Studies of the transient changes in radiocarbon found in various photosynthetic and respiratory intermediates in Scenedesmus, which result when changing from a condition of steady-state photosynthesis in the light to dark and then back to light again, indicate the following metabolic mechanisms: (a) The carboxylation step in the carbon-reduction cycle of photosynthesis results in the formation of two molecules of 3-PGA from one RuDP molecule, one CO{sub 2}, and one H{sub 2}O: (b) This carboxylation reaction proceeds for about thirty seconds in the dark after the light is turned off and its rate is proportional to the falling concentration of …
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Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, CA (United States)
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Berkeley, California
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Studies of the transient changes in radiocarbon found in various photosynthetic and respiratory intermediates in Scenedesmus, which result when changing from a condition of steady-state photosynthesis in the light to dark and then back to light again, indicate the following metabolic mechanisms: (a) The carboxylation step in the carbon-reduction cycle of photosynthesis results in the formation of two molecules of 3-PGA from one RuDP molecule, one CO{sub 2}, and one H{sub 2}O: (b) This carboxylation reaction proceeds for about thirty seconds in the dark after the light is turned off and its rate is proportional to the falling concentration of RuDP, and stops when the latter concentration falls to zero. (c) Turning off the light results in the transfer of radiocarbon from PGA to citric acid, and glutamic acid, whereas turning on the light results in a decrease in radiocarbon in citric acid. These results provide new evidence for the theory that the oxidation of pyruvic acid to acetyl CoA and CO{sub 2} with a subsequent condensation of acetyl CoA with oxaloacetic acid to give citric acid is blocked in the light by reduction of a cofactor, which may be thioctic acid, required for pyruvic acid oxidation. (d) These transients in radioactivity found in Krebs-cycle acids are taken as evidence for the association with the chloroplast of enzymes and intermediates of the Krebs cycle.
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Bassham, James A.; Shibata, Kazuo; Steenberg, Kjell; Bourdon,Jean & Calvin, M.The Photosynthetic Cycle and Respiration: Ligh-DarkTransients,
report,
March 1, 1956;
Berkeley, California.
(https://digital.library.unt.edu/ark:/67531/metadc877458/:
accessed November 18, 2025),
University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu;
crediting UNT Libraries Government Documents Department.
