全文获取类型
收费全文 | 66802篇 |
免费 | 20169篇 |
国内免费 | 15篇 |
出版年
2021年 | 1219篇 |
2020年 | 2581篇 |
2019年 | 4188篇 |
2018年 | 4415篇 |
2017年 | 4570篇 |
2016年 | 5049篇 |
2015年 | 5536篇 |
2014年 | 5255篇 |
2013年 | 6042篇 |
2012年 | 4284篇 |
2011年 | 3853篇 |
2010年 | 4463篇 |
2009年 | 3075篇 |
2008年 | 2530篇 |
2007年 | 2079篇 |
2006年 | 1985篇 |
2005年 | 1839篇 |
2004年 | 1734篇 |
2003年 | 1664篇 |
2002年 | 1593篇 |
2001年 | 1249篇 |
2000年 | 1144篇 |
1999年 | 978篇 |
1998年 | 512篇 |
1997年 | 429篇 |
1996年 | 405篇 |
1995年 | 353篇 |
1994年 | 367篇 |
1993年 | 381篇 |
1992年 | 730篇 |
1991年 | 660篇 |
1990年 | 644篇 |
1989年 | 687篇 |
1988年 | 569篇 |
1987年 | 618篇 |
1986年 | 488篇 |
1985年 | 584篇 |
1984年 | 500篇 |
1983年 | 397篇 |
1982年 | 410篇 |
1981年 | 371篇 |
1980年 | 341篇 |
1979年 | 451篇 |
1978年 | 396篇 |
1977年 | 350篇 |
1976年 | 319篇 |
1975年 | 345篇 |
1974年 | 391篇 |
1973年 | 377篇 |
1972年 | 310篇 |
排序方式: 共有10000条查询结果,搜索用时 687 毫秒
31.
32.
33.
Hemopexin-mediated heme uptake by liver. Characterization of the interaction of heme-hemopexin with isolated rabbit liver plasma membranes 总被引:6,自引:0,他引:6
Plasma membranes isolated from rabbit liver retain the ability to interact specifically with heme-hemopexin. In this system, apohemopexin does not compete effectively with heme-hemopexin for binding. The membranes bind heme-hemopexin complexes with high affinity (KD = 6.8 X 10(-7) M) and with an apparent capacity of 2.3 pmol/mg of membrane protein. These membranes also retain the ability to remove heme from heme-hemopexin. The release of heme reaches a plateau after 15-30 min at 30 degrees C and does not involve metabolic energy, proteolysis of hemopexin or pH gradients. The apohemopexin formed is rapidly released from the membranes. The accumulation of heme is saturable and is affected by pH and temperature with maximum uptake occurring between pH 5.5 and 6.5 and at 30 degrees C. Interestingly, much more heme (approximately 25 pmol/mg of membrane protein) is accumulated than hemopexin at saturation, implying that the receptor can turn over several times and that a heme-binding component exists in the rabbit liver plasma membrane. 相似文献
34.
35.
36.
Thirteen polymorphic microsatellite loci were developed for blowflies for use in studies of genetic differentiation in wild populations of Lucilia illustris, to detect the possible occurrence of bottlenecks and to study changes in genetic variation in laboratory populations of Lucilia sericata following artificial bottlenecks. In this preliminary study it was revealed that heterozygosity was lower than expected in wild populations and genetic variation had been lost in the laboratory population despite being kept at a large size. 相似文献
37.
38.
Thea Kristiansen Jens‐Christian Svenning Wolf L. Eiserhardt Dennis Pedersen Hans Brix Søren Munch Kristiansen Maria Knadel César Grández Henrik Balslev 《Journal of Biogeography》2012,39(7):1318-1332
Aim It is a central issue in ecology and biogeography to understand what governs community assembly and the maintenance of biodiversity in tropical rain forest ecosystems. A key question is the relative importance of environmental species sorting (niche assembly) and dispersal limitation (dispersal assembly), which we investigate using a large dataset from diverse palm communities. Location Lowland rain forest, western Amazon River Basin, Peru. Methods We inventoried palm communities, registering all palm individuals and recording environmental conditions in 149 transects of 5 m × 500 m. We used ordination, Mantel tests and indicator species analysis (ISA) to assess compositional patterns, species responses to geographical location and environmental factors. Mantel tests were used to assess the relative importance of geographical distance (as a proxy for dispersal limitation) and environmental differences as possible drivers of dissimilarity in palm species composition. We repeated the Mantel tests for subsets of species that differ in traits of likely importance for habitat specialization and dispersal (height and range size). Results We found a strong relationship between compositional dissimilarity and environmental distance and a weaker but also significant relationship between compositional dissimilarity and geographical distance. Consistent with expectations, relationships with environmental and geographical distance were stronger for understorey species than for canopy species. Geographical distance had a higher correlation with compositional dissimilarity for small‐ranged species compared with large‐ranged species, whereas the opposite was true for environmental distance. The main environmental correlates were inundation and soil nutrient levels. Main conclusions The assembly of palm communities in the western Amazon appears to be driven primarily by species sorting according to hydrology and soil, but with dispersal limitation also playing an important role. The importance of environmental characteristics and geographical distance varies depending on plant height and geographical range size in agreement with functional predictions, increasing our confidence in the inferred assembly mechanisms. 相似文献
39.
R Smith 《BMJ (Clinical research ed.)》1987,295(6612):1547-1550
40.