全文获取类型
收费全文 | 9523篇 |
免费 | 880篇 |
国内免费 | 5篇 |
出版年
2022年 | 66篇 |
2021年 | 148篇 |
2020年 | 83篇 |
2019年 | 120篇 |
2018年 | 118篇 |
2017年 | 116篇 |
2016年 | 215篇 |
2015年 | 399篇 |
2014年 | 404篇 |
2013年 | 459篇 |
2012年 | 614篇 |
2011年 | 559篇 |
2010年 | 367篇 |
2009年 | 358篇 |
2008年 | 477篇 |
2007年 | 506篇 |
2006年 | 470篇 |
2005年 | 473篇 |
2004年 | 424篇 |
2003年 | 396篇 |
2002年 | 381篇 |
2001年 | 199篇 |
2000年 | 138篇 |
1999年 | 154篇 |
1998年 | 104篇 |
1997年 | 89篇 |
1996年 | 77篇 |
1995年 | 94篇 |
1994年 | 69篇 |
1993年 | 66篇 |
1992年 | 100篇 |
1991年 | 85篇 |
1990年 | 91篇 |
1989年 | 87篇 |
1988年 | 72篇 |
1987年 | 103篇 |
1986年 | 77篇 |
1985年 | 89篇 |
1984年 | 100篇 |
1983年 | 60篇 |
1982年 | 84篇 |
1981年 | 65篇 |
1980年 | 61篇 |
1979年 | 81篇 |
1978年 | 59篇 |
1976年 | 63篇 |
1975年 | 52篇 |
1974年 | 67篇 |
1973年 | 55篇 |
1972年 | 72篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
991.
Zoospores of plant pathogenic oomycetes exhibit distinct swimming speeds and patterns under natural conditions. Zoospore swimming is influenced by ion homeostasis and changes in the ionic composition of media. Therefore, we used video microscopy to investigate swimming patterns of five oomycete species in response to changes in potassium homeostasis. In general, zoospore speed tended to be negatively correlated with zoospore size. Three Phytophthora species (Phytophthora palmivora, Phytophthora megakarya, and Phytophthora infestans) swam in straight patterns with speeds ranging from 50 to 250 microm/s whereas two Pythium species (Pythium aphanidermatum and Pythium dissotocum) swam at similar speeds ranging from 180 to 225 microm/s with a pronounced helical trajectory and varying amplitudes. High external concentrations of potassium salts reduced the swimming speed of Ph. palmivora and induced encystment. This was not observed for Py. aphanidermatum. Application of the potassium ionophores gramicidin, nigericin and valinomycin resulted in reduced swimming speeds and changes in the swimming patterns of the Phytophthora species. Therefore, potassium ions play a key role in regulating zoospore behavior. 相似文献
992.
993.
SUMMARY: Pairwise comparisons of disagreement in phylogenetic datasets offer a powerful tool for isolating historical incongruence for closer analysis. Statistically significant phylogenetic character incongruence may reflect important differences in evolutionary history, such as horizontal gene transfer. Such testing can also be used to specify possible combinations of datasets for further phylogenetic analysis. The process of comparing multiple datasets can be very time consuming, and it is sometimes unclear how to combine data partitions given the observed patterns of incongruence. Here we present an application that automates the process of making pairwise comparisons between large numbers of phylogenetic datasets using the Incongruence Length Difference (ILD) test. The application also implements strategies for data combination based on the patterns of incongruence observed in pairwise comparisons. 相似文献
994.
995.
Williamson NR Simonsen HT Ahmed RA Goldet G Slater H Woodley L Leeper FJ Salmond GP 《Molecular microbiology》2005,56(4):971-989
The biosynthetic pathway of the red-pigmented antibiotic, prodigiosin, produced by Serratia sp. is known to involve separate pathways for the production of the monopyrrole, 2-methyl-3-n-amyl-pyrrole (MAP) and the bipyrrole, 4-methoxy-2,2'-bipyrrole-5-carbaldehyde (MBC) which are then coupled in the final condensation step. We have previously reported the cloning, sequencing and heterologous expression of the pig cluster responsible for prodigiosin biosynthesis in two Serratia sp. In this article we report the creation of in-frame deletions or insertions in every biosynthetic gene in the cluster from Serratia sp. ATCC 39006. The biosynthetic intermediates accumulating in each mutant have been analysed by LC-MS, cross-feeding and genetic complementation studies. Based on these results we assign specific roles in the biosynthesis of MBC to the following Pig proteins: PigI, PigG, PigA, PigJ, PigH, PigM, PigF and PigN. We report a novel pathway for the biosynthesis of MAP, involving PigD, PigE and PigB. We also report a new chemical synthesis of MAP and one of its precursors, 3-acetyloctanal. Finally, we identify the condensing enzyme as PigC. We reassess the existing literature and discuss the significance of the results for the biosynthesis of undecylprodigiosin by the Red cluster in Streptomyces coelicolor A3(2). 相似文献
996.
