全文获取类型
收费全文 | 3405篇 |
免费 | 220篇 |
专业分类
3625篇 |
出版年
2022年 | 16篇 |
2021年 | 32篇 |
2019年 | 28篇 |
2018年 | 37篇 |
2017年 | 35篇 |
2016年 | 76篇 |
2015年 | 92篇 |
2014年 | 130篇 |
2013年 | 133篇 |
2012年 | 221篇 |
2011年 | 247篇 |
2010年 | 186篇 |
2009年 | 140篇 |
2008年 | 186篇 |
2007年 | 187篇 |
2006年 | 166篇 |
2005年 | 171篇 |
2004年 | 150篇 |
2003年 | 174篇 |
2002年 | 157篇 |
2001年 | 40篇 |
2000年 | 29篇 |
1999年 | 55篇 |
1998年 | 61篇 |
1997年 | 45篇 |
1996年 | 58篇 |
1995年 | 40篇 |
1994年 | 44篇 |
1993年 | 31篇 |
1992年 | 52篇 |
1991年 | 32篇 |
1990年 | 42篇 |
1989年 | 28篇 |
1988年 | 20篇 |
1987年 | 16篇 |
1986年 | 15篇 |
1985年 | 18篇 |
1984年 | 27篇 |
1983年 | 23篇 |
1982年 | 30篇 |
1981年 | 32篇 |
1980年 | 21篇 |
1979年 | 34篇 |
1978年 | 23篇 |
1977年 | 19篇 |
1976年 | 20篇 |
1973年 | 14篇 |
1971年 | 15篇 |
1970年 | 16篇 |
1967年 | 14篇 |
排序方式: 共有3625条查询结果,搜索用时 15 毫秒
991.
Theissen U Hoffmeister M Grieshaber M Martin W 《Molecular biology and evolution》2003,20(9):1564-1574
Mitochondria occur as aerobic, facultatively anaerobic, and, in the case of hydrogenosomes, strictly anaerobic forms. This physiological diversity of mitochondrial oxygen requirement is paralleled by that of free-living alpha-proteobacteria, the group of eubacteria from which mitochondria arose, many of which are facultative anaerobes. Although ATP synthesis in mitochondria usually involves the oxidation of reduced carbon compounds, many alpha-proteobacteria and some mitochondria are known to use sulfide (H2S) as an electron donor for the respiratory chain and its associated ATP synthesis. In many eubacteria, the oxidation of sulfide involves the enzyme sulfide:quinone oxidoreductase (SQR). Nuclear-encoded homologs of SQR are found in several eukaryotic genomes. Here we show that eukaryotic SQR genes characterized to date can be traced to a single acquisition from a eubacterial donor in the common ancestor of animals and fungi. Yet, SQR is not a well-conserved protein, and our analyses suggest that the SQR gene has furthermore undergone some lateral transfer among prokaryotes during evolution, leaving the precise eubacterial lineage from which eukaryotes obtained their SQR difficult to discern with phylogenetic methods. Newer geochemical data and microfossil evidence indicate that major phases of early eukaryotic diversification occurred during a period of the Earth's history from 1 to 2 billion years before present in which the subsurface ocean waters contained almost no oxygen but contained high concentrations of sulfide, suggesting that the ability to deal with sulfide was essential for prokaryotes and eukaryotes during that time. Notwithstanding poor resolution in deep SQR phylogeny and lack of a specifically alpha-protebacterial branch for the eukaryotic enzyme on the basis of current lineage sampling, a single eubacterial origin of eukaryotic SQR and the evident need of ancient eukaryotes to deal with sulfide, a process today germane to mitochondrial quinone reduction, are compatible with the view that eukaryotic SQR was an acquisition from the mitochondrial endosymbiont. 相似文献
992.
Wiethölter N Graessner B Mierau M Mort AJ Moerschbacher BM 《Molecular plant-microbe interactions : MPMI》2003,16(10):945-952
Plants possess an efficient nonself surveillance system triggering induced disease resistance mechanisms upon molecular recognition of microbial invaders. Successful pathogens have evolved strategies to evade or counteract these mechanisms, e.g., by the generation of suppressors. Pectic fragments produced during host cell wall degradation can act as endogenous suppressors of the hypersensitive response in wheat leaves. We have isolated and characterized homogalacturonans from cell walls of two wheat cultivars susceptible to the stem rust fungus, Puccinia graminis f. sp. tritici, namely cvs. Prelude and Marquis, and from near-isogenic lines of both cultivars containing the Sr5-gene for hypersensitive rust resistance. Two independent approaches were used to compare their methyl esterification: i) immunochemistry using the monoclonal antibodies JIM5, JIM7, PAM1, and LM7 and ii) chromatography of oligogalacturonides representing stretches of contiguous nonmethyl-esterified GalA residues. The results clearly indicate a significant difference in the homogalacturonans from susceptible and resistant wheat lines. The difference can best be explained by assuming a nonrandom and more blockwise distribution of the methyl esters in the homogalacturonans of susceptible wheat cultivars as compared with a presumably more random distribution in the near-isogenic resistant lines. Possible consequences of this difference for the enzymatic generation of endogenous suppressors are discussed. 相似文献
993.
