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排序方式: 共有50条查询结果,搜索用时 15 毫秒
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DANIEL P. LAVIN CHRISTOS HATZIS FRIEDRICH SRIENC A. G. FREDRICKSON 《The Journal of eukaryotic microbiology》1990,37(3):157-163
Flow cytometry has been used to make direct measurements of rates of uptake of latex microspheres from dilute, monodisperse suspensions by Tetrahymena pyriformis. Measurements were made for five different sizes of microspheres, ranging from 1.09 to 6.17 μm diameter. Fractions of cells in the population that did not ingest the microspheres offered were also determined. In addition, the size distributions, as indicated by the forward angle light scattering intensity which is measured by the instrument, were determined for the whole population and for the subpopulations of cells that did and did not ingest the particles, for each particle size used. It was found that the fraction of cells that did not ingest the particles was small and independent of particle size when this was less than about 2.7 μm, but increased with particle size when particle size was increased above this value. The so-called maximum clearance rate, which can be calculated from the data, was found to increase monotonically with particle size if it were based only on those cells which actually ingested the particles offered. However, a plot of maximum clearance rate vs. particle size exhibited a maximum if the clearance rate were based on all cells present in the population. 相似文献
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Relationships Between Invertebrate Phyla Based on Functional-Mechanical Analysis of the Hydrostatic Skeleton 总被引:1,自引:0,他引:1
SYNOPSIS. The phylogeny of the major groups of deuterostomecoelomatesthe chordates,hemichordates and echinodermsisdiscussed based on a mechanical-functional analysis of the hydrostaticskeleton and associated structures. The basic approach is tofirst establish transformation series of individual featuresand of functional complexes of features and secondto determinetheir "Lesrichtung" by showing the direction of increased economy(i.e., better adaptation) with respect to environmental factors.It is argued that a metameric coelom is primitive with respectto an oligomeric one and that the ancestral form of the deuterostomecoelomates is a metameric, coelomate worm-like animal with acomplex set of circular, transverse andlongitudinal body muscles.The coelom plus the complex body musculature formed the hydrostaticskeleton.The sequence of structural modifications leading to chordatesis: (a) appearance of the notochord; (b) specialization of thedorsal longitudinal muscles with a reduction and disappearanceof the transverse and circular muscles; (c) simultaneous appearanceof the dorsal hollow nerve cord; (d) development of a postanaltail; and (e) appearance and specialization of the branchialbasket with gill slits as a filter feeding apparatus. The primitivechordate would be most similar to the lancelet (Acrania). Tunicatesare advanced chordates specialized forsessile life and lostmost chordate features in the adult, but retained them in thelarvae as adaptations for active dispersal. Enteropneusts (acornworms) are another advanced group specialized for burrowingin fine sediments and that evolved the anterior proboscis asa peristaltic burrowing organ. The notochord was lost as wasthe dorsal nerve cord and segmented conditionof the coelom.A collar originated as a means to prevent discharged water fromre-entering themouth. Pterobranchs arose from enteropneustlikeforms; their major structural changes are reduction of the branchialbasket and modification of the collar into tentacles which areassociated with life in a closed tube. Finally, echinodermsarose from a pterobranch-like ancestor by specializing for sessilelife and feeding with tentacles and by final loss of the branchialbasket. Groups such as the tunicates, hemichordates and echinodermscould be eliminated as ancestral forms within the deuterostomecoelomates because the evolution of acraniates and vertebratesfrom each of these groups would involve the appearance of gillslits before the notochord and/or the evolution of a metamericcoelom from an oligomeric one, both of which are exceedinglyimprobable. Central to the methods used to establish the transformationseries of features and their direction of evolutionary change(Lesrichtung) are functional (mechanical) analysis and adaptiveinterpretation of features; hence, functional-adaptive analysesare an integral and essential part of the methodology of phylogeneticinvestigation. 相似文献
