Physiology: The Science of the Body. Ernest G. Martin
and fats are the same as in starch and sugar, although the proportions are not the same. Weight for weight oil has more than twice the energy value of sugar; in making a given amount of peanut oil the peanut vine used up more than twice the amount of starch or sugar; but since energy value is what counts rather than bulk the plant is just as well off, and perhaps better on account of the smaller bulk occupied by the stored material. One of the very interesting examples of oil storage is found in the very tiny plants, called diatoms, which abound in the water of the ocean. Each tiny diatom stores within itself an even more tiny drop of oil. Although by themselves single oil drops would make no impression, if enough of them could be brought into one place a respectable accumulation of oil would result. This is precisely what the geologists tell us has happened in past ages; the bodies of diatoms have accumulated through thousands of years, and finally the oil accumulations have been covered over with sediment of one kind or another. When we tap through the sediment we strike into the “oil sand,” which contains this residue of the diatoms, and an oil well results.
Since we depend for our food, and so for our life, on the sugar-making activities of green plants, it will be worth our while to think for a moment of the slowness with which the process goes on. The slice of bread which we may eat in a dozen bites represents the result of a season’s growth of several wheat plants, every one of which was absorbing the sun’s energy and laying up starch grains during every daylight minute throughout the growing season. From the standpoint of the plant which does the storing the material which serves us as food is the excess over the plant’s own daily needs. In most cases it would be utilized at the beginning of the next season’s growth before the plant had put out a leaf system, if the course of events were not disturbed to satisfy the needs of man.
In addition to starch, sugar, and fat there is another kind of food material manufactured by plants, known as protein. This substance is much more complex chemically than any of the others; it contains, in addition to the three chemical elements—carbon, oxygen, and hydrogen—that are found in them, the element nitrogen, and usually some phosphorus and sulphur. These materials are dissolved in the soil water in the form of simple chemical substances, and are taken up by the plant along with the water which enters the roots and flows as sap up to the leaves. The same cells of the plant that make sugar have the power to make protein, using as raw materials some of the sugar along with the substances brought in with the soil water. The energy for the manufacture of protein comes from the oxidation of some of the sugar or starch in the leaf. The finished protein has about the same energy value, weight for weight, as has the starch from which it was mainly derived.
When an animal eats a plant or part of one, he is eating for the sake of the sugar or sugar products which the plant has made. There is one sugar product that is useful as food for many animals, but not for man, except possibly to a very slight extent. That is the woody substance, cellulose, which is formed in plants mainly as a support to the delicate living protoplasm. Cotton fiber is nearly pure cellulose. Cellulose is very similar to starch chemically, and is an excellent fuel wherever it can be burned. The human digestive tract is unable to handle it in a manner to make it usable, although grazing animals do so quite efficiently. A good many plants make products that are either disagreeable in flavor or actually poisonous. Of course, in such cases the plants become useless as food unless a treatment can be devised that will remove the objectionable material or convert it into something harmless. The few dozen kinds of plants that we raise for food are those that are free from harmful substances and that yield large quantities of stored food materials, or in some cases that taste especially good, even though they may not have much food value. Tomatoes, lettuce, and the like, come in this latter class. The world has been pretty well ransacked for food grains, fruits, and herbs, but probably there are others yet to be found besides those we now have.
CHAPTER IV
THE USES OF FOOD
WE have had a good deal to say thus far about power development in living animals, and have talked about food in connection with its use as fuel for the purpose. While we are on the topic it may be as well to say something about other uses to which food is put in animals besides that of serving as fuel, and also something about what is done with the power that is developed by the burning of such food as is used for fuel. To begin with, it is evident that one use that is made of food is to build the body itself. The new-born infant usually weighs somewhere between 5 and 12 pounds. From birth until the body gets its growth there is an almost continuous gain in weight until a total which may range anywhere between 90 and 250 pounds is reached. Of course, every bit of this additional material came into the body in the form of food. The whole mass of the body divides itself, as has been said before, into living protoplasm and nonliving substance. We do not know accurately what proportion of the whole weight is made up by protoplasm; it has been estimated at about 60 per cent, but any estimate can be only very rough because about half of the nonliving substance consists of fat deposits which vary greatly in different people.
In any case, that part of the food which goes to make gain in weight is passed over to the living
cells. If we accept the rough estimate given above, about 60 per cent is then used for the actual manufacture of new protoplasm; the remainder is worked over by cells specially devoted for the purpose and put into place to serve as supporting structure, or to be held in reserve as fat. Living protoplasm is chemically a very complex mixture. In consistency it resembles a rather thin, transparent jelly; the thickness of the jelly depends on how much water it contains and this varies greatly in different kinds of protoplasm. The watery part of the protoplasm has dissolved in it several substances; among them may be mentioned ordinary table salt; also salts of potash and lime. Only tiny amounts of these are present, but it is a curious fact that without these tiny amounts of salts protoplasm cannot live. The chief solid substance in protoplasm is protein; this
material, which is one of the most complex substances known to chemistry, has certain peculiarities which seem to fit it specially to serve as the chemical basis of life. Evidently of all the foodstuffs protein is the most important for the manufacture of new protoplasm, in other words for growth. In the case of a tiny one-celled animal, whose body is made up of protoplasm, not much else would be needed, but any animal that has a bony skeleton has to build this up to keep pace with the growth of the soft parts of the body. For this purpose mineral substances are needed, chiefly lime salts.
In addition to these foods which are actually used for making new body substance it has recently been discovered that proper growth in the higher animals, including man, depends on the presence in the diet of certain dietary accessories, whose use is not at all understood, although there is no doubt of their importance. These materials, to which has been given the rather cumbersome name of “growth-promoting vitamines,” are found dissolved in certain food fats. Apparently they are insoluble in water and soluble in oil. Most animal fats appear to contain them in small amounts, while most vegetable fats do not. Milk and eggs, which are