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   47434篇
  免费   418篇
  国内免费   562篇
系统科学   559篇
丛书文集   1025篇
教育与普及   315篇
理论与方法论   276篇
现状及发展   17240篇
研究方法   1557篇
综合类   26940篇
自然研究   502篇
  2014年   437篇
  2013年   594篇
  2012年   863篇
  2011年   1420篇
  2010年   594篇
  2009年   586篇
  2008年   1122篇
  2007年   1136篇
  2006年   1064篇
  2005年   1056篇
  2004年   898篇
  2003年   1002篇
  2002年   1033篇
  2001年   1516篇
  2000年   1348篇
  1999年   1102篇
  1997年   408篇
  1992年   855篇
  1991年   708篇
  1990年   738篇
  1989年   712篇
  1988年   694篇
  1987年   664篇
  1986年   612篇
  1985年   754篇
  1984年   587篇
  1983年   494篇
  1982年   444篇
  1981年   434篇
  1980年   573篇
  1979年   1216篇
  1978年   1031篇
  1977年   1020篇
  1976年   757篇
  1975年   823篇
  1974年   1128篇
  1973年   1010篇
  1972年   1067篇
  1971年   1259篇
  1970年   1631篇
  1969年   1259篇
  1968年   1211篇
  1967年   1240篇
  1966年   1060篇
  1965年   754篇
  1959年   446篇
  1958年   682篇
  1957年   541篇
  1956年   455篇
  1954年   424篇
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
41.
In this paper two alternative loss criteria for the least squares Procrustes problem are studied. These alternative criteria are based on the Huber function and on the more radical biweight function, which are designed to be resistant to outliers. Using iterative majorization it is shown how a convergent reweighted least squares algorithm can be developed. In asimulation study it turns out that the proposed methods perform well over a specific range of contamination. When a uniform dilation factor is included, mixed results are obtained. The methods also yield a set of weights that can be used for diagnostic purposes.  相似文献   
42.
Studies of intracellular traffic in yeast and mammalian systems have implicated members of the Rab family of small GTP-binding proteins as regulators of membrane fusion. We have used the patch clamp technique to measure exocytotic fusion events directly and investigate the role of GTP-binding proteins in regulating exocytosis in mast cells. Intracellular perfusion of mast cells with GTP-gamma S is sufficient to trigger complete exocytotic degranulation in the absence of other intracellular messengers. Here we show that GTP is a potent inhibitor of GTP-gamma S-induced degranulation, indicating that sustained activation of a GTP-binding protein is sufficient for membrane fusion. We have found that synthetic oligopeptides, corresponding to part of the effector domain of Rab3a, stimulate complete exocytotic degranulation, similar to that induced by GTP-gamma S. The response is selective for Rab3a sequence and is strictly dependent on Mg2+ and ATP. This suggests that sustained activation of a Rab3 protein causes exocytotic fusion. The peptide response can be accelerated by GDP-beta S, suggesting that Rab3a peptides compete with endogenous Rab3 proteins for a binding site on a target effector protein, which causes fusion on activation.  相似文献   
43.
44.
Retrograde transport of endocytosed Shiga toxin to the endoplasmic reticulum.   总被引:39,自引:0,他引:39  
K Sandvig  O Garred  K Prydz  J V Kozlov  S H Hansen  B van Deurs 《Nature》1992,358(6386):510-512
Shiga toxin and some other protein toxins that act on targets in the cytosol have previously been shown to enter the trans-Golgi network. Transport by this route may be necessary for translocation of the toxin to the cytosol and for intoxication, but it is not known whether the enzymatically active part of the toxins actually enters the cytosol from the trans-Golgi network. It has been suggested that such toxins are transported in a retrograde manner to the endoplasmic reticulum and that translocation occurs in this organelle, but retrograde transport of endocytosed material beyond the trans-Golgi network has never been demonstrated. Here we show that in butyric acid-treated A431 cells endocytosed Shiga toxin is not only transported to the trans-Golgi network, but also to all Golgi stacks, to the endoplasmic reticulum and to the nuclear envelope. Furthermore, butyric acid sensitizes the cells to Shiga toxin, which is consistent with the possibility that retrograde transport is required for translocation of the toxin to the cytosol.  相似文献   
45.
S J Weintraub  C A Prater  D C Dean 《Nature》1992,358(6383):259-261
  相似文献   
46.
介绍了国内第一个综采放顶煤开采矿区铁路线下压煤试验面的成功经验,总结了试采中对铁路所采取的各种分析和维护措施,分析了该采煤方法下的地在移动和变形特点,为我国“三下”采煤领域提供了具有特色的成功实例。  相似文献   
47.
48.
The glutathione (GSH) monolayer and complex monolayer of GSH-metallic ion on polycrystalline gold electrode were studied by using K3Fe(CN)6 as the redox probe. As for the GSH monolayer, it was found that the metallic ions could open the ion-gate in the monolayer dramatically in the order La3+> Pb2+>>Ba2+>Ca2+ whereas Zn2+ ion closed the ion-gate. The complexes of GSH-metallic ions were capable of self-assembling the different kind of monolayer. All the differences were related to the structural configuration of the anchored GSH molecule, which changed with different metallic ions or pH. Biography:Fang Cheng (1971-) male, Ph. D, research direction: electroanlysis.  相似文献   
49.
50.
Summary Traces of nor-adrenaline restore the vascular action of adrenaline altered in epinephrectomized dogs to the reaction of the normal animal. Therefore it is claimed that the adrenals discharge one or several substances into the blood stream, which are necessary for the usual peripheral vascular action of adrenaline. Further investigations are in progress.  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