全文获取类型
收费全文 | 1465篇 |
免费 | 172篇 |
出版年
2023年 | 6篇 |
2022年 | 15篇 |
2021年 | 41篇 |
2020年 | 21篇 |
2019年 | 17篇 |
2018年 | 34篇 |
2017年 | 16篇 |
2016年 | 58篇 |
2015年 | 116篇 |
2014年 | 106篇 |
2013年 | 140篇 |
2012年 | 142篇 |
2011年 | 137篇 |
2010年 | 78篇 |
2009年 | 57篇 |
2008年 | 100篇 |
2007年 | 103篇 |
2006年 | 74篇 |
2005年 | 77篇 |
2004年 | 74篇 |
2003年 | 77篇 |
2002年 | 61篇 |
2001年 | 10篇 |
2000年 | 7篇 |
1999年 | 12篇 |
1998年 | 12篇 |
1997年 | 6篇 |
1996年 | 7篇 |
1995年 | 9篇 |
1994年 | 7篇 |
1993年 | 5篇 |
1992年 | 3篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1987年 | 3篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
排序方式: 共有1637条查询结果,搜索用时 15 毫秒
121.
Pieter Lemmens Joachim Mergeay Jeroen Van Wichelen Luc De Meester Steven A. J. Declerck 《PloS one》2015,10(9)
Ponds throughout the world are subjected to a variety of management measures for purposes of biodiversity conservation. Current conservation efforts typically comprise a combination of multiple measures that directly and indirectly impact a wide range of organism groups. Knowledge of the relative impact of individual measures on different taxonomic groups is important for the development of effective conservation programs. We conducted a field study of 28 man-made ponds, representing four management types differing in the frequency of periodic pond drainage and the intensity of fish stock management. We disentangled the relative importance of direct and indirect effects of pond management measures on the community composition of phytoplankton, zooplankton, aquatic macro-invertebrates, submerged and emergent vascular plants. With the exception of phytoplankton, pond management had strong effects on the community composition of all investigated biota. Whether management affected communities directly or indirectly through its impact on fish communities or local environmental conditions in the pond varied between organism groups. Overall, the impact of pond drainage regime and fish community characteristics on the community composition of target organism groups were more important than local environmental conditions. The majority of taxa were negatively associated with fish density, whereas multiple emergent plant species and several taxa of aquatic macro-invertebrates were positively affected by increased drainage frequency. The effects of fish community and drainage tended to be largely independent. The present study indicates that pond drainage is an important element for biodiversity conservation in eutrophicated shallow and interconnected man-made ponds. 相似文献
122.
Ana Camejo Daniel A. Gold Diana Lu Kiva McFetridge Lindsay Julien Ninghan Yang Kirk D.C. Jensen Jeroen P.J. Saeij 《International journal for parasitology》2014
The rhoptries are key secretory organelles from apicomplexan parasites that contain proteins involved in invasion and modulation of the host cell. Some rhoptry proteins are restricted to the posterior bulb (ROPs) and others to the anterior neck (RONs). As many rhoptry proteins have been shown to be key players in Toxoplasma invasion and virulence, it is important to identify, understand and characterise the biological function of the components of the rhoptries. In this report, we identified putative novel rhoptry genes by identifying Toxoplasma genes with similar cyclical expression profiles as known rhoptry protein encoding genes. Using this approach we identified two new rhoptry bulb (ROP47 and ROP48) and one new rhoptry neck protein (RON12). ROP47 is secreted and traffics to the host cell nucleus, RON12 was not detected at the moving junction during invasion. Deletion of ROP47 or ROP48 in a type II strain did not show major influence in in vitro growth or virulence in mice. 相似文献
123.
124.
