全文获取类型
收费全文 | 1700篇 |
免费 | 159篇 |
国内免费 | 200篇 |
出版年
2024年 | 4篇 |
2023年 | 29篇 |
2022年 | 56篇 |
2021年 | 82篇 |
2020年 | 67篇 |
2019年 | 76篇 |
2018年 | 82篇 |
2017年 | 77篇 |
2016年 | 99篇 |
2015年 | 134篇 |
2014年 | 152篇 |
2013年 | 143篇 |
2012年 | 156篇 |
2011年 | 142篇 |
2010年 | 101篇 |
2009年 | 69篇 |
2008年 | 89篇 |
2007年 | 73篇 |
2006年 | 67篇 |
2005年 | 38篇 |
2004年 | 46篇 |
2003年 | 42篇 |
2002年 | 35篇 |
2001年 | 19篇 |
2000年 | 23篇 |
1999年 | 31篇 |
1998年 | 11篇 |
1997年 | 5篇 |
1996年 | 7篇 |
1995年 | 8篇 |
1994年 | 14篇 |
1993年 | 6篇 |
1992年 | 16篇 |
1991年 | 10篇 |
1990年 | 4篇 |
1989年 | 5篇 |
1988年 | 5篇 |
1987年 | 8篇 |
1985年 | 6篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1981年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1973年 | 2篇 |
1971年 | 1篇 |
1969年 | 1篇 |
1968年 | 2篇 |
排序方式: 共有2059条查询结果,搜索用时 234 毫秒
171.
In this study, a high-throughput microfluidic system is presented. The system is comprised of seven parallel channels. Each channel contains 32 square-shaped microchambers. After simulation studies on samples loaded into the microchambers, and the solute exchange between the microchambers and channels, the long-term culture of Escherichia coli (E. coli) HB101 in the microchambers is realized. Using the principle that L-arabinose (L-Ara) can induce recombinant E. coli HB101 pGLO to synthesize green fluorescent protein (GFP), the real-time analysis of GFP expression in different initial bacterial densities is performed. The results demonstrate that higher initial loading densities of the bacterial colony cause bacterial cell to enter log-phase proliferation sooner. High or low initial loading densities of the bacterial cell suspension induce the same maximum growth rates during the log-phase. Quantitative on-chip analysis of tetracycline and erythromycin inhibition on bacterial cell growth is also conducted. Bacterial morphology changes during antibiotic treatment are observed. The results show that tetracycline and erythromycin exhibit different inhibition activities in E. coli cells. Concentrations of 3 μg/mL tetracycline can facilitate the formation of long filamentous bacteria with the average length of more than 50 μm. This study provides an on-chip framework for bacteriological research in a high-throughput manner and the development of recombinant bacteria-based biosensors for the detection of specific substances. 相似文献
172.
A novel quantum dots (QDs) based biosensor was developed to monitor physiological pH changes in both fixed and living cells by means of pH-dependent emission color of the QDs. In our system, the nominally single-sized colloidal solution samples of the L-cysteine-capped CdTe QDs with intrinsically broadened size distributions were prepared by employing aqueous synthesis technique. The quench of fluorescence intensities of the QDs with a 16 nm red shift of the emission maximum and a color change from green to yellow was observed with a slight pH decrease (from 7.0 to 6.8) in the system. This pH-dependent emission could be attributed to the efficient exciton energy transfer from smaller QDs to larger ones, which was controlled by electrostatic-tuned aggregation/disaggregation (low/high pH values) processes of the QDs. In addition to high stability, the emission shift of the QDs was reversible for at least one cycle under optimal conditions. Our pH biosensor may find potential application for monitoring the intracellular pH changes in both physiological and pathological conditions. 相似文献
173.
Propionic acid production by Propionibacterium freudenreichii from molasses and waste propionibacterium cells was studied in plant fibrous-bed bioreactor (PFB). With non-treated molasses as carbon source, 12.69 ± 0.40 g l-1 of propionic acid was attained at 120 h in free-cell fermentation, whereas the PFB fermentation yielded 41.22 ± 2.06 g l-1 at 120 h and faster cells growth was observed. In order to optimize the fermentation outcomes, fed-batch fermentation was performed with hydrolyzed molasses in PFB, giving 91.89 ± 4.59 g l-1 of propionic acid at 254 h. Further studies were carried out using hydrolyzed waste propionibacterium cells as substitute nitrogen source, resulting in a propionic acid concentration of 79.81 ± 3.99 g l-1 at 302 h. The present study suggests that the low-cost molasses and waste propionibacterium cells can be utilized for the green and economical production of propionic acid by P. freudenreichii. 相似文献
174.
Wang GZ Haile PA Daniel T Belot B Viet AQ Goodman KB Sha D Dowdell SE Varga N Hong X Chakravorty S Webb C Cornejo C Olzinski A Bernard R Evans C Emmons A Briand J Chung CW Quek R Lee D Gough PJ Sehon CA 《Bioorganic & medicinal chemistry letters》2011,21(24):7291-7294
A series of biarylsulfonamides was identified as hCCR2 receptor antagonist but suffered from high plasma protein binding resulting in a >100 fold shift in activity in a functional GTPγS assay run in tandem in the presence and absence of human serum albumin. Introduction of an aryl amide with ethylenediamine linker led to compounds with reduced shifts and improved activity in whole blood. 相似文献
175.
