全文获取类型
收费全文 | 181篇 |
免费 | 20篇 |
出版年
2022年 | 2篇 |
2021年 | 2篇 |
2020年 | 2篇 |
2019年 | 3篇 |
2018年 | 5篇 |
2017年 | 5篇 |
2016年 | 2篇 |
2015年 | 6篇 |
2014年 | 12篇 |
2013年 | 5篇 |
2012年 | 10篇 |
2011年 | 14篇 |
2010年 | 8篇 |
2009年 | 4篇 |
2008年 | 12篇 |
2007年 | 5篇 |
2006年 | 7篇 |
2005年 | 5篇 |
2004年 | 5篇 |
2003年 | 9篇 |
2002年 | 4篇 |
2001年 | 5篇 |
2000年 | 3篇 |
1999年 | 4篇 |
1998年 | 9篇 |
1997年 | 4篇 |
1996年 | 1篇 |
1995年 | 3篇 |
1992年 | 4篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 5篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1977年 | 4篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1973年 | 4篇 |
1971年 | 2篇 |
1970年 | 1篇 |
1969年 | 2篇 |
1954年 | 1篇 |
1941年 | 1篇 |
排序方式: 共有201条查询结果,搜索用时 179 毫秒
61.
62.
Christian Michel Simon Herzog Christian de Capitani Patricia Burkhardt-Holm Constanze Pietsch 《PloS one》2014,9(7)
Worldwide increases in fluvial fine sediment are a threat to aquatic animal health. Fluvial fine sediment is always a mixture of particles whose mineralogical composition differs depending on the sediment source and catchment area geology. Nonetheless, whether particle impact in aquatic organisms differs between mineral species remains to be investigated. This study applied an in vitro approach to evaluate cytotoxicity and uptake of four common fluvial mineral particles (quartz, feldspar, mica, and kaolin; concentrations: 10, 50, 250 mg L−1) in the rainbow trout epithelial gill cell line RTgill-W1. Cells were exposed for 24, 48, 72, and 96 h. Cytotoxicity assays for cell membrane integrity (propidium iodide assay), oxidative stress (H2DCF-DA assay), and metabolic activity (MTT assay) were applied. These assays were complemented with cell counts and transmission electron microscopy. Regardless of mineral species, particles ≤2 µm in diameter were taken up by the cells, suggesting that particles of all mineral species came into contact and interacted with the cells. Not all particles, however, caused strong cytotoxicity: Among all assays the tectosilicates quartz and feldspar caused sporadic maximum changes of 0.8–1.2-fold compared to controls. In contrast, cytotoxicity of the clay particles was distinctly stronger and even differed between the two particle types: mica induced concentration-dependent increases in free radicals, with consistent 1.6–1.8-fold-changes at the 250 mg L−1 concentration, and a dilated endoplasmic reticulum. Kaolin caused concentration-dependent increases in cell membrane damage, with consistent 1.3–1.6-fold increases at the 250 mg L−1 concentration. All effects occurred in the presence or absence of 10% fetal bovine serum. Cell numbers per se were marginally affected. Results indicate that (i.) natural mineral particles can be cytotoxic to gill epithelial cells, (ii.) their cytotoxic potential differs between mineral species, with clay particles being more cytotoxic, and (iii.) some clays might induce effects comparable to engineered nanoparticles. 相似文献
63.