The biosynthesis of the volatiles 2,5‐ and 2,6‐diisopropylpyrazine ( 2 and 3 , resp.) released by the myxobacteria Nannocystis exedens subsp. cinnabarina (Na c29) and Chondromyces crocatus (strains Cm c2 and Cm c5) was studied. Isotopically labeled precursors and proposed pathway intermediates were fed to agar plate cultures of the myxobacteria. Subsequently, the volatiles were collected by use of a closed loop stripping apparatus (CLSA), and incorporation into the pyrazines was followed by GC/MS analysis. [2H8]Valine was smoothly incorporated into both pyrazines clearly establishing their origin from the amino acid pool. The cyclic dipeptide valine anhydride ( 16 ) – a potential intermediate on the biosynthetic pathway to branched dialkylpyrazines – was synthesized containing 2H1 labels in specific positions. Feeding of [2H16]‐ 16 and [2H12]‐ 16 in both valine subunits mainly resulted in the formation of pyrazines derived from only one labeled amino acid, whereas only traces of the expected pyrazines with two labeled subunits were found. To investigate the origin of nitrogen in the pyrazines, a feeding experiment with [15N]valine was performed, resulting in the incorporation of the 15N label. The results contradict a biosynthetic pathway via cyclic dipeptides, but rather point to a pathway on which valine is reduced to valine aldehyde. Its dimerization to 2,5‐diisopropyldihydropyrazine 36 and subsequent oxidation results in 2 . The proposed biosynthetic pathway neatly fits the results of earlier labeling studies and also explains the formation of the regioisomer 2,6‐diisopropylpyrazine 3 by isomerization during the first condensation step of two molecules valine aldehyde. A general biosynthetic pathway to different classes of pyrazines is presented. 相似文献
125.
Mycobacterium avium subsp. paratuberculosis is a pathogen which causes a debilitating chronic enteritis in ruminants. Unfortunately, the mechanisms that control M. avium subsp. paratuberculosis persistence during infection are poorly understood and the key steps for developing Johne's disease remain elusive. A proteomic
analysis approach, based on one dimensional polyacrylamide gel electrophoresis (SDS-PAGE) followed by LC-MS/MS, was used to
identify and characterize the cell wall associated proteins of M. avium subsp. paratuberculosis K10 and an cell surface enzymatic shaving method was used to determine the surface-exposed proteins. 309 different proteins
were identified, which included 101 proteins previously annotated as hypothetical or conserved hypothetical. 38 proteins were
identified as surface-exposed by trypsin treatment. To categorize and analyze these proteomic data on the proteins identified
within cell wall of M. avium subsp. paratuberculosis K10, a rational bioinformatic approach was followed. The analyses of the 309 cell wall proteins provided theoretical molecular
mass and p I distributions and determined that 18 proteins are shared with the cell surface-exposed proteome. In short, a comprehensive
profile of the M. avium subsp. paratuberculosis K10 cell wall subproteome was created. The resulting proteomic profile might become the foundation for the design of new
preventive, diagnostic and therapeutic strategies against mycobacterial diseases in general and M. avium subsp. paratuberculosis in particular. 相似文献
126.
De Schrijver A Geudens G Augusto L Staelens J Mertens J Wuyts K Gielis L Verheyen K 《Oecologia》2007,153(3):663-674
Converting deciduous forests to coniferous plantations and vice versa causes environmental changes, but till now insight into
the overall effect is lacking. This review, based on 38 case studies, aims to find out how coniferous and deciduous forests
differ in terms of throughfall (+stemflow) deposition and seepage flux to groundwater. From the comparison of coniferous and
deciduous stands at comparable sites, it can be inferred that deciduous forests receive less N and S via throughfall (+stemflow)
deposition on the forest floor. In regions with relatively low open field deposition of atmospheric N (<10 kg N ha−1 year−1), lower NH4+ mean throughfall (+stemflow) deposition was, however, reported under conifers compared to deciduous forest, while in regions
with high atmospheric N pollution (>10 kg N ha−1 year−1), the opposite could be concluded. The higher the open field deposition of NH4+, the bigger the difference between the coniferous and deciduous throughfall (+stemflow) deposition. Furthermore, it can be
concluded that canopy exchange of K+, Ca2+ and Mg2+ is on average higher in deciduous stands. The significantly higher stand deposition flux of N and S in coniferous forests
is reflected in a higher soil seepage flux of NO3−, SO42−, K+, Ca2+, Mg2+ and Al(III). Considering a subset of papers for which all necessary data were available, a close relationship between throughfall
(+stemflow) deposition and seepage was found for N, irrespective of the forest type, while this was not the case for S. This
review shows that the higher input flux of N and S in coniferous forests clearly involves a higher seepage of NO3− and SO42− and accompanying cations K+, Ca2+, Mg2+ and Al(III) into the groundwater, making this forest type more vulnerable to acidification and eutrophication compared to
the deciduous forest type. 相似文献
127.
van Uchelen J Houpt P 《Plastic and reconstructive surgery》2007,119(7):2323; author reply 2323-2323; author reply 2324
128.
