全文获取类型
收费全文 | 2921篇 |
免费 | 241篇 |
出版年
2023年 | 16篇 |
2022年 | 6篇 |
2021年 | 44篇 |
2020年 | 34篇 |
2019年 | 39篇 |
2018年 | 60篇 |
2017年 | 49篇 |
2016年 | 73篇 |
2015年 | 174篇 |
2014年 | 170篇 |
2013年 | 217篇 |
2012年 | 271篇 |
2011年 | 246篇 |
2010年 | 150篇 |
2009年 | 122篇 |
2008年 | 197篇 |
2007年 | 179篇 |
2006年 | 175篇 |
2005年 | 153篇 |
2004年 | 140篇 |
2003年 | 115篇 |
2002年 | 115篇 |
2001年 | 34篇 |
2000年 | 30篇 |
1999年 | 32篇 |
1998年 | 42篇 |
1997年 | 17篇 |
1996年 | 17篇 |
1995年 | 19篇 |
1994年 | 27篇 |
1993年 | 20篇 |
1992年 | 19篇 |
1991年 | 18篇 |
1990年 | 12篇 |
1989年 | 21篇 |
1988年 | 6篇 |
1987年 | 16篇 |
1986年 | 8篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1983年 | 8篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 6篇 |
1978年 | 4篇 |
1976年 | 3篇 |
1964年 | 3篇 |
1940年 | 2篇 |
1912年 | 2篇 |
排序方式: 共有3162条查询结果,搜索用时 322 毫秒
1.
Saturated fatty acids like palmitate induce endoplasmic reticulum (ER) stress in pancreatic beta‐cells, an event linked to apoptotic loss of β‐cells in type 2 diabetes. Sustained activation of the ER stress response leads to expression of growth arrest and DNA damage‐inducible protein 34 (GADD34), a regulatory subunit of protein phosphatase 1. In the present study, we have used small interfering RNA in order to knockdown GADD34 expression in insulin‐producing MIN6 cells prior to induction of ER stress by palmitate and evaluated its consequences on RNA‐activated protein kinase‐like ER‐localized eIF2alpha kinase (PERK) signalling and apoptosis. Salubrinal, a specific inhibitor of eukaryotic initiation factor 2α (eIF2α) dephosphorylation, was used as a comparison. Salubrinal treatment augmented palmitate‐induced ER stress and increased GADD34 levels. Both GADD34 knockdown and salubrinal treatment potentiated the cytotoxic effects of palmitate as evidenced by increased DNA fragmentation and activation of caspase 3, with the fundamental difference that the former did not involve enhanced levels of GADD34. The data from this study suggest that sustained activation of PERK signalling and eIF2α phosphorylation sensitizes insulin‐producing MIN6 cells to lipoapoptosis independently of GADD34 expression levels. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
2.
3.
4.
Thea Kristiansen Jens‐Christian Svenning Wolf L. Eiserhardt Dennis Pedersen Hans Brix Søren Munch Kristiansen Maria Knadel César Grández Henrik Balslev 《Journal of Biogeography》2012,39(7):1318-1332
Aim It is a central issue in ecology and biogeography to understand what governs community assembly and the maintenance of biodiversity in tropical rain forest ecosystems. A key question is the relative importance of environmental species sorting (niche assembly) and dispersal limitation (dispersal assembly), which we investigate using a large dataset from diverse palm communities. Location Lowland rain forest, western Amazon River Basin, Peru. Methods We inventoried palm communities, registering all palm individuals and recording environmental conditions in 149 transects of 5 m × 500 m. We used ordination, Mantel tests and indicator species analysis (ISA) to assess compositional patterns, species responses to geographical location and environmental factors. Mantel tests were used to assess the relative importance of geographical distance (as a proxy for dispersal limitation) and environmental differences as possible drivers of dissimilarity in palm species composition. We repeated the Mantel tests for subsets of species that differ in traits of likely importance for habitat specialization and dispersal (height and range size). Results We found a strong relationship between compositional dissimilarity and environmental distance and a weaker but also significant relationship between compositional dissimilarity and geographical distance. Consistent with expectations, relationships with environmental and geographical distance were stronger for understorey species than for canopy species. Geographical distance had a higher correlation with compositional dissimilarity for small‐ranged species compared with large‐ranged species, whereas the opposite was true for environmental distance. The main environmental correlates were inundation and soil nutrient levels. Main conclusions The assembly of palm communities in the western Amazon appears to be driven primarily by species sorting according to hydrology and soil, but with dispersal limitation also playing an important role. The importance of environmental characteristics and geographical distance varies depending on plant height and geographical range size in agreement with functional predictions, increasing our confidence in the inferred assembly mechanisms. 相似文献
5.
