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   289844篇
  免费   35796篇
  国内免费   198篇
  325838篇
  2016年   2790篇
  2015年   4162篇
  2014年   4855篇
  2013年   6893篇
  2012年   7771篇
  2011年   7710篇
  2010年   5292篇
  2009年   4990篇
  2008年   7014篇
  2007年   7226篇
  2006年   6844篇
  2005年   6800篇
  2004年   6617篇
  2003年   6727篇
  2002年   6398篇
  2001年   11042篇
  2000年   11163篇
  1999年   9190篇
  1998年   3561篇
  1997年   3733篇
  1996年   3718篇
  1995年   3458篇
  1994年   3463篇
  1993年   3429篇
  1992年   8170篇
  1991年   7963篇
  1990年   7758篇
  1989年   7687篇
  1988年   7251篇
  1987年   7199篇
  1986年   6667篇
  1985年   6812篇
  1984年   5712篇
  1983年   5129篇
  1982年   4094篇
  1981年   3939篇
  1980年   3576篇
  1979年   5928篇
  1978年   4635篇
  1977年   4443篇
  1976年   4236篇
  1975年   4583篇
  1974年   5038篇
  1973年   4932篇
  1972年   4588篇
  1971年   4137篇
  1970年   3645篇
  1969年   3667篇
  1968年   3274篇
  1967年   2824篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The number of catalytic sites in acetylcholinesterase   总被引:3,自引:2,他引:1       下载免费PDF全文
By using two methods of titration, the number of active sites in acetylcholinesterase was determined. Either stepwise inhibition of the enzyme by an irreversible inhibitor, namely di-isopropyl phosphorofluoridate, or direct measurement of the concentration of active sites by titration with o-nitrophenyl dimethylcarbamate yielded an equivalent weight of approx. 130000 for an active site in acetylcholinesterase. This indicates two sites per molecule, since the native enzyme has a molecular weight of 260000.  相似文献   
992.
Synthetic lysophospholipids represent a variety of analogs of the naturally occurring 2-lysophosphatidylcholine. Some of these compounds showed significant therapeutic effects on the survival of mice following radiation injury when administered after various doses of whole-body X irradiation. Such therapeutic effects were discernible even when the treatment was given 6 hr after irradiation, and both intravenous and oral application were effective. Intravenous application of 2 X 25 mg/kg lysophospholipid after whole-body X irradiation around the LD50 resulted in significantly higher numbers of surviving animals. The mode of action remains speculative.  相似文献   
993.
994.
The histologic properties of pleural adipose organs were studied in 14 newborns. These organs contain milky spots, in which lymphocytes, macrophages and plurivacuolated fat cells are present. The milky spots have a mesothelial covering, persist to the age of 9 months and seem to act as defence devices and a site of fluid exchange.  相似文献   
995.
A program is presented for calculating s20,w values from data obtained by zone sedimentation in linear sucrose or salt gradients in a variety of rotors at temperatures ranging from 0° to 20°C. The program can be either run with the use of high-speed computers or performed with the aid of a small calculator in a reasonably short time.  相似文献   
996.
Studies on the enzymic synthesis of glutamine   总被引:17,自引:9,他引:8  
  相似文献   
997.
998.
999.
This paper examines the relationship between protected and endangered riverine species (target species) and hydrodynamics in river-floodplain ecosystems, combining ecological and policy-legal aspects of biodiversity conservation in river management. The importance of different hydrodynamic conditions along a lateral gradient was quantified for various taxonomic groups. Our results show that (i) target species require ecotopes along the entire hydrodynamic gradient; (ii) different parts of the hydrodynamic gradient are important to different species, belonging to different taxonomic groups; (iii) in particular low-dynamic parts are important for many species and (iv) species differ in their specificity for hydrodynamic conditions. Many species of higher plants, fish and butterflies have a narrow range for hydrodynamics and many species of birds and mammals use ecotopes along the entire gradient. Even when focussing only on target species, the entire natural hydrodynamic gradient is important. This means that the riverine species assemblage as a whole can benefit from measures focussing on target species only. River reconstruction and management should aim at re-establishing the entire hydrodynamic gradient, increasing the spatial heterogeneity of hydrodynamic conditions.  相似文献   
1000.
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