Werij JS Kloosterman B Celis-Gamboa C de Vos CH America T Visser RG Bachem CW 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》2007,115(2):245-252
Enzymatic discoloration (ED) of potato tubers was investigated in an attempt to unravel the underlying genetic factors. Both
enzyme and substrate concentration have been reported to influence the degree of discoloration and as such this trait can
be regarded as polygenic. The diploid mapping population C × E, consisting of 249 individuals, was assayed for the degree
of ED and levels of chlorogenic acid and tyrosine. Using this data, Quantitative Trait Locus (QTL) analysis was performed.
Three QTLs for ED have been found on parental chromosomes C3, C8, E1, and E8. For chlorogenic acid a QTL has been identified
on C2 and for tyrosine levels, a QTL has been detected on C8. None of the QTLs overlap, indicating the absence of genetic
correlations between these components underlying ED, in contrast to earlier reports in literature. An obvious candidate gene
for the QTL for ED on Chromosome 8 is polyphenol oxidase (PPO), which was previously mapped on chromosome 8. With gene-specific
primers for PPO gene POT32 a CAPS marker was developed. Three different alleles (POT32-1, -2, and -3) could be discriminated.
The segregating POT32 alleles were used to map the POT32 CAPS marker and QTL analysis was redone, showing that POT32 coincides
with the QTL peak. A clear correlation between allele combinations and degree of discoloration was observed. In addition,
analysis of POT32 gene expression in a subset of genotypes indicated a correlation between the level of gene expression and
allele composition. On average, genotypes having two copies of allele 1 had both the highest degree of discoloration as well
as the highest level of POT32 gene expression. 相似文献
129.
GxxxG motif of severe acute respiratory syndrome coronavirus spike glycoprotein transmembrane domain is not involved in trimerization and is not important for entry 总被引:1,自引:0,他引:1 下载免费PDF全文
Recently, a paper was published in which it was proposed that the GxxxG motif of the severe acute respiratory syndrome (SARS) coronavirus spike (S) protein transmembrane domain plays a vital role in oligomerization of the protein (E. Arbely, Z. Granot, I. Kass, J. Orly, and I. T. Arkin, Biochemistry 45:11349-11356, 2006). Here, we show that the GxxxG motif is not involved in SARS S oligomerization by trimerization analysis of S GxxxG mutant proteins. In addition, the capability of S to mediate entry of SARS S-pseudotyped particles overall was affected moderately in the mutant proteins, also arguing for a nonvital role for the GxxxG motif in SARS coronavirus entry. 相似文献
130.
Juan Zhang Rui-Yan Fu Jeroen Hugenholtz Yin Li Jian Chen 《Applied microbiology》2007,73(16):5268-5275
Previously we showed that glutathione (GSH) can protect Lactococcus lactis against oxidative stress (Y. Li et al., Appl. Environ. Microbiol. 69:5739-5745, 2003). In the present study, we show that the GSH imported by L. lactis subsp. cremoris SK11 or produced by engineered L. lactis subsp. cremoris NZ9000 can protect both strains against a long-term mild acid challenge (pH 4.0) and a short-term severe acid challenge (pH 2.5). This shows for the first time that GSH can protect a gram-positive bacterium against acid stress. During acid challenge, strain SK11 containing imported GSH and strain NZ9000 containing self-produced GSH exhibited significantly higher intracellular pHs than the control. Furthermore, strain SK11 containing imported GSH had a significantly higher activity of glyceraldehyde-3-phosphate dehydrogenase than the control. These results suggest that the acid stress resistance of starter culture can be improved by selecting L. lactis strains capable of producing or importing GSH. 相似文献