Carbon Dioxide Assimilation by Leaves, Isolated Chloroplasts, and Ribulose Bisphosphate Carboxylase from Spinach

Carbon Dioxide Assimilation by Leaves, Isolated Chloroplasts, and Ribulose Bisphosphate... The relationship between rate of photosynthesis and CO 2 concentration has been reinvestigated using isolated spinach ( Spinacia oleracea ) chloroplasts. The apparently low CO 2 concentration required for half-maximal photosynthesis is shown to result partly from a ceiling imposed by electron transport. In double reciprocal plots of rate against CO 2 concentration, this ceiling results in departures from linearity at high CO 2 concentrations. If these rate limitations are disregarded in extrapolation the “true” CO 2 concentration required for half maximal carboxylation by intact chloroplasts is approximately 46 μ m (CO 2 ). When assayed under comparable conditions, ribulose bisphosphate carboxylase from these chloroplasts also shows an apparent K m (CO 2 ) of approximately 46 μ m , suggesting that its characteristics are not modified by extraction. An improved assay for ribulose bisphosphate carboxylase yielded rates of carboxylation considerably higher than those previously reported, the highest maximal velocities recorded approaching 1000 μmoles CO 2 fixed mg −1 chlorophyll hr −1 at 20 C. With such K m and V max , values the carboxylase would be able to achieve, at concentrations of CO 2 less than atmospheric, rates of CO 2 fixation equal to those displayed by the parent tissue or by the average plant under favorable conditions in its natural environment. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png

Carbon Dioxide Assimilation by Leaves, Isolated Chloroplasts, and Ribulose Bisphosphate Carboxylase from Spinach

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Publisher
American Society of Plant Biologist
Copyright
Copyright © 1975 by the American Society of Plant Biologists
ISSN
1532-2548
eISSN
0032-0889
DOI
10.1104/pp.55.6.1087
Publisher site
See Article on Publisher Site

Abstract

The relationship between rate of photosynthesis and CO 2 concentration has been reinvestigated using isolated spinach ( Spinacia oleracea ) chloroplasts. The apparently low CO 2 concentration required for half-maximal photosynthesis is shown to result partly from a ceiling imposed by electron transport. In double reciprocal plots of rate against CO 2 concentration, this ceiling results in departures from linearity at high CO 2 concentrations. If these rate limitations are disregarded in extrapolation the “true” CO 2 concentration required for half maximal carboxylation by intact chloroplasts is approximately 46 μ m (CO 2 ). When assayed under comparable conditions, ribulose bisphosphate carboxylase from these chloroplasts also shows an apparent K m (CO 2 ) of approximately 46 μ m , suggesting that its characteristics are not modified by extraction. An improved assay for ribulose bisphosphate carboxylase yielded rates of carboxylation considerably higher than those previously reported, the highest maximal velocities recorded approaching 1000 μmoles CO 2 fixed mg −1 chlorophyll hr −1 at 20 C. With such K m and V max , values the carboxylase would be able to achieve, at concentrations of CO 2 less than atmospheric, rates of CO 2 fixation equal to those displayed by the parent tissue or by the average plant under favorable conditions in its natural environment.

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