白马雪山国家级自然保护区典型森林生态系统服务 总被引:1,自引:0,他引:1
生态系统服务是近年来生态学研究的热点领域,对关键区域生态系统服务的研究具有重要意义.云南省白马雪山国家级自然保护区地处青藏高原南延部分,拥有独特的地理位置,是生物多样性保护的热点区域.本文对该保护区森林生态系统的生物量与生产力、水源涵养、营养物质循环等3项服务的功能量进行了评估.结果表明:保护区森林总生物量2215.86×104t,生产力171.84×104t·a-1;水源涵养量11964.56×104m3;N、P、K年吸收量分别为26025.94t、2638.57t、12016.85 t.研究表明,保护区森林生态效益显著,对于维持当地以及周边地区的生态安全具有重要意义. 相似文献
176.
177.
The prognosis of patients with myocardial infarction (MI) and resultant chronic heart failure remains extremely poor despite advances in optimal medical therapy and interventional procedures. Animal experiments and clinical trials using adult stem cell therapy following MI have shown a global improvement of myocardial function. Bone marrow-derived mesenchymal stem cells (MSCs) hold promise for cardiac repair following MI, due to their multilineage, self-renewal and proliferation potential. In addition, MSCs can be easily isolated, expanded in culture, and have immunoprivileged properties to the host tissue. Experimental studies and clinical trials have revealed that MSCs not only differentiate into cardiomyocytes and vascular cells, but also secrete amounts of growth factors and cytokines which may mediate endogenous regeneration via activation of resident cardiac stem cells and other stem cells, as well as induce neovascularization, anti-inflammation, anti-apoptosis, anti-remodelling and cardiac contractility in a paracrine manner. It has also been postulated that the anti-arrhythmic and cardiac nerve sprouting potential of MSCs may contribute to their beneficial effects in cardiac repair. Most molecular and cellular mechanisms involved in the MSC-based therapy after MI are still unclear at present. This article reviews the potential repair mechanisms of MSCs in the setting of MI. 相似文献
178.
Three new sesquiterpene acids, xylaric acids A-C (1-3, resp.), and a new tetralone (=3,4-dihydronaphthalen-1(2H)-one) derivative, 4, along with nine known compounds, xylaric acid D (5), hydroheptelidic acid (6), gliocladic acid (7), chlorine heptelidic acid (8), trichoderonic acid A (9), 16-(α-D-mannopyranosyloxy)isopimar-7-en-19-oic acid (10), 16-(α-D-glucopyranosyloxy)isopimar-7-en-19-oic acid (11), 5-carboxymellein (12), and naphthalen-1,8-diol 1-O-α-D-glucopyranoside (13) have been isolated from the solid culture of the ascomycete fungus Xylaria sp. associated with termite nest. The structures of these compounds were elucidated primarily by NMR experiments. The absolute configurations of compounds 1-3 and 5-9 were determined by combination of X-ray data and CD spectral analysis. The absolute configuration of 4 was assigned by Snatzke's method. Compounds 8 and 11 showed slight cytotoxicities against two cell lines A549 and SGC7901. 相似文献
179.
A highly selective sucrose isomerase (SIase) was purified to homogeneity from the cell-free extract of Erwinia rhapontici NX-5 with a recovery of 27.7% and a fold purification of 213.6. The purified SIase showed a high specific activity of 427.1 U mg−1 with molecular weight of 65.6 kDa. The K
m for sucrose was 222 mM while V
max was 546 U mg−1. The optimum pH and temperature for SIase activity were 6.0 and 30 °C, respectively. The purified SIase was stable in the
temperature range of 10–40 °C and retained 65% of the enzyme activity after 2 weeks’ storage at 30 °C. The SIase activity
was enhanced by Mg2+ and Mn2+, inhibited by Ca2+, Cu2+, Zn2+, and Co2+, completely inhibited by Hg2+ and Ag2+. The purified SIase was strongly inhibited by SDS, while partially inhibited by dimethylformamide, tetrahydrofuran, and PMSF.
Additionally, glucose and fructose acted as competitive inhibitors for purified SIase. 相似文献
180.
Hui Li Hong Xu Hao Xu Sha Li Han-Jie Ying Ping-Kai Ouyang 《Bioprocess and biosystems engineering》2011,34(1):95-102
Batch fermentative production of welan gum by Alcaligenes sp. CGMCC2428 was investigated under various oxygen supply conditions using regulating agitation speed. Based on a three
kinetic parameters analysis that includes specific cell growth rate (μ), specific glucose consumption rate (q
s), and specific welan formation rate (q
p), a two-stage agitation speed control strategy was proposed to achieve high concentration, high yield, and high viscosity
of welan. During the first 22 h, the agitation speed in 7.5 L fermenter was controlled at 800 rpm to maintain high μ for cell growth. The agitation was then reduced step-wise to 600 rpm to maintain a changing profile with stable dissolved
oxygen levels and obtain high qp for high welan accumulation. Finally, the maximum concentration of welan was reached at 26.3 ± 0.89 g L−1 with a yield of 0.53 ± 0.003 g g−1 and the welan gum viscosity of 3.05 ± 0.10 Pa s, which increased by an average of 15.4, 15.2, and 20.1% over the best results
controlled by constant agitation speeds. 相似文献