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Effects of cations on the adhesion between membrane vesicles obtained by digitonin fractionation of spinach chloroplasts.
No full textThe heavy fraction obtained by digitonin treatment of stacked spinach chloroplasts, suspended in media with different ionic composition, was examined by electron microscopy. In the presence of 5 mM MgCl2 the thylakoid fragments adhere to one another in a ‘stacked configuration,’ while, in the presence of 10 mM NaCl, mainly only single ‘unstacked’ vesicles are present, which, upon addition of 5 mM MgCl2, completely revert to the stacked configuration. As previously reported (Chow, W.S. and Barber, J. (1980) Biochim. Biophys. Acta 593, 149–157), no difference in fractionation of chlorophyll between light and heavy fractions was seen after a second digitonin treatment of this fraction suspended in media containing different cation concentrations. From these results it was concluded: (1) that for the unstacking process the movement of proteins or complexes from the stromal to the granal lamellae is not required. Upon lowering the screening by cations of the surface negative charges, the membranes separate from one another; (2) that, under these conditions, as in others (Jennings, R.C., Gerola, P.D., Garlaschi, F.M. and Forti, G. (1980) FEBS Lett. 115, 39–42), digitonin fractionation is not a tool to investigate the degree of membrane stacking
Inhibition of photosynthesis by azide and cyanide and the role of oxygen in photosynthesis.
No full textCyanide and azide inhibit photosynthesis and catalase activity of isolated, intact spinach (Spinacia oleracea) chloroplasts. When chloroplasts are illuminated in the presence of CN− or N3−, accumulation of H2O2 is observed, parallel to inhibition of photosynthesis. Photosynthetic O2 evolution is inhibited to the same extent, under saturating light, whether CO2 or phosphoglycerate is present as electron acceptor.
The illumination of chloroplasts with CN− or N3− inactivates the NADPH- and ATP-dependent phosphoglycerate reduction. This enzyme system can be reactivated by dithiothreitol. In reconstituted, envelope-less chloroplasts, the phosphoglycerate-dependent and the ribose 5-phosphate-dependent O2 evolution are inhibited to the same extent, while electron transport to NADP is unaffected.
It is concluded that the inhibition of photosynthesis by CN− and N3− is due to H2O2 accumulation, which is a consequence of catalase inhibition.
The inhibition of phosphoglycerate reduction, but not of CO2 reduction, is abolished under conditions where ATP is available in excess of NADPH (low light, supply of ATP). This is taken as an indication that electron flow from photosystem I is diverted to O2 (Mehler reaction, which produces H2O2) when the unavailability of ATP is limiting the rate of reoxidation of NADPH. The Mehler reaction is considered a physiological process supplying ATP for photosynthesis
Fonti di energia per i viventi e loro utilizzazione: evoluzione delle strutture per la trasduzione di energia
No full textSi osserva che non vi è stato aumento nella efficienza termodinamica nei processi fermentativi nè respiratori dagli orgnaismi più primitivi a quelli comparsi più recentemente. Così nella fotosintesi, anossigenica ed ossigenica, attraverso oltre tre miliardi di anni di evoluzione. Inoltre, i meccanismi di accoppiamento chimico della trasduzione di energia che sono probabilmente più antichi, sono termodinamicamente più efficienti, sebbene meno versatili, dei più recenti meccanismi chemiosmotici. Si conclude che la ottimizzazione dell'efficienza termodinamica non è mai stata un fattore di selezione positiva per l'evoluzione darwiniana. Questo era d'altronde d'attendersi, data la enorme quantità di energia che fluisce dal sole alla biosfera del nostro pianeta, in grande eccesso su quanto richiesto per i processi vitali
Influence of the interaction of ferredoxin NADP reductase with thylakoids on the catalysis of NADP reduction
No full text
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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