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1.
Concentration factor and biological half-life of 54Mn were determined in three species representing an ecologically and economically important food chain. Green algae (Chlorella spp.), Daphnia magna and yellow perch (Perca flavescens) were exposed to 54Mn in water and assayed for 54Mn uptake. Steady state concentration factors computed from the laboratory data for algae, Daphnia and perch were 4230, 17 000 and 11, respectively. Respective biological half-lives were 1.6, 1.2 and 8.3 days. 相似文献
2.
High resolution nuclear magnetic resonance study of base pairing in four purified transfer RNA molecules 总被引:3,自引:0,他引:3
The high resolution nuclear magnetic resonance spectra of hydrogen bonded protons in four purified tRNA molecules are reported. From the temperature and concentration dependence it is shown that these resonances arise from intramolecular hydrogen bonds. 相似文献
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Thomas C. Vogelmann Alan K. Knapp Therese M. McClean William K. Smith 《Physiologia plantarum》1988,72(3):623-630
Vogelmann, T. C., Knapp, A. K., McClean, T. M. and Smith, W. K. 1988. Measurement of light within thin plant tissues with fiber optic microprobes. - Physiol. Plant. 72: 623–630.
The measurement of light with fiber optic microprobes has been extended to thin (200–300 μm) plant tissue samples. To test the method, light measurements were made in thin aqueous films and paradermal sections from 10-day-old etiolated Cucurbita pepo L. cv. Fordhook cotyledons. The measurements obtained were highly reproducible. Paradermal sections of spongy mesophyll that were irradiated with collimated light scattered light more effectively than the palisade layer of intact cotyledons. These results demonstrate that different plant tissues have different light scattering characteristics. The successful extension of the fiber optic microprobe technique to thin systems makes it possible to examine the optical properties of different cell layers within leaves and other plant organs. 相似文献
The measurement of light with fiber optic microprobes has been extended to thin (200–300 μm) plant tissue samples. To test the method, light measurements were made in thin aqueous films and paradermal sections from 10-day-old etiolated Cucurbita pepo L. cv. Fordhook cotyledons. The measurements obtained were highly reproducible. Paradermal sections of spongy mesophyll that were irradiated with collimated light scattered light more effectively than the palisade layer of intact cotyledons. These results demonstrate that different plant tissues have different light scattering characteristics. The successful extension of the fiber optic microprobe technique to thin systems makes it possible to examine the optical properties of different cell layers within leaves and other plant organs. 相似文献
6.
A 2H-NMR study of the A-DNA conformation in films of oriented Na-DNA: evidence of a disordered B-DNA contribution 总被引:3,自引:0,他引:3
Solid-state 2H-nmr spectra have been obtained from folded films of oriented Li- and Na-DNA molecules with the purine bases selectively deuterium labeled at the 8 position. From line shape simulations, we find that the Na-DNA sample at 75% relative humidity (rh) contains both A-DNA and surprisingly large amounts of B-DNA 1 Here, B-DNA refers to “B-DNA family” (i.e. B- or C-DNA). (57%). For the A-DNA component the average base tilt is 23°, and the total distribution width of tilt angles and helix axis orientations is ~ 4° (standard deviation). In the B-DNA component the base tilt is ~ 0° and the total distribution width is ~ 20°. In contrast, films of Li-DNA only exhibit the B-form line shape, consistent with a base tilt of ~ 0° and a total distribution width of base tilt angles and helix axis orientations of 9°. The nmr results that demonstrate the presence of large amounts of B-DNA in the Na-DNA sample contrast with the x-ray diffraction measurements that indicated mainly A-form. The nmr spectra are used to monitor the B-DNA content in the Na-films and to evaluate procedures for increasing the A-DNA fraction. 相似文献
7.
