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

BOOK 1

starch in potatoes, for example, or tablesugar—which are more complex in their composition must be broken down to glucose before they enter our real interiors. Similarly the fats are broken down to their simpler factors, glycerine and fatty acids.

Assisted by bacteria in his intestine, the mouse can also turn cellulose, the main substance of ordinary vegetable tissue, into glucose, but in that he has the advantage of man.

In digestion all the three important classes of organic foods—proteins, carbohydrates, and fats—are thus treated according to the same general plan ; the molecules are broken up into simpler constituent ones, absorbed as such into the blood, and then rebuilt by the tissue-cells into other arrangements which suit their individual needs. Water and the mineral salts that we require are not dissected in this way; their molecules are already as simple as need be, and no adjustment is necessary.

In addition to taking the food apart into units for protoplasmic reconstruction, digestion also serves to get the food from the open passage of the gut into the real interior of the body. Insoluble food like starch must be turned into soluble compounds to do this; and when molecules are large like those of the proteins they must be broken up into smaller ones, which can pass more easily through the walls of the intestine.

All this disintegration involves operations carried out with great delicacy and precision. It is not a senseless shattering, like the blowing up of a house by a bomb ; it is a methodical taking apart of the molecule, a division of it into certain units that we need, and units which have to be handled with care lest they themselves be broken.

The reader will be familiar with the way in which mass-produced motor-cars are assembled. ‘There is a long moving platform on to which the bare chassis is put. As it is carried steadily along it passes a series of skilled mechanics, each of whom slips one particular part into its place. Each man has his own special operation that he performs on the cars that pass him—a part to insert, a particular bolt to tighten. The process is so organized that finally, at the other end of the platform, the car stands completed. Now imagine the motion of the platform reversed ; imagine the finished car standing at one end and moving past a file of highly-skilled dismemberers ; each man lifts out one particular part or loosens a particular nut and so helps the disintegration one stage further; finally, when the car

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THE SCIENCE OF LIFE

CHAPTER 2

reaches the other end, it has been carefully and completely taken to bits. This reversed process would be exactly parallel to digestion. Food is passed from chamber to chamber along the digestive tube, and in each region it undergoes special and appropriate stages of break-down.

The important point about the image is that of the workmen, standing beside the moving belt and each a specialist in some particular operation, for in our digestive tubes the food is attacked and modified by a chain of chemical workmen—the substances known to the physiological chemist as enzymes—each of which makes its own adjustment, and leaves its own mark on the food.

Consider, for example, the digestion of starch. Starch molecules, as we have seen, are complexes which have to be broken up into the simpler unit called glucose. As a mouthful of bread is chewed it is mixed with saliva, and the saliva contains the first chemical operator, an enzyme known as ptyalin. The ptyalin begins to act on the bread at once, and, being swallowed at the same time, continues its work in the stomach. Ptyalin does not completely digest the starch molecule; it breaks it into parts which are simpler than the original molecule, but nevertheless more complicated than the final product. On leaving the stomach, the bread is handed over to two more enzymes, amylopsin and maltase. The former works on the pieces that the ptyalin left, and breaks them into simpler structures, disaccharides, each of which is the equivalent of two glucose molecules stuck together. The latter completes the process by splitting these dual structures into glucose. Thus the starch molecule is not suddenly blasted ; it is pulled to pieces with precision and method in a series of graded stages. So with fat, and so with protein.

The enzymes are pre-eminently specialists; maltase, which specializes in the handling of disaccharides, can do nothing to intact starch molecules, and ptyalin, which performs the earlier stages, cannot carry the dissection beyond a certain point. The two have to co-operate. Moreover, like human experts, they are particular about their working conditions. Thus maltase refuses to work in an acid medium, and we shall later note what precautions are taken to neutralize in the intestine the acidity of the. gastric juice. These substances are peculiar in that they act in very small concentration ; they are consequently hard to analyse, and, beyond the fact that their molecules rival those of the proteins in