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• W2003194976 abstract "Some aspects of photosynthesis in the brown alga Giffordia mitchellae (Harv.) Ham, were studied. The primary objective of this study was the comparison of in vitro RuDP-carboxylase-mediated CO2-mixation with in vivo photoassimilation measured as 14CO2-incorporation. The data on photosynthesis were also compared to long-term assimilation of carbon as calculated from growth-curves. For these comparisons it was necessary to demonstrate that total protein, dry weight and chlorophyll were equally valid as reference systems for the biological system “Giffordia”. In detail, the results were as follows: Linear Lineweaver-Burk plots were obtained for all substrate saturation-curves of RuDP-carboxylase. The apparent Michaelis-Menten constants were 2.6 · 10-4 for RuDP, 1.8 · 10-3 for Mg++, and 8.8 · 10-3 for HCO3 (=6 · 10-5 for CO2). Excess MG++ and bicarbonate concentrations strongly inhibit the carboxylating reaction. Optimum pH for enzyme extraction was 7.2 while it was 8.5 for carboxylation of RuDP. The kinetic data were determined from crude enzyme extracts only, because enzyme purification would have required much more plant material, than presently available. RuDP-carboxylase from Griffordia exhibits light activation: in the dark twothirds of the enzyme activity was lost within 30 minutes; it was restored within a similar period after reillumination of the alga. Activation energy of the enzyme reaction was calculated from ARRHENIUS-plots as 14.4 Kcal/mol; the optimum temperature was 35°C. 35°C is also the threshold temperature beyond which RuDP-carboxylase is rapidly inactivated in vivo. Under saturating conditions of light and CO2 the 14CO3-assimilation of the alga has nearly the same temperature coefficient (14.2 Kcal/mol) as the carboxylating reaction itself. The optimum temperature of CO2-fixation, however, is 25°C, which is 10°C lower than for the enzyme. The CO2-concentration for saturation of photosynthesis is not markedly different from the concentration which saturates RuDP-carboxylase, the values being 15 and 20 mM, respectively. The absolute enzyme activity (at 20°C) of 18–20 μmol CO2/mg chl · h accounts for about 50% of the carboxylation rate in vivo. This, in turn, accounts for the rate of dry-weight accumulation of the algae-cultures during the phase of linear growth. The low maximum CO2-fixation rate of 40 μmol CO2/mg chl · h is a reflection of the low light saturation of photosynthesis (2500 lux). At higher light intensities, photodynamic inhibition of photosynthesis takes place. The labelling pattern of short-term 14CO3-fixation shows high activities in mannitol, in sugar-phosphates and some amino acids (alanine, glycine, serine, aspartic acid, glutamic acid), in conformity with results for other brown algae. CO2-fixation in the dark amounts to only 3–5% of that in the light, as does the activity of phosphoenolpyruvate-carboxy-kinase when compared to RuDP-carboxylase. Substantial uptake of 14C-glucose and 14C-fructose exogenously applied to the algae could be demonstrated, light exerting a strong stimulating influence. A high percentage of the hexoses incorporated were transformed into mannitol and were also to some extend interconverted. These results confirm the hypothesis that hexoses (i.e. hexosephospates) are direct mannitol precursors. Except for AMP, the adenylates and phosphoglycerate show in Giffordia the well-known light dark kinetics of photophosphorylation. Similarly to ATP, AMP, however, rises to a sharp peak after the onset of the light period; at the beginning of the dark period AMP increase parallels ADP increase." @default.
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• W2003194976 date "1975-11-01" @default.
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• W2003194976 title "Untersuchungen zur Photosynthese bei Giffordia mitchellae Photosynthesis in Giffordia mitchellae" @default.
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