全文获取类型
收费全文 | 2818篇 |
免费 | 216篇 |
国内免费 | 2篇 |
专业分类
3036篇 |
出版年
2023年 | 14篇 |
2022年 | 37篇 |
2021年 | 61篇 |
2020年 | 38篇 |
2019年 | 42篇 |
2018年 | 53篇 |
2017年 | 53篇 |
2016年 | 95篇 |
2015年 | 149篇 |
2014年 | 166篇 |
2013年 | 184篇 |
2012年 | 233篇 |
2011年 | 253篇 |
2010年 | 149篇 |
2009年 | 135篇 |
2008年 | 179篇 |
2007年 | 169篇 |
2006年 | 156篇 |
2005年 | 143篇 |
2004年 | 120篇 |
2003年 | 129篇 |
2002年 | 94篇 |
2001年 | 30篇 |
2000年 | 14篇 |
1999年 | 23篇 |
1998年 | 26篇 |
1997年 | 22篇 |
1996年 | 18篇 |
1995年 | 10篇 |
1994年 | 14篇 |
1993年 | 9篇 |
1992年 | 8篇 |
1990年 | 10篇 |
1988年 | 6篇 |
1987年 | 10篇 |
1986年 | 7篇 |
1985年 | 11篇 |
1984年 | 7篇 |
1983年 | 5篇 |
1982年 | 19篇 |
1981年 | 11篇 |
1980年 | 14篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1977年 | 7篇 |
1976年 | 8篇 |
1974年 | 6篇 |
1951年 | 9篇 |
1950年 | 5篇 |
1948年 | 5篇 |
排序方式: 共有3036条查询结果,搜索用时 15 毫秒
61.
Adult neurogenesis within the subgranular zone (SGZ) of the hippocampal dentate gyrus and the subventricular zone (SVZ) of the lateral ventricle (LV) has been most intensely studied within the brains of rodents such as mice and rats. However, little is known about the cell types and processes involved in adult neurogenesis within primates such as the common marmoset (Callithrix jacchus). Moreover, substantial differences seem to exist between the neurogenic niche of the LV between rodents and humans. Here, we set out to use immunohistochemical and autogradiographic analysis to characterize the anatomy of the neurogenic niches and the expression of cell type-specific markers in those niches in the adult common marmoset brain. Moreover, we demonstrate significant differences in the activity of neurogenesis in the adult marmoset brain compared to the adult mouse brain. Finally, we provide evidence for ongoing proliferation of neuroblasts within both the SGZ and SVZ of the adult brain and further show that the age-dependent decline of neurogenesis in the hippocampus is associated with a decrease in neuroblast cells. 相似文献
62.
von Stetten D Seibeck S Michael N Scheerer P Mroginski MA Murgida DH Krauss N Heyn MP Hildebrandt P Borucki B Lamparter T 《The Journal of biological chemistry》2007,282(3):2116-2123
The mutants H250A and D197A of Agp1 phytochrome from Agrobacterium tumefaciens were prepared and investigated by different spectroscopic and biochemical methods. Asp-197 and His-250 are highly conserved amino acids and are part of the hydrogen-bonding network that involves the chromophore. Both substitutions cause a destabilization of the protonated chromophore in the Pr state as revealed by resonance Raman and UV-visible absorption spectroscopy. Titration experiments demonstrate a lowering of the pK(a) from 11.1 (wild type) to 8.8 in H250A and 7.2 in D197A. Photoconversion of the mutants does not lead to the Pfr state. H250A is arrested in a meta-Rc-like state in which the chromophore is deprotonated. For H250A and the wild-type protein, deprotonation of the chromophore in meta-Rc is coupled to the release of a proton to the external medium, whereas the subsequent proton re-uptake, linked to the formation of the Pfr state in the wild-type protein, is not observed for H250A. No transient proton exchange with the external medium occurs in D197A, suggesting that Asp-197 may be the proton release group. Both mutants do not undergo the photo-induced protein structural changes that in the wild-type protein are detectable by size exclusion chromatography. These conformational changes are, therefore, attributed to the meta-Rc --> Pfr transition and most likely coupled to the transient proton re-uptake. The present results demonstrate that Asp-197 and His-250 are essential for stabilizing the protonated chromophore structure in the parent Pr state, which is required for the primary photochemical process, and for the complete photo-induced conversion to the Pfr state. 相似文献
63.
