全文获取类型
收费全文 | 4767篇 |
免费 | 337篇 |
出版年
2023年 | 26篇 |
2022年 | 45篇 |
2021年 | 80篇 |
2020年 | 60篇 |
2019年 | 60篇 |
2018年 | 70篇 |
2017年 | 79篇 |
2016年 | 111篇 |
2015年 | 242篇 |
2014年 | 263篇 |
2013年 | 309篇 |
2012年 | 427篇 |
2011年 | 377篇 |
2010年 | 230篇 |
2009年 | 199篇 |
2008年 | 303篇 |
2007年 | 313篇 |
2006年 | 303篇 |
2005年 | 230篇 |
2004年 | 226篇 |
2003年 | 221篇 |
2002年 | 230篇 |
2001年 | 41篇 |
2000年 | 41篇 |
1999年 | 54篇 |
1998年 | 50篇 |
1997年 | 30篇 |
1996年 | 36篇 |
1995年 | 49篇 |
1994年 | 20篇 |
1993年 | 36篇 |
1992年 | 21篇 |
1991年 | 21篇 |
1990年 | 23篇 |
1989年 | 17篇 |
1988年 | 15篇 |
1987年 | 15篇 |
1986年 | 22篇 |
1985年 | 18篇 |
1984年 | 20篇 |
1983年 | 22篇 |
1982年 | 21篇 |
1981年 | 10篇 |
1979年 | 13篇 |
1978年 | 14篇 |
1977年 | 20篇 |
1976年 | 11篇 |
1975年 | 12篇 |
1974年 | 11篇 |
1973年 | 9篇 |
排序方式: 共有5104条查询结果,搜索用时 15 毫秒
101.
Jonathan C Lansing Morten Hohwy Christopher P Jaroniec A F L Creemers Johan Lugtenburg Judith Herzfeld Robert G Griffin 《Biochemistry》2002,41(2):431-438
In recent years, structural information about bacteriorhodopsin has grown substantially with the publication of several crystal structures. However, precise measurements of the chromophore conformation in the various photocycle states are still lacking. This information is critical because twists about the chromophore backbone chain can influence the Schiff base nitrogen position, orientation, and proton affinity. Here, we focus on the C14-C15 bond, using solid-state nuclear magnetic resonance spectroscopy to measure the H-C14-C15-H dihedral angle. In the resting state (bR(568)), we obtain an angle of 164 +/- 4 degrees, indicating a 16 degrees distortion from a planar all-trans chromophore. The dihedral angle is found to decrease to 147 +/- 10 degrees in the early M intermediate (M(o)) and to 150 +/- 4 degrees in the late M intermediate (M(n)). These results demonstrate changes in the chromophore conformation undetected by recent X-ray diffraction studies. 相似文献
102.
Nilsson J Persson B Von Heijne G 《Protein science : a publication of the Protein Society》2002,11(12):2974-2980
We have developed a method to reliably identify partial membrane protein topologies using the consensus of five topology prediction methods. When evaluated on a test set of experimentally characterized proteins, we find that approximately 90% of the partial consensus topologies are correctly predicted in membrane proteins from prokaryotic as well as eukaryotic organisms. Whole-genome analysis reveals that a reliable partial consensus topology can be predicted for approximately 70% of all membrane proteins in a typical bacterial genome and for approximately 55% of all membrane proteins in a typical eukaryotic genome. The average fraction of sequence length covered by a partial consensus topology is 44% for the prokaryotic proteins and 17% for the eukaryotic proteins in our test set, and similar numbers are found when the algorithm is applied to whole genomes. Reliably predicted partial topologies may simplify experimental determinations of membrane protein topology. 相似文献
103.