Finn?DanielsenEmail author Neil?D.?Burgess Andrew?Balmford 《Biodiversity and Conservation》2005,14(11):2507-2542
Monitoring of biodiversity and resource use by professional scientists is often costly and hard to sustain, especially in
developing countries, where financial resources are limited. Moreover, such monitoring can be logistically and technically
difficult and is often perceived to be irrelevant by resource managers and the local communities. Alternatives are emerging,
carried out at a local scale and by individuals with little formal education. The methods adopted span a spectrum, from participatory
monitoring where aims and objectives are defined by the community, to ranger-based monitoring in protected areas. What distinguishes
these approaches is that local people or local government staff are directly involved in data collection and (in most instances)
analysis. In this issue of Biodiversity and Conservation, 15 case studies examine whether these new approaches can address
the limitations of professional monitoring in developing countries. The case studies evaluate ongoing locally-based monitoring
schemes involving more than 1500 community members in 13 countries. The papers are based on a symposium held in Denmark in
April 2004 (www. monitoringmatters.org). Here, we review how the case studies shed light on the following key issues concerning
locally-based methods: cost, sustainability, their ability to detect true local or larger-scale trends, their links to management
decisions and action, and the empowerment of local constituencies. Locally-based monitoring appears to be consistently cheap
relative to the costs of management and of professional monitoring, even though the start-up costs can be high. Most local
monitoring schemes are still young and thus their chances of being sustained over the longer term are not yet certain. However,
we believe their chances of surviving are better than many professional schemes, particularly when they are institutionalised
within existing management structures, and linked to the delivery of ecosystem goods or services to local communities. When
properly designed, local schemes yield locally relevant results that can be as reliable as those derived from professional
monitoring. Many management decisions emanate from local schemes. The decisions appear to be taken promptly, in response to
immediate threats to the environment, and often lead to community-based actions to protect habitats, species or the local
flow of ecosystem benefits; however, few local schemes have so far led to actions beyond the local scale. Locally-based monitoring
schemes often reinforce existing community-based resource management systems and lead to change in the attitude of locals
towards more environmentally sustainable resource management. Locally-derived data have considerable unexplored potential
to elucidate global patterns of change in the status of populations and habitats, the services they provide, and the threats
they face, but more effort is needed to develop effective modalities for feeding locally-derived data up to national and international
levels. 相似文献
997.
Protein N-glycosylation in eukaryotes and peptidoglycan biosynthesis in bacteria are both initiated by the transfer of a D-N-acetylhexosamine 1-phosphate to a membrane-bound polyprenol phosphate. These reactions are catalyzed by a family of transmembrane proteins known as the UDP-D-N-acetylhexosamine: polyprenol phosphate D-N-acetylhexosamine 1-phosphate transferases. The sole eukaryotic member of this family, the d-N-acetylglucosamine 1-phosphate transferase (GPT), is specific for UDP-GlcNAc as the donor substrate and uses dolichol phosphate as the membrane-bound acceptor. The bacterial translocases, MraY, WecA, and WbpL, utilize undecaprenol phosphate as the acceptor substrate, but differ in their specificity for the UDP-sugar donor substrate. The structural basis of this sugar nucleotide specificity is uncertain. However, potential carbohydrate recognition (CR) domains have been identified within the C-terminal cytoplasmic loops of MraY, WecA, and WbpL that are highly conserved in family members with the same UDP-N-acetylhexosamine specificity. This review focuses on the catalytic mechanism and substrate specificity of these bacterial UDP-D-N-acetylhexosamine: polyprenol phosphate D-N-acetylhexosamine 1-P transferases and may provide insights for the development of selective inhibitors of cell wall biosynthesis. 相似文献
998.
999.
Wyatt PG Allen MJ Borthwick AD Davies DE Exall AM Hatley RJ Irving WR Livermore DG Miller ND Nerozzi F Sollis SL Szardenings AK 《Bioorganic & medicinal chemistry letters》2005,15(10):2579-2582
This paper covers efforts to discover orally active potent and selective oxytocin antagonists. Screening pooled libraries identified a novel series of 2,5-diketopiperazine derivatives with antagonist activity at the human oxytocin receptor. We report the initial structure-activity relationship investigations and the determination of the stereochemistry of the most potent compounds. 相似文献
1000.
Li JJ Wang H Qu F Musial C Tino JA Robl JA Slusarchyk D Golla R Seethala R Dickinson K Giupponi L Grover G Sleph P Flynn N Murphy BJ Gordon D Kung M Stoffel R 《Bioorganic & medicinal chemistry letters》2005,15(7):1799-1802
Several novel series of tetrahydroisoquinoline 1-carboxamides were prepared and shown to be potent growth hormone (GH) secretagogues. Among them, carbamate 12a-E2 displays excellent in vivo activity by increasing plasma GH 10-fold in an anesthetized IV rat model. 相似文献