Intake of Food

It is rather difficult to discuss the intake of food in any broad fashion and, indeed, the subject is not one of particular interest to the general physiologist. At any rate, the ingestion of food by animals is of more concern to the naturalist who is interested in describing the habits and behavior of various forms than it is to the physiologist. One may distinguish between the entrance of food into the body of an animal and the entrance of food materials into the cells which compose the living part of the animal. Obviously in protozoa there is no distinction between these two processes, for the cell is the animal. In metazoa the food materials typically find their way into an alimentary canal or intestine, and pass from there to the surrounding cells or to the blood. The mechanism of such passage will be considered in a later section under the head of Absorption.

In plants, food enters the cells by diffusion. There are a few higher plants which trap and digest insects, but even in these forms as well as in all other plants, the food materials that are taken into the cells are either gaseous, liquid or in solution. In higher plants, carbon dioxide enters through the leaves, water and dissolved salts through the roots. The rate of entrance through the root hairs and the cells of the root is a function of the permeability of the plasma membrane. The accumulation of substances taken up by plant cells from their environment is dependent not only on permeability, but also on various other factors. One such factor is the chemical combination which may occur between the entering food substance and the protoplasm of the cell. Thus, some seaweeds contain very appreciable percentages of iodine, although the sea water in which they live contains only traces of this element. In this instance it is possible that iodine combines with the cell protoplasm, and is thus trapped within the cell. The factors underlying the accumulation of salts in plant cells are many and complicated.