Tourette's syndrome (TS) is a neurodevelopmental disorder characterized by the presence of chronic vocal and motor tics. Tics are sudden, highly stereotyped, movements that can be simple or complex in appearance. Since patients with TS have difficulties preventing unwanted movements, one might expect that their ability to voluntarily control goal-directed movements would be similarly poor. Indeed, it has been suggested that TS sufferers are impaired at inhibiting reflexively triggered movements and in rapidly selecting or switching between different motor sets. This idea is consistent with current views on the neurological basis of TS that posit a dysfunction of the neural circuits linking the frontal lobes and the striatum. These circuits are known to be involved in the voluntary control of action. By using an oculomotor switching task, we show for the first time that young people with TS exhibit paradoxically greater levels of cognitive control over their movements than their age-matched controls. This finding is consistent with an increased need to monitor and control movements and may indicate a subcortical locus for the triggering of tics. It also suggests that the constant need to suppress tics could have resulted in an enhancement of the executive processes involved in inhibitory control. 相似文献
64.
65.
Dynamics and functional relevance of ammonia-oxidizing archaea in two agricultural soils 总被引:4,自引:0,他引:4
Kristina Schauss reas Focks Sven Leininger Anja Kotzerke Holger Heuer Sören Thiele-Bruhn Shilpi Sharma Berndt-Michael Wilke Michael Matthies Kornelia Smalla Jean Charles Munch Wulf Amelung Martin Kaupenjohann Michael Schloter Christa Schleper 《Environmental microbiology》2009,11(2):446-456
Crucial steps in geochemical cycles are in many cases performed by more than one group of microorganisms, but the significance of this functional redundancy with respect to ecosystem functioning is poorly understood. Ammonia-oxidizing archaea (AOA) and their bacterial counterparts (AOB) are a perfect system to address this question: although performing the same transformation step, they belong to well-separated phylogenetic groups. Using pig manure amended with different concentrations of sulfadiazine (SDZ), an antibiotic that is frequently used in veterinary medicine, it was possible to affect AOB and AOA to different degrees. Addition of manure stimulated growth of AOB in both soils and, interestingly, also growth of AOA was considerably stimulated in one of the soils. The antibiotic treatments decreased the manure effect notably on AOB, whereas AOA were affected to a lower extent. Model calculations concerning the respective proportions of AOA and AOB in ammonia oxidation indicate a substantial contribution of AOA in one of the soils that further increased under the influence of SDZ, hence indicating functional redundancy between AOA and AOB. 相似文献
66.
Sven Erik Godtfredsen Anne Munk Rasmussen Martin Ottesen Peer Rafn Nicolai Peitersen 《Applied microbiology and biotechnology》1984,20(1):23-28
Summary Acetolactate decarboxylase activity has been detected among three genera, nine species and 263 strains of lactic acid bacteria tested in the course of a screening for acetolactate decarboxylases amenable for use in brewing as maturation aid. Streptococcus diacetylactis strain FD-64-D was found to generate a decarboxylase exhibiting a satisfactory activity and an excellent stability at the pH prevailing in beer and wort. This decarboxylase could not be solubilized but enzymatically active, freeze-dried cells were effective for satisfactory flavour maturation of beer although difficulties were encountered during attempts to remove the applied cell material by filtration of the beer. Lactobacillus casei DSM 2547 was likewise found to produce a decarboxylase exhibiting a satisfactory activity and stability at the low pH of beer and which, in addition, was readily solubilized. A method has been developed for pilot scale production of preparations of this decarboxylase suitable for use in brewing.Abbreviations DSM
Deutsche Sammlung von Microorganismen
- EDTA
Ethylene diaminetetra-acetic acid 相似文献
67.
68.