Marie-José Gaillard Shinya Sugita M. Jane Bunting Richard Middleton Anna Broström Christopher Caseldine Thomas Giesecke Sophie E. V. Hellman Sheila Hicks Kari Hjelle Catherine Langdon Anne-Birgitte Nielsen Anneli Poska Henrik von Stedingk Sim Veski 《Vegetation History and Archaeobotany》2008,17(5):419-443
Information on past land cover in terms of absolute areas of different landscape units (forest, open land, pasture land, cultivated
land, etc.) at local to regional scales is needed to test hypotheses and answer questions related to climate change (e.g.
feedbacks effects of land-cover change), archaeological research, and nature conservancy (e.g. management strategy). The palaeoecological
technique best suited to achieve quantitative reconstruction of past vegetation is pollen analysis. A simulation approach
developed by Sugita (the computer model POLLSCAPE) which uses models based on the theory of pollen analysis is presented together
with examples of application. POLLSCAPE has been adopted as the central tool for POLLANDCAL (POLlen/LANdscape CALibration),
an international research network focusing on this topic. The theory behind models of the pollen–vegetation relationship and
POLLSCAPE is reviewed. The two model outputs which receive greatest attention in this paper are the relevant source area of
pollen (RSAP) and pollen loading in mires and lakes. Six examples of application of POLLSCAPE are presented, each of which
explores a possible use of the POLLANDCAL tools and a means of validating or evaluating the models with empirical data. The
landscape and vegetation factors influencing the size of the RSAP, the importance of pollen productivity estimates (PPEs)
for the model outputs, the detection of small and rare patches of plant taxa in pollen records, and quantitative reconstructions
of past vegetation and landscapes are discussed on the basis of these examples. The simulation approach is seen to be useful
both for exploring different vegetation/landscape scenarios and for refuting hypotheses. 相似文献
6.
Pollen productivity estimates of key European plant taxa for quantitative reconstruction of past vegetation: a review 总被引:2,自引:2,他引:0
Anna Broström Anne Birgitte Nielsen Marie-José Gaillard Kari Hjelle Florence Mazier Heather Binney Jane Bunting Ralph Fyfe Viveca Meltsov Anneli Poska Satu Räsänen Welmoed Soepboer Henrik von Stedingk Henna Suutari Shinya Sugita 《Vegetation History and Archaeobotany》2008,17(5):461-478
Information on the spatial distribution of past vegetation on local, regional and global scales is increasingly used within
climate modelling, nature conservancy and archaeology. It is possible to obtain such information from fossil pollen records
in lakes and bogs using the landscape reconstruction algorithm (LRA) and its two models, REVEALS and LOVE. These models assume
that reliable pollen productivity estimates (PPEs) are available for the plant taxa involved in the quantitative reconstructions
of past vegetation, and that PPEs are constant through time. This paper presents and discusses the PPEs for 15 tree and 18
herb taxa obtained in nine study areas of Europe. Observed differences in PPEs between regions may be explained by methodological
issues and environmental variables, of which climate and related factors such as reproduction strategies and growth forms
appear to be the most important. An evaluation of the PPEs at hand so far suggests that they can be used in modelling applications
and quantitative reconstructions of past vegetation, provided that consideration of past environmental variability within
the region is used to inform selection of PPEs, and bearing in mind that PPEs might have changed through time as a response
to climate change. Application of a range of possible PPEs will allow a better evaluation of the results. 相似文献
7.
Bodil Kjær Yean-Sung Jung Lian Yu John H. Golbeck Henrik Vibe Scheller 《Photosynthesis research》1994,41(1):105-114
The photosynthetic reaction center complex from the green sulfur bacteriumChlorobium vibrioforme has been isolated under anaerobic conditions. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals polypeptides with apparent molecular masses of 80, 40, 30, 18, 15, and 9 kDa. The 80- and 18-kDa polypeptides are identified as the reaction center polypeptide and the secondary donor cytochromec
551 encoded by thepscA andpscC genes, respectively. N-terminal amino acid sequences identify the 40-kDa polypeptide as the bacteriochlorophylla-protein of the baseplate (the Fenna-Matthews-Olson protein) and the 30-kDa polypeptide as the putative 2[4Fe-4S] protein encoded bypscB. Electron paramagnetic resonance (EPR) analysis shows the presence of an iron-sulfur cluster which is irreversibly photoreduced at 9K. Photoaccumulation at higher temperature shows the presence of an additional photoreduced cluster. The EPR spectra of the two iron-sulfur clusters resemble those of FA and FB of Photosystem I, but also show significantly differentg-values, lineshapes, and temperature and power dependencies. We suggest that the two centers are designated Center I (with calculatedg-values of 2.085, 1.898, 1.841), and Center II (with calculatedg-values of 2.083, 1.941, 1.878). The data suggest that Centers I and II are bound to thepscB polypeptide. 相似文献
8.
Susanne Berglund Bryan J. Egner Henrik Gradén Joakim Gradén David G.A. Morgan Tord Inghardt Fabrizio Giordanetto 《Bioorganic & medicinal chemistry letters》2009,19(15):4268-4273
Herein, we disclose the discovery and optimization of 2-piperidin-4-yl-acetamide derivatives as MCH-R1 antagonists. Structural investigation of piperidin-4-yl-amide and piperidin-4-yl-ureas identified 2-piperidin-4-yl-acetamide-based MCH-R1 antagonists with outstanding in vivo efficacy but flawed with high affinity towards the hERG potassium channel. While existing hERG SAR information was employed to discover highly potent MCH-R1 antagonists with minimized hERG inhibition, additional hurdles prevented their subsequent clinical exploration. 相似文献
9.
10.