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   848835篇
  免费   93723篇
  国内免费   974篇
  2018年   8064篇
  2017年   8075篇
  2016年   12076篇
  2015年   18794篇
  2014年   20633篇
  2013年   26905篇
  2012年   29268篇
  2011年   27640篇
  2010年   19255篇
  2009年   17865篇
  2008年   23350篇
  2007年   23936篇
  2006年   22475篇
  2005年   27621篇
  2004年   26263篇
  2003年   23663篇
  2002年   20252篇
  2001年   34312篇
  2000年   33801篇
  1999年   27680篇
  1998年   10282篇
  1997年   10500篇
  1996年   9947篇
  1995年   9421篇
  1994年   9112篇
  1993年   9252篇
  1992年   23851篇
  1991年   23717篇
  1990年   23046篇
  1989年   22497篇
  1988年   20818篇
  1987年   19956篇
  1986年   18721篇
  1985年   18693篇
  1984年   15450篇
  1983年   13279篇
  1982年   10023篇
  1981年   9081篇
  1980年   8569篇
  1979年   14632篇
  1978年   11762篇
  1977年   10508篇
  1976年   9984篇
  1975年   11293篇
  1974年   12162篇
  1973年   11969篇
  1972年   10790篇
  1971年   10029篇
  1970年   8435篇
  1969年   8243篇
排序方式: 共有10000条查询结果,搜索用时 406 毫秒
111.
112.
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   
113.
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   
114.
Experiments were made on rabbit fed an atherogenic diet (0.5 g/kg cholesterol) singly for 15 and 24 h and repeatedly for 3, 9 and 30 days. At early stages of lipid metabolism distress the interrelationship was established between blood rheological disorders and microcirculatory abnormalities. The dependence of the initial reaction of some rheological characteristics on their initial level was marked.  相似文献   
115.
116.
With DNA sequencing now getting cheaper more quickly than data storage or computation, the time may have come for genome informatics to migrate to the cloud.  相似文献   
117.
OBJECTIVE--To test the hypothesis that farmers are at high risk of hip osteoarthritis and to investigate possible causes for such a hazard. DESIGN--Cross sectional survey. SETTING--Five rural general practices. SUBJECTS--167 male farmers aged 60-76 and 83 controls from mainly sedentary jobs. All those without previous hip replacement underwent radiography of the hip. MAIN OUTCOME MEASURES--Hip replacement for osteoarthritis or radiological evidence of hip osteoarthritis. RESULTS--Prevalence of hip osteoarthritis was higher in farmers than controls and especially in those who had farmed for over 10 years (odds ratio 9.3, 95% confidence interval 1.9 to 44.5). The excess could not be attributed to any one type of farming, and heavy lifting seems the likely explanation. CONCLUSIONS--Manual handling in agriculture should be limited where possible. Consideration should be given to making hip osteoarthritis a prescribed industrial disease in farmers. There may be wider implications for the prevention of hip osteoarthritis in the general population.  相似文献   
118.
119.
Synthesis of prenylquinones in chloroplasts   总被引:3,自引:0,他引:3  
  相似文献   
120.
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