he all-important
respiration-regulating pigment in the blood of animals. Haemoglobin is a
complex compound, consisting of about 96 per cent of albumin (a protein,
see Chapter XIII) united with about 4 per cent of _haematin_, a brilliant
red pigment which has the formula FeClC_{32}H_{32}O_{4}N_{4}. When treated
with acids, the iron (and its accompanying Cl) is removed, and
haematoporphyrin, C_{32}H_{36}O_{4}N_{4}, is obtained. When either haematin,
or haematoporphyrin is oxidized, haematinic acid imide identical with that
obtained from aetioporphyrin is obtained. Also, when haematoporphyrin is
reduced, haemopyrrole identical with that from aetioporphyrin is obtained.
Thus, it would appear that the unit structural groups in haematin and in
chlorophyll are identical; although chlorophyll may exhibit more variations
in isomeric arrangement of these structural units than have been found in
haematin. Hence, it is apparent that the only essential difference in
composition between chlorophyll and haematin is that in the former the
structural units are linked together by iron, while in the latter, the same
units are united through magnesium as the linking element. Further, it is
known that while iron is not a constituent element in the chlorophyll
molecule, it is, in some unknown way, absolutely essential to the
production of chlorophyll in plants; plants furnished with an iron-free
nutrient solution rapidly become etiolated and photosynthesis stops.

The following skeleton formulas have been suggested to indicate the way in
which these elements are linked between the structural units in their
respective compounds.

-C C- -C C-
\ / \ /
N N N N
/ \ / \ / \ / \
-C \ / C- -C \ / C-
M g F e
-C / \ C- -C / | \ C-
\ / \ / \ / | \ /
N N N C l N
/ \ / \
-C C- -C C-

Chlorophyll Haematin

It is understood, of course, that the mineral element does not furnish the
definite means of holding the structural units together as otherwise it
would not be possible to remove the