Studies on the Efficiency of Energy Conservation in the Initial Stages of Photophosphorylation
Flores, Susan Cecille
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https://hdl.handle.net/2142/77654
Description
Title
Studies on the Efficiency of Energy Conservation in the Initial Stages of Photophosphorylation
Author(s)
Flores, Susan Cecille
Issue Date
1983
Department of Study
Plant Biology
Discipline
Botany
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Plant Physiology
Language
eng
Abstract
The first section of the thesis is an investigation of the accessibility of the thylakoid lumen to exogenous buffers, and the effects of added buffers on presteady state phosphorylation. Using the method of silicon oil centrifugation to measure uptake of radioactively labelled compounds, we estimated permeability coefficients for sorbitol, sucrose, and Tris, Tricine, and MOPS buffers and found these compounds to be only slowly permeant. However, incubation of thylakoids in Tricine for one hour allowed enough buffer to be taken up to generate a predictable lag in the onset of ATP synthesis. When membranes so incubated were illuminated by a train of single turnover flashes, ATP synthesis began long before the added Tricine could have been titrated.
The second section deals with the initial stages of flash-induced ATP synthesis associated with photosystem I and II partial reactions. It was found that, as in continuous light, system II reactions achieve only half the predicted maximum efficiency, whereas PS I or PS I and PS II together are nearly perfectly coupled. In flashing light, the liphophilic electron acceptors required for PS II partial reactions in continuous light were not necessary, indicating that inhibitory effects of these compounds are not the cause of the poor efficiency. Measurement of proton uptake in flashing light suggested that rapid loss of protons to the outer aqueous phase was also not the explanation. High PS II efficiencies could be attained if a subthreshold pH difference was imposed prior to the flash sequence, or if the membranes were preilluminated. Therefore, PS II most likely contributes half the ATP made during whole chain electron transport.
The final section deals with the effects of DCCD and triphenyltin chloride, two energy transfer inhibitors, on ATP synthesis in flashing light. Both compounds were less inhibitory in flashing light than in continuous light, which is evidence for a somewhat delocalized high energy intermediate. Additionally, all partial reactions were inhibited to a similar extent, indicating that there is most likely not a special association between a particular electron transport complex and the coupling factor complex.
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