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   425296篇
  免费   47872篇
  国内免费   162篇
  2018年   3907篇
  2016年   5304篇
  2015年   6926篇
  2014年   8174篇
  2013年   11268篇
  2012年   12831篇
  2011年   13274篇
  2010年   9099篇
  2009年   8445篇
  2008年   12138篇
  2007年   12643篇
  2006年   11860篇
  2005年   11334篇
  2004年   11362篇
  2003年   10652篇
  2002年   10471篇
  2001年   17434篇
  2000年   17450篇
  1999年   13930篇
  1998年   5026篇
  1997年   5276篇
  1996年   4915篇
  1995年   4622篇
  1994年   4489篇
  1993年   4537篇
  1992年   11615篇
  1991年   11539篇
  1990年   11299篇
  1989年   10916篇
  1988年   10500篇
  1987年   10084篇
  1986年   9366篇
  1985年   9240篇
  1984年   7754篇
  1983年   6707篇
  1982年   5162篇
  1981年   4621篇
  1980年   4466篇
  1979年   7422篇
  1978年   5866篇
  1977年   5400篇
  1976年   5205篇
  1975年   5596篇
  1974年   6302篇
  1973年   6172篇
  1972年   5768篇
  1971年   5235篇
  1970年   4636篇
  1969年   4576篇
  1968年   4426篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
82.
From a variety of localities 14 strains of Beggiatoa, 1 ¼–3μ wide, were isolated in axenic heterotrophic culture. Most of these were freshwater forms, 2 were from brackish water, 1 was marine. The widths of the individual strains were constant, independent of conditions. The nutritional requirements of most of the strains are simple. Acetate at low concentrations, an ammonium salt as nitrogen source and the usual inorganic salts including trace elements supported growth. A few strains did not grow well without addition of an amino acid, and 2 (identical) strains required peptone or beef extract. Lactate, succinate, or pyruvate could often replace acetate. Multiplication was in most cases also possible with amino acids alone, without a further organic substrate. The appearance of the various strains on agar plates differs characteristically. Two types could be discerned: one forms spirals and one grows in tongues. These 2 types are not homogeneous for there are within them differences in width, growth rate, nutrition, and salt tolerance, so that a considerable number of independent forms exist even within the narrow limits in width of trichomes to which the investigations were restricted.  相似文献   
83.
84.
85.
86.
87.
88.
89.
90.
The cell surface of the parasitic protozoan Leishmania mexicana is coated by glycosylphosphatidylinositol (GPI)-anchored glycoproteins, a GPI-anchored lipophosphoglycan and a class of free GPI glycolipids. To investigate whether the anchor or free GPIs are required for parasite growth we cloned the L.mexicana gene for dolichol-phosphate-mannose synthase (DPMS) and attempted to create DPMS knockout mutants by targeted gene deletion. DPMS catalyzes the formation of dolichol-phosphate mannose, the sugar donor for all mannose additions in the biosynthesis of both the anchor and free GPIs, except for a alpha1-3-linked mannose residue that is added exclusively to the free GPIs and lipophosphoglycan anchor precursors. The requirement for dolichol-phosphate-mannose in other glycosylation pathways in L.mexicana is minimal. Deletion of both alleles of the DPMS gene (lmdpms) consistently resulted in amplification of the lmdpms chromosomal locus unless the promastigotes were first transfected with an episomal copy of lmdpms, indicating that lmdpms, and possibly GPI biosynthesis, is essential for parasite growth. As evidence presented in this and previous studies indicates that neither GPI-anchored glycoproteins nor lipophosphoglycan are required for growth of cultured parasites, it is possible that the abundant and functionally uncharacterized free GPIs are essential membrane components.  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