首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   3787篇
  免费   473篇
  国内免费   294篇
  2024年   4篇
  2023年   167篇
  2022年   149篇
  2021年   292篇
  2020年   245篇
  2019年   292篇
  2018年   202篇
  2017年   191篇
  2016年   163篇
  2015年   191篇
  2014年   270篇
  2013年   300篇
  2012年   174篇
  2011年   209篇
  2010年   169篇
  2009年   241篇
  2008年   249篇
  2007年   208篇
  2006年   153篇
  2005年   120篇
  2004年   111篇
  2003年   78篇
  2002年   67篇
  2001年   59篇
  2000年   24篇
  1999年   31篇
  1998年   35篇
  1997年   24篇
  1996年   16篇
  1995年   24篇
  1994年   19篇
  1993年   16篇
  1992年   16篇
  1991年   8篇
  1990年   6篇
  1989年   1篇
  1988年   3篇
  1986年   3篇
  1985年   3篇
  1984年   7篇
  1983年   1篇
  1982年   4篇
  1981年   1篇
  1980年   1篇
  1976年   1篇
  1974年   1篇
  1973年   2篇
  1972年   1篇
  1971年   1篇
  1969年   1篇
排序方式: 共有4554条查询结果,搜索用时 15 毫秒
1.
2.
3.
Protein collective motions play a critical role in many biochemical processes. How to predict the functional motions and the related key residue interactions in proteins is important for our understanding in the mechanism of the biochemical processes. Normal mode analysis (NMA) of the elastic network model (ENM) is one of the effective approaches to investigate the structure-encoded motions in proteins. However, the motion modes revealed by the conventional NMA approach do not necessarily correspond to a specific function of protein. In the present work, a new analysis method was proposed to identify the motion modes responsible for a specific function of proteins and then predict the key residue interactions involved in the functional motions by using a perturbation approach. In our method, an internal coordinate that accounts for the specific function was introduced, and the Cartesian coordinate space was transformed into the internal/Cartesian space by using linear approximation, where the introduced internal coordinate serves as one of the axes of the coordinate space. NMA of ENM in this internal/Cartesian space was performed and the function-relevant motion modes were identified according to their contributions to the specific function of proteins. Then the key residue interactions important for the functional motions of the protein were predicted as the interactions whose perturbation largely influences the fluctuation along the internal coordinate. Using our proposed methods, the maltose transporter (MalFGK2) from E. Coli was studied. The functional motions and the key residue interactions that are related to the channel-gating function of this protein were successfully identified.  相似文献   
4.
目的:探究尖波间期和无尖波恢复期颞叶癫痫大鼠模型的海马CA1网络theta节律随癫痫发展进程的变化规律。方法:14只成年雄性Wistar大鼠(200~250 g)麻醉后开颅,在海马的背侧埋入一个双极性钢电极(直径小于1 mm),在颅骨上埋入三个不锈钢皮质电极,在左和右额皮质内分别植入两个电极,在小脑埋入参考电极,粘合颅骨,检测、记录大鼠脑电图;之后腹腔注射癫痫诱发药物,匹罗卡品氢氯化物(pilocarpine hydrochloride,310 mg/kg),30 min后给予大鼠莨菪碱 (scopolamine, 1 mg/kg);借助14只大鼠在嗅行(exploration)时脑深部记录(SEEG),应用Gabor小波时频能量分析估算断裂段数(以350 ms为1段),与总段数的比值定义为断裂比,用来衡量theta节律断裂程度,分别计算了癫痫发展进程的早期(D7)和晚期(D25)中两个邻近尖波之间时间段(定义为尖波间期)和随后无尖波恢复期theta节律断裂比,与注射前脑电图比较。结果:① 与对照脑电图比较,癫痫诱发大鼠的D7和D25的theta节律断裂比显著升高(P<0.05),并且D7远高于晚期D25;② 在无尖波恢复期,theta节律断裂比与尖波间期相当 (P<0.05)。结论:癫痫尖波直接导致了theta节律断裂,断裂程度将随癫痫发展进程而动态变化,早期伤害尤其严重。  相似文献   
5.
The hierarchical branching nature of river networks can have a strong influence on the assembly of freshwater communities. This unique structure has spurred the development of the network position hypothesis (NPH), which states that the strength of different assembly processes depends on the community position in the river network. Specifically, it predicts that 1) headwater communities should be exclusively controlled by the local environment given that they are more isolated and environmentally heterogeneous relative to downstream reaches. In contrast, 2) downstream communities should be regulated by both environmental and dispersal processes due to increased connectivity given their central position in the riverscape. Although intuitive, the NPH has only been evaluated on a few catchments and it is not yet clear whether its predictions are generalizable. To fill this gap, we tested the NPH on river dwelling fishes using an extensive dataset from 28 French catchments. Stream and climatic variables were assembled to characterize environmental conditions and graph theory was applied on river networks to create spatial variables. We tested both predictions using variation partitioning analyses separately for headwater and downstream sites in each catchment. Only 10 catchments supported both predictions, 11 failed to support at least one of them, while in 7 the NPH was partially supported given that spatial variables were also significant for headwater communities. We then assembled a dataset at the catchment scale (e.g. topography, environmental heterogeneity, network connectivity) and applied a classification tree analysis (CTA) to determine which regional property could explain these results. The CTA showed that the NPH was not supported in catchments with high heterogeneity in connectivity among sites. In more homogeneously connected catchments, the NPH was only supported when headwaters were more environmentally heterogeneous than downstream sites. We conclude that the NPH is context dependent even for taxa dispersing exclusively within streams.  相似文献   
6.
