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   20416篇
  免费   1871篇
  国内免费   1436篇
  23723篇
  2023年   243篇
  2022年   541篇
  2021年   914篇
  2020年   617篇
  2019年   702篇
  2018年   743篇
  2017年   550篇
  2016年   732篇
  2015年   1221篇
  2014年   1336篇
  2013年   1446篇
  2012年   1672篇
  2011年   1589篇
  2010年   1060篇
  2009年   889篇
  2008年   982篇
  2007年   937篇
  2006年   847篇
  2005年   722篇
  2004年   677篇
  2003年   607篇
  2002年   567篇
  2001年   452篇
  2000年   445篇
  1999年   413篇
  1998年   191篇
  1997年   196篇
  1996年   176篇
  1995年   139篇
  1994年   171篇
  1993年   105篇
  1992年   191篇
  1991年   176篇
  1990年   154篇
  1989年   118篇
  1988年   102篇
  1987年   122篇
  1986年   93篇
  1985年   113篇
  1984年   55篇
  1983年   59篇
  1982年   57篇
  1981年   40篇
  1980年   47篇
  1979年   62篇
  1978年   49篇
  1977年   54篇
  1976年   38篇
  1975年   41篇
  1974年   50篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Binding and energy transfer of an erbium ion with rhaponticin (RH) was investigated and a simple spectrofluorimetric method was developed for determination of RH using an Er3+ probe. Results using spectra, chromatography and density functional theory (DFT) indicated that binding of Er3+ to RH occurred at the hydroxyl group of C‐11 and at the methoxy group at C‐12 of the conjugated aromatic ring through hydrogen bonding. Resonance energy transfer (RET) occurred from intramolecular charge transfers (ICT), the binding of RH to Er3+ ion induced fluorescence enhancement (FE) of the Er3+ ion. This method was applied to the pharmacokinetic detection of RH. The main advantages of the proposed method compared with previously reported ones are its simplicity and lower cost. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   
992.
Two novel 1D complexes, [Ni(hto){Au(CN)2}2][Ni(hto)](ClO4)2 · 4H2O (hto = 1,3,6,9,11,14-hexaazatricyclo[12.2.1.16,9]octadecane, in Scheme 1.) (1) and [Co(dien)2][{Au(CN)2}3] · 2H2O (dien = diethylenetriamine) (2) have been synthesized and characterized. The crystal structure of 1 confirmed the presence of nickel (II) chains via Au-Au interactions. The coordination geometry around the six-coordinate nickel (II) ion is slightly distorted octahedral with four nitrogen atoms of the macrocycle and two nitrogen atoms of the cyanides. The presence of Au-Au interactions have effectively increased the zero dimensional coordination polymer to a one dimensional system. In the complex 2, there are one cation, three anions, and two water molecules in the asymmetric unit. These components are interconnected through a combination of aurophilic attractions and hydrogen bonds and formed 1D chains.  相似文献   
993.
994.
Mutations in the ATM gene (mutated in ataxia telangiectasia) in both humans and mice predispose to lymphoid tumors. A defect in this gene also causes neurodegeneration in humans and a less severe neurological phenotype in mice. There is some evidence that oxidative stress contributes to these defects, suggesting that antioxidants could alleviate the phenotype. We demonstrate here that the antioxidant 5-carboxy-1,1,3,3-tetramethylisoindolin-2-yloxyl (CTMIO) dramatically delays the onset of thymic lymphomas in Atm(-/-) mice which is not due to an enhancement of apoptosis by CTMIO. We also show that this compound corrects neurobehavioral deficits in these mice and reduces oxidative damage to Purkinje cells. The likely mechanism of action of CTMIO is due to a reduction in oxidative stress, which is protective against both the tumor progression and the development of neurological abnormalities. These data suggest that antioxidant therapy has considerable potential in the management of ataxia telangiectasia and possibly other neurodegenerative disorders where oxidative stress is implicated.  相似文献   
995.
