(NOTE: Since the page has proved somewhat
popular, for fear of link rot I have excerpted some of the material in the links
and posted it
here.)
Hemoglobin and its
Substitutes
One of the great
turning points in the history of science was the decision of Galileo,
Descartes, and others to do science at a very primitive, physical level,
stripping the subject matter of almost all of its symbolic, religious,
philosophical, and theological significance and dealing only with the
immediately available data. (In the received histories of science, Descartes
counts as an idealist and a rationalist -- in contrast to Locke, for example
--
but if you read his brief Discourse on Method, you will find that he
was of a strongly empiricist bent too).
The
enormous symbolic structures of alchemy and astrology (as exemplified by
Bruno and Paracelsus) mixed what were eventually to become the foundations
of modern chemistry and astronomy in with a fiendish stew of magic,
mysticism, philosophy, numerology, color-symbolism, and Biblical exegesis,
and the founding early modern scientists rejected their method. They also rejected the
skepticism of the humanists such as Montaigne or (more strikingly) Robert
Burton, whose works often seem merely to mumble around dozens of classical
citations without ever reaching any resolution. The scientists' goal was to
reach truth, and their method was simplification.
That said, one relic of alchemy
actually does have a scrap of scientific validity. In alchemical color
symbolism, green represents life, etc., and red represents Mars, war, and
blood. As it happens, the redness of blood, the planet Mars, and
(rusted) iron all do come from the same source (iron oxide), and the
greenness of life comes from chlorophyll. Furthermore, the chlorophyll
molecule and the hemoglobin molecule are somewhat similiar, and play
complementary oxidation/reduction roles, chlorophyll fixing
energy from the sun and hemoglobin helping oxidise and release it.
Alchemy is not
completely dead yet, and the similiarities between chlorophyll and
hemoglobin have led to considerable
metaphysical
speculation in organic-gardening circles about the significance of these
similiarities. It is sometimes claimed that they are the same, except
that magnesium plays the role in chlorophyll that iron that iron plays in hemoglobin. However, this
is not the only difference between the two molecules, and chlorophyll is
actually more similar to
heme, a component of hemoglobin (both being porphyrins), than to
hemoglobin itself.
“Chlorophyll performs
almost the same function as Cytochrome c, absorbing light to produce two
high-energy electrons, which are transmitted through an electron transfer
chain to produce useful energy for the cell. Hemoglobin's function is also
related: it carries oxygen from the lungs to the tissues by using heme to
bind two electrons which happen to be attached to the oxygen molecule. So
chlorophyll and hemoglobin represent new ways for organisms to use old
materials.”
"The iron-porphyrin in
hemoglobin accounts for the red color of blood, and the magnesium-porphyrin
in chlorophyll is responsible for the green of
plants."
(Source)
As always in natural history, we
have exceptions. Even among the vertebrates, there are several species of
fish, called
icefish, which have no hemoglobin at all. These fish live in
oxygen-saturated Antarctic waters and have enormous hearts which pump large
amounts of serum through their bodies. One assumes that they are
rather torpid, and since some species are virtually transparent, I would
also guess is
that they often just lie with their mouths open waiting for their prey to
swim in. (A second unique trait of these fish is that their blood includes
an antifreeze making it possible for them to survive at temperatures as low
as -2° C,)
Oxygen-carrying
molecules all are pigments (as is chlorophyll), presumably having to do with
chlorophyll's capturing the energy of sunlight. Thus octopi and arthropods have blue
blood relying on copper-based
hemocyanin ; the priapalids or "penis worms" have purple blood using
hemerythrin, which
is not a true heme; and annelids such as
bristleworms
have green blood using
chlorocruorins. Ascidian
tunicates
(sea squirts) have vanadium-rich green blood;
however, the vanadium is probably not used for oxygen transport.
Internet research on green blood is made
extremely difficult by all the Star Trek references.
New:
The Coming of the Age of Iron.
Red ochre and several other iron ores were used as pigments in paint during
the Stone Age, and the gathering of ochre was a precursor of iron-smelting.
