全文获取类型
收费全文 | 9883篇 |
免费 | 879篇 |
国内免费 | 837篇 |
专业分类
11599篇 |
出版年
2024年 | 26篇 |
2023年 | 155篇 |
2022年 | 333篇 |
2021年 | 572篇 |
2020年 | 403篇 |
2019年 | 437篇 |
2018年 | 423篇 |
2017年 | 307篇 |
2016年 | 433篇 |
2015年 | 612篇 |
2014年 | 787篇 |
2013年 | 759篇 |
2012年 | 925篇 |
2011年 | 778篇 |
2010年 | 542篇 |
2009年 | 423篇 |
2008年 | 460篇 |
2007年 | 495篇 |
2006年 | 408篇 |
2005年 | 341篇 |
2004年 | 303篇 |
2003年 | 234篇 |
2002年 | 201篇 |
2001年 | 175篇 |
2000年 | 157篇 |
1999年 | 153篇 |
1998年 | 71篇 |
1997年 | 69篇 |
1996年 | 73篇 |
1995年 | 52篇 |
1994年 | 63篇 |
1993年 | 54篇 |
1992年 | 76篇 |
1991年 | 60篇 |
1990年 | 39篇 |
1989年 | 53篇 |
1988年 | 29篇 |
1987年 | 33篇 |
1986年 | 24篇 |
1985年 | 25篇 |
1984年 | 8篇 |
1983年 | 11篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 5篇 |
1979年 | 6篇 |
1978年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
911.
912.
Fumi Shima Yuichi Ijiri Shin Muraoka Jingling Liao Min Ye Mitsugu Araki Kousuke Matsumoto Naoki Yamamoto Takeshi Sugimoto Yoko Yoshikawa Takashi Kumasaka Masaki Yamamoto Atsuo Tamura Tohru Kataoka 《The Journal of biological chemistry》2010,285(29):22696-22705
Ras family small GTPases assume two interconverting conformations, “inactive” state 1 and “active” state 2, in their GTP-bound forms. Here, to clarify the mechanism of state transition, we have carried out x-ray crystal structure analyses of a series of mutant H-Ras and M-Ras in complex with guanosine 5′-(β,γ-imido)triphosphate (GppNHp), representing various intermediate states of the transition. Crystallization of H-RasT35S-GppNHp enables us to solve the first complete tertiary structure of H-Ras state 1 possessing two surface pockets unseen in the state 2 or H-Ras-GDP structure. Moreover, determination of the two distinct crystal structures of H-RasT35S-GppNHp, showing prominent polysterism in the switch I and switch II regions, reveals a pivotal role of the guanine nucleotide-mediated interaction between the two switch regions and its rearrangement by a nucleotide positional change in the state 2 to state 1 transition. Furthermore, the 31P NMR spectra and crystal structures of the GppNHp-bound forms of M-Ras mutants, carrying various H-Ras-type amino acid substitutions, also reveal the existence of a surface pocket in state 1 and support a similar mechanism based on the nucleotide-mediated interaction and its rearrangement in the state 1 to state 2 transition. Intriguingly, the conformational changes accompanying the state transition mimic those that occurred upon GDP/GTP exchange, indicating a common mechanistic basis inherent in the high flexibility of the switch regions. Collectively, these results clarify the structural features distinguishing the two states and provide new insights into the molecular basis for the state transition of Ras protein. 相似文献
913.
914.
915.
First report of a novel plant lysozyme with both antifungal and antibacterial activities 总被引:6,自引:0,他引:6
Wang S Ng TB Chen T Lin D Wu J Rao P Ye X 《Biochemical and biophysical research communications》2005,327(3):820-827
A novel lysozyme exhibiting antifungal activity and with a molecular mass of 14.4kDa in SDS-polyacrylamide gel electrophoresis was isolated from mung bean (Phaseolus mungo) seeds using a procedure that involved aqueous extraction, ammonium sulfate precipitation, ion exchange chromatography on CM-Sephadex, and high-performance liquid chromatography on POROS HS-20. Its N-terminal sequence was very different from that of hen egg white lysozyme. Its pI was estimated to be above 9.7. The specific activity of the lysozyme was 355U/mg at pH 5.5 and 30 degrees C. The lysozyme exhibited a pH optimum at pH 5.5 and a temperature optimum at 55 degrees C. It is reported herein, for the first time, that a novel plant lysozyme exerted an antifungal action toward Fusarium oxysporum, Fusarium solani, Pythium aphanidermatum, Sclerotium rolfsii, and Botrytis cinerea, in addition to an antibacterial action against Staphylococcus aureus. 相似文献
916.
