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12.
A model is presented describing the relationship between chlorophyll fluorescence quenching and photoinhibition of Photosystem (PS) II-dependent electron transport in chloroplasts. The model is based on the hypothesis that excess light creates a population of inhibited PS II units in the thylakoids. Those units are supposed to posses photochemically inactive reaction centers which convert excitation energy to heat and thereby quench variable fluorescence. If predominant photoinhibition of PS II and cooperativity in energy transfer between inhibited and active units are presumed, a quasi-linear correlation between PS II activity and the ratio of variable to maximum fluorescence, FVFM, is obtained. However, the simulation does not result in an inherent linearity of the relationship between quantum yield of PS II and FVFM ratio. The model is used to fit experimental data on photoinhibited isolated chloroplasts. Results are discussed in view of current hypotheses of photoinhibition.Abbreviations FM
maximum total fluorescence
- F0
initial fluorescence
- FV
maximum variable fluorescence
- PS
Photosystem
- QA, QB
primary and secondary electron acceptors of Photosystem II 相似文献
13.
The maturation of zygotes formed by the fusion of two gametes is the essential part of the diploid phase of the Chlamydomonas reinhardtii sexual life cycle and results in mature zygotes competent to germinate. To understand the molecular mechanisms underlying zygote maturation and the attainment of competence for germination we isolated genomic clones representing three different genes that are specifically expressed in Chlamydomonas reinhardtii zygotes. Accumulation of the RNAs started more than 24 h after mating, setting these genes apart from genes expressed in young zygotes [9]. Upon light-induced germination of zygotes, the mRNAs disappeared. The patterns of RNA accumulation and disappearance were gene-specific and suggested a function of these genes in maturation and/or in initial steps of germination. 相似文献
14.
A long-range repeat family of more than 50 kb repeat size is clustered in Chromosomes (Chr) 1 of Mus musculus and M. spretus. In M. musculus this long-range repeat family shows considerable variation of copy-number frequency and contains coding regions for at least two genes. In an intron of a gene, which is part of the repeat, a B2 small interspersed repetitive element (SINE) is inserted at identical positions. The B2 element is present in all copies of the long-range repeat family; it was presumably a component of the ancestral single-copy precursor sequence that gave rise by amplification to the repeat family. Copies of the long-range repeat family vary with respect to the number of TAAA tandem repeats in the A-rich 3 end region of the B2 element. As inferred from polymerase chain reaction (PCR) data, presence and frequency of repeat number variants in the (TAAA)n block are strain and species specific. The B2 element and its flanking regions were sequenced from two copies of the long-range repeat family. Sequence divergence between the two copies (only non-CG base substitutions and deletions/insertions) was determined to be 2.6%. Based on the drift rate in human Alu elements and a correction for the higher drift rates in rodents, and estimate for the divergence time of 1.7 million years was calculated. Since the long-range repeat family is present in M. musculus and M. spretus, it must have evolved by amplification before the separation of the two species about 1–4 million years ago. 相似文献
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16.
Christoph Aly 《Entomologia Experimentalis et Applicata》1988,46(1):55-61
Filtration rates of fourth instars of Aedes aegypti L., Anopheles albimanus Wiedemann, Anopheles quadrimaculatus Say and Culex quinquefasciatus Say (Diptera: Culicidae) were determined by quantifying removal rates of suspended latex microspheres or yeast cells. Average filtration rates were 33–34 l/larva/h (An. quadrimaculatus), 49–55 (An. albimanus), 490–590 (C. quinquefasciatus) or 590–690 l/larva/h (Ae. aegypti) for larvae exposed to latex beads suspended in phagostimulant yeast extract solutions. In suspensions of yeast cells, filtration rates of Ae. aegypti and C. quinquefasciatus were not significantly different from filtration rates in latex bead suspensions. Larval density, ranging from 0.3 to 2.4 individuals/ml in tests with Ae. aegypti and C. quinquefasciatus and up to 4.8 larvae/ml in tests with Anopheles, did not influence filtration rates.
