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   752247篇
  免费   81008篇
  国内免费   511篇
  2016年   7539篇
  2015年   10505篇
  2014年   12421篇
  2013年   18339篇
  2012年   19997篇
  2011年   20575篇
  2010年   13881篇
  2009年   12858篇
  2008年   18355篇
  2007年   19393篇
  2006年   18311篇
  2005年   17684篇
  2004年   17627篇
  2003年   17075篇
  2002年   16873篇
  2001年   32156篇
  2000年   32645篇
  1999年   26235篇
  1998年   9066篇
  1997年   9552篇
  1996年   9004篇
  1995年   8531篇
  1994年   8388篇
  1993年   8387篇
  1992年   22057篇
  1991年   21578篇
  1990年   21383篇
  1989年   20848篇
  1988年   19666篇
  1987年   18572篇
  1986年   17565篇
  1985年   17975篇
  1984年   14942篇
  1983年   12864篇
  1982年   10068篇
  1981年   9188篇
  1980年   8483篇
  1979年   14422篇
  1978年   11489篇
  1977年   10664篇
  1976年   10130篇
  1975年   11190篇
  1974年   12270篇
  1973年   12076篇
  1972年   11181篇
  1971年   10158篇
  1970年   8808篇
  1969年   8803篇
  1968年   8105篇
  1967年   6927篇
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
131.
Monosomy 21: a new case confirmed by in situ hybridization   总被引:4,自引:0,他引:4  
Summary A new case of total monosomy 21 in a newborn is described. The diagnosis was first made using the cytogenetic data; it was then confirmed by the dosage of copper-superoxide dismutase (SOD-1) which showed a 50% decrease. In situ hybridization with a probe previously assigned to chromosome 21 was used to rule out the possibility of a partial monosomy with an unbalanced reciprocal translocation.  相似文献   
132.
A series of measurements of blood pressure in normotensive and hypertensive subjects showed that measurements made with a sphygmomanometer with the arm dependent by the side were consistently higher than those made with the arm horizontal at heart level. The mean difference in a group of 90 hypertensive outpatients was 11/12 mm Hg. Failure to appreciate the importance of arm position may lead to erroneous measurements of blood pressure. This has important implications for clinical practice and research.  相似文献   
133.
134.
The design and characteristics of inexpensive and simply constructed equal-energy response photosynthetic irradiance sensors is described for use particularly where several cells are required in comparative ecological studies either above or below water. The dimensions of the sensors can be changed proportionally to suit different applications or components. The response of the sensor to irradiance at varying angles corresponds very closely to that required by the cosine law. The sensor is comparatively insensitive to other environmental variables in field use and gave a stable output; the long term drift was proportional to electrical output but in continuous use, drift is regular and could reach -0.08 year-1 of the total. The spectral range and cosine response is discussed in comparison to other more expensive (x 5–10) commercially available, sensors and to local standards.  相似文献   
135.
136.
137.
Spinosyns A and D are the active ingredients in an insect control agent produced by fermentation of Saccharopolyspora spinosa. Spinosyns are macrolides with a 21-carbon, tetracyclic lactone backbone to which the deoxysugars forosamine and tri-O-methylrhamnose are attached. The spinosyn biosynthesis genes, except for the rhamnose genes, are located in a cluster that spans 74 kb of the S. spinosa genome. DNA sequence analysis, targeted gene disruptions and bioconversion studies identified five large genes encoding type I polyketide synthase subunits, and 14 genes involved in sugar biosynthesis, sugar attachment to the polyketide or cross-bridging of the polyketide. Four rhamnose biosynthetic genes, two of which are also necessary for forosamine biosynthesis, are located outside the spinosyn gene cluster. Duplication of the spinosyn genes linked to the polyketide synthase genes stimulated the final step in the biosynthesis — the conversion of the forosamine-less pseudoaglycones to endproducts. Duplication of genes involved in the early steps of deoxysugar biosynthesis increased spinosyn yield significantly. Journal of Industrial Microbiology & Biotechnology (2001) 27, 399–402. Received 31 May 2001/ Accepted in revised form 09 July 2001  相似文献   
138.
139.
140.
The effect of glutathione (GSH) depletion by L-buthionine-[S,R]-sulphoximine (BSO) on tumor necrosis factor-alpha (TNF-alpha)-induced adhesion molecule expression and mononuclear leukocyte adhesion to human umbilical vein endothelial cells (HUVECs) was investigated. Cells with marked depletion of cytoplasmic GSH, but with an intact pool of mitochondrial GSH, only slightly enhanced TNF-alpha-induced E-selectin and vascular cell adhesion molecule-1 (VCAM-1) expression, compared with the control. However, TNF-a-induced expression of both molecules was markedly enhanced when the mitochondrial GSH pool was diminished to <15% of the control. In contrast, TNF-alpha-induced intercellular adhesion molecule-1 (ICAM-1) expression was not affected by the depletion of either cytoplasmic or mitochondrial GSH. Marked enhancement of TNF-alpha-induced adhesion molecule expression by the depletion of mitochondrial GSH resulted in increased in mononuclear leukocyte adhesion to treated HUVECs, compared with the control. These effects parallel reactive oxygen species (ROS) formation by the depletion of mitochondrial but not cytoplasmic GSH. Our findings demonstrate that depletion of mitochondrial GSH renders more ROS generation in HUVECs, and mitochondrial GSH modulates TNF-alpha-induced adhesion molecule expression and mononuclear leukocyte adhesion in HUVECs.  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