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Let us first consider how the body works. It does its work by use of
special molecules, molecules that have specific properties suitable to the use
to which they are put. A few of these molecules are simple ones, representing
ordinary chemical substances, such as water, oxygen, carbon dioxide. Others
are more complicated, with ten or twenty or thirty atoms per molecule.
These include the necessary food substances called the vitamins-vitamin A,
vitamin B, vitamin C, and so on, with formulas such as Cz,H3o0 (Vitamin A),
and special foods such as sugar, C1,H22011. And then there are the giant
molecules, containing tens of thousands or hundreds of thousands of atoms,
and with certain well-defined properties-the ability to do very special jobs
that serve the purposes of the organism. Thus in living we make use of
oxygen of the air, to burn certain materials in our tissues, and in this way to
obtain heat and energy to keep our bodies warm and to permit us to do work,
and also at the same time to get rid of some unwanted materials by burning
them to water and carbon dioxide. The oxygen that we inhale goes into
the blood in the lungs. It does this by combining with a special substance
in the blood, the red substance called hemoglobin. Hemoglobin is an
extremely interesting substance. It is practically the only substance known
that has the property of combining easily with oxygen from the air, and then
of giving up the oxygen under slightly changed conditions. A molecule of
hemoglobin contains about 10,000 atoms-atoms of carbon, nitrogen,
oxygen, sulfur, and four atoms of iron. It is these four iron atoms that are
directly concerned in the job of carrying oxygen from the lungs to the tissues;
each of the iron atoms attaches to itself one molecule of oxygen, 02, and
carries it along in the arterial bloodstream to the tissues, where it gives it up.
Ordinary iron atoms, in ordinary compounds of iron, do not have this
property. It is the special structure of the hemoglobin molecule, the special
arrangement of other atoms, of nitrogen, carbon, hydrogen, and sulfur, in the
neighbourhood of the iron atom in the hemoglobin molecule, that gives to this
molecule this special property. Then in the muscles and other tissues another
protein, myoglobin, only one-quart•: r as large as hemoglobin, takes the
oxygen from the hemoglobin and carries it around within the muscle. Here
still other special substances, giant molecules, begin to work. Under
ordinary circumstances the presence of oxygen molecules, in the air, in the
neighbourhood of a food or other combustible material does not lead to the
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