CHAPTER LX. CHEMISTRY OF LIFE.

335. Growth.--The chemistry of organic life is very complex, and
not well understood. A few of the principal points of distinction
between the two great classes of living organisms, plants and
animals, are all that can be noted here. Minerals grow by
accretion, i.e. by the external addition of molecules of the same
material as their interior. A crystal of quartz grows by the
addition of successive molecules of SiO2, arranged in a
symmetrical manner around its axis. The growth of crystals can be
seen by suspending a string in a saturated solution of CuSO4, or
of sugar. In plants and animals the growth is very much more
complex, but is from the interior, and is produced by the
multiplication of cells. To produce this cell-growth and
multiplication, food-materials must be furnished and assimilated.
In plants, sap serves to carry the food-materials to the parts
where they are needed. In the higher animals, vari- ous fluids,
the most important of which is the blood, serve the same purpose.

336. Chemistry of Plants.--In ultimate analysis, plants consist
mainly of C, H, O, N, P, K. In proximate analysis, as it is
called, they are found to contain these elements combined to form
substances like starch, sugar, etc. Water is the leading compound
in both animals and plants. One of the most important differences
between animals and plants is, that all plants, except parasitic
ones, are capable of building up such compounds as starch from
mineral food-stuffs, while animals have not that power, but must
have the products of proximate analysis ready prepared, as it
were, by the plant. Hence plants thrive on minerals, whereas
animals feed on plants or on other animals. The power which
plants have of transforming mineral matter is largely due to
sunlight, the action of which in separating CO, was described.
The reaction in the synthesis of starch from CO2 and H2O in the
leaf, is thought to be as follows: 6 CO2 + 5 H2O = C6H10O5 + 12
O. C6H10O5 is taken into the tree as starch; 12 O is given back
to the air. All the constituents, except CO2 and a very small
quantity of H2O, are absorbed by the roots, from the soil, from
which they are soon withdrawn by vegetation. To renew the supply,
fertilizers or manures are applied to the soil. These must
contain compounds of N, P, and K. N is usually applied in the
form of ammonium compounds, e.g. (NH4)2SO4, (NH4)2CO3, and
NH4NO3. The reduction and application of Cas(PO4)2 for this
purpose was described. K is usually applied in the form of KCl
and K2SO4.

337. Food of Man.--In the higher animals the object is not so much
to increase the size as to supply the waste of the system. The
principal elements in man's body are C, H, O, N, S, P.

An illustration of the transformation of mineral foods by plants
before they can be used by animals is found in the Ca3(PO4)2 of
bones. This is rendered soluble; plants absorb and transform it;
animals eat the plants and obtain the phosphates. Thus man is
said to "eat his own bones." The food of mankind may be divided
into four classes (1) proteids, which contain C, H, O, N, and
often S and P; (2) fats, and (3) amyloids, both of which contain
C, H, O; (4) minerals. Examples of the first class are the gluten
of flour, the albumen of the white of egg, and the casein of
cheese. To the second class belong fats and oils; to the third,
starch, sugar, and gums; to the fourth, H2O, NaCl and other
salts. Since only proteids contain all the requisite elements,
they are essential to human food, and are the only absolutely
essential ones, except minerals; but since they do not contain
all the elements in the proportion needed by the system, a mixed
diet is indispensable. Milk, better than any other single food,
supplies the needs of the system. The digestion and assimilation
of these food-stuffs and the composition of the various tissues
is too complicated to be taken up here; for their discussion the
reader is referred to works on physiological chemistry.

338. Conservation.--Plants, in growing, decompose CO2, and
thereby store up energy, the energy derived from the light and
heat of the sun. When they decay, or are burned, or are eaten by
animals, exactly the same amount of energy is liberated, or
changed from potential to kinetic, and the same amount of CO2 is
restored to the air. The tree that took a hundred years to
complete its growth may be burned in an hour, or be many years in
decaying; but in either case it gives back to its mother Nature,
all the matter and energy that it originally borrowed. The ash
from burning plants represents the earthy matter, or salts, which
the plant assimilated during its growth; the rest is volatile. In
the growth and destruction of plants or of animals, both energy
and matter have undergone transformation. Animals, in feeding on
plants, transform the energy of sunlight into the energy of
vitality. Thus "we are children of the sun."