S?ren Franzenburg Sebastian Fraune Philipp M Altrock Sven Künzel John F Baines Arne Traulsen Thomas CG Bosch 《The ISME journal》2013,7(4):781-790
Animals are colonized by complex bacterial communities. The processes controlling community membership and influencing the establishment of the microbial ecosystem during development are poorly understood. Here we aimed to explore the assembly of bacterial communities in Hydra with the broader goal of elucidating the general rules that determine the temporal progression of bacterial colonization of animal epithelia. We profiled the microbial communities in polyps at various time points after hatching in four replicates. The composition and temporal patterns of the bacterial communities were strikingly similar in all replicates. Distinct features included high diversity of community profiles in the first week, a remarkable but transient adult-like profile 2 weeks after hatching, followed by progressive emergence of a stable adult-like pattern characterized by low species diversity and the preponderance of the Betaproteobacterium Curvibacter. Intriguingly, this process displayed important parallels to the assembly of human fecal communities after birth. In addition, a mathematical modeling approach was used to uncover the organizational principles of this colonization process, suggesting that both, local environmental or host-derived factor(s) modulating the colonization rate, as well as frequency-dependent interactions of individual bacterial community members are important aspects in the emergence of a stable bacterial community at the end of development. 相似文献
69.
Bo Fernholm Michael Norén Sven O. Kullander Andrea M. Quattrini Vincent Zintzen Clive D. Roberts Hin‐Kiu Mok Chien‐Hsien Kuo 《Journal of Zoological Systematics and Evolutionary Research》2013,51(4):296-307
A recent phylogenetic analysis of the Myxinidae based on the 16S rRNA gene resulted in synonymization of Paramyxine with Eptatretus. This created homonymy of Paramyxine fernholmi with Eptatretus fernholmi and Paramyxine wisneri with Eptatretus wisneri. In order to resolve this nomenclatural dilemma, we made a more extensive phylogenetic assessment of the Myxinidae and examined the nomenclature of the family. We used 75 sequences (37 of which new for this study) of a 561 bp fragment of the 16S rRNA gene, representing 33 species, and 72 sequences (37 of which new for this study) of a 687 bp fragment of the cytochrome c oxidase subunit I (COI) gene, representing 23 species, to reconstruct the phylogeny of Myxinidae. The monophyly of the subfamily Myxininae, traditionally characterized by having a single pair of external gill openings, was rejected (0.50 Bayesian posterior probability) by the 16S analysis, but supported by the COI and combined COI+16S analyses (0.99 and 0.81 Bpp, respectively). The monophyly of the subfamily Eptatretinae, characterized by having several pairs of external gill openings, was not supported by the 16S analysis and rejected by the COI and combined COI+16S analysis due to the placement of Eptatretus lopheliae as the earliest branch of Myxinidae (0.71 and 0.57 Bpp, respectively). Eptatretus lopheliae and Eptatretus rubicundus formed a monophyletic group and were allocated to a new genus, Rubicundus, characterized by the presence of an elongated tubular nostril and reddish coloration. A new monotypic subfamily, Rubicundinae, was proposed for Rubicundus. The synonymy of the genera Paramyxine and Quadratus with Eptatretus was confirmed. E. fernholmi is renamed Eptatretus luzonicus. Eptatretus wisneri was renamed Eptatretus bobwisneri. Petromyzon cirrhatus Forster, 1801, Homea banksii Fleming, 1822, and Bdellostoma forsteri Müller, 1836 are synonyms, but no type specimens are known to exist. Petromyzon cirrhatus was designated as type species of Eptatretus, conserving present usage. Gastrobranchus dombeyi Shaw, 1804 has priority over other names for Chilean myxinids. Bdellostoma stoutii was designated as type species of Polistotrema Gill. The validity of the Western Atlantic Myxine limosa as distinct from the Eastern Atlantic Myxine glutinosa was confirmed. 相似文献
70.
Marco Dollinger René Müller-Wille Florian Zeman Michael Haimerl Christoph Niessen Lukas P. Beyer Sven A. Lang Andreas Teufel Christian Stroszczynski Philipp Wiggermann 《PloS one》2015,10(8)