Pctaire1, a member of the cyclin-dependent kinase (Cdk)-related family, has recently been shown to be phosphorylated and regulated by Cdk5/p35. Although Pctaire1 is expressed in both neuronal and non-neuronal cells, its precise functions remain elusive. We performed a yeast two-hybrid screen to identify proteins that interact with Pctaire1. N-Ethylmaleimide-sensitive fusion protein (NSF), a crucial factor in vesicular transport and membrane fusion, was identified as one of the Pctaire1 interacting proteins. We demonstrate that the D2 domain of NSF, which is required for the oligomerization of NSF subunits, binds directly to and is phosphorylated by Pctaire1 on serine 569. Mutation of this phosphorylation site on NSF (S569A) augments its ability to oligomerize. Moreover, inhibition of Pctaire1 activity by transfecting its kinase-dead (KD) mutant into COS-7 cells enhances the self-association of NSF. Interestingly, Pctaire1 associates with NSF and synaptic vesicle-associated proteins in adult rat brain. To investigate whether Pctaire1 phosphorylation of NSF is involved in regulation of Ca(2+)-dependent exocytosis, we examined the effect of expressing Pctaire1 or NSF phosphorylation mutants on the regulated secretion of growth hormone from PC12 cells. Interestingly, expression of either Pctaire1-KD or NSF-S569A in PC12 cells significantly increases high K(+)-stimulated growth hormone release. Taken together, our findings provide the first demonstration that Pctaire1 phosphorylation of NSF regulates the ability of NSF to oligomerize, implicating an unexpected role of this kinase in modulating exocytosis. These findings open a new avenue of research in studying the functional roles of Pctaire1 in the nervous system.  相似文献   
996.
金城 《微生物学通报》2016,43(9):2114-2114
正香菇(Lentinula edodes)是产于北半球温带与亚热带地区的一种腐生真菌,不仅营养丰富,而且具有较高的保健价值,自古以来深受人们的喜爱[1]。但香菇属中低温型变温结实性菇类,在福建、浙江等香菇主产区,因夏季温度较高,能够高温出菇的香菇品种少,导致夏季鲜香菇供应不足。目前可用于夏季香菇栽培的方法主要有两种:一是在海拔500 m以上的高山上栽培;二是利用林下或山沟出菇、覆土栽培出菇等。但这两种栽培方法也有其局限性。因此,要从根本上解决香菇高温栽培的问  相似文献   
997.
滨海白首乌(耳叶牛皮消)药理活性研究进展   总被引:1,自引:0,他引:1  
滨海白首乌(耳叶牛皮消)是一种传统中药材,为萝藦科鹅绒藤属植物耳叶牛皮消的块根。耳叶牛皮消具有抗肿瘤、调节免疫、抗氧化、抗衰老、调血脂、促进毛发生长、保护脏器等多种药理活性。本文分析了耳叶牛皮消药理活性,提出了今后耳叶牛皮消研究与开发利用方向。  相似文献   
998.
本实验通过扫描电镜等方法研究豚鼠卵巢囊及卵巢囊淋巴孔的结构,并探讨了卵巢囊及其淋巴孔的种属差异.实验结果首次报道豚鼠卵巢囊内、外层上皮均存在淋巴孔;豚鼠卵巢囊结构在输卵管走行、囊闭合程度及囊表面超微结构等方面与金仓鼠存在差异.结果提示豚鼠卵巢囊内、外层淋巴孔是沟通卵巢囊腔、卵巢囊淋巴系及腹膜腔的形态基础,可能是三者间物质转运的重要途径.卵巢囊淋巴孔可能影响物种的繁殖并参与囊腔内局部免疫反应.卵巢囊结构和发育程度与对应的输卵管伞端等结构及其他生殖特点相适应,研究结果丰富了生殖形态学和比较解剖学资料.  相似文献   
999.
1000.
We have constructed a genetic linkage map for Nicotiana plumbaginifolia/Nicotiana longiflora (2n= 2x=20), based on the segregation of 69 RFLP and 102 RAPD loci in 99 F2 plants from the cross N. plumbaginifolia×N. longiflora. The map consists of nine major linkage groups, each containing more than nine marker loci, and spans 1062 cM. Twenty of the RFLP markers were mapped previously to Nicotiana sylvestris (2n=2x=24) chromosomes using monosomic alien addition lines. Taxonomically, N. plumbaginifolia and N. sylvestris belong to the same section, namely the Alatae; however, cytogenetic evidence indicates that they are not closely related. Comparison of the distribution of markers common to both maps suggests that genome reorganization has occurred during the evolution of these two species. Evidence is also presented that genome reorganization may be accompanied by gain and loss of specific classes of DNA sequences in their genomes.  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