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   3170篇
  免费   362篇
  国内免费   7篇
  2021年   32篇
  2019年   34篇
  2018年   40篇
  2017年   41篇
  2016年   54篇
  2015年   75篇
  2014年   97篇
  2013年   108篇
  2012年   131篇
  2011年   137篇
  2010年   102篇
  2009年   84篇
  2008年   134篇
  2007年   137篇
  2006年   113篇
  2005年   147篇
  2004年   102篇
  2003年   102篇
  2002年   118篇
  2001年   93篇
  2000年   110篇
  1999年   73篇
  1998年   34篇
  1997年   41篇
  1996年   36篇
  1995年   33篇
  1994年   36篇
  1993年   39篇
  1992年   49篇
  1991年   58篇
  1990年   51篇
  1989年   50篇
  1988年   48篇
  1987年   48篇
  1986年   47篇
  1985年   37篇
  1984年   53篇
  1983年   42篇
  1982年   40篇
  1981年   38篇
  1980年   33篇
  1979年   36篇
  1978年   29篇
  1976年   35篇
  1975年   44篇
  1974年   31篇
  1973年   46篇
  1972年   35篇
  1971年   31篇
  1968年   29篇
排序方式: 共有3539条查询结果,搜索用时 15 毫秒
71.
72.
An additive relationship of lethality between purified protease and haemolysin of the extracellular products (ECP) of Aeromonas salmonicida was demonstrated by i.p. injection in Atlantic salmon (Salmo salar L.). The lethal toxicity of the combinations of protease and haemolysin follow a linear regression line y = -54.54x + 2400. The LD50 of protease and haemolysin when injected separately was 2400 ng/g fish and 44 ng protein/g fish, respectively.  相似文献   
73.
Human hepatoma HepG2 cells were used to demonstrate coordinate regulation of three enzymes of cholesterol synthesis under a variety of conditions. Addition of either delipidized serum and mevinolin or low density lipoprotein, 25-hydroxycholesterol, or mevalonic acid to HepG2 cells resulted in rapid changes both in the levels of the mRNAs and in the rates of synthesis of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) synthase, HMG-CoA reductase, and farnesyl pyrophosphate synthetase (prenyltranferase). In all cases, the changes in mRNA levels were paralleled by changes in the rates of specific protein synthesis. Pulse-chase techniques were used to determine the half-lives of all three proteins. Addition of low density lipoprotein to the media during the chase increased the rate of degradation of HMG-CoA reductase 4.6-fold but had no affect on the half-lives of HMG-CoA synthase or prenyltransferase. Therefore, we conclude that the coordinate regulation of these three enzymes under a variety of conditions occurs at the level of enzyme synthesis and not at the level of protein stability.  相似文献   
74.
There is evidence that three inhibitors of Na,K-ATPase activity--ouabain, K-free extracellular fluid, and vanadate--inhibit renin secretion by increasing Ca2+ concentration in juxtaglomerular cells, but in the case of vanadate, it is uncertain whether the increase in Ca2+ is due to a decrease in Ca2+ efflux (inhibition of Ca-ATPase activity, or inhibition of Na,K-ATPase activity, followed by an increase in intracellular Na+ and a decrease in Na-Ca exchange) or to an increase in Ca2+ influx through potential operated Ca channels (inhibition of electrogenic Na,K transport, followed by membrane depolarization and activation of Ca channels). In the present experiments, the rat renal cortical slice preparation was used to compare and contrast the effects of ouabain, of K-free fluid, and of vanadate on renin secretion, in the absence and presence of methoxyverapamil, a Ca channel blocker. Basal renin secretory rate averaged 7.7 +/- 0.3 GU/g/60 min, and secretory rate was reduced to nearly zero by 1 mM ouabain, by K-free fluid, by 0.5 mM vanadate, and by K-depolarization (increasing extracellular K+ to 60 mM). Although 0.5 microM methoxyverapamil completely blocked the inhibitory effect of K-depolarization, it failed to antagonize the inhibitory effects of ouabain, of K-free fluid, and of vanadate. A concentration of methoxyverapamil two hundred times higher (100 microM) completely blocked the inhibitory effects of vanadate, but still failed to antagonize the effects of ouabain and of K-free fluid. Collectively, these observations demonstrate that vanadate-induced inhibition of renin secretion cannot be attributed entirely to Na,K-ATPase inhibition, since in the presence of methoxyverapamil, the effect of vanadate differed from the effects of either ouabain (a specific Na,K-ATPase inhibitor) or K-free fluid. Moreover, it cannot be attributed entirely to a depolarization-induced influx of Ca2+ through potential-operated Ca channels, since methoxyverapamil antagonized K-depolarization-induced inhibition of renin secretion much more effectively than it antagonized vanadate-induced inhibition.  相似文献   
75.