Xiaofang Ye Li Peng Haidong Kan Weibing Wang Fuhai Geng Zhe Mu Ji Zhou Dandan Yang 《PloS one》2016,11(3)
Introduction
Evidence based on ecological studies in China suggests that short-term exposure to particulate matter (PM) is associated with cardiovascular mortality. However, there is less evidence of PM-related morbidity for coronary heart disease (CHD) in China. This study aims to investigate the relationship between acute PM exposure and CHD incidence in people aged above 40 in Shanghai.Methods
Daily CHD events during 2005–2012 were identified from outpatient and emergency department visits. Daily average concentrations for particulate matter with aerodynamic diameter less than 10 microns (PM10) were collected over the 8-year period. Particulate matter with aerodynamic diameter less than 2.5 microns (PM2.5) were measured from 2009 to 2012. Analyses were performed using quasi-poisson regression models adjusting for confounders, including long-term trend, seasonality, day of the week, public holiday and meteorological factors. The effects were also examined by gender and age group (41–65 years, and >65 years).Results
There were 619928 CHD outpatient and emergency department visits. The average concentrations of PM10 and PM2.5 were 81.7μg/m3 and 38.6μg/m3, respectively. Elevated exposure to PM10 and PM2.5 was related with increased risk of CHD outpatients and emergency department visits in a short time course. A 10 μg/m3 increase in the 2-day PM10 and PM2.5 was associated with increase of 0.23% (95% CI: 0.12%, 0.34%) and 0.74% (95% CI: 0.44%, 1.04%) in CHD morbidity, respectively. The associations appeared to be more evident in the male and the elderly.Conclusion
Short-term exposure to high levels of PM10 and PM2.5 was associated with increased risk of CHD outpatient and emergency department visits. Season, gender and age were effect modifiers of their association. 相似文献917.
In recent years, animal ethics issues have led researchers to explore nondestructive methods to access materials for genetic studies. Cicada exuviae are among those materials because they are cast skins that individuals left after molt and are easily collected. In this study, we aim to identify the most efficient extraction method to obtain high quantity and quality of DNA from cicada exuviae. We compared relative DNA yield and purity of six extraction protocols, including both manual protocols and available commercial kits, extracting from four different exoskeleton parts. Furthermore, amplification and sequencing of genomic DNA were evaluated in terms of availability of sequencing sequence at the expected genomic size. Both the choice of protocol and exuvia part significantly affected DNA yield and purity. Only samples that were extracted using the PowerSoil DNA Isolation kit generated gel bands of expected size as well as successful sequencing results. The failed attempts to extract DNA using other protocols could be partially explained by a low DNA yield from cicada exuviae and partly by contamination with humic acids that exist in the soil where cicada nymphs reside before emergence, as shown by spectroscopic measurements. Genomic DNA extracted from cicada exuviae could provide valuable information for species identification, allowing the investigation of genetic diversity across consecutive broods, or spatiotemporal variation among various populations. Consequently, we hope to provide a simple method to acquire pure genomic DNA applicable for multiple research purposes. 相似文献
918.
Hydrolysis of lignocellulosic materials for ethanol production: a review 总被引:85,自引:0,他引:85
Lignocellulosic biomass can be utilized to produce ethanol, a promising alternative energy source for the limited crude oil. There are mainly two processes involved in the conversion: hydrolysis of cellulose in the lignocellulosic biomass to produce reducing sugars, and fermentation of the sugars to ethanol. The cost of ethanol production from lignocellulosic materials is relatively high based on current technologies, and the main challenges are the low yield and high cost of the hydrolysis process. Considerable research efforts have been made to improve the hydrolysis of lignocellulosic materials. Pretreatment of lignocellulosic materials to remove lignin and hemicellulose can significantly enhance the hydrolysis of cellulose. Optimization of the cellulase enzymes and the enzyme loading can also improve the hydrolysis. Simultaneous saccharification and fermentation effectively removes glucose, which is an inhibitor to cellulase activity, thus increasing the yield and rate of cellulose hydrolysis. 相似文献
919.
To better understand the relationship between salinity and the carbon stable isotope composition (expressed as δ13C) of mangrove plants and to test whether the patterns of variation in δ13C of mangrove plants differ from those of nonhalophytes as response to salinity, the effect of salinity on leaf δ13C in two dominant mangrove species, Aegiceras corniculatum and Kandelia candel, was studied. Furthermore, to determine whether the variation in δ13C of mangrove species is adjusted by stomatal conductance, K. candel was selected as an example, and leaf gas-exchange characteristics of the seedlings were measured. It was observed that both
mangrove species had a lower leaf δ13C under their optimum salinity (1.50% for Ae. corniculatum and 2.00% for K. candel). This variation in δ13C of mangrove plants was attributable largely to stomatal adjustment as for nonhalophytes in which a strong correlation between
δ13C and relevant photosynthetic properties is observed. This result suggests that the different response pattern in δ13C was a consequence of the variation in stomata in relation to the different tolerance to salinity. The optimum salinity inferred
by leaf δ13C provides a feasible method for comparing salt tolerance between mangrove plants belonging to different species, which is
useful for mangrove restoration. 相似文献
920.