Lass-Flörl C Ledochowski M Fuchs D Speth C Kacani L Dierich MP Fuchs A Würzner R 《FEMS immunology and medical microbiology》2003,35(1):11-15
This study investigated whether the interaction between isolates of Candida albicans (n=7), Candida parapsilosis (n=3), Candida krusei (n=2), Candida dubliniensis (n=1) and sertraline, a typical selective serotonin reuptake inhibitor, alters candidal virulence. Sertraline treatment of Candida spp. significantly (P<0.05) affected hyphal elongation, phospholipase activity, production of secreted aspartyl proteinases and fungal viability. In addition, monocyte-derived macrophages (MDMs) treated with sertraline reduced inhibition of blastoconidia germination in comparison to MDMs alone. In conclusion, our findings suggest that the interaction between sertraline and Candida spp. may also diminish the virulence properties of this fungal pathogen in vivo. 相似文献
994.
995.
996.
Bone-targeted 2,6,9-trisubstituted purines: novel inhibitors of Src tyrosine kinase for the treatment of bone diseases 总被引:2,自引:0,他引:2
Wang Y Metcalf CA Shakespeare WC Sundaramoorthi R Keenan TP Bohacek RS van Schravendijk MR Violette SM Narula SS Dalgarno DC Haraldson C Keats J Liou S Mani U Pradeepan S Ram M Adams S Weigele M Sawyer TK 《Bioorganic & medicinal chemistry letters》2003,13(18):3067-3070
Novel bone-targeted 2,6,9-trisubstituted purine template-based inhibitors of Src tyrosine kinase are described. Drug design studies of known purine compounds revealed that both positions-2 and -6 were suitable for incorporating bone-seeking moieties. A variety of bone-targeting groups with different affinity to hydroxyapatite were utilized in the study. Compound 3d was determined to be a potent Src inhibitor and was quite selective against a panel of other protein kinases. 相似文献
997.
Effect of methylation on the stability and solvation free energy of amylose and cellulose fragments: a molecular dynamics study 总被引:1,自引:0,他引:1
Yu H Amann M Hansson T Köhler J Wich G van Gunsteren WF 《Carbohydrate research》2004,339(10):1697-1709
Molecular dynamics (MD) simulations were used to study the stability and solvation of amylose and cellulose fragments. The recently developed gromos carbohydrate force field was further tested by simulating maltose, cellobiose, and maltoheptaose. The MD simulations reproduced fairly well the favorable conformations of disaccharides defined by the torsional angles related with the glycosidic bond and the radius gyration of maltoheptaose. The effects of methylation at different hydroxyl groups on the stability of amylose and cellulose fragments were investigated. The methylations of O-2 and O-3 reduce the stability of a single helix more than methylation at O-6, while the latter reduces the stability of a double helix more. Solvation free-energy differences between the unsubstituted amylose and cellulose fragments and the methylated species were studied using the single-step perturbation method. It was found that methylation at O-2 has the biggest effect, in agreement with experiment. 相似文献
998.
Lo?c?VillierEmail author Daniel?B.?Blake John?W.?M.?Jagt Manfred?Kutscher 《Pal?ontologische Zeitschrift》2004,78(2):281-299
The Pterasteridae comprises a diversified group of extant largely deep-sea starfishes. Generic diagnoses have been based classically
on soft tissue characters and skeletal architecture. A preliminary phylogeny of sixteen extant species is here worked out
by cladistic analysis. The resulting tree suggests monophyly of extant genera and the validity of dissociated plates for identification
of genera. Fossil remains of Pterasteridae are here described for the first time. By comparison with extant species, all the
skeletal remains from the lower Upper Campanian of Belgium and from the lower Maastrichtian of Germany are tentatively assigned
to the genusPteraster. The fossil record of starfishes is poor, but the present Late Cretaceous pterasterids provide one more piece of evidence
of the high diversity of starfishes during the Mesozoic. Known Late Cretaceous and Paleogene fossils are broadly similar,
which suggests the end-Cretaceous extinction event did not cause major turnover in asteroid faunal composition. As suggested
for other starfish groups, both the fossil record of deep-sea Pterasteridae in shelf settings and tree topology imply an onshore-offshore
evolutionary trend.
相似文献
999.
1000.
Simultaneous determination of ketamine and xylazine in canine plasma by liquid chromatography with ultraviolet absorbance detection 总被引:2,自引:0,他引:2
Niedorf F Bohr HH Kietzmann M 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》2003,791(1-2):421-426
An isocratic reversed-phase high-performance liquid chromatographic method for the simultaneous determination of ketamine and xylazine in canine plasma is described. Plasma samples (500 microl) are cleaned up via liquid-liquid extraction. The analytes and the internal standard clonidine are separated on a cyano (CN) column using a mobile phase containing acetonitrile-0.005 M phosphate buffer adjusted to pH 5.5 (3:2) at a detection wavelength of 215 nm. The method was validated according to specificity, sensitivity, accuracy and reproducibility and was used to determine the plasma concentrations of both compounds in dogs after intramuscular injection. 相似文献