全文获取类型
收费全文 | 5352篇 |
免费 | 438篇 |
国内免费 | 1篇 |
专业分类
5791篇 |
出版年
2023年 | 20篇 |
2022年 | 54篇 |
2021年 | 112篇 |
2020年 | 64篇 |
2019年 | 85篇 |
2018年 | 81篇 |
2017年 | 81篇 |
2016年 | 148篇 |
2015年 | 269篇 |
2014年 | 281篇 |
2013年 | 362篇 |
2012年 | 467篇 |
2011年 | 427篇 |
2010年 | 301篇 |
2009年 | 218篇 |
2008年 | 366篇 |
2007年 | 328篇 |
2006年 | 307篇 |
2005年 | 290篇 |
2004年 | 288篇 |
2003年 | 272篇 |
2002年 | 230篇 |
2001年 | 62篇 |
2000年 | 51篇 |
1999年 | 50篇 |
1998年 | 55篇 |
1997年 | 47篇 |
1996年 | 48篇 |
1995年 | 37篇 |
1994年 | 25篇 |
1993年 | 31篇 |
1992年 | 31篇 |
1991年 | 21篇 |
1990年 | 11篇 |
1989年 | 13篇 |
1988年 | 8篇 |
1987年 | 12篇 |
1986年 | 11篇 |
1985年 | 15篇 |
1984年 | 18篇 |
1983年 | 12篇 |
1982年 | 13篇 |
1981年 | 12篇 |
1979年 | 8篇 |
1978年 | 14篇 |
1977年 | 13篇 |
1973年 | 10篇 |
1972年 | 8篇 |
1968年 | 7篇 |
1880年 | 8篇 |
排序方式: 共有5791条查询结果,搜索用时 11 毫秒
81.
Reconstructing the function and behaviour of extinct groups of echinoderms is problematic because there are no modern analogues for their aberrant body plans. Cinctans, an enigmatic group of Cambrian echinoderms, exemplify this problem: their asymmetrical body plan differentiates them from all living species. Here, we used computational fluid dynamics to analyse the functional performance of cinctans without assuming an extant comparative model. Three-dimensional models of six species from across cinctan phylogeny were used in computer simulations of water flow. The results demonstrate that cinctans with strongly flattened bodies produced much less drag than species characterized by dorsal protuberances or swellings, suggesting the former were more stable on the seafloor. However, unlike the flattened forms, cinctans with high-relief bodies were able to passively direct flow towards the mouth and associated food grooves, indicating that they were capable of more efficient feeding on particles suspended in the water. This study provides evidence of a previously unknown evolutionary trade-off between feeding and stability in Cambrian cinctan echinoderms. 相似文献
82.
83.
Palanivel Mathangi Mac Aogáin Micheál Purbojati Rikky W. Uchida Akira Aung Ngu War Lim Serene B. Y. Putra Alexander Drautz-Moses Daniela I. Seaton Shila Rogers Thomas R. Schuster Stephan C. Chotirmall Sanjay H. 《Mycopathologia》2020,185(2):405-408
Mycopathologia - Aspergillus terreus species complex is an opportunistic fungal pathogen increasingly implicated in invasive infection, as well as chronic respiratory disease. Currently, an... 相似文献
84.
85.
Elaine T. Lim Soumya Raychaudhuri Stephan J. Sanders Christine Stevens Aniko Sabo Daniel G. MacArthur Benjamin M. Neale Andrew Kirby Douglas M. Ruderfer Menachem Fromer Monkol Lek Li Liu Jason Flannick Stephan Ripke Uma Nagaswamy Donna Muzny Jeffrey G. Reid Alicia Hawes Mark J. Daly 《Neuron》2013,77(2):235-242
86.
