there
are significant differences between the lipoids of plants and those of
animal origin. However, most investigators use the same methods of study
and the same systems of nomenclature for these fat-like substances,
regardless of their origin.

LECITHIN

This phosphatide is by far the best-known lipoid. It occurs in the brain,
the heart, the liver, and in the yolk of the eggs of many animals; and
either lecithin or a substance so nearly like it in character as to be
regarded by most investigators as identical with it, is present in small,
but constant, quantities in nearly all seeds, especially those of
leguminous plants. In many legume seeds, it constitutes from 50 to 60 per
cent of the "ether extract," or "crude fat," which can be extracted from
the crushed seeds, using ether as the solvent.

Lecithin is a glyceride. Only two of the (OH) groups of the glycerol are
replaced by fatty acids, however; the third being replaced by phosphoric
acid, H_{3}PO_{4}, or PO(OH)_{3}, which, in turn, has one of its hydrogen
atoms replaced by the base _choline_. Choline is a nitrogenous base, or
amine, which may be regarded as ammonium hydroxide with three of its
hydrogen atoms replaced by methyl groups and the fourth by the ethoxyl
group, the latter being the ethyl group with an OH in place of one of its
hydrogens. Thus,

Ammonium hydride Choline

H H CH_{3} C_{2}H_{4}OH
\ / \ /
H-N CH_{3}-N
/ \ / \
H OH CH_{3} OH

Without the choline, lecithin would be a di-fatty acid derivative of
glycero-phosphoric acid. These relations may be seen in the following
formulas:

Glycerol Glycero-phosphoric acid Fatty Acid

CH_{2}OH CH_{2}OH OH HOOC.R
| | /
| | /
CHOH CH--O--P=O
| | \
| | \
CH_{2}OH CH_{2}OH OH

Choline Lecithin

HOC_{2}H_{4} CH_{2}OOC.R OH
\ | /
\ | /
(CH_{3})_{3}[trb]N CH---O---P=O C_{2}H_{4}