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Changes in nitrogen cycling and retention processes in soils under spruce forests along a nitrogen enrichment gradient in Germany 总被引:5,自引:0,他引:5
MARIFE D. CORRE RAINER BRUMME EDZO VELDKAMP FRIEDRICH O. BEESE 《Global Change Biology》2007,13(7):1509-1527
A network of long-term monitoring sites on nitrogen (N) input and output of forests across Germany showed that a number of Germany's forests are subject to or are experiencing N saturation and that spruce (Picea abies) stands have high risk. Our study was aimed at (1) quantifying the changes in gross rates of microbial N cycling and retention processes in forest soils along an N enrichment gradient and (2) relating the changes in soil N dynamics to N losses. We selected spruce sites representing an N enrichment gradient (indicated by leaching : throughfall N ratios) ranging from 0.04–0.13 (low N),≤0.26 (intermediate N enrichment) to≥0.42 (highly N enriched). To our knowledge, our study is the first to report on mechanistic changes in gross rates of soil N cycling and abiotic NO3− retention under ambient N enrichment gradient. Gross N mineralization, NH4+ immobilization, gross nitrification, and NO3− immobilization rates increased up to intermediate N enrichment level and somewhat decreased at highly N-enriched condition. The turnover rates of NH4+ and microbial N pools increased while the turnover rates of the NO3− pool decreased across the N enrichment gradient. Abiotic immobilization of NH4+ did not differ across sites and was lower than that of NO3−. Abiotic NO3− immobilization decreased across the N enrichment gradient. Microbial assimilation and turnover appeared to contribute largely to the retention of NH4+. The increasing NO3− deposition and decreasing turnover rates of the NO3− pool, combined with decreasing abiotic NO3− retention, possibly contributed to increasing NO3− leaching and gaseous emissions across the N enrichment gradient. The empirical relationships of changes in microbial N cycling across the N enrichment gradient may be integrated in models used to predict responses of forest ecosystems (e.g. spruce) to increasing N deposition. 相似文献
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FRIEDRICH BEERMANN ERIKA SCHMID RUTH GANSS GÜNTHER SCHÜTZ SIEGFRIED RUPPERT 《Pigment cell & melanoma research》1992,5(5):295-299
Tyrosinase is the key enzyme in melanin synthesis, and is expressed in the pigment epithelium of the retina, a cell layer derived from the optic cup; and in neural crest-derived melanocytes of skin, hair follicle, choroid, and iris. The tyrosinase gene has been cloned and shown to map to the well-characterized c-locus (albino locus) of the mouse. Subsequent studies demonstrated that a functional tyrosinase minigene was able to rescue the albino phenotype in transgenic mice. The transgene was expressed in a cell type-specific manner in skin and eye. During development of the mouse, the tyrosinase gene is expressed in the pigment epithelium of the retina as early as day 10.5 of gestation. In the hair follicle, tyrosinase gene expression is detected from day 16.5 onwards. This cell-type–specific expression is largely reproduced in transgenic mice. Our results suggest that sequences in the immediate vicinity of the mouse tyrosinase gene are sufficient to provide cell type-specificity and developmental regulation in melanocytes and the pigment epithelium. 相似文献
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L-DOPA had no effect on the endogenous phosphorylation of proteins after extraction with 1% Triton X-100 from hamster melanoma. When proteins were purified further by wheat germ-agglutinin chromatography, however, a dramatic and dose-dependent inhibitory effect of DOPA on glycoprotein phosphorylation was observed in the presence of Mn +2. 相似文献
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Induction of the Feline Oncornavirus Associated Cell Membrane Antigen in Human Cells 总被引:1,自引:0,他引:1
MYRON ESSEX GEORGE KLEIN FRIEDRICH DEINHARDT LAUREN G. WOLFE WILLIAM D. HARDYjun. GORDAN H. THEILEN LEONARD D. PEARSON 《Nature: New biology》1972,238(84):187-189
BY N-terminal amino-acid sequence analysis, Glenner et al.1 have shown that amyloid fibrils are very similar to the variable part of an immunoglobulin kappa light chain (Bence-Jones protein Ker) and they suggested that amyloid is constituted by immunoglobulin light chains. 相似文献