全文获取类型
收费全文 | 2513篇 |
免费 | 166篇 |
国内免费 | 4篇 |
专业分类
2683篇 |
出版年
2023年 | 19篇 |
2022年 | 38篇 |
2021年 | 57篇 |
2020年 | 43篇 |
2019年 | 38篇 |
2018年 | 63篇 |
2017年 | 74篇 |
2016年 | 102篇 |
2015年 | 146篇 |
2014年 | 174篇 |
2013年 | 164篇 |
2012年 | 195篇 |
2011年 | 195篇 |
2010年 | 142篇 |
2009年 | 96篇 |
2008年 | 144篇 |
2007年 | 127篇 |
2006年 | 134篇 |
2005年 | 128篇 |
2004年 | 116篇 |
2003年 | 103篇 |
2002年 | 77篇 |
2001年 | 24篇 |
2000年 | 17篇 |
1999年 | 28篇 |
1998年 | 22篇 |
1997年 | 17篇 |
1996年 | 13篇 |
1995年 | 13篇 |
1994年 | 18篇 |
1993年 | 3篇 |
1992年 | 19篇 |
1991年 | 14篇 |
1990年 | 9篇 |
1989年 | 4篇 |
1988年 | 7篇 |
1987年 | 14篇 |
1986年 | 7篇 |
1985年 | 6篇 |
1984年 | 3篇 |
1983年 | 6篇 |
1982年 | 9篇 |
1981年 | 6篇 |
1980年 | 4篇 |
1979年 | 6篇 |
1978年 | 11篇 |
1977年 | 4篇 |
1976年 | 4篇 |
1975年 | 4篇 |
1974年 | 7篇 |
排序方式: 共有2683条查询结果,搜索用时 15 毫秒
11.
Rafael Jaime Julio M. Alc ntara Jesú s M. Bastida Pedro J. Rey 《Journal of Plant Ecology》2015,8(5):457
Aims This study explores the patterns of niche differentiation in a group of seven closely related columbines (genus Aquilegia, Ranunculaceae) from the Iberian Peninsula. Populations of these columbines are subject to complex patterns of divergent selection across environments, which partly explain the taxonomic structure of the group. This suggests the hypothesis that niche divergence must have occurred along the process of diversification of the group.Methods We used MaxEnt to build environmental niche models of seven subspecies belonging to the three species of Aquilegia present in the Iberian Peninsula. From these models, we compared the environmental niches through two different approaches: ENMtools and multivariate methods.Important findings MaxEnt distributions conformed closely to the actual distribution of the study taxa. ENMtools methods failed to uncover any clear patterns of niche differentiation or conservatism in Iberian columbines. Multivariate analyses indicate the existence of differentiation along altitudinal gradients and along a gradient of climatic conditions determined by the summer precipitation and temperatures. However, climatic conditions related to winter temperature and precipitation, as well as soil properties, were equally likely to show conservatism or divergence. The complex patterns of niche evolution we found suggest that Iberian Columbines have not been significantly constrained by forces of niche conservatism, so they could respond adaptively to the fast and profound climate changes in the Iberian Peninsula through the glacial cycles of the Pleistocene. 相似文献
12.
Athanasios Paschalis Simone Fatichi Jakob Zscheischler Philippe Ciais Michael Bahn Lena Boysen Jinfeng Chang Martin De Kauwe Marc Estiarte Daniel Goll Paul J. Hanson Anna B. Harper Enqing Hou Jaime Kigel Alan K. Knapp Klaus S. Larsen Wei Li Sebastian Lienert Yiqi Luo Patrick Meir Julia E. M. S. Nabel Rom Ogaya Anthony J. Parolari Changhui Peng Josep Peuelas Julia Pongratz Serge Rambal Inger K. Schmidt Hao Shi Marcelo Sternberg Hanqin Tian Elisabeth Tschumi Anna Ukkola Sara Vicca Nicolas Viovy Ying‐Ping Wang Zhuonan Wang Karina Williams Donghai Wu Qiuan Zhu 《Global Change Biology》2020,26(6):3336-3355
Changes in rainfall amounts and patterns have been observed and are expected to continue in the near future with potentially significant ecological and societal consequences. Modelling vegetation responses to changes in rainfall is thus crucial to project water and carbon cycles in the future. In this study, we present the results of a new model‐data intercomparison project, where we tested the ability of 10 terrestrial biosphere models to reproduce the observed sensitivity of ecosystem productivity to rainfall changes at 10 sites across the globe, in nine of which, rainfall exclusion and/or irrigation experiments had been performed. The key results are as follows: (a) Inter‐model variation is generally large and model agreement varies with timescales. In severely water‐limited sites, models only agree on the interannual variability of evapotranspiration and to a smaller extent on gross primary productivity. In more mesic sites, model agreement for both water and carbon fluxes is typically higher on fine (daily–monthly) timescales and reduces on longer (seasonal–annual) scales. (b) Models on average overestimate the relationship between ecosystem productivity and mean rainfall amounts across sites (in space) and have a low capacity in reproducing the temporal (interannual) sensitivity of vegetation productivity to annual rainfall at a given site, even though observation uncertainty is comparable to inter‐model variability. (c) Most models reproduced the sign of the observed patterns in productivity changes in rainfall manipulation experiments but had a low capacity in reproducing the observed magnitude of productivity changes. Models better reproduced the observed productivity responses due to rainfall exclusion than addition. (d) All models attribute ecosystem productivity changes to the intensity of vegetation stress and peak leaf area, whereas the impact of the change in growing season length is negligible. The relative contribution of the peak leaf area and vegetation stress intensity was highly variable among models. 相似文献
13.
