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   285932篇
  免费   31554篇
  国内免费   257篇
  2016年   3254篇
  2015年   4604篇
  2014年   5141篇
  2013年   7754篇
  2012年   8385篇
  2011年   8615篇
  2010年   5697篇
  2009年   5290篇
  2008年   7778篇
  2007年   7957篇
  2006年   7541篇
  2005年   7287篇
  2004年   7144篇
  2003年   6917篇
  2002年   6678篇
  2001年   12014篇
  2000年   12055篇
  1999年   9600篇
  1998年   3523篇
  1997年   3726篇
  1996年   3642篇
  1995年   3238篇
  1994年   3226篇
  1993年   3230篇
  1992年   8075篇
  1991年   8047篇
  1990年   7705篇
  1989年   7664篇
  1988年   7038篇
  1987年   6783篇
  1986年   6186篇
  1985年   6398篇
  1984年   5317篇
  1983年   4465篇
  1982年   3497篇
  1981年   3249篇
  1980年   3052篇
  1979年   5115篇
  1978年   3943篇
  1977年   3828篇
  1976年   3564篇
  1975年   3904篇
  1974年   4281篇
  1973年   4216篇
  1972年   3792篇
  1971年   3545篇
  1970年   3191篇
  1969年   3077篇
  1968年   2828篇
  1967年   2451篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
942.
943.
944.
The localization of the elongation factor Tu (EF-Tu) in ultrathin cryosections of E. coli cells was determined with the electron microscope using a highly specific immunological labelling technique. EF-Tu is distributed almost homogeneously throughout the cytoplasm. Although it has often been suggested that EF-Tu could be part of a putative prokaryotic cytoskeleton, we did not find any evidence for supramolecular assemblies, such as fibres or filaments, containing a large amount of EF-Tu. EF-Tu was not observed in association with the outer cell membrane and periplasmic space. A topological relationship with the inner membrane is not apparent in our micrographs. In cells in which the EF-Tu level is raised significantly, the protein piles up in discrete cell regions.  相似文献   
945.
946.
Specific antibodies and protein proteinase inhibitors will inhibit cell-surface proteinase activity on human fibroblasts and cause a concomitant inhibition of DNA synthesis and of cell multiplication. An insolubilized proteinase inhibitor also inhibits cell multiplication. The same reagents partially inhibit the multiplication of mouse L cells, both in monolayer and suspension culture, and inhibit the mitogenic effect of epidermal growth factor (EGF) on both types of cell.  相似文献   
947.
Four of six human monoclonal IgM proteins were found to react best with Klebsiella polysaccharides containing 3,4py beta DGal (pyruvic acetalated D-galactopyranose), one with Klebsiella polysaccharides with 4,6pyDGlc; the sixth is uncharacterized. The combining sites of two of these (IgMWEA and IgMNAE) were essentially indistinguishable by quantitative precipitin studies at varying pH and by quantitative precipitin inhibition assays, but the other two differed in specificity of their combining sites from these and from each other. These differences were detected by precipitin inhibition assays with 3,4py beta DGal-containing oligosaccharide alditols, the R and S isomers of methyl 4,6py alpha DGal, the R isomer of methyl 4,6py beta DGal, or the R and S isomers of methyl 4,6py alpha DGlc, and -beta DGlc. In all of these except the S isomer of methyl 4,6pyDGal and R isomer of methyl 4,6pyDGlc, the carboxyl group is axial to the plane of the acetal ring. Their specificity appears to be determined by the nonreducing ends of chains and is considered to be cavity-type.  相似文献   
948.
The amount of radioactivity which derived from 14C-labeled pollutants was determined in liver, kidney, intestine, blood, muscle and gills of carp, exposed for 6, 24 and 72 hr to high external concentrations of urea, methanol, atrazine and PCP. The results allowed one to calculate roughly the uptake rate for these compounds. It was low for urea (0.055 micrograms/g per hr), higher for methanol (0.12) and atrazine (0.16) and highest for PCP (1.5). The bioaccumulation factors (BFs) were determined for the different substances and organs. They correlated with the hydrophilic-lipophilic nature of the chemicals. The more lipophilic the substances the more accumulation occurred in the liver. PCP accumulated the most. BF was 300-400 in most tissues except muscle where it was quite low. The BF was 3-4 for atrazine in liver, kidney and intestine, but just 1 in blood, muscle and gills. There is some evidence that the BF for methanol equals 1 in liver, kidney, gills and intestine. It is less than 1 in blood and muscle. Urea was equally distributed in all organs and in the external medium.  相似文献   
949.
Crude extracts of Streptomyces griseus grown on soybean flour-enriched medium contain high levels of cytochrome P-450. The cytochrome P-450-enriched fractions, obtained by ammonium sulfate fractionation (30-50% saturation), catalyze the NADPH-dependent oxidation of a variety of xenobiotics when complemented with both spinach ferredoxin:NADP+ oxidoreductase and spinach ferredoxin. Reactions observed are aromatic, benzylic and alicyclic hydroxylations, O-dealkylation, non-aromatic double bond epoxidation, N-oxidation and N-acetylation.  相似文献   
950.
In this paper, we describe an efficient procedure for the purification of yeast phosphofructokinase. This procedure eliminates any time delay and enables to obtain an enzyme with minimum proteolytic alterations. The molecular weights of the oligomeric enzyme and of its constitutive subunits were both evaluated by means of several independent methods. However, the accuracy of each measurement was not sufficient to discriminate between an hexameric and an octameric structure of the enzyme oligomer. On the other hand, crosslinking experiments demonstrated the octameric structure of yeast phosphofructokinase. Obviously, some methods of molecular weight determination have led to erroneous results. In particular, our experiments show that the reliability of molecular weight determinations performed by gel filtration of native proteins must be considered with caution.  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