Digestion--Enzymes

Whenever an organism consumes food in the solid state, this must be brought into solution before it can be utilized. It is necessary, therefore, that such solid food be digested. In some organisms, as pointed out in the last chapter, digestion may occur outside of the body, and this may constitute an important adaptation for those animals which are in the habit of eating animals larger than themselves. Ordinarily, however, food is taken into the body and digested there. Digestion may occur in cavities of special organs such as the stomach or intestine, or it may occur within the protoplasm of cells. The latter type of digestion obviously takes place in protozoa. In organisms such as paramecium or ameba the ingested food is enclosed in a food vacuole, which serves the same purpose as the stomach or intestine of a complicated metazoan. Within the food vacuole the solid food particle is brought into solution. It must not be thought, however, that intracellular digestion is confined to protozoa. In sponges, coelenterates, and flatworms, much of the solid food taken into the body is ingested by ameboid cells lining the walls of the digestive tract, and digestion takes place within the protoplasm of these cells rather than in the lumen of the digestive tube or cavity. Moreover, in some animals rather higher in the evolutionary scale, there is also a certain amount of intracellular digestion. In spiders and in arachnids generally, the digestion of protein, begun either outside the animal or in the digestive tract, is finally completed within the cells lining the tract. In clams and other lamellibranchs, the digestion of protein and fat has been thought to occur exclusively within cells. Both in lamellibranchs and echinoderms, amebocyte cells play an important role. These phagocytic cells even enter into the lumen of the stomach or intestine, ingest particles of food there and then carry these food particles back into the tissues and digest them there. Such intracellular digestion, however, is a primitive character and it does not occur to any extent in higher animals such as the insects and vertebrates.

In the conversion of solid food to a state of solution, enzymes play the leading role. It will be necessary, therefore, to consider the subject of enzymes and the nature of enzyme action. It should be strongly emphasized, however, that enzymes are not concerned only with digestion, but that they are essential factors in all of the chemical activities of the organism. Our discussion at this point will be somewhat parenthetical. It will also be brief; indeed, it would scarcely be possible to present an up to date summary of enzyme chemistry in the space of a single chapter. The subject has grown so rapidly that it has become a science by itself (enzymology), and the modern books on enzymes are heavy with information.