Katrijn Van Laere Soraya C. França Hein Vansteenkiste Johan Van Huylenbroeck Kathy Steppe Marie-Christine Van Labeke 《Acta Physiologiae Plantarum》2011,33(4):1149-1156
In this study, we analysed morphological, anatomical and physiological effects of polyploidisation in Spathiphyllum
wallisii in order to evaluate possible interesting advantages of polyploids for ornamental breeding. Stomatal density was negatively
correlated with increased ploidy level. Stomatal size increased in polyploids. Tetraploid Spathiphyllum plants had more ovate and thicker leaves. The inflorescence of tetraploids had a more ovate and thicker spathum, a more cylindrical
spadix and a thicker but shorter flower stalk. Biomass production of the tetraploids was reduced, as expressed by lower total
dry weights, and tetraploids produced fewer shoots and leaves compared with their diploid progenitors. Furthermore, tetraploid
Spathiphyllum plants were more resistant to drought stress compared with diploid plants. After 15 days of drought stress, diploids showed
symptoms of wilting, while the tetraploids showed almost no symptoms. Further, measurements of stomatal resistance, leaf water
potential, relative water content and proline content indicated that the tetraploid genotypes were more resistant to drought
stress compared with the diploids. 相似文献
104.
Using the ecosystem services approach for better planning and conservation of urban green spaces: a Finland case study 总被引:6,自引:0,他引:6
Jari Niemelä Sanna-Riikka Saarela Tarja Söderman Leena Kopperoinen Vesa Yli-Pelkonen Seija Väre D. Johan Kotze 《Biodiversity and Conservation》2010,19(11):3225-3243
Ecosystem services are vital for humans in urban regions. However, urban development poses a great risk for the ability of
ecosystems to provide these services. In this paper we first address the most important ecosystem services in functional urban
regions in Finland. Well accessible and good quality recreational ecosystem services, for example, provided by urban nature,
are an important part of a high-quality living environment and important for public health. Vegetation of urban regions can
have a role in carbon dioxide sequestration and thus in climate change mitigation. For instance, estimates of carbon sinks
can be compared to total CO2 emissions of an urban region, and the municipality can aim at both increasing carbon sinks and decreasing CO2 emissions with proper land-use planning. Large and contiguous core nature areas, smaller green areas and ecological connections
between them are the essence of regional ecological networks and are essential for maintaining interconnected habitats for
species and thus biological diversity. Thus, both local and regional level ecological networks are vital for maintaining ecosystem
services in urban regions. The impacts of climate change coupled with land-use and land cover change will bring serious challenges
for maintaining ecosystem services in urban areas. Although not yet widely used in planning practices, the ecosystem services
approach can provide an opportunity for land-use planning to develop ecologically sustainable urban regions. Currently, information
on ecosystem services of urban regions is lacking and there is a need to improve the knowledge base for land-use planning. 相似文献
105.
Jinshu Chi Mats B. Nilsson Hjalmar Laudon Anders Lindroth Jrgen Wallerman Johan E. S. Fransson Natascha Kljun Tomas Lundmark Mikaell Ottosson Lfvenius Matthias Peichl 《Global Change Biology》2020,26(4):2353-2367
The boreal biome exchanges large amounts of carbon (C) and greenhouse gases (GHGs) with the atmosphere and thus significantly affects the global climate. A managed boreal landscape consists of various sinks and sources of carbon dioxide (CO2), methane (CH4), and dissolved organic and inorganic carbon (DOC and DIC) across forests, mires, lakes, and streams. Due to the spatial heterogeneity, large uncertainties exist regarding the net landscape carbon balance (NLCB). In this study, we compiled terrestrial and aquatic fluxes of CO2, CH4, DOC, DIC, and harvested C obtained from tall‐tower eddy covariance measurements, stream monitoring, and remote sensing of biomass stocks for an entire boreal catchment (~68 km2) in Sweden to estimate the NLCB across the land–water–atmosphere continuum. Our results showed that this managed boreal forest landscape was a net C sink (NLCB = 39 g C m?2 year?1) with the landscape–atmosphere CO2 exchange being the dominant component, followed by the C export via harvest and streams. Accounting for the global warming potential of CH4, the landscape was a GHG sink of 237 g CO2‐eq m?2 year?1, thus providing a climate‐cooling effect. The CH4 flux contribution to the annual GHG budget increased from 0.6% during spring to 3.2% during winter. The aquatic C loss was most significant during spring contributing 8% to the annual NLCB. We further found that abiotic controls (e.g., air temperature and incoming radiation) regulated the temporal variability of the NLCB whereas land cover types (e.g., mire vs. forest) and management practices (e.g., clear‐cutting) determined their spatial variability. Our study advocates the need for integrating terrestrial and aquatic fluxes at the landscape scale based on tall‐tower eddy covariance measurements combined with biomass stock and stream monitoring to develop a holistic understanding of the NLCB of managed boreal forest landscapes and to better evaluate their potential for mitigating climate change. 相似文献
106.