76.
Genetic approaches for studying rhizosphere colonization   总被引:3,自引:0,他引:3  
Most bacterial traits involved in colonization of plant roots are yet to be defined. Studies were initiated to identify genes in Pseudomonas which play significant roles in this process. The general approach is to use transposons to construct collections of insertion mutants, each of which is then screened for alterations in its interactions with the host plant. In one study a Tn5 derivative containing a constitutively expressed -galactosidase (lacZ) gene was used to generate a collection of insertion mutants which could be distinguished from the wild-type parent on X-gal plates. Each mutant was examined for its ability to colonize wheat seedlings in the presence of the wild-type parent. Mutants which gave wild-type:mutant ratio of 20:1 or greater were obtained. In a second study a Tn5 derivative which carries a promoterless lacZ gene located near one end of the transposon was constructed. Expression of the lacZ gene depends on the presence of an active promoter outside of the transposon in the correct orientation. Insertion mutants generated with this transposon were examined for changes in -galactosidase expression in the presence and absence of plant root exudate. A number of mutants which showed differential lacZ expression have been identified.  相似文献   
77.
Summary Aneuploid stocks, which included Triticum aestivum/alien, disomic, chromosome addition lines, wheat/alien, ditelosomic, chromosome addition lines, and the available aneuploids of Chinese Spring wheat, were used to locate genes that influence milling energy requirement (ME). Genes that affected ME were found on all seven homoeologous chromosome groups. The addition of complete wheat chromosomes 1B, 1D, 2A, 2D, 5B, 6B, 7B and 7D increased ME. Positive effects were also found in specific chromosome arms: 1BS, 2DS, 5AS, 5BS and 6BL. Wheat chromosome 3B conditioned low ME and the gene(s) responsible was located on the short arm. Other negative effects were attributed to wheat chromosome arms 4BL, 4DL, 5DS and 6DS. Alien chromosome additions that conferred high ME included 2H, 5H, 6H and 7H of barley, Hordeum vulgare and 2R, 2R, 4R, 4RL, 6R, 6RL and 7RL of rye, Secale cereale. Those that conferred a low ME included 1H ch of H. chilense, and 6u and 7u of Aegilops umbellulata, 5R and 5RS of S. cereale and 5R m and 5R mS of S. montanum. Although the control of ME is polygenic, there is a major effect of genes located on the short arms of homoeologous group 5 chromosomes.  相似文献   
78.
Splenocytes derived from mice inoculated with a commercial cellulase preparation or purified cellulases were fused with a stable myeloma cell line (SP2/0). Specific monoclonal antibodies to cellobiohydrolases I and II and endoglucanases I and II were established. In addition to specific monoclonal antibodies, we were also able to establish stable hybridoma cell lines which produced monoclonal antibodies that recognized similar epitopes possessed by two or more of the above cellulases. By obtaining monospecific antibodies for all four individual cellulases, the role and function of the individual cellulases can thus be studied in greater detail.  相似文献   
79.
The grass flora of Namibia (374 species in 110 genera) shows surprisingly little variation in 13C values along a rainfall gradient (50–600 mm) and in different habitat conditions. However, there are significant differences in the 13C values between the metabolic types of the C4 photosynthetic pathway. NADP-ME-type C4 species exhibit the highest 13C values (–11.7 ) and occur mainly in regions with high rainfall. NAD-ME-type C4 species have significantly lower 13C values (–13.4 ) and dominate in the most arid part of the precipitation regime. PCK-type C4 species play an intermediate role (–12.5 ) and reach a maximum abundance in areas of intermediate precipitation. This pattern is also evident in genera containing species of different metabolic types. Within the same genus NAD species reach more negative 13C values than PCK species and 13C values decreased with rainfall. Also in Aristida, with NADP-ME-type photosynthesis, 13C values decreased from –11 in the inland region (600 mm precipitation) to –15 near the coast (150 mm precipitation), which is a change in discrimination which is otherwise associated by a change in metabolism. The exceptional C3 species Eragrostis walteri and Panicum heterostachyum are coastal species experiencing 50 mm precipitation only. Many of the rare species and monotypic genera grow in moist habitats rather than in the desert, and they are not different in their carbon isotope ratios from the more common flora. The role of species diversity with respect to habitat occupation and carbon metabolism is discussed.  相似文献   
80.
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