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   724047篇
  免费   81131篇
  国内免费   899篇
  806077篇
  2018年   6512篇
  2017年   6222篇
  2016年   8908篇
  2015年   12685篇
  2014年   14795篇
  2013年   20325篇
  2012年   23610篇
  2011年   23934篇
  2010年   16091篇
  2009年   14623篇
  2008年   21180篇
  2007年   21958篇
  2006年   20542篇
  2005年   19629篇
  2004年   19493篇
  2003年   18476篇
  2002年   18168篇
  2001年   33352篇
  2000年   33568篇
  1999年   26475篇
  1998年   9162篇
  1997年   9319篇
  1996年   8855篇
  1995年   8393篇
  1994年   8088篇
  1993年   7928篇
  1992年   21858篇
  1991年   21554篇
  1990年   20894篇
  1989年   20127篇
  1988年   18655篇
  1987年   17484篇
  1986年   16456篇
  1985年   16200篇
  1984年   13272篇
  1983年   11569篇
  1982年   8639篇
  1981年   7690篇
  1980年   7145篇
  1979年   12585篇
  1978年   9899篇
  1977年   8841篇
  1976年   8201篇
  1975年   9482篇
  1974年   10171篇
  1973年   9942篇
  1972年   8960篇
  1971年   8124篇
  1970年   7075篇
  1969年   6820篇
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
961.
During an HIV-1 prevention clinical trial in East Africa, we observed 16 cases of primary HIV-1 infection in women coincident with pregnancy or breastfeeding. Nine of eleven pregnant women initiated rapid combination antiretroviral therapy (ART), despite having CD4 counts exceeding national criteria for ART initiation; breastfeeding women initiated ART or replacement feeding. Rapid ART initiation during primary HIV-1 infection during pregnancy and breastfeeding is feasible in this setting.  相似文献   
962.
A Gram-positive Rhodococcus erythropolis strain S1 was shown to assimilate aromatic amino acids such as L-phenylalanine, L-tyrosine, L-tryptophan, D-phenylalanine, D-tyrosine and D-tryptophan, which were utilized not only as the sole carbon source but also as a suitable nitrogen source. The highest growth on these aromatic amino acids occurred at a temperature of 30°C. L-Phenylalanine, L-tyrosine and L-tryptophan degradative pathways would appear to be independent, and to be induced alternatively. The strain S1 also showed the ability to assimilate peptides which consisted of only L-phenylalanine and L-tyrosine.  相似文献   
963.
964.
965.
966.
The biting midge Culicoides sonorensis Wirth and Jones (Diptera: Ceratopogonidae) transmits pathogens to both livestock and wildlife. Biting midge surveillance relies heavily on light traps for collection; however, little is known about the light spectra preferences of C. sonorensis midges. A light assay arena was constructed and light‐emitting diodes (LEDs) of various light spectra were used as light sources to evaluate midge photoattraction. A comparison of responses to light spectra indicated the highest proportions of C. sonorensis were attracted to ultraviolet (UV) light and that midges differentiated 10‐nm differences in wavelength. Stronger intensities of UV light resulted in greater attraction. Midges exhibited both sugar‐seeking and escape behaviours under different conditions of sugar supplementation before and during the experiment. These behaviours occurred with lights of 355 nm and 365 nm in wavelength. Based on the results of this study, the attraction of C. sonorensis to light traps can be improved through the use of bright LEDs at 355 nm or 365 nm.  相似文献   
967.
968.
Development of national research of cellulose-degrading microorganisms and enzymes is reviewed, with emphasis on the prospects of producing ethanol from cellulose materials using cellulolytic enzymes. Leading Russian research groups in this field are introduced. A section of the review analyzes problems and prospects of setting up environmentally friendly production of motor biofuels from renewable raw materials of plant origin (an approach developed in Russia).  相似文献   
969.
Metabolic and antiproliferative effects of methylglyoxal bis(butylamidinohydrazone) (MGBB) and methylglyoxal bis(cyclopentylamidinohydrazone) (MGBCP), inhibitors for polyamine biosynthetic pathway, on Escherichia coli, Shigella sonnei, Aeromonas sobria, Aeromonas hydrophila and Vibrio cholerae were investigated. MGBB at the concentration of 100 mumol/l depleted intracellular putrescine and spermidine concentrations of E. coli to 25 and 20% of the controls, respectively, while MGBCP depressed their concentrations to 38 and 24%, respectively. In these polyamine-depleted E. coli cells the syntheses of RNA, DNA and protein decreased to 13, 54 and 29% of the control, respectively, with MGBB and to 23, 71 and 55%, respectively, with MGBCP. The minimum inhibitory concentrations (MIC) of MGBB for the growth of A. sobria, E. coli, A. hydrophila, V. cholerae and Sh. sonnei were estimated to be 50, 160, 240, 285 and 320 mumol/l, respectively, whereas those of MGBCP were slightly higher for respective bacteria.  相似文献   
970.
Recent reports suggest that prostaglandins, rather than cAMP, play a major role in mediating cholera toxin-induced water and electrolyte secretion from rabbit intestinal loops. We examined the role of prostaglandins in mediating toxin-induced pancreatic and gastric exocrine secretion. In these tissues, indomethacin, a potent inhibitor of prostaglandin synthesis, did not alter the stimulatory effects of cholera toxin on increases in cellular cAMP or enzyme secretion. Moreover, the addition of cholera toxin did not alter prostaglandin E2 release from either tissue. In contrast to their effects in rabbit intestinal loops, prostaglandins do not regulate cholera toxin-induced enzyme secretion from the guinea pig pancreas or stomach.  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