Page:Popular Science Monthly Volume 82.djvu/140

136 cell-membranes are to be regarded as essentially surface-films, or haptogen membranes. Not only do such thin films form about the reconstituting nuclei of dividing cells, but they are also deposited about various cell-inclusions, and even about division-spheres, chromatophores and other cell structures under certain conditions. It is well known that portions of protoplasm cut off from living cells—such as egg-cells, protozoa, root-hairs, etc.—exhibit the same osmotic properties as the intact cells, showing that new semi-permeable membranes are quickly formed at the cut-surfaces.

The surface of contact of the living substance with its medium thus becomes the seat of deposition of certain protoplasmic constituents or products which form membranes, often of a high degree of impermeability. This impermeability is a property of fundamental physiological importance. Speculation on the evolutionary origin of living cells usually leads to little result, but we may at least infer that the early protoplasmic systems which survived and became the ancestors of living organisms must have consisted in part of colloids like proteins and lipoids which had the property of forming surface-films sufficiently impermeable to limit or prevent free diffusive interchange with the surroundings. Only systems thus isolated to a sufficient degree from the surroundings could preserve the requisite complexity and constancy of composition, and hence be enabled to develop the properties of so-called living beings—properties which are so widely different from those shown by other natural systems. The surface-films, or plasma-membranes, of living cells at the present time are in fact typically characterized by a remarkably high impermeability to simple crystalloid substances like sugars, neutral salts and amino-acids, all of which are important constituents of protoplasm. Zangger expresses the situation concisely when he says that living cells can contain as permanent constituents only such substances as are not free to diffuse into the surrounding medium. The existence of this diffusion-preventing or insulating surface-film, the plasma-membrane, is thus a necessary condition of the stability of the living system and hence of the continuance of the life-processes. The living condition is in fact incompatible with marked and permanent increase in surface-permeability. During life the semi-permeable condition is retained; on death there is always a marked increase in the permeability of the plasma-membrane; the cell then undergoes a ready and rapid dissolution or cytolysis, and the constituents serve as food to bacteria. It is probable that the various intracellular membranes—nuclear membranes, vacuole-membranes, sphere-membranes, chromatophore-membranes—subserve a similar insulating or differentiating function. Hofmeister has indeed conceived of the protoplasm of living cells as subdivided in this manner into a many-chambered system, which accordingly permits of a high degree of chemical differentiation. A variety of independent processes