Zusammenfassung Die Filtrierraten von Viertlarven der Stechmückenarten Aedes aegypti, Anopheles albimanus, Anopheles quadrimaculatus und Culex quinquefasciatus wurden in Laborversuchen bestimmt. Dabei wurde die Filtrierrate definiert als ein Wasservolumen, welches pro Stunde von einer Larve von den Testpartikeln (Hefezellen oder Latexkugeln mit einem Durchmesser von 2 m) befreit wurde. Nach Exposition der Larven in Dichten zwischen 0.15 und 2.4 Larven/ml (Ae. aegypti und C. quinquefasciatus), oder zwischen 0.6 und 4.8 Larven/ml (Anopheles) wurde der Partikelgehalt der Suspensionen in einem elektronischen Partikelzähler bestimmt. Die Filtrierraten wurden über die Verringerung der Partikeldichte mit zunehmender Larvendichte entsprechend einer in der Literatur angegebenen Formel berechnet.Suspensionen von Hefezellen wurden von Ae. aegypti Larven mit einer Leistung von 680±220 l/Larve/h gefiltert. Bei Larven von C. quinquefasciatus wurden Filtrierraten von 600±120 l/Larve/h gemessen. Die Filtrierraten beider Arten waren unabhängig von der Larvendichte. In Suspensionen von Latexpartikeln (zur Phagostimulation der Larven wurden diese Partikel in Lösungen von Hefeextrakt angeboten) wurden die folgenden Filtrierraten festgestellt: An quadrimaculatus: 33–34, An. albimanus: 49–55, C. quinquefasciatus: 490–590, und Ae. aegypti: 560–690 l/Larve/h. Die Larvendichte hatte auch hier keinen Einfluss auf die Filtrierrate. Hefezellen und Latexpartikel wurden von Ae. aegypti und C. quinquefasciatus mit statistisch nicht signifikant verschiedenen Filtrierraten aufgenommen. Die Filtrierraten der Anopheles Larven waren um mehr als eine Zehnerpotenz kleiner als die Filtrierraten von Culex und Aedes, und auch untereinander signifikant verschieden. Der Einfluss der Filtrierraktivität von Stechmückenlarven auf das Seston der Brutgewässer wird diskutiert.相似文献
17.
Summary 1-Methylguanine and 7-methylguanine, both metabolic products of tRNA degradation, are known to induce transformation of Chinese hamster fibroblasts in culture. The effects of these compounds on the cell membrane have been studied by the method of Concanavalin A-mediated hemadsorption. 1-Methylguanine or 7-methylguanine induced a 50% increase of Con A-mediated hemadsorption within 20 hours of exposure of the cells to the agent at a concentration of 10-5 M. This alteration was reversed within 13 days when the cells were grown in the control medium. Prolonged treatment with 1-methylguanine or 7-methylguanine resulted in changes which were only slowly reversed during growth of the cells in the control medium. The effect of the methylated purines on the cell membrane could be completely inhibited by simultaneous addition of dibutyryl-cAMP at a concentration of 10-5 M. The possible mechanism of cell membrane alteration by methylated purines and its relevance to transformation in vitro are discussed. 相似文献
18.
Dr. Christoph Schubert 《Cell and tissue research》1974,156(1):103-112
Summary Dorsal tubercle and skin of Mertensiella caucasica have been investigated with the electron microscope and enzyme histochemical methods. The epidermis of the tubercle consists of 8–9 cell layers, that of normal dorsal skin of 5–6. The tubercle is filled with large mucous glands which are surrounded by an almost complete layer of smooth muscle cells (myoepithelial cells). Their glandular cells undergo cyclical changes and are characterized by specific secretory granules, which differ from those of the relatively small mucous glands of the normal dorsal skin.In the connective tissue of the tubercle a relatively rich supply of nerve fibres has been found, which in part contain synaptic and dense core vesicles or accumulations of mitochondria. In the normal dorsal skin nerve fibres occur less frequently.The following enzymes have been demonstrated in the mucous glands of the tubercle: SDH, acid phosphatase, unspecific esterases, E 600 resistant esterase.The tubercle seems to stimulate the female cloaca chemically and mechanically. 相似文献
19.
The aims, content, and organisatory structure of a proposed interdisciplinary ecosystem research project in the Wadden Sea
of Schleswig-Holstein (W. Germany) are briefly presented. The project will include research on both fundamental as well as
applied aspects of the Wadden Sea ecosystems and their interaction with local human activities. In contrast to most of the
other completed or currently running ecosystem research projects on tidal coasts, a considerable part of the scientific work
will also deal with aspects of ecosystem management and protection of the various marine and semiterrestrial habitats of the
Wadden Sea. Considerable attention is paid to theoretical and methodological aspects of research on ecosystems and landscape
units. In particular, the adoption of a hierarchical view of complex biological and environmental systems is recommended.
Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List,
FRG, 1–4 November 1988) 相似文献
20.