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PHOSPHATIDES. 241
Attention has been called in Chapter I to the importance of the phos-
phatides (lipoids) for the limiting layer of the cells as well as for the
osmotic processes and for the metabolism of the cells. The unsat-
urated, readily oxidizable phosphatides also play a possible role as oxygen
carriers and the phosphatides are undoubtedly of great importance as
constituents of the food-stuffs. There is also no doubt that they are
very important for development and growth. It has been found that
the amount of phosphatides is especially abundant in the new-born,
and that these latter, to a certain extent, bring into the world a store of
phosphatides and this store diminishes during growth.1
The phosphatides seem to be closely related to one another; they
influence the solubility and precipitation properties of one another, and
are generally precipitated as mixtures which are extremely difficult
to separate into individual constituents. They are also amorphous,
and readily oxidized, and it is easy to understand why their preparation
in a pure state is so extremely difficult. Under these circumstances
we have no sufficient guarantee as to their chemical individuality, and
the description of their properties and composition must be accepted
with some reservation.
The phosphatides are generally amorphous, colorless or yellowish;
they melt, on warming, and burn. As a rule they are insoluble in water
and swell up therein forming colloidal solutions, which are precipitated by
certain salts. The phosphatides as above stated, belong to the lipoids
and it is for this reason that each phosphatide is dissolved by at least one
of the solvents for fats (alcohol, ether, benzene, petroleum ether, etc.).
The lipoid group cannot be otherwise characterized. Originally we
included in this group, bodies similar to fat or in certain respects related
to the fats such as phosphatides, cholesterin and cerebrosides, but later
the conception has been developed and now we consider as lipoids
those bodies that are soluble in ether or equivalent. Under these
circumstances, as the diverse known and unknown bodies, such as lactic
acid, phenols, alkaloids and extractive bodies of various kinds may
belong to the lipoid group there does not seem to be any sense in speaking
of a special lipoid group, and especially from a chemical standpoint it
would be better to drop the name entirely.
The various phosphatides show a different behavior toward the
solvents for lipoids, namely some are soluble in ether while others
are insoluble therein, etc., and these differences are important for their
1
In regard to the quantity and importance of the phosphatides (the lecithins)
see Siwertzow, Bioch. Centralbl., 2, 310; Glikin, Bioch. Zeitschr., 4 and 7; Nerking,
ibid., 10; Stoklasa, Ber. d. d. chem. Gesellsch., 29, Wien. Sitz. Ber., 104; Zeitschr.
f. physiol. Chem. 25; Danilewsky, Compt. Rend., 121 and 123 and \Y. Koch., Zeitschr.
f. physiol. Chem., 37; Kyes, ibid., 41 and Berl. klin. Wochenschr., 1904.
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