14.
One of the most important scientific challenges today is the quantitative and predictive understanding of biological function. Classical mathematical and computational approaches have been enormously successful in modeling inert matter, but they may be inadequate to address inherent features of biological systems. We address the conceptual and methodological obstacles that lie in the inverse problem in biological systems modeling. We introduce a full Bayesian approach (FBA), a theoretical framework to study biological function, in which probability distributions are conditional on biophysical information that physically resides in the biological system that is studied by the scientist. 相似文献
15.
Hernández-SanMiguel E Vay L Santo-Domingo J Lobatón CD Moreno A Montero M Alvarez J 《Cell calcium》2006,40(1):53-61
There is increasing evidence that mitochondria play an important role in the control of cytosolic Ca2+ signaling. We show here that the main mitochondrial Ca2+-exit pathway, the mitochondrial Na+/Ca2+ exchanger, controls the pattern of cytosolic Ca2+ oscillations in non-excitable cells. In HeLa cells, the inhibitor of the mitochondrial Na+/Ca2+ exchanger CGP37157 changed the pattern of the oscillations induced by histamine from a high-frequency irregular one to a lower frequency baseline spike type, surprisingly with little changes in the average Ca2+ values of a large cell population. In human fibroblasts, CGP37157 increased the frequency of the baseline oscillations in cells having spontaneous activity and induced the generation of oscillations in cells without spontaneous activity. This effect was dose-dependent, disappeared when the inhibitor was washed out and was not mimicked by mitochondrial depolarization. CGP37157 increased mitochondrial [Ca2+] and ATP production in histamine-stimulated HeLa cells, but the effect on ATP production was only transient. CGP37157 also activated histamine-induced Ca2+ release from the endoplasmic reticulum and increased the size of the cytosolic Ca2+ peak induced by histamine in HeLa cells. Our results suggest that the mitochondrial Na+/Ca2+ exchanger directly modulates inositol 1,4,5-trisphosphate-induced Ca2+ release and in that way controls cytosolic Ca2+ oscillations. 相似文献
16.
Sébastien Chanfreau Maria Mena Jaime R. Porras-Domínguez Mariana Ramírez-Gilly Miquel Gimeno Pedro Roquero Alberto Tecante Eduardo Bárzana 《Bioprocess and biosystems engineering》2010,33(5):629-638
The syntheses of poly-l-lactide (PLLA) and poly-l-lactide-co-glycolide (PLLGA) is reported in the ionic liquid 1-hexyl-3-methylimidazolium hexafluorophosphate [HMIM][PF6] mediated by the enzyme lipase B from Candida antarctica (Novozyme 435). The highest PLLA yield (63%) was attained at 90 °C with a molecular weight (M
n
) of 37.8 × 103 g/mol determined by size exclusion chromatography. This procedure produced relatively high crystalline polymers (up to 85%
PLLA) as determined by DSC. In experiments at 90 °C product synthesis also occurred without biocatalyst, however, PLLA synthesis
in [HMIM][PF6] at 65 °C followed only the enzymatic mechanism as ring opening was not observed without the enzyme. In addition, the enzymatic
synthesis of PLLGA is first reported here using Novozyme 435 biocatalyst with up to 19% of lactyl units in the resulting copolymer
as determined by NMR. Materials were also characterized by TGA, MALDI-TOF–MS, X-ray diffraction, polarimetry and rheology. 相似文献
17.
Jaime Fábregas Vicente Vázquez Buenaventura Cabezas Ana Otero 《Journal of applied phycology》1993,5(5):543-545
Tris (Tris(hydroxymethyl)amino methane), a compound often used as a buffer in microalgal culture media, sustains active bacterial growth in non-axenic microalgal cultures when sodium phosphate is present. The low pH levels caused by bacterial growth and probably the depletion of phosphorus in the medium caused the collapse ofPhaeodactylum tricornutum cultures resulting in a reduction of microalgal growth from 32 x 106 to 1.1 x 106 cells ml–1. This emphasizes the need for care when interpreting the results of non-axenic microalgae cultures in which Tris or other organic buffer is added. 相似文献
18.
