| Abstract: | This account presents the views of the author on the functional and regulatory aspects of respiratory metabolism in higher plants: Control of metabolism (by enzymes) and the interaction of respiration with the other physiological functions in the living plant (metabolic control). This concept, formulated in the early fifties (ref. 47), was presented in part in 1965 (ref. 2) based on experiments performed mostly by the author and his colleagues and by his co-workers in this country. After an interruption of a decade, during which his work was discontinued, a more complete formulation of his views are given here based on results reported by workers in this field in other countries during that period. The more complete view can now be 'summarized as follows: Respiratory metabolism is the process whereby a part of the material stored in the plant (organism) is converted into biological work (function) for maintaining its state of being alive, while the other part of the same material is converted into substances of higher degrees of orderliness (negative entropy) in the form of structure and organization. Within limits imposed by the genetic potential, these processes are controlled by enzymes which in turn are regulated by internal and external factors. The above statement is essentially a special expression of a general view on the functional aspects of living organisms given in the author’s earlier book, Green Thral- dom (Alien & Unwin, London, 1949). If the above theme finds acceptance, it follows, as stated earlier (ref. 14), that: 1. Respiratory metabolic pathways must be multiple ("multilineal") and multi- directional; 2. They must be interacting, not only with themselves, but also with other functions in the plant, alternatingly in time and separately in space (compartmentation); 3. There must be mutual interactions among the pathways and func- tions regulated by enzymes which in turn are regulated through external and internal factors. This functional and regulatory concept of respiratory metabolism in higher plants are now summarized by the following expressions: 1. CH2O + O2→>Xl→X2→H2O + CO2 + E ↓↓ Y1 Y2 in which E = Energy, X1, X2 etc. represent intermediate products, and Y1, Y2 etc. represent anabolic products of different composition and different degrees of complexity. 2. Borrowing from the second law of thermodynamics, the free energy △G deri- ved from process 1 is used for performance of physiological work (function) during which part of the energy is given off in the form of heat (△H), and the other part is concerned with the change of materials of lower orderliness into form and structure with a decrease in entropy (△S): △G = △H - T△S in which T is temperature (in K). This equation may or may not be directly applicable without qualifications in our case. But the decrease in entropy with the change of degree of orderliness in the process of tissue and organ formation from formless materials holds true. 3. The third expression presents the fundamental aspects of our concept of control of metabolism by enzymes and metabolic control of physiological functions. This may be given as: Fuction Gene→ Enzymes→Metabolism→ Structurc State→ Time cource (Solid arrows denote Control) .Experimental evidences selected from the numerous published experimental results, mostly from those of our own, in support of the above scheme at the substrate level oxidation in addition to those given in an earlier account (ref. 2) are presented here. Evidences based on experiments during the past decade on multiple pathways in NADH oxidation through the electron transport chain gathered in the literature (ref. 37) during the period when our work was interrupted completes the formula- tion of our concept on respiratory metabolism at both the substrate and terminal oxidation levels. The use of this generalized concept on the functional and regulatory aspects of respiratory metabolism in higher plants for guiding further research on plant respiration and on other physiological processes, as well as the application of this concept to practical physiological and biological problems are discussed. |
