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   1081369篇
  免费   113699篇
  国内免费   588篇
  2018年   9676篇
  2016年   13034篇
  2015年   17114篇
  2014年   20408篇
  2013年   29414篇
  2012年   32819篇
  2011年   33708篇
  2010年   23084篇
  2009年   21344篇
  2008年   30192篇
  2007年   31523篇
  2006年   29538篇
  2005年   28503篇
  2004年   28222篇
  2003年   27154篇
  2002年   26662篇
  2001年   46264篇
  2000年   46354篇
  1999年   37256篇
  1998年   13603篇
  1997年   13818篇
  1996年   13073篇
  1995年   12169篇
  1994年   11812篇
  1993年   11882篇
  1992年   30535篇
  1991年   29937篇
  1990年   29218篇
  1989年   28547篇
  1988年   26397篇
  1987年   25238篇
  1986年   23739篇
  1985年   23814篇
  1984年   19581篇
  1983年   17083篇
  1982年   13139篇
  1981年   12060篇
  1980年   11120篇
  1979年   18705篇
  1978年   14920篇
  1977年   13558篇
  1976年   12961篇
  1975年   14487篇
  1974年   15680篇
  1973年   15527篇
  1972年   14253篇
  1971年   12845篇
  1970年   11201篇
  1969年   11056篇
  1968年   9835篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
Auxotrophic mutants of Candida albicans FC18 were induced by a combination of treatments with nitrous acid and UV irradiation. Arginine (Arg-), histidine (His-) and methionine/cysteine (MetA-) auxotrophs were recovered by this means. The Arg- auxotrophs lacked active argininosuccinate lyase (EC 4.3.2.1), the enzyme catalysing the final step in arginine biosynthesis. Thus the locus may be designated arg-4. The mutant strains bearing this mutation did not form germ tubes unless the germination medium contained arginine.  相似文献   
142.
It has been found by gel-penetrating chromatography and quantitative 13C NMR spectroscopy that mechanochemical processing of arabinogalactan isolated from the wood of the Siberian larch changes the molecular mass distribution, monosaccharide composition, and the degree of branching of its macromolecules. This is due to the partial destruction of polysaccharide macromolecules and subsequent recombination of resulting fragments. The intensity of these processes depends on the conditions of mechanochemical processing. Based on the data of IR and 13C NMR spectra, the conclusion has been drawn that no functionalization of arabinogalactan macromolecules occurs under these conditions. The toxicopharmacological properties of mechanically processed arabinogalactan have been studied. Its acute toxicity is >5000 mg/kg.  相似文献   
143.
144.
Protein metal-binding sites.   总被引:2,自引:0,他引:2  
Metal ions have a role in a variety of important functions in proteins including protein folding, assembly, stability, conformational change, and catalysis. The presence or absence of a given metal ion is crucial to the conformation or activity of over one third of all proteins. Recent developments have been made in the understanding and design of metal-binding sites in proteins, an important and rapidly advancing area of protein engineering.  相似文献   
145.
Maleylated bovine serum albumin (maleyl-BSA) and other polyanionic polymers that are recognized by cell surface receptors on macrophages have been shown to induce chemotaxis, protease secretion, and tumoricidal function in this cell type. In this paper the effect of maleyl-BSA on Ia antigen expression has been evaluated. In a fashion similar to LPS, maleyl-BSA suppressed IFN-gamma-induced expression of Ia in a time- and dose-dependent manner. Also like LPS, maleyl-BSA stimulated the production and secretion of substantial amounts of PGE2 over a 24-hr period. This did not, however, appear to be the primary mechanism by which expression of Ia was suppressed, because co-treatment of the cells with indomethacin, which totally inhibited the production of PGE2, only minimally affected the suppressive activity. Surprisingly, the suppressive activity of both maleyl-BSA and LPS could be largely abrogated by co-treatment of the cells with cyclohexamide during the time period when Ia expression was sensitive to suppression. This effect was selective in that PGE2- or dibutyryl cyclic AMP-induced suppression of Ia expression was not affected by cyclohexamide treatment. The data support the concept that there are multiple molecular mechanisms involved in the negative regulation of IFN-gamma-induced Ia expression in macrophages. Such mechanisms may include, in addition to the synthesis of PGE2 and consequent elevation in intracellular levels of cyclic AMP, one or more proteins made early after treatment with either maleyl-BSA or LPS. Thus the function of some of these early gene products may be to regulate expression of functional genes such as that encoding Ia antigen.  相似文献   
146.
147.
148.
Electron microscopy of the adrenal medulla and cortex during the intermediate period of endotoxin shock has revealed severe destructive changes in parenchymal cells. These changes are the reason for synthetic, secretory and trophic disturbances of glandular functions. Ultrastructural lesions indicate that stress exposure exceeds the adaptive capacity of cells, the majority of which are exhausted and killed.  相似文献   
149.
150.
The metabolism of the cholestatic triterpene acid reduced lantadene A has been studied in susceptible and resistant rats and in sheep which are susceptible to intoxication. Sheep and susceptible female rats produced a similar major metabolite and rats produced a second metabolite which was a glucuronide. These metabolites were also observed in extracts of bile canalicular membranes prepared from intoxicated rats. Resistant male and female rats produced a similar major metabolite which was different to those synthesized by susceptible animals. It is concluded that in rats and sheep there is a correlation between the type of metabolites produced in the liver and the susceptibility to intoxication by reduced lantadene A.  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