全文获取类型
收费全文 | 3302篇 |
免费 | 266篇 |
国内免费 | 2篇 |
出版年
2023年 | 17篇 |
2022年 | 40篇 |
2021年 | 61篇 |
2020年 | 38篇 |
2019年 | 41篇 |
2018年 | 51篇 |
2017年 | 48篇 |
2016年 | 87篇 |
2015年 | 145篇 |
2014年 | 164篇 |
2013年 | 218篇 |
2012年 | 248篇 |
2011年 | 261篇 |
2010年 | 182篇 |
2009年 | 177篇 |
2008年 | 242篇 |
2007年 | 231篇 |
2006年 | 201篇 |
2005年 | 199篇 |
2004年 | 203篇 |
2003年 | 166篇 |
2002年 | 177篇 |
2001年 | 53篇 |
2000年 | 31篇 |
1999年 | 33篇 |
1998年 | 35篇 |
1997年 | 26篇 |
1996年 | 19篇 |
1995年 | 25篇 |
1994年 | 12篇 |
1993年 | 15篇 |
1992年 | 9篇 |
1991年 | 10篇 |
1990年 | 9篇 |
1989年 | 7篇 |
1988年 | 10篇 |
1987年 | 4篇 |
1986年 | 6篇 |
1985年 | 9篇 |
1984年 | 5篇 |
1983年 | 11篇 |
1982年 | 12篇 |
1981年 | 8篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1976年 | 4篇 |
1973年 | 2篇 |
1968年 | 1篇 |
排序方式: 共有3570条查询结果,搜索用时 46 毫秒
31.
Heat-stable somatic antigen (HSSA) variants were isolated from Bacillus thuringiensis strain T84A1-A by antiserum-mediated selection. Substantial differences in HSSAs were clearly shown between variants and the parent strain by HSSA agglutination tests, Ouchterlony tests and precipitin-halo-formation (PHF) tests of extracellular HSSAs, although a 'one way'cross-reaction was evident. The frequency of HSSA variants in the population was < 10-4 , as determined by PHF tests on antiserum-agar plates. Heat-stable somatic antigen variants showed no alteration in flagellar antigenic structure and in 30 phenotypic characteristics. The insecticidal activity of HSSA variants was on the level with that of the parent strain, when tested with larvae of three lepidopterous species. 相似文献
32.
The ability of the benthic cyanobacterium Lyngbya wollei to fix nitrogen was studied using field samples and axenic cultures. L. wollei was collected and isolated from Lake Okeechobee, Florida, where it forms extensive mats. Rates of acetylene reduction up
to 39.1 nmol mg dry wt−1 h−1 were observed for field samples. The maximum observed rate of acetylene reduction in axenic laboratory cultures was 200 nmol
mg dry wt−1 h−1. Aerobic conditions limited nitrogen fixation activity, but dark/light cycles promoted the development of activity. Reduced
oxygen levels appeared to be required for the development of significant levels of nitrogenase activity. The level of irradiance
also had a significant impact on the level of activity. The potential significance of nitrogen fixation to Lyngbya production is discussed. 相似文献
33.
A variety of preparative methods for in situ X-ray energy dispersive analysis were tested to determine their effects on the elemental composition of polyphosphate bodies in P. boryanum. The bodies were found to contain large amounts of P and K and small amounts of Ca and Mg. Air drying, freeze-drying and freeze-drying from a liquid nitrogen slush all gave similar results. Fixation of the cells in glutaraldehyde and/or OsO4 resulted in loss of the K and enhancement of the Ca peak. Magnesium was lost during embedding in epoxy. 相似文献
34.
35.
36.
Benjamin Feit Tim Dempster Tim S. Jessop Jonathan K. Webb Mike Letnic 《Global Change Biology》2020,26(5):2829-2840
Invasive vertebrates are frequently reported to have catastrophic effects on the populations of species which they directly impact. It follows then, that if invaders exert strong suppressive effects on some species then other species will indirectly benefit due to ecological release from interactions with directly impacted species. However, evidence that invasive vertebrates trigger such trophic cascades and alter community structure in terrestrial ecosystems remains rare. Here, we ask how the cane toad, a vertebrate invader that is toxic to many of Australia's vertebrate predators, influences lizard assemblages in a semi‐arid rangeland. In our study area, the density of cane toads is influenced by the availability of water accessible to toads. We compared an index of the abundance of sand goannas, a large predatory lizard that is susceptible to poisoning by cane toads and the abundances of four lizard families preyed upon by goannas (skinks, pygopods, agamid lizards and geckos) in areas where cane toads were common or rare. Consistent with the idea that suppression of sand goannas by cane toads initiates a trophic cascade, goanna activity was lower and small lizards were more abundant where toads were common. The hypothesis that suppression of sand goannas by cane toads triggers a trophic cascade was further supported by our findings that small terrestrial lizards that are frequently preyed upon by goannas were more affected by toad abundance than arboreal geckos, which are rarely consumed by goannas. Furthermore, the abundance of at least one genus of terrestrial skinks benefitted from allogenic ecosystem engineering by goannas where toads were rare. Overall, our study provides evidence that the invasion of ecosystems by non‐native species can have important effects on the structure and integrity of native communities extending beyond their often most obvious and frequently documented direct ecological effects. 相似文献
37.
