Chapter 97

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except CO2 and a very small quantity of H2O, are absorbed by the roots, from the soil, from which they are soon withdrawn by vegetation.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."