Johanna Blomqvist Thomas Eberhard Johan Schnürer Volkmar Passoth 《Applied microbiology and biotechnology》2010,87(4):1487-1497
The influence of pH, temperature and carbon source (glucose and maltose) on growth rate and ethanol yield of Dekkera bruxellensis was investigated using a full-factorial design. Growth rate and ethanol yield were lower on maltose than on glucose. In controlled
oxygen-limited batch cultivations, the ethanol yield of the different combinations varied from 0.42 to 0.45 g (g glucose)−1 and growth rates varied from 0.037 to 0.050 h−1. The effect of temperature on growth rate and ethanol yield was negligible. It was not possible to model neither growth rate
nor ethanol yield from the full-factorial design, as only marginal differences were observed in the conditions tested. When
comparing three D. bruxellensis strains and two industrial isolates of Saccharomyces cerevisiae, S. cerevisiae grew five times faster, but the ethanol yields were 0–13% lower. The glycerol yields of S. cerevisiae strains were up to six-fold higher compared to D. bruxellensis, and the biomass yields reached only 72–84% of D. bruxellensis. Our results demonstrate that D. bruxellensis is robust to large changes in pH and temperature and may have a more energy-efficient metabolism under oxygen limitation
than S. cerevisiae. 相似文献
107.
Facilitation in plant communities: the past, the present, and the future 总被引:24,自引:11,他引:24
Rob W. Brooker Fernando T. Maestre Ragan M. Callaway Christopher L. Lortie Lohengrin A. Cavieres Georges Kunstler Pierre Liancourt Katja Tielbörger Justin M. J. Travis Fabien Anthelme Cristina Armas Lluis Coll Emmanuel Corcket Sylvain Delzon Estelle Forey Zaal Kikvidze Johan Olofsson Francisco Pugnaire Constanza L. Quiroz Patrick Saccone Katja Schiffers Merav Seifan Blaize Touzard Richard Michalet 《Journal of Ecology》2008,96(1):18-34
108.
Shared signals -'alarm calls' from plants increase apparency to herbivores and their enemies in nature 总被引:1,自引:0,他引:1
The attraction of natural enemies of herbivores by volatile organic compounds as an induced indirect defence has been studied in several plant systems. The evidence for their defensive function originates mainly from laboratory studies with trained parasitoids and predators; the defensive function of these emissions for plants in natural settings has been rarely demonstrated. In native populations and laboratory Y-tube choice experiments with transgenic Nicotiana attenuata plants unable to release particular volatiles, we demonstrate that predatory bugs use terpenoids and green leaf volatiles (GLVs) to locate their prey on herbivore-attacked plants. By attracting predators with volatile signals, this native plant reduces its herbivore load – demonstrating the defensive function of herbivore-induced volatile emissions. However, plants producing GLVs are also damaged more by flea beetles. The implications of these conflicting ecological effects for the evolution of induced volatile emissions and for the development of sustainable agricultural practices are discussed. 相似文献
109.
110.
Sara Kanje Raminta Venskutonytė Julia Scheffel Johan Nilvebrant Karin Lindkvist-Petersson Sophia Hober 《Journal of molecular biology》2018,430(18):3427-3438
Presented here is an engineered protein domain, based on Protein A, that displays a calcium-dependent binding to antibodies. This protein, ZCa, is shown to efficiently function as an affinity ligand for mild purification of antibodies through elution with ethylenediaminetetraacetic acid. Antibodies are commonly used tools in the area of biological sciences and as therapeutics, and the most commonly used approach for antibody purification is based on Protein A using acidic elution. Although this affinity-based method is robust and efficient, the requirement for low pH elution can be detrimental to the protein being purified. By introducing a calcium-binding loop in the Protein A-derived Z domain, it has been re-engineered to provide efficient antibody purification under mild conditions. Through comprehensive analyses of the domain as well as the ZCa–Fc complex, the features of this domain are well understood. This novel protein domain provides a very valuable tool for effective and gentle antibody and Fc-fusion protein purification. 相似文献