THE ORGANIZATION OF LIVING MATTER

.-Living systems consist of relatively few common chemical elements,' yet have unique properties which set them aside from the rest of matter: they are able to control their exchanges with the surrounding medium and to respond to changes therein; they can transform energy and metabolize matter; and especially they are capable of self-duplication. These remarkable properties, expressed in one form or another, have intrigued and challenged man's mind at least since the beginning of history. The answers have varied over the ages, but for a long time have retained an element of reverence and have assumed the involvement of extraordinary forces. Now, after millenia of illusions, doubts, and probing, it turns out that these unique properties are due to the way in which common chemical elements are put together in time and space. For our times, lifehuman life included-is the outcome of an elaborate organization based on trivial ingredients and ordinary forces. Historically speaking, this has been a drastic readjustment which is still affecting, sometimes with devastating force, whole fields of human endeavor. The Structural Hierarchy of Living Systems.-The elaborate organization just mentioned consists of a hierarchy of structural patterns which begins with relatively simple molecules and proceeds, step by step-first to polymeric macromolecules; then to molecular or macromolecular aggregates, which often take the form of elementary structures2 such as fibrils, membranes, and particles; then to aggregates of such structures which turn out to be cell organs; then to cells; and beyond cells to tissues, organs, and organisms. Life with its full complement of attributes, life self-sustained and self-reproducible emerges at the cell level within the hierarchy. The sufficiency of this stage is attested by the fact that so many living forms do not go beyond it, and by the ease with which cells of metazoic origin revert to an independent state under culture in vitro. Below this level there are self-reproducible forms, like the viruses, but they are not self-sustained; they subsist only as cell symbionts or cell parasites. Hence our discussion can be limited to living matter at the cellular and subcellular level. Already at this level, life depends on an extensive organization in depth, on a superimposition of patterns which amount to infinitely more order than matter usually tolerates. This thermodynamically improbable situation is achieved by continuously supplying energy to a pre-existing structural framework. It is this framework, the hierarchy of patterns already mentioned, that captures energy and matter from outside sources and channels them into a complex series of reactions whose outcome is the maintenance of the system and its eventual duplication. As far as we know, at present the framework is always inherited, never created