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   418942篇
  免费   35636篇
  国内免费   301篇
  454879篇
  2018年   4877篇
  2017年   4667篇
  2016年   6090篇
  2015年   6359篇
  2014年   8134篇
  2013年   11604篇
  2012年   13173篇
  2011年   14107篇
  2010年   9706篇
  2009年   8590篇
  2008年   12427篇
  2007年   12840篇
  2006年   12263篇
  2005年   11576篇
  2004年   11468篇
  2003年   11123篇
  2002年   10977篇
  2001年   18988篇
  2000年   18845篇
  1999年   14580篇
  1998年   4462篇
  1997年   4460篇
  1996年   4288篇
  1995年   3910篇
  1994年   3814篇
  1992年   11405篇
  1991年   11319篇
  1990年   11308篇
  1989年   11097篇
  1988年   10316篇
  1987年   9704篇
  1986年   8745篇
  1985年   9268篇
  1984年   7341篇
  1983年   6227篇
  1982年   4341篇
  1981年   3912篇
  1980年   3728篇
  1979年   6792篇
  1978年   5174篇
  1977年   4781篇
  1976年   4664篇
  1975年   5475篇
  1974年   5949篇
  1973年   5965篇
  1972年   5278篇
  1971年   4966篇
  1970年   4306篇
  1969年   4092篇
  1968年   3865篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
993.
994.
The frequency of spontaneous reversion in r-mutants of phage T4 is determined in order to study the influence of mutant RNA polymerase on the level of spontaneous mutations in phage T4. The rate of spontaneous mutagenesis in the phage is found to be increased in strains with mutant RNA polymerase.  相似文献   
995.
996.
Modifications of existing methods have allowed for the isolation and purification of various species of plasma glycosaminoglycans on the basis of their sulfate content and molecular size. All of the preparations precipitated human plasma low density lipoproteins (LDL); maximal precipitation occurred with amounts of glycans corresponding to 50 mug of hexuronate and 12 mg of LDL. The interaction of glycans with pyrene-labeled lipoproteins was also studied, measuring variations of the fluorescence emitted by the monomer (M) and excimer (E) species of the bound pyrene. The ratio IE/IM is proportional to c/eta, where c is the microscopic concentration of the pyrene confined to the hydrocarbon region of the lipoprotein and eta is the microviscosity of that region. To 0.12 mg of pyrene-labeled LDL, very low density lipoproteins (VLDL) or high density lipoproteins (HDL) were added increasing amounts of the various glycan preparations. The sulfate-rich species decreased the IE/IM ratio of LDL and HDL but not that of VLDL. This finding suggests that the glycan caused a change in lipoprotein conformation associated with either an increased volume or increased microscopic viscosity of the hydrocarbon region. The modification of LDL conformation could be prevented by proteolytic treatment of the sulfate-rich species or by addition to the system of suitable amounts of sulfate-poor species or of chrondroitin-4-sulfate, but could not be prevented by increased ionic concentration. These results suggest that the two main species of plasma glycans are important in maintaining adequate rheological properties of plasma lipoproteins.  相似文献   
997.
Evolutionary stability of the histone genes of sea urchins   总被引:1,自引:0,他引:1  
M N Farguhar  B J McCarthy 《Biochemistry》1973,12(21):4113-4122
  相似文献   
998.
999.
The paper studies mycobiota of the dead insects in West Siberia, Primorsky krai, and Kyrgyzstan. Ascomycetes anamorphs of 13 genera are revealed. In all regions Beauveria bassiana (Bals.-Criv.) Vuill. dominated comprising on average 68% of the total number of isolates. The fungus hosts list the insects of 7 orders and 32 families with Coleoptera, Lepidoptera, and Hemiptera dominating. The rarely found entomopathogens include Tolypocladium inflatum Gams (primarily on Lepidoptera), Metarhizium anisopliae (Metschn.) Sorokin (on Coleoptera). The mortality rate of the insects due to micromycetes is observed mainly on enzootic level. The study of the pathogenic properties of the dominating species (B. bassiana) show the absence of specificity of its environmental isolates for a number of representatives of Orthoptera, Lepidoptera, Coleoptera, and Diptera.  相似文献   
1000.
The influence of several polymeric additives on specific glucose uptake rate of flocs of a S. cerevisiae strain — S. cerevisiae NRRLY 265 was studied. A special continuous membrane microreactor was used to measure glucose uptake on the presence of calcium and of the tested additives — two cationic polymers — bis(polyoxyethylene-bis(amine)) 20,000 and BPA 1,000 and one anionic polymer — Magna Floc LT25.An increase on glucose uptake rate was always observed when comparing with calcium bound flocs. For bis(polyoxyethylene-bis(amine)) 20,000 the increase was only 19% but for BPA 1,000 a value of more than 50% was observed. For Magna Floc LT25 a two fold increase was measured.The determination of floc size and porosity in the presence of the additives indicated that, on the basis of these parameters, it was not possible to explain the observed glucose uptake rates. The floc porosites in additive bound flocs were similar and 10% larger than for calcium bound flocs and glucose uptake rate was larger for the largest flocs — Magna Floc LT25 bound flocs were the largest followed by BPA 1,000, bis(polyoxyethylene-bis(amine)) 20,000 and calcium bound flocs. These values disagree with what should be expected in diffusion controlled processes.The calculation of intercellular floc distance indicated that polymeric additives act on the reduction of diffusional limitations by increasing the available flux area for glucose inside the flocs. By analysing different kinds of packings, it was also observed that the packing arrangement for yeast cells in flocs is close to the cubic packing. The simulation of this arrangement for the obtained floc sizes confirmed that the 10% increase in floc porosity is sufficient to explain the increase in the available flux area.  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