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PENTOSES. 207
is known as to how this cleavage occurs. According to Buchner and
Meissenheimer 1
the fermentation with the enzymes contained in the
bacteria produces chiefly the racemic, inactive form of the acid. This
also occurs as a rule by the action of living bacteria. In reference to the
formation of lactic acid within the organism see Chapter X.
The monosaccharides are colorless and odorless bodies, neutral in
reaction, with a sweet taste, readily soluble in water, generally soluble
with difficulty in absolute alcohol, and insoluble in ether. Some of them
crystallize well in the pure state. They are strong reducing substances.
They reduce metallic silver from ammoniacal silver solutions and they
also reduce other metallic oxides such as copper, bismuth and mercury
oxides, on heating in alkaline solution. This behavior is of great
importance in the detection and quantitative estimation of the sugars.
The simple varieties of sugar occur in part in nature as such, already
formed, which is the case with both of the very important sugars, glucose
and fructose. They also occur in great abundance in nature as more
complex carbohydrates (di- and polysaccharides); also as ester-like
combinations with different substances, as so-called glucosides.
Among the groups of monosaccharides known at the present time,
those containing less than five and more than six carbon atoms in the
molecule have no great importance in biochemistry, although they are
of high scientific interest. Of the two groups the hexoses are the more
abundant and are cf special interest. The pentoses are becoming cf
greater importance, net only for the chemistry of plants, but also for
the chemical processes in the animal body.
4
Pentoses (C5H10O5).
As a rule the pentoses do not occur as such in nature. They are
obtained from animal tissues, organs and fluids as cleavage products
of the nucleic acids, or nucleoproteins. The pentoses are chiefly obtained
from the plant kingdom, where nucleic acids also occur, by the hydro-
lytic cleavage with dilute mineral acids, of more complex carbohydrates,
the so-called pentosans. The pentosans exist very widely distributed
in the plant kingdom, and are of especially great importance in the build-
ing up of certain plant constituents. Methyl pentosans and methyl
pentoses also occur in the plants, and of these, the methyl pentose, rham-
nose, which occurs in several glucosides, must be specially mentioned.
The pentoses were first found in the animal kingdom by Salkowski
and Jastrowitz in the urine of a person addicted to the morphine habit,
1
Ann. d. Chem. u. Pharm., 349, 125 (1906).
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