全文获取类型
收费全文 | 18856篇 |
免费 | 1633篇 |
国内免费 | 4篇 |
专业分类
20493篇 |
出版年
2023年 | 69篇 |
2022年 | 153篇 |
2021年 | 323篇 |
2020年 | 176篇 |
2019年 | 233篇 |
2018年 | 324篇 |
2017年 | 282篇 |
2016年 | 408篇 |
2015年 | 805篇 |
2014年 | 858篇 |
2013年 | 1075篇 |
2012年 | 1497篇 |
2011年 | 1462篇 |
2010年 | 883篇 |
2009年 | 871篇 |
2008年 | 1218篇 |
2007年 | 1230篇 |
2006年 | 1118篇 |
2005年 | 1128篇 |
2004年 | 1002篇 |
2003年 | 940篇 |
2002年 | 921篇 |
2001年 | 206篇 |
2000年 | 154篇 |
1999年 | 194篇 |
1998年 | 273篇 |
1997年 | 162篇 |
1996年 | 156篇 |
1995年 | 149篇 |
1994年 | 129篇 |
1993年 | 121篇 |
1992年 | 120篇 |
1991年 | 102篇 |
1990年 | 116篇 |
1989年 | 103篇 |
1988年 | 88篇 |
1987年 | 70篇 |
1986年 | 88篇 |
1985年 | 88篇 |
1984年 | 115篇 |
1983年 | 84篇 |
1982年 | 94篇 |
1981年 | 115篇 |
1980年 | 99篇 |
1979年 | 64篇 |
1978年 | 75篇 |
1977年 | 63篇 |
1976年 | 47篇 |
1975年 | 46篇 |
1974年 | 67篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Olivia Sackett Katherina Petrou Brian Reedy Ross Hill Martina Doblin John Beardall Peter Ralph Philip Heraud 《The ISME journal》2016,10(2):416-426
Diatoms, an important group of phytoplankton, bloom annually in the Southern Ocean, covering thousands of square kilometers and dominating the region''s phytoplankton communities. In their role as the major food source to marine grazers, diatoms supply carbon, nutrients and energy to the Southern Ocean food web. Prevailing environmental conditions influence diatom phenotypic traits (for example, photophysiology, macromolecular composition and morphology), which in turn affect the transfer of energy, carbon and nutrients to grazers and higher trophic levels, as well as oceanic biogeochemical cycles. The paucity of phenotypic data on Southern Ocean phytoplankton limits our understanding of the ecosystem and how it may respond to future environmental change. Here we used a novel approach to create a ‘snapshot'' of cell phenotype. Using mass spectrometry, we measured nitrogen (a proxy for protein), total carbon and carbon-13 enrichment (carbon productivity), then used this data to build spectroscopy-based predictive models. The models were used to provide phenotypic data for samples from a third sample set. Importantly, this approach enabled the first ever rate determination of carbon productivity from a single time point, circumventing the need for time-series measurements. This study showed that Chaetoceros simplex was less productive and had lower protein and carbon content during short-term periods of high salinity. Applying this new phenomics approach to natural phytoplankton samples could provide valuable insight into understanding phytoplankton productivity and function in the marine system. 相似文献
22.
23.
24.
25.
26.
Affinity labeling studies of NADP(+)-glutamate dehydrogenase from Salmonella typhimurium have shown that the peptide Leu-282-Lys-286 is located near the coenzyme site [Haeffner-Gormley et al. (1991) J. Biol. Chem. 266, 5388-5394]. The present study was undertaken to evaluate the role of lysine-286. The mutant enzymes K286R, K286Q, and K286E were prepared by site-directed mutagenesis, expressed in Escherichia coli, and purified. The Vmax values (micromoles of NADPH per minute per milligram of protein) were similar for WT (270), K286R (529), K296Q (409), and K286E (382) enzymes. As measured at pH 7.9, the Km value for NADPH was much greater for K286E (280 microM) than for WT (9.8 microM), K286R (30 microM), or K286Q (66 microM) enzymes. The efficiencies (kcat/Km) of the WT and K286R mutant were similar (1.2 x 10(3) min-1 microM-1 and 1.0 x 10(3) min-1 microM-1, respectively) while those of K286Q (0.30 x 10(3) min-1 microM-1) and K286E (0.07 x 10(3) min-1 microM-1) were greatly reduced. The decreased efficiency of the K286E mutant results from the increase in Km-NADPH, consistent with a role for a basic residue at position 286 which enhances the binding of NADPH. Plots of Vmax vs pH showed the pH optima to be 8.1-8.3 for all enzymes at saturating NADPH concentrations. A 40-fold increase in Km-NADPH for K286E was observed as the pH increased from 5.98 to 8.08, from which a unique pKe of 6.5 was calculated.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
27.
28.
29.
Kevin Dougherty Brian A. Smith Autumn F. Moore Shannon Maitland Chris Fanger Rachel Murillo David A. Baltrus 《PloS one》2014,9(7)
Horizontal gene transfer often leads to phenotypic changes within recipient organisms independent of any immediate evolutionary benefits. While secondary phenotypic effects of horizontal transfer (i.e., changes in growth rates) have been demonstrated and studied across a variety of systems using relatively small plasmids and phage, little is known about the magnitude or number of such costs after the transfer of larger regions. Here we describe numerous phenotypic changes that occur after a large-scale horizontal transfer event (∼1 Mb megaplasmid) within Pseudomonas stutzeri including sensitization to various stresses as well as changes in bacterial behavior. These results highlight the power of horizontal transfer to shift pleiotropic relationships and cellular networks within bacterial genomes. They also provide an important context for how secondary effects of transfer can bias evolutionary trajectories and interactions between species. Lastly, these results and system provide a foundation to investigate evolutionary consequences in real time as newly acquired regions are ameliorated and integrated into new genomic contexts. 相似文献
30.