The science of life : fully illustrated in tone and line and including many diagrams

BOOK 1

his needs. ‘This assimilation involves very complicated changes, for the stuff of tissues of dead or dying animals and plants is quite unsuitable for immediate entry into his blood. To explain how it is made suitable involves a certain use of chemical terms, and it will make things clearer if we remind the reader of the exact significance of a few expressions that are unavoidable in the discussion, that are always being heard when food is under consideration and about which most of us are apt to be a little vague and wanting in precision. Nowadays we read so much of the ‘‘ breakup of the atom,” and so forth, that some people seem to be in doubt whether they may still think and speak about atoms. Thirty or forty years ago the atom was

5000 7 4000 3000 2006 1000

O 1 4

WOODCUTTER | CARPENTER | BOOKBINDER Fig. 24. Energy requirements from food.

The diagram shows approximately (measured in calories) the energy needed in twenty-four hours by men in various occupations. =

STONEMASON

supposed to be a very simple little particle, the smallest quantity, the unit amount of an element that could go into combination. To-day we understand the atom has an extremely complicated structure; but the fact remains that an atom is the smallest quantity of an element that can go into chemical combination. And a “ molecule ” still means the smallest quantity of a chemical substance that can exist separately. It may consist of from one to a vast number of atoms of the same or different elements ; but though there are large molecules as well as small ones, the largest molecule is still ultra-microscopic and minute beyond our everyday imagination. A water molecule contains three atoms; a molecule of common salt two; but in many of the

48

THE SCIENCE OF LIFE

CHAPTER 2

substances we shall next deal with- the atoms may run to thousands or tens of thousands.

In these discussions on digestion we are always hearing the terms carbohydrates, fats, and proteins. Carbohydrates and fats are made up of carbon, hydrogen, and oxygen. They differ from each other in the proportions of these three elements that they contain. In carbohydrates there are two atoms of hydrogen and one of oxygen to every carbon atom (hence their name, for water is H,O), while in fats the proportion of oxygen is much lower. They differ also in the pattern on which the atoms are fitted together. The best-known carbohydrates are sugar and starch. The term fats as used in physiological discussion

includes the vegetable oils (such as olive oil) in addition to the

more obvious fatty substances. Both carbohydrates and fais

are readily combustible—hence their importance to the living body. They are the chief fuels that it consumes. They can be substituted for one another. The Eskimos get their energy chiefly from oils and fats, while the inhabitants | of a tropical climate substitute carbohydrates to a large extent. The third class of food-stufis, the proteins, are a vast variety of much more complicated substances, containing not only carbon, hydrogen and oxygen, but nitrogen and often other elements, such as sulphur and phosphorus and iron. ‘Their relatively gigantic molecules contain hundreds and even thousands of atoms. Gelatin and ege white are proteins unmixed with any other food substances: of the commoner foods, meat, cheese, and vegetable seeds (such as peas, beans, and lentils) are richest in protein. Proteins, like fats or carbohydrates, can be burnt as fuel, and a large proportion of the proteins we eat is used up in this way. But they have another and more essential réle, one which they alone can perform. For the actual fabric of the living cell is built largely of proteins. Not only can they be burnt by our engines ; they