全文获取类型
收费全文 | 71760篇 |
免费 | 6372篇 |
国内免费 | 131篇 |
出版年
2022年 | 362篇 |
2021年 | 992篇 |
2020年 | 654篇 |
2019年 | 797篇 |
2018年 | 1031篇 |
2017年 | 961篇 |
2016年 | 1635篇 |
2015年 | 2746篇 |
2014年 | 2924篇 |
2013年 | 3792篇 |
2012年 | 4797篇 |
2011年 | 4775篇 |
2010年 | 3107篇 |
2009年 | 2806篇 |
2008年 | 3993篇 |
2007年 | 4033篇 |
2006年 | 3760篇 |
2005年 | 3760篇 |
2004年 | 3682篇 |
2003年 | 3445篇 |
2002年 | 3388篇 |
2001年 | 1843篇 |
2000年 | 1506篇 |
1999年 | 1436篇 |
1998年 | 1076篇 |
1997年 | 817篇 |
1996年 | 773篇 |
1995年 | 736篇 |
1994年 | 722篇 |
1993年 | 700篇 |
1992年 | 900篇 |
1991年 | 784篇 |
1990年 | 671篇 |
1989年 | 622篇 |
1988年 | 598篇 |
1987年 | 475篇 |
1986年 | 486篇 |
1985年 | 505篇 |
1984年 | 506篇 |
1983年 | 437篇 |
1982年 | 487篇 |
1981年 | 437篇 |
1980年 | 384篇 |
1979年 | 278篇 |
1978年 | 322篇 |
1977年 | 304篇 |
1976年 | 268篇 |
1975年 | 251篇 |
1974年 | 259篇 |
1973年 | 258篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
The phylogeny of the fungus gnat family Mycetophilidae (Diptera) is reconstructed with a focus on the species‐rich and taxonomically difficult subfamilies Gnoristinae and Mycomyinae. The multigene phylogenetic analyses are based on five nuclear (18S, 28S, CAD, MCS, ITS2) and four mitochondrial (12S, 16S, COI, CytB) gene markers. The analyses strongly support the monophyly of Mycetophilidae and the subfamilies Manotinae, Sciophilinae, Leiinae, and Mycomyinae, although Gnoristinae is paraphyletic with respect to Mycetophilinae. All the genera and groups of genera included are supported as monophyletic, except for Acomoptera Vockeroth, Boletina Staeger, Dziedzickia Johannsen, Ectrepesthoneura Enderlein, and Neoempheria Osten Sacken. Ancestral character state reconstructions were applied to two morphological features present in Gnoristinae and Mycomyinae (i.e. presence of setae on wing membrane and wing vein R4) in order to assess their evolution. The wing vein R4 appears as an unstable character, spread throughout different clades. A dated phylogeny of the family Mycetophilidae showed that most of the subfamilies of Mycetophilidae originated and diversified during the Cretaceous. The youngest subfamilies, originated in the Paleogene, appear to be Mycomyinae and Mycetophilinae. 相似文献
2.
The effect of the alkyl side chain of the ubiquinone molecule on the electron-transfer activity of ubiquinone in mitochondrial succinate-cytochrome c reductase is studied by using synthetic ubiquinone derivatives that possess the basic ubiquinone structure of 2,3-dimethoxy-5-methyl-1,4-benzoquinone with different alkyl side chains at the 6-position. The alkyl side chains vary in chain length, degree of saturation, and location of double bonds. When a ubiquinone derivative is used as an electron acceptor for succinate-ubiquinone reductase, an alkyl side chain of six carbons is needed to obtain the maximum activity. However, when it serves as an electron donor for ubiquinol-cytochrome c reductase or as a mediator in succinate-cytochrome c reductase, an alkyl side chain of 10 carbons gives maximal efficiency. Introduction of one or two isolated double bonds into the alkyl side chain of the ubiquinone molecule has little effect on electron-transfer activity. However, a conjugated double bond system in the alkyl side chain drastically reduces electron-transfer efficiency. The effect of the conjugated double bond system on the electron-transferring efficiency of ubiquinone depends on its location in the alkyl side chain. When location is far from the benzoquinone ring, the effect is minimal. These observations together with the results obtained from photoaffinity-labeling studies lead us to conclude that flexibility in the portion of the alkyl side chain immediately adjacent to the benzoquinone ring is required for the electron-transfer activity of ubiquinone. 相似文献
3.
This paper is concerned with gene survival in a population which may increase without density dependence according to a generalization of the Moran model for haploid individuals. A selective advantage to one allele and the possibility of differential reproductive rates are allowed. Simple conditions are given for ultimate homozygosity to be certain and for the possibility of ultimate polymorphism. The results complement and extend those of Heyde (1981, 1982). 相似文献
4.
5.
6.
7.
8.
9.
Peter Buchwald 《Theoretical biology & medical modelling》2009,6(1):5-13
Background
The function and viability of cultured, transplanted, or encapsulated pancreatic islets is often limited by hypoxia because these islets have lost their vasculature during the isolation process and have to rely on gradient-driven passive diffusion, which cannot provide adequate oxygen transport. Pancreatic islets (islets of Langerhans) are particularly susceptible due to their relatively large size, large metabolic demand, and increased sensitivity to hypoxia. Here, finite element method (FEM) based multiphysics models are explored to describe oxygen transport and cell viability in avascular islets both in static and in moving culture media. 相似文献10.
The reaction of oxygen with hemoglobin and the kinetic basis of the effect of salt on binding of oxygen 总被引:7,自引:0,他引:7
Q H Gibson 《The Journal of biological chemistry》1970,245(13):3285-3288