Photosynthesis isotope effect

Kinetic isotope effects are an important factor in the biology of deuterium. Isotopic fractionation of hydrogen and deuterium in plants occurs in photosynthesis. The lighter isotope is preferentially incorporated from water into carbohydrates and tipids formed by photosynthesis. Hydrogen isotopic fractionation has thus become a valuable tool in the elucidation of plant biosynthetic pathways (42,43). 

Kinetic isotope effects during photosynthesis concentrate the light isotope 12C in the synthesized organic material. 

The reasons for isotope discrimination are isotope effects which are caused by both kinetic and thermodynamic factors. Especially the kinetic isotope effect during primary C02-fixation in photosynthesis is relevant for the source-specific discrimination of compounds from C3 and C4 plants. 

The natural cycles of the bioelements carbon, oxygen, hydrogen, nitrogen and sulphur) are subjected to various discrimination effects, such as thermodynamic isotope effects during water evaporation and condensation or isotope equilibration between water and CO2. On the other hand, the processes of photosynthesis and secondary plant metabolism are characterised by kinetic isotope effects, caused by defined enzyme-catalysed reactions [46]. 

In the case of the stable isotopes of carbon, and C, two isotope effects are noticeable the kinetic isotope effect in photosynthesis, leading to an enrichment of in plants, and the equilibrium isotope effect in the exchange reaction... 

Carbon isotope effects associated with photosynthesis... 

Isotope effects and mass balance. An arbitrary sequence of reactions is indicated schematically in Figure 2. Analysis of this system will provide a useful introduction to studies of real organisms. Reactant A could, for example, represent a particular carbon position in a two- or three-carbon product of metabolism or photosynthesis. If there were no isotope effect associated with Reaction (1) (si = 0), then the isotopic composition of the carbon being transmitted by Reaction (1) would be equal to that of reactant A. This follows from a general relationship which can be written as... 

Calvin M, Bassham JA (1962) The Photosynthesis of Carbon Compounds. W A Benjamin, New York Cavalier-Smith T (2000) Membrane heredity and early chloroplast evolution. Trends Plant Sci 5 174-182 Cleland WW, O Leary MH, Northrop DB (eds) (1977) Isotope Effects on Enzyme-Catalyzed Reactiorrs. Appendix A A note on the use of fractionation factors versus isotope effects on rate corrstants. University Park Press, Baltimore, Maryland... 

Nelson BK, DeNiro MJ, Schoeninger MJ (1986) Effects of diagenesis on strontium, carbon, nitrogen, and oxygen concentration and isotopie eomposition of bone. Geoehim Cosmoehim Acta 50 1941 -1949 O Leary MH (1988) Carbon isotopes in photosynthesis. Biosei 38 328-336... 

Fractionation 1) Selective separation of chemical elements or isotopes through physical, chemical or biochemical processes. For example, the fractionation of carbon isotopes of the naturally occurring carbon isotopes -98.9% is C, 1.1% is and only 1 part in 10 % is In nature, however, a fractionation of this ratio occurs (e.g., photosynthesis results in an enrichment of relative to the other isotopes in most plant tissues). Based on thermodynamical laws, which show that the heavier isotope " C is twice as enriched as C, radiocarbon laboratories correct for the probable effects of fractionation. C can be measured in a sub-sample of the material to be dated and the C C ratio is then compared with a standard (PDB) and published as deviation from this standard. See also normalization. 2) In physical and chemical processes involving the isotopes of a particular element, the relative abundance of the isotopes may change between the initial substance (the substrate ) and its product. In isotope geochemistry, this change is commonly referred to as fractionation, (see also fractionation factor isotope effects). 

The Cl + H2 HCl + H reaction has a particularly long and interesting history, including thermal and photochemical studies of the H2-CI2 system, molecular beam experiments, measurements of perhaps the largest number of kinetic isotope effects known for any system, and a host of theoretical treatments. We will mention only a few particularly relevant studies. Early experimental studies of this reaction were often initiated by photodissociation of CI2, and some of the papers are entitled Photosynthesis of Hydrogen Chloride (a translation of die Photochemische... 

Isotopic variation in European human remains is caused by a combination of environmental, biological and cultural factors. For carbon, the main influence is the dominance of C3 photosynthesis in all vegetation types. The factors that can cause deviations from this general trend are the climatic effect,... 

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