EMMA L. WRIGHT COLIN R. BLACK ALEXANDER W. CHEESMAN TREVOR DRAGE DAVID LARGE BENJAMIN L. TURNER SOFIE SJÖGERSTEN 《Global Change Biology》2011,17(9):2867-2881
Tropical peatlands play an important role in the global carbon cycling but little is known about factors regulating carbon dioxide (CO2) and methane (CH4) fluxes from these ecosystems. Here, we test the hypotheses that (i) CO2 and CH4 are produced mainly from surface peat and (ii) that the contribution of subsurface peat to net C emissions is governed by substrate availability. To achieve this, in situ and ex situ CO2 and CH4 fluxes were determined throughout the peat profiles under three vegetation types along a nutrient gradient in a tropical ombrotrophic peatland in Panama. The peat was also characterized with respect to its organic composition using 13C solid state cross‐polarization magic‐angle spinning nuclear magnetic resonance spectroscopy. Deep peat contributed substantially to CO2 effluxes both with respect to actual in situ and potential ex situ fluxes. CH4 was produced throughout the peat profile with distinct subsurface peaks, but net emission was limited by oxidation in the surface layers. CO2 and CH4 production were strongly substrate‐limited and a large proportion of the variance in their production (30% and 63%, respectively) was related to the quantity of carbohydrates in the peat. Furthermore, CO2 and CH4 production differed between vegetation types, suggesting that the quality of plant‐derived carbon inputs is an important driver of trace gas production throughout the peat profile. We conclude that the production of both CO2 and CH4 from subsurface peat is a substantial component of the net efflux of these gases, but that gas production through the peat profile is regulated in part by the degree of decomposition of the peat. 相似文献
64.
Pietsch J Sickmann A Weber G Bauer J Egli M Wildgruber R Infanger M Grimm D 《Proteomics》2011,11(10):2095-2104
The human cell lines FTC-133 and CGTH W-1, both derived from patients with thyroid cancer, assemble to form different types of spheroids when cultured on a random positioning machine. In order to obtain a possible explanation for their distinguishable aggregation behaviour under equal culturing conditions, we evaluated a proteomic analysis emphasising cytoskeletal and membrane-associated proteins. For this analysis, we treated the cells by ultrasound, which freed up some of the proteins into the supernatant but left some attached to the cell fragments. Both types of proteins were further separated by free-flow IEF and SDS gel electrophoresis until their identity was determined by MS. The MS data revealed differences between the two cell lines with regard to various structural proteins such as vimentin, tubulins and actin. Interestingly, integrin α-5 chains, myosin-10 and filamin B were only found in FTC-133 cells, while collagen was only detected in CGTH W-1 cells. These analyses suggest that FTC-133 cells express surface proteins that bind fibronectin, strengthening the three-dimensional cell cohesion. 相似文献
65.
Pietsch D Bernát G Kahmann U Staiger D Pistorius EK Michel KP 《Photosynthesis research》2011,108(2-3):121-132
Iron limitation has a strong impact on electron transport reactions of the unicellular fresh water cyanobacterium Synechococcus elongatus PCC 7942 (thereafter referred to as S.?elongatus). Among the various adaptational processes on different cellular levels, iron limitation induces a strongly enhanced expression of IdiC (iron-deficiency-induced protein C). In this article, we show that IdiC is loosely attached to the thylakoid and to the cytoplasmic membranes and that its expression is enhanced during conditions of iron starvation and during the late growth phase. The intracellular IdiC level was even more increased when additional iron was replenished in the late growth phase. On the basis of its amino acid sequence and of its absorbance spectrum, IdiC can be classified as a member of the family of thioredoxin (TRX)-like (2Fe-2S) ferredoxins. The presence of an iron cofactor in IdiC was detected by inductive coupled plasma optical emission spectrometry (ICP-OES). Comparative measurements of electron transport activities of S.?elongatus wild type (WT) and an IdiC-merodiploid mutant called MuD, which contained a strongly reduced IdiC content under iron-sufficient as well as iron-deficient growth conditions, were performed. The results revealed that MuD had a strongly increased light sensitivity, especially under iron limitation. The measurements of photosystem II (PS II)-mediated electron transport rates in WT and MuD strain showed that PS II activity was significantly lower in MuD than in the WT strain. Moreover, P(700) (+) re-reduction rates provided evidence that the respiratory activities, which were very low in the MuD strain in the presence of iron, significantly increased in iron-starved cells. Thus, an increase in respiration may compensate for the drastic decrease of photosynthetic electron transport activity in MuD grown under iron starvation. Based on the similarity of the S. elongatus IdiC to the NuoE subunit of the NDH-1 complex in Escherichia coli, it is likely that IdiC has a function in the electron transport processes from NAD(P)H to the plastoquinone pool. This is in agreement with the up-regulation of IdiC in the late growth phase as well as under stress conditions when PS II is damaged. As absence or high reduction of the IdiC level would prevent or reduce the formation of functional NDH-1 complexes, under such conditions electron transport routes via alternative substrate dehydrogenases, donating electrons to the plastoquinone pool, can be assumed to be up-regulated. 相似文献
66.
