Excerpt from Some Fundamental Problems of Cellular PhysiologyLet us consider for a moment certain activities which are common to all living things. All cells synthesize certain foods which exist in soluble form. What prevents these substances from dif fusing out of the cell into the surrounding liquid, thereby depriving it of materials essen tial to its existence? Experiments have Shown that the protoplasm is sufficiently impermeable to such materials to prevent serious loss and that it also possesses the power to exclude various substances, some of Which are deleterious. At the same time it must admit raw materials out of which it manufactures food. Hence the proto plasm has selective permeability, permitting certain things to go in and out while barring the passage of others.In order to understand how materials are ab sorbed and given out, we may consider first of all the structure of the cell. A typical plant cell consists of a cell wall (of non-living material) within which is a layer of. Living protoplasm, often so thin as to be little more than a delicate membrane. Within this is a large central vacuole filled With cell sap, and in many cases plastids (such as the chloroplasts) are present. A simple animal cell differs in several respects: of especial significance for our present purpose is the ab sence of the cell wall and of the central vacuole.About the PublisherForgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.comThis book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully, any imperfections that remain are intentionally left to preserve the state of such historical works.