Structural Relations in Cell Respiration

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T HE living cell exhibits two striking and apparently contradictory properties. In the first place, the substances that make up living protoplasm endow it with a certain morphological stability, which expresses itself grossly in the well-known intracellular structures such as the nucleus, chondriosomes, plastids and so on, and more finely in the persistence of the physical and chemical properties of protoplasm through the environmental, developmental, and evolutionary changes to which it is subject. This stability of living organisms, and the relative independence of the cell from alterations in its environmental conditions (within certain limits), was one of the first of its properties to be recognized by biologists and led many to suppose that such effects could only be accounted for by a supernatural and directive vital force. On the other hand, it was soon made abundantly clear that the apparent stability of the cell and of the whole organism is contingent upon a continuous interchange of matter between the cell and its environment-that life is characterized by the assimilation of substances from the outside world and the concommitant destruction and excretion of intracellular material and of protoplasm itself. Thus, morphological stability and metabolic activity, while apparently antagonistic, are together the essential properties of all living things. To date it has been impossible to separate these characteristics, and the smallest independent unit of structure, the cell, is also the smallest unit which is capable of maintaining complete metabolic activity. However, in recent years with the development of analytical biochemistry, it has been possible to break down the cell, extract certain of its components, and reproduce in-vitro various fragments of the metabolic process. This type of work has accumulated a large body of data which are in the main limited to the destructive processes alone. Perhaps the best known and most important of these catabolic processes are those involving the oxidative degradation of organic metabolites, which in series make up respiration. The very accumulation of these data has, in the last few years, forced upon research workers the problem of integrating this information and relating the separate reactions to the entire respiratory process in the intact cell. It has become generally recognized that the separate in-vitro data on the various extracted parts of the respiratory system do not in themselves provide an adequate picture of the behavior of this system in the cell itself. But there is a good deal of disagreement as to the manner in which these analytical data are to be related to the properties of the entire cell. It has frequently been suggested, for example, that the total living system can be reconstructed by fitting together the various reactions which have been studied in extracts, as one would a jig-saw puzzle. Thus, according to D. E. Green (1) the biochemist must resort to the disorganization of the cell in order to puzzle out the mechanisms of reaction. The validity of this reconstruction method has been severely taxed by the evidence obtained from the extracts themselves. As will be shown below, these in-vitro data themselves necessitate the conclusion that the protoplasmic structure (which is destroyed in obtaining such data) plays the major role in orienting the metabolic reactions within the cell. Further, it will be seen that data obtained from intact cells often disagree with the behavior of reconstituted enzyme systems in-vitro. On the other hand, a number of workers have suggested, on the basis of specific evidence, that the properties of cellular metabolic systems must