Effects of hydration on purine motion in solid DNA 总被引:1,自引:0,他引:1
Deuterium quadrupole echo spectra and spin-lattice relaxation rates measured at 76.8 and 38.4 MHz as a function of relative humidity are reported for calf thymus DNA deuterated at positions A8 and G8. The amplitude of base pair motion is observed to increase slightly with increasing degree of hydration (up to approximately 20 mol of H2O/nucleotide), and the onset of motion is associated with a more than 100-fold drop in T1. This observed decrease in T1 parallels that observed previously for the phosphate backbone and appears to be characteristic of collective modes of motion. Above approximately 20 mol of H2O/nucleotide, the amplitude of the base motion increases substantially up to a point where slow components of motion lead to a complete loss of the quadrupole echo. 相似文献
8.
Decidual cells are direct descendants of endometrial stromal cells and the ultimate progeny of bone marrow-derived precursors. In view of their bone marrow genealogy and demonstrated immunoregulatory role during pregnancy, this study attempted to identify a lineage-specific differentiation marker(s) on murine decidual cells with the hope of tracing their developmental pathway and exploring their familial relationship to other lymphomyeloid cells. Two protein A-binding, IgG2b isotype monoclonal antibodies (secreted by clones 16F12 and 2G4F8) were raised by immunizing virgin CBA mice with syngeneic decidual cells. The presence and the density of the antigenic marker(s) recognized by these antibodies were examined by radioautography on various cell types in single cell suspensions of the decidua, placenta, and lymphomyeloid organs after a sandwich labeling with hybridoma supernatants followed by 125I-protein A. Both antibodies appeared to recognize antigen(s) unique for the decidual cell lineage in mice, humans, and rats. The incidence of antigen-bearing decidual cells increased with gestational age in CBA, C3H, and CD1 mice between days 8 and 14, and in humans between 6 and 10.5 wk; in rats, however, some decline was noted between days 8 and 14. The binding was always higher with 16F12 than with 2G4F8 supernatants. No significant binding of either antibody to trophoblast cells of the placenta or leukocytes within the decidua was noted in any of the above mouse strains or species. Little or no labeling of any cell type was seen on lymphomyeloid cells of the virgin or pregnant CBA mice, but a consistent labeling of a rare blast-type cell in the blood was observed with both antibodies, raising the possibility that this cell may represent the circulating precursor of the decidual cell lineage. It remains to be investigated whether these antibodies are recognizing the same or different differentiation antigen(s) on the decidual cells, and whether a conservation of this antigen(s) during speciation signifies its functional importance. 相似文献
9.
Phytoplankton and periphyton responses to in situ experimental nutrient enrichment in a shallow subtropical lake 总被引:2,自引:0,他引:2
Havens Karl E.; East Therese L.; Meeker Richard H.; Davis William P.; Steinman Alan D. 《Journal of plankton research》1996,18(4):551-566
Experiments were performed in situ in shallow, subtropical LakeOkeechobee (Florida. USA) to quantify and compare the responsesof phytoplanklon (in 20 I clear polycarbonate carboys) and periphyton(on nutrient-diffusing clay substrates) to additions of nitrogenand/or phosphorus. During early and late summer. 1994, bothassemblages were nitrogen limited or co-limited by nitrogenand phosphorus, indicating the potential for competition betweenbenthic and planktonic communities. During late summer, therewas evidence that high phytoplankton biomass reduced light penetrationthrough the water column and may have suppressed periphytongrowth. The similar phytoplankton and periphyton taxonomic structures,both dominated by Lyngbya sp. and pennate diatoms, suggestedthat in shallow regions of this lake, resuspended meroplanktonmight account for a large portion of phytoplankton biomass.This phenomenon has been observed in other shallow, wind-drivenFlorida lakes. 相似文献
10.
The Yeast Bsd2-1 Mutation Influences Both the Requirement for Phosphatidylinositol Transfer Protein Function and Derepression of Phospholipid Biosynthetic Gene Expression in Yeast 总被引:1,自引:0,他引:1
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S. Kagiwada B. G. Kearns T. P. McGee M. Fang K. Hosaka V. A. Bankaitis 《Genetics》1996,143(2):685-697