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

THE GOMPLEX BODY-MACHINE AND HOW IT WORKS

are the steel of which these engines are made. Herein lies the importance of proteins in our diet. Any of the three kinds of food-stufts wil serve as an energy-source, but protein is the only one that can be used for body building—for growth and for keeping the tissue-machines in repair.

There is just one other chemical term we shall have to use here—amino-acid, because these amino-acids (there are a number of them) lie at the base of protein structure. An amino-acid molecule is simple compared with a protein, complex compared with water. It contains from ten to twenty atoms of carbon, hydrogen, oxygen, nitrogen and sulphur, which may be put together in various ways. The '!00 | number of different arrangements—in other words, of different amino-acids—that 80 | have been found in living tissue is small; 70 + there are about twenty-five. Now 60 these amino-acid | moleculescancombine 0 | together, and it is possible for greatnum- 49 | bers of them to unite to form a single giant molecule. It is these giant molecules which are the proteins.

There are twentyfive different aminoacids, but countless

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And now we are able to describe the essential process of protein digestion. The body-cells require proteins, but they require particular ones. The proteins in our food are different from those in our flesh. It is not possible, for example, that the proteins of a cabbage-leaf could play any part in the activity of a muscle-cell; its amino-acids are fitted together in the wrong way. There is indeed no way of eating the correct kind of protein except cannibalism, and even then there exists no way of getting it direct to the blood without unmaking and making it again. It is necessary thus to convert a great variety of different food-proteins into flesh-proteins. The method adopted is as

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numbers of proteins. © That is because Bees

CHEESE MILK POTATOES BREAD

there is an infinity Fig. 25. The percentage composition of some common food-stuffs. of ways in which the Protein, cross-hatched; fat, black ; carbohydrate, stippled ; water and mineral salts, white.

amino-acids can be

built together. Imagine a box of building blocks, containing twenty kinds of bricks, and about ten or fifteen of each kind. How many different houses could you build, using from fifty to a hundred bricks at a time? An incalculable number. The bricks correspond to amino-acid molecules, the houses to protein molecules. Like the houses, proteins differ from each other in two ways—in the proportions in which the different aminoacids are present, and in the geometrical plan on which these acids are arranged. As a further complication, a few proteins include factors that are not really aminoacids at all; thus haemoglobin contains iron and the nuclear proteins contain phosphorus.

follows : We eat proteins of all kinds; we break them up in our bellies into their constituent amino-acids; we absorb the amino-acids into our blood and thus distribute them to our tissues ; finally, the cells take up these amino-acids and fit them together to make up the particular proteins that they require.

The work that is done upon the carbohydrates and fats is quite parallel to what is done upon proteins. The most familiar simple carbohydrate is glucose, the chief sugar of many fruits, and especially of grapes, and the sweetening constituent of common jams and sweets. ‘This is the form of carbohydrate which circulates in the blood, and all other carbohydrates—the