38.
Karsten Schnatbaum Victor Solis‐Mezarino Daniil Pokrovsky Frederike Schfer Dennis Nagl Lars Hornberger Johannes Zerweck Tobias Knaute Julia Avramova‐Nehmer Mike Schutkowski Veit Hornung Holger Wenschuh Moritz Carl Vlker‐Albert Axel Imhof Ulf Reimer 《Proteomics》2020,20(10)
Targeted proteomics depends on the availability of stable isotope labeled (SIL) peptide standards, which for absolute protein quantification need to be absolutely quantified. In the present study, three new approaches for absolute quantification of SIL peptides are developed. All approaches rely on a quantification tag (Qtag) with a specific UV absorption. The Qtag is attached to the peptide during synthesis and is removed by tryptic digestion under standard proteomics workflow conditions. While one quantification method (method A) is designed to allow the fast and economic production of absolutely quantified SIL peptides, two other methods (methods B and C) are developed to enable the straightforward re‐quantification of SIL peptides after reconstitution to control and monitor known problems related to peptide solubility, precipitation, and adhesion to vials. All methods yield consistent results when compared to each other and when compared to quantification by amino acid analysis. The precise quantitation methods are used to characterize the in vivo specificity of the H3 specific histone methyltransferase EZH2. 相似文献
39.
Matthew N. Mendoza Mike Jian Moeko T. King Cory L. Brooks 《Protein science : a publication of the Protein Society》2020,29(4):990-1003
A distinguishing feature of camel (Camelus dromedarius) VHH domains are noncanonical disulfide bonds between CDR1 and CDR3. The disulfide bond may provide an evolutionary advantage, as one of the cysteines in the bond is germline encoded. It has been hypothesized that this additional disulfide bond may play a role in binding affinity by reducing the entropic penalty associated with immobilization of a long CDR3 loop upon antigen binding. To examine the role of a noncanonical disulfide bond on antigen binding and the biophysical properties of a VHH domain, we have used the VHH R303, which binds the Listeria virulence factor InlB as a model. Using site directed mutagenesis, we produced a double mutant of R303 (C33A/C102A) to remove the extra disulfide bond of the VHH R303. Antigen binding was not affected by loss of the disulfide bond, however the mutant VHH displayed reduced thermal stability (Tm = 12°C lower than wild‐type), and a loss of the ability to fold reversibly due to heat induced aggregation. X‐ray structures of the mutant alone and in complex with InlB showed no major changes in the structure. B‐factor analysis of the structures suggested that the loss of the disulfide bond elicited no major change on the flexibility of the CDR loops, and revealed no evidence of loop immobilization upon antigen binding. These results suggest that the noncanonical disulfide bond found in camel VHH may have evolved to stabilize the biophysical properties of the domain, rather than playing a significant role in antigen binding. 相似文献
40.
Sophia E. Kimmig Joscha Beninde Miriam Brandt Anna Schleimer Stephanie Kramer‐Schadt Heribert Hofer Konstantin Brner Christoph Schulze Ulrich Wittstatt Mike Heddergott Tanja Halczok Christoph Staubach Alain C. Frantz 《Molecular ecology》2020,29(3):466-484
Urbanization affects key aspects of wildlife ecology. Dispersal in urban wildlife species may be impacted by geographical barriers but also by a species’ inherent behavioural variability. There are no functional connectivity analyses using continuous individual‐based sampling across an urban‐rural continuum that would allow a thorough assessment of the relative importance of physical and behavioural dispersal barriers. We used 16 microsatellite loci to genotype 374 red foxes (Vulpes vulpes) from the city of Berlin and surrounding rural regions in Brandenburg in order to study genetic structure and dispersal behaviour of a mobile carnivore across the urban‐rural landscape. We assessed functional connectivity by applying an individual‐based landscape genetic optimization procedure. Three commonly used genetic distance measures yielded different model selection results, with only the results of an eigenvector‐based multivariate analysis reasonably explaining genetic differentiation patterns. Genetic clustering methods and landscape resistance modelling supported the presence of an urban population with reduced dispersal across the city border. Artificial structures (railways, motorways) served as main dispersal corridors within the cityscape, yet urban foxes avoided densely built‐up areas. We show that despite their ubiquitous presence in urban areas, their mobility and behavioural plasticity, foxes were affected in their dispersal by anthropogenic presence. Distinguishing between man‐made structures and sites of human activity, rather than between natural and artificial structures, is thus essential for better understanding urban fox dispersal. This differentiation may also help to understand dispersal of other urban wildlife and to predict how behaviour can shape population genetic structure beyond physical barriers. 相似文献