Steffen Willwacher Katina Mira Fischer Rita Benker Stephan Dill Gert–Peter Brüggemann 《Journal of biomechanics》2013
The purpose of the present study was to identify kinetic responses to running on mediolaterally elevated (cross-sloped) running surfaces. Ground reaction forces (GRFs), GRF lever arms and joint moment characteristics of 19 male runners were analyzed when running at 3.5 m/s on a custom-made, tiltable runway. Tilt angles of 3° and 6° for medial and lateral elevation were analyzed using a 10 camera Vicon Nexus system and a force platform. The point of force application of the GRF showed a systematic shift in the order of 1–1.5 cm to either the lateral or medial aspect of the foot for lateral or medial inclinations, respectively. Consequently, the strongest significant effects of tilt orientation and level on joint kinetics and ground reaction force lever arms were identified at the ankle, knee and hip joint in the frontal plane of movement. External eversion moments at the ankle were significantly increased by 35% for 6° of lateral elevation and decreased by 16% for 6° of medial elevation. Altering the cross-slope of the running surface changed the pattern of ankle joint moments in the transversal plane. Effect sizes were on average larger for laterally elevated conditions, indicating a higher sensitivity of kinetic parameters to this kind of surface tilt. These alterations in joint kinetics should be considered in the choice of the running environment, especially for specific risk groups, like runners in rehabilitation processes. 相似文献
87.
Kerstin Vocke Kristin Dauner Anne Hahn Anne Ulbrich Jana Broecker Sandro Keller Stephan Frings Frank M?hrlen 《The Journal of general physiology》2013,142(4):381-404
Calcium-dependent chloride channels serve critical functions in diverse biological systems. Driven by cellular calcium signals, the channels codetermine excitatory processes and promote solute transport. The anoctamin (ANO) family of membrane proteins encodes three calcium-activated chloride channels, named ANO 1 (also TMEM16A), ANO 2 (also TMEM16B), and ANO 6 (also TMEM16F). Here we examined how ANO 1 and ANO 2 interact with Ca2+/calmodulin using nonstationary current analysis during channel activation. We identified a putative calmodulin-binding domain in the N-terminal region of the channel proteins that is involved in channel activation. Binding studies with peptides indicated that this domain, a regulatory calmodulin-binding motif (RCBM), provides two distinct modes of interaction with Ca2+/calmodulin, one at submicromolar Ca2+ concentrations and one in the micromolar Ca2+ range. Functional, structural, and pharmacological data support the concept that calmodulin serves as a calcium sensor that is stably associated with the RCBM domain and regulates the activation of ANO 1 and ANO 2 channels. Moreover, the predominant splice variant of ANO 2 in the brain exhibits Ca2+/calmodulin-dependent inactivation, a loss of channel activity within 30 s. This property may curtail ANO 2 activity during persistent Ca2+ signals in neurons. Mutagenesis data indicated that the RCBM domain is also involved in ANO 2 inactivation, and that inactivation is suppressed in the retinal ANO 2 splice variant. These results advance the understanding of Ca2+ regulation in anoctamin Cl− channels and its significance for the physiological function that anoctamin channels subserve in neurons and other cell types. 相似文献
88.
Barbara U. Metzler-Zebeli Evelyne Mann Stephan Schmitz-Esser Martin Wagner Mathias Ritzmann Qendrim Zebeli 《Applied and environmental microbiology》2013,79(23):7264-7272
Several dietary ingredients may affect the bacterial community structure and metabolism in the porcine gut and may therefore influence animals'' health and performance. This study investigated the effects of cereal source and calcium-phosphorus (CaP) level in the diet on bacterial microbiota and metabolites, nutrient intake, and gut environment in weaned pigs. Pigs (n = 8/treatment) were fed wheat-barley- or corn-based diets with an adequate or high CaP level for 14 days. Effects on microbiota in the stomach, ileum, and midcolon were assessed using quantitative PCR. Data showed that Enterobacteriaceae, Campylobacter spp., and Helicobacter spp., which all contain highly immune reactive lipopolysaccharide (LPS), were abundant at all gut sites. Diet effects on bacteria and metabolites were moderate and occurred mainly in the upper gut, whereas no effects on bacteria, fermentation products, and LPS could be observed in the colon. Differences in carbohydrate intake with corn versus wheat-barley diets selectively stimulated Bifidobacterium in the stomach and ileum. There was a growth advantage for a few bacterial groups in the stomach and ileum of pigs fed the high versus adequate CaP level (i.e., gastric Enterobacteriaceae and ileal Enterococcus, Bacteroides-Prevotella-Porphyromonas, and Campylobacter). Interestingly, gastrointestinal pH was not affected by dietary CaP level. The present findings demonstrate the stability of the bacterial community and gut environment toward dietary changes even in young pigs. The results on stimulation of gastric and ileal Bifidobacterium by corn diets may be employed in nutritional strategies to support gut health after weaning. 相似文献
89.
