the occurrence or functions of these compounds in plant
tissues, but for the purpose of giving to the student a graphic conception
of the structure and properties of these simple carbohydrates, as a basis
for the understanding of the nature, properties, possible chemical
reactions, syntheses, etc., of the more complex types of carbohydrates,
which, along with these simple monosaccharides, constitute the most
important single group of organic components of plants.

THE OCCURRENCE AND PROPERTIES OF MONOSACCHARIDES

Only two monosaccharides occur as such in plants. These are glucose and
fructose. All the other hexoses, whose structure is shown on pages 37 and
38, occur in plants only as constituents of the more complex saccharides,
in glucoside-formations, or as the corresponding polyatomic alcohols.

The aldo-hexoses which occur most commonly in plants, either free or in
combination, are _d_-glucose, _d_-mannose, and _d_-galactose; while
_d_-fructose and _d_-sorbose are the common keto-hexoses.

=Glucose= (often called also dextrose, fruit sugar, or grape sugar) occurs
widely distributed in plants, most commonly in the juices of ripening
fruits, where it is usually associated with fructose and sucrose, the two
hexoses being easily derived from sucrose by hydrolysis. Glucose is also
produced by the hydrolysis of many of the more complex carbohydrates, by
the action either of enzymes or of dilute acids; lactose, maltose,
raffinose, starch, and cellulose, as well as many glucosides all yielding
glucose as one of the products of their hydrolysis. In all such cases, it
is _d_-glucose which is obtained.

Glucose is a crystalline solid (although it does not form such sharply
defined crystals as does sucrose, or "granulated sugar"), which is easily
soluble in water. It usually appears on the market in the form of thick
syrups, which are produced commercially by the hydrolysis of starch with
dilute sulfuric acid, removal of the acid after the hydrolysis is complete,
and evaporation of the resulting solution to the desired syrupy
co