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   835296篇
  免费   99828篇
  国内免费   517篇
  935641篇
  2016年   8394篇
  2015年   12133篇
  2014年   14414篇
  2013年   20554篇
  2012年   22739篇
  2011年   22934篇
  2010年   15202篇
  2009年   14381篇
  2008年   20446篇
  2007年   21561篇
  2006年   20311篇
  2005年   19650篇
  2004年   19475篇
  2003年   18776篇
  2002年   18343篇
  2001年   36125篇
  2000年   36925篇
  1999年   29608篇
  1998年   10458篇
  1997年   11019篇
  1996年   10470篇
  1995年   10044篇
  1994年   9975篇
  1993年   9914篇
  1992年   25193篇
  1991年   24696篇
  1990年   24187篇
  1989年   23495篇
  1988年   22048篇
  1987年   21165篇
  1986年   19950篇
  1985年   20146篇
  1984年   16879篇
  1983年   14822篇
  1982年   11559篇
  1981年   10778篇
  1980年   10046篇
  1979年   16711篇
  1978年   13224篇
  1977年   12134篇
  1976年   11474篇
  1975年   12849篇
  1974年   13683篇
  1973年   13534篇
  1972年   12581篇
  1971年   11245篇
  1970年   9804篇
  1969年   9434篇
  1968年   8625篇
  1967年   7416篇
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
81.
82.
83.
Isopropanol administered in a large (6 g/kg, orally) as well as in a lower dose (1 g/kg, I.P.) is slowly oxidized into acetone by the intact rat. Using two inhibitors, 3 amino-1,2,4-triazole and pyrazole, investigations on the hepatic enzymatic system involved in the oxidation of isopropanol show that catalase does not play an important part in this pathway, contrary to alcohol dehydrogenase which is the major enzyme responsible for this oxidation. Although isopropanol oxidation is mainly catalysed in the liver through alcohol dehydrogenase, no alteration of the hepatic extramitochondrial redox state occurs after the administration of a large as well as of a lower dose of isopropanol. From these experiments it may be concluded that alterations of the liver NAD+/NADH ratio, which seem to play an important part in the ethanol induced fatty liver, are not involved in the isopropanol induced one.  相似文献   
84.
Tansley Review No. 112   总被引:4,自引:0,他引:4  
  相似文献   
85.
An agar-degrading marine bacterium identified as a Microscilla species was isolated from coastal California marine sediment. This organism harbored a single 101-kb circular DNA plasmid designated pSD15. The complete nucleotide sequence of pSD15 was obtained, and sequence analysis indicated a number of genes putatively encoding a variety of enzymes involved in polysaccharide utilization. The most striking feature was the occurrence of five putative agarase genes. Loss of the plasmid, which occurred at a surprisingly high frequency, was associated with loss of agarase activity, supporting the sequence analysis results.  相似文献   
86.
Expression of the glycoprotein clusterin is markedly increased following tissue injury. One function of clusterin is to promote cell interactions which are perturbed in these pathologic settings. Clusterin causes cell aggregation and adhesion in vitro yet the molecular mechanism for this effect is not known. In order to identify the active site(s) of clusterin, 34 peptides, each 15 amino acid residues in length, were synthesized from hydrophilic regions of human clusterin. When studied individually, none of the peptides caused aggregation of LLC-PK1 cells, a porcine renal epithelial cell line. However, two out of the 34 peptides inhibited clusterin-induced cell aggregation in a dose-dependent manner. Scrambled versions of these two 'active' peptides did not inhibit cell aggregation. Seven peptides promoted cell adhesion. In conclusion, these findings provide evidence for novel amino acid sequences mediating clusterin-induced renal cell interactions.  相似文献   
87.
The cysteine-rich region (CRR) of the β2 integrin subunit was replaced by that of β1 to give the chimera β2NV1. β2NV1 can combine with αL to form a variant leukocyte-function-associated antigen (LFA)-1 on COS cell surface, suggesting that the specificity of the β2 interaction with αL does not lie in the CRR. Unlike those expressing wild-type LFA-1, COS cells expressing αLβ2NV1 are constitutively active in intercellular adhesion molecule (ICAM)-1 adhesion. These results suggest that activation of LFA-1 involves the release of an intramolecular constraint, which is maintained, in part, by the authentic β2 CRR.  相似文献   
88.
89.
The Caenorhabditis elegans excretory cell extends tubular processes, called canals, along the basolateral surface of the epidermis. Mutations in the exc-5 gene cause tubulocystic defects in this canal. Ultrastructural analysis suggests that exc-5 is required for the proper placement of cytoskeletal elements at the apical epithelial surface. exc-5 encodes a protein homologous to guanine nucleotide exchange factors and contains motif architecture similar to that of FGD1, which is responsible for faciogenital dysplasia. exc-5 interacts genetically with mig-2, which encodes Rho GTPase. These results suggest that EXC-5 controls the structural organization of the excretory canal by regulating Rho family GTPase activities.  相似文献   
90.
A thrombin (E.C. 3.4.21.5) inhibitor, savignin, was isolated from the salivary glands of Ornithodoros savignyi by a combination of size exclusion, anion-exchange, and reversed-phase chromatography. The inhibitor has a molecular mass of 12,430.4 Da as determined by electrospray mass spectrometry. The behavior of savignin during anion-exchange chromatography indicated that it has an acidic pI. The available N-terminal sequence (residues 1-11) differed from that of ornithodorin with only one residue. Savignin inhibits thrombin-induced platelet aggregation, but has no effect on ADP- or collagen-induced aggregation. Kinetic studies indicated that savignin is a competitive, slow-, tight-binding inhibitor of alpha-thrombin (K(i) = 4.89 +/- 1.39 pM). Tight-binding kinetics showed that the inhibitor has a lower affinity for gamma-thrombin (K(i) = 22.3 +/- 5.9 nM). Plasmin, factor Xa, and trypsin are not inhibited by savignin.  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