1. Benthic macroinvertebrates (MI) are commonly used to assess freshwater ecosystems with the reference condition approach. Such assessments necessitate control for natural community variation, either by categorical typologies or by predictive models that have been widely and successfully developed for running water biota but not previously for lake profundal invertebrates. 2. We evaluated four modelling techniques [multivariate regression tree (MRT), limiting environmental differences, nonparametric multiplicative regression (NPMR) and River Invertebrate Prediction And Classification System (RIVPACS) and the operative Finnish lake typology for assessing taxonomic completeness (observed‐to‐expected number of taxa, O/E) of profundal MI assemblages. We used data from 74 and 33 minimally disturbed reference lake basins for calibration and validation of the approaches, respectively, and 72 test basins subject to various anthropogenic pressures to evaluate sensitivity to detect impact. Either all predicted taxa (threshold probability of capture Pt = 0+) or only those predicted to be captured with ≥0.25 probability were used to calculate O/E. 3. With Pt = 0.25, all four modelling approaches were accurate (mean O/E = 0.966–1.053) but imprecise (SD of O/E = 0.279–0.304) in predicting the fauna actually observed in validation sites. All models were subtly more precise than a null model (mean 1.038, SD 0.343) or the typology (1.046, 0.327). The taxon‐specific NPMR model was slightly more precise than the other three models based on site groupings. 4. The O/E values correlated relatively weakly (r = 0.55–0.86) among the approaches, which thus produced contrasting lake‐specific assessments, despite their seemingly comparable performances. Indeed, typology, suggesting that MI assemblages were impaired in 56% of test sites, was more sensitive than the other approaches (26–46%) as an indicator of human‐induced deterioration. However, this greater ostensible sensitivity seemed to be biased, as lake morphometry, a main driver of natural community variation, remained uncontrolled by the typology. 5. Generally, our exercise illustrates the inconclusiveness of the common validation criteria for the assessment methods. The apparent poor predictability of the profundal fauna, irrespective of the method, may partly stem from large observation error, which could be alleviated by more intensive sampling. However, instead of an O/E‐taxa index, some other metric encompassing quantitative aspects might be preferable for assessing these species‐poor communities. 相似文献
67.
Myzostoma cirriferum Leuckart, 1836 possesses five paired, serially arranged, blindending nephridial organs which are described for the first time. Ultrastructural investigations reveal that each nephridium is composed of three terminal cells and one tubular cell that forms the emission tubule. The central lumen of the individual terminal cells contains six to nine flagella, each of which is surrounded regularly by cytoplasmic rods arranged in parallel. Weir-like fenestrations in the peripheral wall of the terminal cells make up the connection between the central lumina and the extracellular space around the nephridial organ. The canal of the emission tubule possesses cilia, microvilli and cytoplasmic structures, suggesting involvement of this cell with active transport and storage. It opens into the cuticle at the ventral surface of the animal. 相似文献
68.
Hayley M Bennett Hoi Ping Mok Effrossyni Gkrania-Klotsas Isheng J Tsai Eleanor J Stanley Nagui M Antoun Avril Coghlan Bhavana Harsha Alessandra Traini Diogo M Ribeiro Sascha Steinbiss Sebastian B Lucas Kieren SJ Allinson Stephen J Price Thomas S Santarius Andrew J Carmichael Peter L Chiodini Nancy Holroyd Andrew F Dean Matthew Berriman 《Genome biology》2014,15(11)
69.
70.
Chien-Hsing Lin Ling-Hui Li Sheng-Feng Ho Tzu-Po Chuang Jer-Yuarn Wu Yuan-Tsong Chen Cathy SJ Fann 《BMC genetics》2008,9(1):1-9