Archiv der bisher stattgefundenen Seminare

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Photosynthesis light-use efficiency is fundamentally constrained by the light-limited quantum yield for carbon dioxide fixation, which under non-photorepiratory conditionsis as good as light-use efficiency gets. Normally assumed to be 0.125 - the magic number in photosynthetic quantum yields, it is actually less than this,and shows wavelength dependency. Despite its importance, this limiting quantum yield is not much studied or quantitatively understood. Some years ago we analysed the spectral-dependency of the light-limited yield for carbon dioxide fixation in cucumber, and we have repeated and extended this work for tomato.In addition to simple absolute quantum yield measurements we also, for tomato,examined the impact of state-transitions on carbon fixation yields, something which has not so far (to the best of our knowledge been) done. In addition to carbon dioxide fixation under light-limited conditions we also measured parameters associated with PSII and PSI efficiency. These results will be reported.  We have also developed equipment that allows us under a range of controlled conditions (varying irradiance, carbon dioxide and oxygen concentrations, and temperature) to measure, more or less concurrently, parameters associated with efficiency and regulation of light-use efficiency by PSI and PSII, and carbon dioxide fixation. With this we can build quite a complete picture of how photosynthesis in vivo proceeds and is regulated as a multicomponent system.Some of the results we have obtained do not seem to be consistent with the simple model(s) of regulation that have been developed over the last few years.Data will be presented to illustrate some of the anomalies we feel we have uncovered. [mehr]

More than a billion years of evolution shaped and tuned the photosynthetic apparatus harbored in thylakoid membranes to make energy conversion both efficient and robust in an often-unpredictable ever-changing nature. This success story of biological energy conversion is based on built-in structural flexibilities of the photosynthetic machinery that allows for a dynamic response on environmental cues. These structural alterations of the thylakoid membrane network occur on three different length scales ranging from the molecular level (Å - few 10 nm), the meosocopic level (several 10 nm – several 100 nm), to the overall membrane level (m). Classification into these three structural levels turned out to be extremely helpful since different physicochemical principles are realized at different length scales which require different methodical approaches to study them. The talk surveys examples for structural alterations on all three levels. In detail, for the molecular level, data on lipid-protein interactions will be presented that show the impact of highly abundant non-bilayer lipids for the structure and function of light-harvesting protein complexes. For the mesoscopic level, the significance of a supra-molecular protein reorganization from disordered to highly ordered semicrystalline arrays will be unraveled. Finally, it will be demonstrated that dynamic swelling and shrinkage of the entire membrane system is a crucial structural alteration for the control of diffusion-dependent electron transport and protein repair processes. Unraveling the design principals for the three structural levels and their interdependency is indispensible for a holistic understanding and modeling of photosynthetic energy conversion. [mehr]