Oscillatory Nature of Metabolism and Carbon Isotope Distribution in Photosynthesizing Cells

Abstract

A study of carbon isotopic characteristics of plants and animals, such as, shifts in carbon isotope ratio of plant biomass relative to environmental CO2, 13C values of biochemical fractions and individual metabolites, different isotopic patterns of biomolecules and diurnal isotopic changes of respired CO2, evidences that in a living cell carbon isotope fractionation takes place. The above characteristics might be the source of valuable information on cell metabolism and regulation of metabolic processes, on assimilate transport, and different aspects of “organism – habitat” interactions. The efficiency of the involving this information in living organism studies greatly depends on the validity of carbon isotope fractionation model used for the interpretation. The validity of the model first of all is determined by the adopted view on the nature of isotope effect origin. Two alternative points of view have been suggested in the literature. One of them asserts (Galimov, 1985; Schmidt, 2003) that carbon isotope effect and isotope distribution in biomolecules are of thermodynamic order. It means that isotope distribution of metabolites doesn’t depend on biosynthesis pathway but is determined by the properties of the molecules themselves, i.e. by their structure and energy characteristics. According to the second point, supported by most of the researchers, the metabolic isotope effects are of the kinetic nature and carbon isotope distributions in metabolites are determined by mechanisms and pathways of their formation. A lot of facts accumulated till now allow saying with confidence that thermodynamic concept is erroneous and the rare casual coincidences only simulate thermodynamic equilibrium (O’Leary & Yapp, 1978; Monson & Hayes, 1982; Ivlev, 2004). In some publications it was shown that the “thermodynamic” idea is “incompatible with the concept of life as a fundamental phenomenon” (Varshavski, 1988; Buchachenko, 2003). So we’ll concentrate on the kinetic concept. Within the frame of the “kinetic” concept two different approaches have been developed. The first is the steady-state model assumes that all the processes in a living cell during photosynthesis proceed simultaneously in stationary conditions. It also means that carbon isotope fractionation proceeds in stationary conditions too. The approach was put forward by Park and Epstein (Park & Epstein, 1960, 1961) and was developed by Farquhar et al.