Amy E. Howard Jaime C. Fox Kevin C. Slep 《The Journal of biological chemistry》2015,290(16):10149-10162
Microtubule-associated proteins regulate microtubule (MT) dynamics spatially and temporally, which is essential for proper formation of the bipolar mitotic spindle. The XMAP215 family is comprised of conserved microtubule-associated proteins that use an array of tubulin-binding tumor overexpressed gene (TOG) domains, consisting of six (A–F) Huntingtin, elongation factor 3, protein phosphatase 2A, target of rapamycin (HEAT) repeats, to robustly increase MT plus-end polymerization rates. Recent work showed that TOG domains have differentially conserved architectures across the array, with implications for position-dependent TOG domain tubulin binding activities and function within the XMAP215 MT polymerization mechanism. Although TOG domains 1, 2, and 4 are well described, structural and mechanistic information characterizing TOG domains 3 and 5 is outstanding. Here, we present the structure and characterization of Drosophila melanogaster Mini spindles (Msps) TOG3. Msps TOG3 has two unique features as follows: the first is a C-terminal tail that stabilizes the ultimate four HEAT repeats (HRs), and the second is a unique architecture in HR B. Structural alignments of TOG3 with other TOG domain structures show that the architecture of TOG3 is most similar to TOG domains 1 and 2 and diverges from TOG4. Docking TOG3 onto recently solved Stu2 TOG1· and TOG2·tubulin complex structures suggests that TOG3 uses similarly conserved tubulin-binding intra-HEAT loop residues to engage α- and β-tubulin. This indicates that TOG3 has maintained a TOG1- and TOG2-like TOG-tubulin binding mode despite structural divergence. The similarity of TOG domains 1–3 and the divergence of TOG4 suggest that a TOG domain array with polarized structural diversity may play a key mechanistic role in XMAP215-dependent MT polymerization activity. 相似文献
19.
Lázaro-Diéguez F Jiménez N Barth H Koster AJ Renau-Piqueras J Llopis JL Burger KN Egea G 《Cell motility and the cytoskeleton》2006,63(12):778-791
Here we examine the contribution of actin dynamics to the architecture and pH of the Golgi complex. To this end, we have used toxins that depolymerize (cytochalasin D, latrunculin B, mycalolide B, and Clostridium botulinum C2 toxin) or stabilize (jasplakinolide) filamentous actin. When various clonal cell lines were examined by epifluorescence microscopy, all of these actin toxins induced compaction of the Golgi complex. However, ultrastructural analysis by transmission electron microscopy and electron tomography/three-dimensional modelling of the Golgi complex showed that F-actin depolymerization first induces perforation/fragmentation and severe swelling of Golgi cisternae, which leads to a completely disorganized structure. In contrast, F-actin stabilization results only in cisternae perforation/fragmentation. Concomitantly to actin depolymerization-induced cisternae swelling and disorganization, the intra-Golgi pH significantly increased. Similar ultrastructural and Golgi pH alkalinization were observed in cells treated with the vacuolar H+ -ATPases inhibitors bafilomycin A1 and concanamycin A. Overall, these results suggest that actin filaments are implicated in the preservation of the flattened shape of Golgi cisternae. This maintenance seems to be mediated by the regulation of the state of F-actin assembly on the Golgi pH homeostasis. 相似文献
20.
Daniella Braz Parente Fernando Fernandes Paiva Jaime Araújo Oliveira Neto Lilian Machado-Silva Fatima Aparecida Ferreira Figueiredo Valeria Lanzoni Carlos Frederico Ferreira Campos Pedro Emmanuel Alvarenga Americano do Brasil Marilia de Brito Gomes Renata de Mello Perez Rosana Souza Rodrigues 《PloS one》2015,10(5)
ObjectiveTo evaluate the capability of intravoxel incoherent motion (IVIM) diffusion-weighted imaging (DWI) to assess steatohepatitis and fibrosis determined by histopathology in type 2 diabetic patients.MethodsFifty-nine type 2 diabetic patients (49 women, 10 men; mean age, 54 ± 9 years) were submitted to liver biopsy for the evaluation of non-alcoholic fatty liver disease (NAFLD) and underwent DWI on a 3.0T MR system using 10 b values. Institutional approval and patient consent were obtained. Pure molecular-based (D), perfusion-related (D*), and vascular fraction (f) were calculated using a double exponential model and least squares curve fitting. D, D*, and f were compared between patients with and without steatohepatitis and between patients with and without fibrosis. The variables were compared by using the Ranksum test and Student t-test.ResultsSteatohepatitis was observed in 22 patients and fibrosis in 16 patients. A lower D median (0.70 s/mm2 vs. 0.83 s/mm2, p<0.05) and a lower D* median (34.39 s/mm2 vs. 45.23 s/mm2, p<0.05) were observed among those with steatohepatitis. A lower D median (0.70 s/mm2 vs. 0.82 s/mm2, p<0.05) and a lower D* median (35.01 s/mm2 vs. 44.76 s/mm2, p=0.05) were also observed among those with fibrosis.ConclusionIVIM-DWI has the potential to aid in the characterization of steatohepatitis and fibrosis. 相似文献