全文获取类型
收费全文 | 6173篇 |
免费 | 31篇 |
国内免费 | 62篇 |
专业分类
系统科学 | 124篇 |
丛书文集 | 274篇 |
教育与普及 | 461篇 |
理论与方法论 | 92篇 |
现状及发展 | 395篇 |
研究方法 | 555篇 |
综合类 | 4361篇 |
自然研究 | 4篇 |
出版年
2023年 | 39篇 |
2022年 | 48篇 |
2021年 | 36篇 |
2020年 | 36篇 |
2019年 | 55篇 |
2018年 | 54篇 |
2017年 | 21篇 |
2016年 | 25篇 |
2015年 | 51篇 |
2014年 | 135篇 |
2013年 | 118篇 |
2012年 | 418篇 |
2011年 | 456篇 |
2010年 | 185篇 |
2009年 | 128篇 |
2008年 | 422篇 |
2007年 | 408篇 |
2006年 | 375篇 |
2005年 | 434篇 |
2004年 | 334篇 |
2003年 | 293篇 |
2002年 | 289篇 |
2001年 | 195篇 |
2000年 | 252篇 |
1999年 | 106篇 |
1998年 | 29篇 |
1996年 | 32篇 |
1995年 | 31篇 |
1994年 | 21篇 |
1992年 | 25篇 |
1991年 | 19篇 |
1990年 | 27篇 |
1989年 | 36篇 |
1988年 | 51篇 |
1987年 | 55篇 |
1986年 | 50篇 |
1985年 | 44篇 |
1984年 | 56篇 |
1983年 | 37篇 |
1982年 | 50篇 |
1981年 | 35篇 |
1980年 | 25篇 |
1979年 | 38篇 |
1959年 | 75篇 |
1958年 | 123篇 |
1957年 | 81篇 |
1956年 | 73篇 |
1955年 | 72篇 |
1954年 | 88篇 |
1948年 | 27篇 |
排序方式: 共有6266条查询结果,搜索用时 31 毫秒
101.
102.
Martinez Molina D Wetterholm A Kohl A McCarthy AA Niegowski D Ohlson E Hammarberg T Eshaghi S Haeggström JZ Nordlund P 《Nature》2007,448(7153):613-616
Cysteinyl leukotrienes are key mediators in inflammation and have an important role in acute and chronic inflammatory diseases of the cardiovascular and respiratory systems, in particular bronchial asthma. In the biosynthesis of cysteinyl leukotrienes, conversion of arachidonic acid forms the unstable epoxide leukotriene A4 (LTA4). This intermediate is conjugated with glutathione (GSH) to produce leukotriene C4 (LTC4) in a reaction catalysed by LTC4 synthase: this reaction is the key step in cysteinyl leukotriene formation. Here we present the crystal structure of the human LTC4 synthase in its apo and GSH-complexed forms to 2.00 and 2.15 A resolution, respectively. The structure reveals a homotrimer, where each monomer is composed of four transmembrane segments. The structure of the enzyme in complex with substrate reveals that the active site enforces a horseshoe-shaped conformation on GSH, and effectively positions the thiol group for activation by a nearby arginine at the membrane-enzyme interface. In addition, the structure provides a model for how the omega-end of the lipophilic co-substrate is pinned at one end of a hydrophobic cleft, providing a molecular 'ruler' to align the reactive epoxide at the thiol of glutathione. This provides new structural insights into the mechanism of LTC4 formation, and also suggests that the observed binding and activation of GSH might be common for a family of homologous proteins important for inflammatory and detoxification responses. 相似文献
103.
104.
105.
Watson DM Bohac CJ Hull C Forrest WJ Furlan E Najita J Calvet N d'Alessio P Hartmann L Sargent B Green JD Kim KH Houck JR 《Nature》2007,448(7157):1026-1028
Class 0 protostars, the youngest type of young stellar objects, show many signs of rapid development from their initial, spheroidal configurations, and therefore are studied intensively for details of the formation of protoplanetary disks within protostellar envelopes. At millimetre wavelengths, kinematic signatures of collapse have been observed in several such protostars, through observations of molecular lines that probe their outer envelopes. It has been suggested that one or more components of the proto-multiple system NGC 1333-IRAS 4 (refs 1, 2) may display signs of an embedded region that is warmer and denser than the bulk of the envelope. Here we report observations that reveal details of the core on Solar System dimensions. We detect in NGC 1333-IRAS 4B a rich emission spectrum of H2O, at wavelengths 20-37 microm, which indicates an origin in extremely dense, warm gas. We can model the emission as infall from a protostellar envelope onto the surface of a deeply embedded, dense disk, and therefore see the development of a protoplanetary disk. This is the only example of mid-infrared water emission from a sample of 30 class 0 objects, perhaps arising from a favourable orientation; alternatively, this may be an early and short-lived stage in the evolution of a protoplanetary disk. 相似文献
106.
107.
108.
Chan CS Guzman JN Ilijic E Mercer JN Rick C Tkatch T Meredith GE Surmeier DJ 《Nature》2007,447(7148):1081-1086
Why dopamine-containing neurons of the brain's substantia nigra pars compacta die in Parkinson's disease has been an enduring mystery. Our studies suggest that the unusual reliance of these neurons on L-type Ca(v)1.3 Ca2+ channels to drive their maintained, rhythmic pacemaking renders them vulnerable to stressors thought to contribute to disease progression. The reliance on these channels increases with age, as juvenile dopamine-containing neurons in the substantia nigra pars compacta use pacemaking mechanisms common to neurons not affected in Parkinson's disease. These mechanisms remain latent in adulthood, and blocking Ca(v)1.3 Ca2+ channels in adult neurons induces a reversion to the juvenile form of pacemaking. Such blocking ('rejuvenation') protects these neurons in both in vitro and in vivo models of Parkinson's disease, pointing to a new strategy that could slow or stop the progression of the disease. 相似文献
109.
110.