Many of the chemical changes which involve either the break down or the synthesis of the compounds discussed in the preced ing paragraphs can be made to occur in vitro extremely slowly or under conditions (e.g., high temperature) which cannot possibly obtain in living protoplasm. It is known, however, that certain substances, called enzymes, have been extracted from animal and plant tissues, which enable reactions to occur at a relatively high velocity and under conditions probably existing in living proto plasm. Many inorganic substances, called catalysts, are known whose effects are analogous to those of enzymes.
In order to account for the fact that the protoplasm maintains its integrity we must assume either that the entire mass of proto plasm is practically insoluble in its surrounding medium or that its immiscibility is due to an enveloping layer or membrane. There is considerable proof that the second assumption is the correct one. For example, when the protoplasmic surface of a variety of living cells is torn with microneedles (see MICROMANIPULATION) the interior protoplasm tends to flow out and dissipate in the sur rounding aqueous medium. The material, as it flows out, espe cially when the surrounding medium has the proper salt content, frequently forms a new surface membrane which encloses it and keeps it intact. Another experiment which indicates the existence of a differentiated membrane is the microinjection of coloured solutions which normally do not penetrate the protoplasm from without. The colour quickly spreads through the protoplasm but remains within its boundary.
Differences in the penetrating powers of various chemical sub stances have given us some idea as to the nature of this mem brane. Water passes in freely especially when it contains certain salts in solution, viz., NaC1 and KC1, to pure solutions of which the plasma membrane, in many instances, appears to be relatively permeable. Other salts, e.g., and possible in pure
solutions have no appreciable powers of penetration. The rate of penetration of acids and bases appears to be a function of their degrees of ionization, viz., those which are the most highly ionized appear to be least able to penetrate. The most universally pene trating substances seem to be those which are lipoid or organo soluble. These facts have convinced many investigators that the surface membrane of protoplasm is at least largely lipoid or fatty in nature, an assumption which is supported by the immiscibility of protoplasm with water.
In all probability we must consider the physiologically active part of the membrane as a shifting, dynamic structure resulting from interactions of protoplasmic components with the environ ment and undergoing reversible reactions with the various chemi cal substances dissolved in the surrounding medium. Non-living membranes which exhibit rapidly reversible reactions of this type are well known.
Some substances to which the protoplasmic membrane appears to be freely permeable are known to be unequally distributed between the interior of the protoplasm and its environment. This is accounted for in part by the chemical combinations of the freely penetrating substance with nonpenetrating substances pres ent on one or the other side of the membrane.
Mention must also be made of a different type of penetration, viz., that by which particulate matter may enter protoplasm. In soluble particles may adhere to, or be wetted by, and consequently sink into the protoplasmic surface layer after which they may gain access into the interior by the reactive changes occurring between the interior and the surface membrane. Space permits only the recital of a list of the possible factors which play a part in the passage of substances through membranes, viz., simple diffusion, osmosis, electro-endosmose, surface tension and solubility in the material of the membrane.
(b) The Internal Protoplasm.—The necessity of the nucleus for the life of the protoplasm of a cell has been so well recognized that the term "protoplasm" is generally applied to an organized system consisting of a differentiated nucleus contained within a mass of so-called cytoplasm.