Food–web complexity often hinders disentangling functionally relevant aspects of food–web structure and its relationships to biodiversity. Here, we present a theoretical framework to evaluate food–web complexity in terms of biodiversity. Food network unfolding is a theoretical method to transform a complex food web into a linear food chain based on ecosystem processes. Based on this method, we can define three biodiversity indices, horizontal diversity (DH), vertical diversity (DV) and range diversity (DR), which are associated with the species diversity within each trophic level, diversity of trophic levels, and diversity in resource use, respectively. These indices are related to Shannon's diversity index (H′), where H′ = DH + DV ? DR. Application of the framework to three riverine macroinvertebrate communities revealed that D indices, calculated from biomass and stable isotope features, captured well the anthropogenic, seasonal, or other within‐site changes in food–web structures that could not be captured with H′ alone.  相似文献   
7.
Some social species exhibit high levels of fission–fusion dynamics (FFD) that improve foraging efficiency. In this study, we shed light on the way that FFD allows animal groups to cope with fluctuations in fruit availability. We explore the relative contribution of fruit availability and social factors like sex in determining association and proximity patterns in spider monkeys. We tested the influence of fruit availability and social factors on the association and proximity patterns using three-year data from a group of spider monkeys in the Yucatan Peninsula of Mexico. We identified subgroup members and estimated their Interindividual distances through instantaneous scan sampling. We evaluated fruit availability by monitoring the phenology of the 10 most important food tree species for spider monkeys in the study site. Social network analyses allowed us to evaluate association and proximity patterns in subgroups. We showed that association patterns vary between seasons, respond to changes in fruit availability, and are influenced by the sex of individuals, likely reflecting biological and behavioral differences between sexes and the interplay between ecological and social factors. In contrast, proximity patterns were minimally affected by changes in fruit availability, suggesting that social factors are more important than food availability in determining cohesion within subgroups.  相似文献   
8.
郭慧  王兵  牛香 《生态学报》2015,35(20):6829-6837
森林生态系统定位观测研究站可为森林经营和生态效益评估提供基础数据。以湖北省为例,设计森林生态系统定位观测研究网络的指标体系,基于球状模型进行普通克里格插值,与GIS的空间叠置分析相耦合,生成湖北省生态地理区划和生态功能区划;建立森林生态站网络规划的有效分区,进行森林生态站站点布设,构建了湖北省森林生态系统定位观测研究网络;总结讨论了该网络的合理性和保障措施、与其它生态站网络的比较和网络规划的局限性。结果表明:该网络将湖北省划分成12个分区,共布设16个森林生态站,其中计划建设12个生态站,已经建设4个生态站;不仅可以监测湖北省81.8%的森林面积,88.9%的生态功能区面积,98.2%的重点生态功能区面积和87.5%的生物多样性保护优先区面积,而且9个森林生态站分布与湖北省4个重点生态功能区和3个生物多样性保护优先区相匹配。该网络可以实现森林生态系统生态要素的连续观测与清查,为森林生态服务功能和生态效益评估,以及重大生态工程提供数据支撑和辅助决策分析依据。  相似文献   
9.
Meir Davis  Dror Tobi 《Proteins》2014,82(9):2097-2105
Gaussian network model (GNM) modes of motion are calculated to a dataset of h emoglobin (Hb) structures and modes with dynamics similarity to the T state are multiply aligned. The sole criterion for the alignment is the mode shape itself and not sequence or structural similarity. Standard deviation (SD) of the GNM value score along the alignment is calculated, regions with high SD are defined as dynamically variable. The analysis shows that the α1β1/α2β2 interface is a dynamically variable region but not the α1β2/α2β1 and the α1α2/β1β2 interfaces. The results are in accordance with the T → R2 transition of Hb. We suggest that dynamically variable regions are regions that are likely to undergo structural change in the protein upon binding, conformational transition, or any other relevant chemical event. The represented technique of multiple dynamics ‐ based alignment of modes is novel and may offer a new insight in proteins ' dynamics to function relation. Proteins 2014; 82:2097–2105. © 2014 Wiley Periodicals, Inc.  相似文献   
10.
Italy counts several sheep breeds, arisen over centuries as a consequence of ancient and recent genetic and demographic events. To finely reconstruct genetic structure and relationships between Italian sheep, 496 subjects from 19 breeds were typed at 50K single nucleotide polymorphism loci. A subset of foreign breeds from the Sheep HapMap dataset was also included in the analyses. Genetic distances (as visualized either in a network or in a multidimensional scaling analysis of identical by state distances) closely reflected geographic proximity between breeds, with a clear north–south gradient, likely because of high levels of past gene flow and admixture all along the peninsula. Sardinian breeds diverged more from other breeds, a probable consequence of the combined effect of ancient sporadic introgression of feral mouflon and long‐lasting genetic isolation from continental sheep populations. The study allowed the detection of previously undocumented episodes of recent introgression (Delle Langhe into the endangered Altamurana breed) as well as signatures of known, or claimed, historical introgression (Merino into Sopravissana and Gentile di Puglia; Bergamasca into Fabrianese, Appenninica and, to a lesser extent, Leccese). Arguments that would question, from a genomic point of view, the current breed classification of Bergamasca and Biellese into two separate breeds are presented. Finally, a role for traditional transhumance practices in shaping the genetic makeup of Alpine sheep breeds is proposed. The study represents the first exhaustive analysis of Italian sheep diversity in an European context, and it bridges the gap in the previous HapMap panel between Western Mediterranean and Swiss breeds.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号