首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   9254篇
  免费   704篇
  国内免费   990篇
  10948篇
  2024年   24篇
  2023年   124篇
  2022年   312篇
  2021年   509篇
  2020年   378篇
  2019年   451篇
  2018年   376篇
  2017年   275篇
  2016年   408篇
  2015年   587篇
  2014年   701篇
  2013年   784篇
  2012年   888篇
  2011年   767篇
  2010年   492篇
  2009年   464篇
  2008年   531篇
  2007年   468篇
  2006年   387篇
  2005年   297篇
  2004年   293篇
  2003年   260篇
  2002年   213篇
  2001年   146篇
  2000年   128篇
  1999年   129篇
  1998年   82篇
  1997年   62篇
  1996年   53篇
  1995年   60篇
  1994年   64篇
  1993年   40篇
  1992年   36篇
  1991年   43篇
  1990年   31篇
  1989年   23篇
  1988年   11篇
  1987年   10篇
  1986年   12篇
  1985年   12篇
  1984年   4篇
  1983年   5篇
  1982年   3篇
  1981年   2篇
  1980年   3篇
排序方式: 共有10000条查询结果,搜索用时 13 毫秒
991.
The process of initiation of host invasion and survival of some foliar phytopathogenic fungi in the absence of external nutrients on host leaf surfaces remains obscure. Here, we demonstrate that gluconeogenesis plays an important role in the process and nutrient‐starvation adaptation before the pathogen host invasion. Deletion of phosphoenolpyruvate c arboxyk inase gene BcPCK1 in gluconeogenesis in Botrytis cinerea, the causative agent of grey mould, resulted in the failure of the ΔBcpck1 mutant conidia to germinate on hard and hydrophobic surface and penetrate host cells in the absence of glucose, reduction in conidiation and slow conidium germination in a nutrient‐rich medium. The wild‐type and ΔBcpck1 conidia germinate similarly in the presence of glucose (higher concentration) as the sole carbon source. Conidial glucose‐content should reach a threshold level to initiate germination and host penetration. Infection structure formation by the mutants displayed a glucose‐dependent fashion, which corresponded to the mutant virulence reduction. Exogenous glucose or complementation of BcPCK1 completely rescued all the developmental and virulence defects of the mutants. Our findings demonstrate that BcPCK1 plays a crucial role in B. cinerea pathogenic growth and virulence, and provide new insights into gluconeogenesis mediating pathogenesis of plant fungal pathogens via initiation of conidial germination and host penetration.  相似文献   
992.
Electrochemical reduction of CO2 provides an opportunity to reach a carbon‐neutral energy recycling regime, in which CO2 emissions from fuel use are collected and converted back to fuels. The reduction of CO2 to CO is the first step toward the synthesis of more complex carbon‐based fuels and chemicals. Therefore, understanding this step is crucial for the development of high‐performance electrocatalyst for CO2 conversion to higher order products such as hydrocarbons. Here, atomic iron dispersed on nitrogen‐doped graphene (Fe/NG) is synthesized as an efficient electrocatalyst for CO2 reduction to CO. Fe/NG has a low reduction overpotential with high Faradic efficiency up to 80%. The existence of nitrogen‐confined atomic Fe moieties on the nitrogen‐doped graphene layer is confirmed by aberration‐corrected high‐angle annular dark‐field scanning transmission electron microscopy and X‐ray absorption fine structure analysis. The Fe/NG catalysts provide an ideal platform for comparative studies of the effect of the catalytic center on the electrocatalytic performance. The CO2 reduction reaction mechanism on atomic Fe surrounded by four N atoms (Fe–N4) embedded in nitrogen‐doped graphene is further investigated through density functional theory calculations, revealing a possible promotional effect of nitrogen doping on graphene.  相似文献   
993.
To develop novel anti-inflammatory agents, a series of new pentadienone oxime ester compounds were designed and synthesized. The structures were determined by IR, 1H NMR, 13?C NMR, and HRMS. All compounds have been screened for their anti-inflammatory activity by evaluating their inhibition against LPS-induced nitric oxide (NO) release in RAW 264.7 cell. Among them, compound 5j was found to be one of the most potent compounds in inhibiting NO and IL-6 (IC50 values were 6.66?µM and 5.07?µM, respectively). Preliminary mechanism studies show that title compound 5j could significantly suppress expressions of nitric oxide synthase, COX-2, and NO, IL-6 through Toll-like receptor 4/mitogen-activated protein kinases/NF-κB signalling pathway. These data support further studies to assess rational design of more efficient pentadienone oxime ester derivatives with anti-inflammatory activity in the future.  相似文献   
994.