Hiroaki Iwaki Stephan Grosse Hélène Bergeron Hannes Leisch Krista Morley Yoshie Hasegawa Peter C. K. Lau 《Applied and environmental microbiology》2013,79(10):3282-3293
Whereas the biochemical properties of the monooxygenase components that catalyze the oxidation of 2,5-diketocamphane and 3,6-diketocamphane (2,5-DKCMO and 3,6-DKCMO, respectively) in the initial catabolic steps of (+) and (−) isomeric forms of camphor (CAM) metabolism in Pseudomonas putida ATCC 17453 are relatively well characterized, the actual identity of the flavin reductase (Fred) component that provides the reduced flavin to the oxygenases has hitherto been ill defined. In this study, a 37-kDa Fred was purified from a camphor-induced culture of P. putida ATCC 17453 and this facilitated cloning and characterization of the requisite protein. The active Fred is a homodimer with a subunit molecular weight of 18,000 that uses NADH as an electron donor (Km = 32 μM), and it catalyzes the reduction of flavin mononucleotide (FMN) (Km = 3.6 μM; kcat = 283 s−1) in preference to flavin adenine dinucleotide (FAD) (Km = 19 μM; kcat = 128 s−1). Sequence determination of ∼40 kb of the CAM degradation plasmid revealed the locations of two isofunctional 2,5-DKCMO genes (camE25–1 for 2,5-DKCMO-1 and camE25–2 for 2,5-DKCMO-2) as well as that of a 3,6-DKCMO-encoding gene (camE36). In addition, by pulsed-field gel electrophoresis, the CAM plasmid was established to be linear and ∼533 kb in length. To enable functional assessment of the two-component monooxygenase system in Baeyer-Villiger oxidations, recombinant plasmids expressing Fred in tandem with the respective 2,5-DKCMO- and 3,6-DKCMO-encoding genes in Escherichia coli were constructed. Comparative substrate profiling of the isofunctional 2,5-DCKMOs did not yield obvious differences in Baeyer-Villiger biooxidations, but they are distinct from 3,6-DKCMO in the stereoselective oxygenations with various mono- and bicyclic ketone substrates. 相似文献
90.
Christian Berger Sandra Berndt Annelie Pichert Stephan Theisgen Daniel Huster 《Biotechnology and bioengineering》2013,110(6):1681-1690
A protocol for the efficient isotopic labeling of large G protein‐coupled receptors with tryptophan in Escherichia coli as expression host was developed that sufficiently suppressed the naturally occurring L‐tryptophan indole lyase, which cleaves tryptophan into indole, pyruvate, and ammonia resulting in scrambling of the isotopic label in the protein. Indole produced by the tryptophanase is naturally used as messenger for cell–cell communication. Detailed analysis of different process conducts led to the optimal expression strategy, which mimicked cell–cell communication by the addition of indole during expression. Discrete concentrations of indole and 15N2‐L‐tryptophan at dedicated time points in the fermentation drastically increased the isotopic labeling efficiency. Isotope scrambling was only observed in glutamine, asparagine, and arginine side chains but not in the backbone. This strategy allows producing specifically tryptophan labeled membrane proteins at high concentrations avoiding the disadvantages of the often low yields of auxotrophic E. coli strains. In the fermentation process carried out according to this protocol, we produced ~15 mg of tryptophan labeled neuropeptide Y receptor type 2 per liter medium. Biotechnol. Bioeng. 2013; 110: 1681–1690. © 2013 Wiley Periodicals, Inc. 相似文献