Protein tyrosine phosphatase 1B (PTP1B) is an attractive molecular target for anti-diabetes, anti-obesity, and anti-cancer drug development. From the seeds of Silybum marianum, nine flavonolignans, namely, silybins A, B (1, 2), isosilybins A, B (3, 4), silychristins A, B (5, 6), isosilychristin A (7), dehydrosilychristin A (8), and silydianin (11) were identified as a novel class of natural PTP1B inhibitors (IC50 1.3 7–23.87?µM). Analysis of structure–activity relationship suggested that the absolute configurations at C-7" and C-8" greatly affected the PTP1B inhibitory activity. Compounds 15 were demonstrated to be non-competitive inhibitors of PTP1B based on kinetic analyses. Molecular docking simulations resulted that 15 docked into the allosteric site, including α3, α6, and α7 helix of PTP1B. At a concentration inhibiting PTP1B completely, compounds 15 moderately inhibited VHR and SHP-2, and weakly inhibited TCPTP and SHP-1. These results suggested the potentiality of these PTP1B inhibitors as lead compounds for further drug developments.  相似文献   
995.
Quantitative evidence of sudden shifts in ecological structure and function in large shallow lakes is rare, even though they provide essential benefits to society. Such ‘regime shifts’ can be driven by human activities which degrade ecological stability including water level control (WLC) and nutrient loading. Interactions between WLC and nutrient loading on the long‐term dynamics of shallow lake ecosystems are, however, often overlooked and largely underestimated, which has hampered the effectiveness of lake management. Here, we focus on a large shallow lake (Lake Chaohu) located in one of the most densely populated areas in China, the lower Yangtze River floodplain, which has undergone both WLC and increasing nutrient loading over the last several decades. We applied a novel methodology that combines consistent evidence from both paleolimnological records and ecosystem modeling to overcome the hurdle of data insufficiency and to unravel the drivers and underlying mechanisms in ecosystem dynamics. We identified the occurrence of two regime shifts: one in 1963, characterized by the abrupt disappearance of submerged vegetation, and another around 1980, with strong algal blooms being observed thereafter. Using model scenarios, we further disentangled the roles of WLC and nutrient loading, showing that the 1963 shift was predominantly triggered by WLC, whereas the shift ca. 1980 was attributed to aggravated nutrient loading. Our analysis also shows interactions between these two stressors. Compared to the dynamics driven by nutrient loading alone, WLC reduced the critical P loading and resulted in earlier disappearance of submerged vegetation and emergence of algal blooms by approximately 26 and 10 years, respectively. Overall, our study reveals the significant role of hydrological regulation in driving shallow lake ecosystem dynamics, and it highlights the urgency of using multi‐objective management criteria that includes ecological sustainability perspectives when implementing hydrological regulation for aquatic ecosystems around the globe.  相似文献   
996.
Aromatase is the key enzyme responsible for catalyzing the conversion of C19 steroids to estrogens. Its inhibitors are widely used in breast cancer therapy. The CH2Cl2 partition of a crude ethanolic extract from the roots of Flemingia philippinensis showed potent inhibitory activity of aromatase. The constituents of the extract were analyzed and identified by liquid chromatography tandem mass spectrometry. Five purified prenylated isoflavones were evaluated for aromatase inhibition and their IC50 values ranged between 2.98 and 58.08 μm . In kinetic studies, all tested compounds behaved as reversible competitive inhibitors and their Ki values were calculated by Dixon plots. The most potent inhibitor (6,8‐diprenylorobol) had a Ki value of 1.42 μm . Furthermore, using UPLC and LC/MS, 6,8‐diprenylorobol was proven to be present in the native roots in high quantities.  相似文献   
997.
接种外生菌根对辽东栎幼苗生长的影响   总被引:13,自引:0,他引:13       下载免费PDF全文
 辽东栎(Quercus liaotungensis)是中国特有的栎林树种,也是中国暖温带落叶阔叶林的主要优势树种之一。铆钉菇(Gomphidius viscidus)和臭红菇(Russula foetens)是在自然环境中与其共生形成外生菌根的真菌。在温室花盆中播种辽东栎种子获得辽东栎幼苗,并对幼苗接种铆钉菇和臭红菇合成外生菌根,比较了有菌根和无菌根辽东栎幼苗生长、光合蒸腾特性、氮磷含量的差异。外生菌根对辽东栎幼苗的生长有明显的促进作用,有菌根幼苗的生物量、株高、净光合速率和水分利用效率高于无菌根幼苗,蒸腾速率则相反。有菌根幼苗的氮磷含量分别为无菌根幼苗的1.7倍和2.2倍,外生菌根的合成还改变了氮磷在幼苗器官间的分配比例,与无菌根幼苗相比,有菌根幼苗茎中的氮磷减少,而叶片中的磷显著增加。同时接种铆钉菇和臭红菇的生长促进效果优于单独接种。  相似文献   
998.
999.
建立了纯化光合膜蛋白质复合体Cytb6f的新方法。与现有方法相比,新方法简单明了,只包括透析离心和用硫酸铵分级沉淀两个步骤,而且适于大批量制备。按此方法从菠菜叶绿体分离纯化的制剂含9.8nmolCytf·mg-1;SDSPAGE显示4条主要区带及1条低分子量区带,背景干净;Cytb6(b型血红素)∶Cytf=2∶1(mol∶mol),Chla∶Cytf=1∶1(mol∶mol);酶活性(PQ2H2→Cytc)80μmolCytc·nmolCytf-1·h-1左右;产率可达38%。  相似文献   
1000.
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   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号