全文获取类型
收费全文 | 7623篇 |
免费 | 570篇 |
国内免费 | 607篇 |
专业分类
8800篇 |
出版年
2024年 | 17篇 |
2023年 | 114篇 |
2022年 | 273篇 |
2021年 | 469篇 |
2020年 | 268篇 |
2019年 | 334篇 |
2018年 | 303篇 |
2017年 | 254篇 |
2016年 | 330篇 |
2015年 | 461篇 |
2014年 | 544篇 |
2013年 | 569篇 |
2012年 | 675篇 |
2011年 | 579篇 |
2010年 | 346篇 |
2009年 | 358篇 |
2008年 | 393篇 |
2007年 | 312篇 |
2006年 | 287篇 |
2005年 | 211篇 |
2004年 | 225篇 |
2003年 | 219篇 |
2002年 | 139篇 |
2001年 | 137篇 |
2000年 | 131篇 |
1999年 | 148篇 |
1998年 | 87篇 |
1997年 | 76篇 |
1996年 | 82篇 |
1995年 | 75篇 |
1994年 | 61篇 |
1993年 | 31篇 |
1992年 | 62篇 |
1991年 | 44篇 |
1990年 | 40篇 |
1989年 | 22篇 |
1988年 | 24篇 |
1987年 | 30篇 |
1986年 | 16篇 |
1985年 | 33篇 |
1984年 | 8篇 |
1983年 | 5篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1979年 | 2篇 |
排序方式: 共有8800条查询结果,搜索用时 0 毫秒
51.
52.
53.
54.
Correlation of structure to antitumor activities of five derivatives of a beta-glucan from Poria cocos sclerotium 总被引:2,自引:0,他引:2
A water-insoluble (1-->3)-beta-D-glucan isolated from fresh sclerotium of Poria cocos was, respectively, sulfated, carboxymethylated, methylated, hydroxyethylated, and hydroxypropylated, to afford five water-soluble derivatives. Their weight-average molecular masses (Mw) and intrinsic viscosities ([eta]) were determined by size-exclusion chromatography combined with laser light scattering (SEC-LLS), LLS, and viscometry in phosphate buffer solution (PBS) at 37 degrees C. The antitumor activities, against Sarcoma 180 tumor cell (S-180) and gastric carcinoma cell strain (MKN-45 and SGC-7901) of the native beta-glucan and the five derivatives, were tested in vitro and in vivo. The Mw values of the five derivatives in PBS were determined to be 3.8 x 10(4), 18.9 x 10(4), 16.0 x 10(4), 76.8 x 10(4), and 224.3 x 10(4), respectively. The high Mw values of the hydroxyethylated and hydroxypropylated derivatives in aqueous solution resulted from aggregation, and their true Mw values obtained in dimethyl sulfoxide were 20.1 x 10(4) and 19.1 x 10(4). The sulfated and carboxymethylated derivatives having DS of 1.0-1.3 show good water solubility, and exist as relatively expanded chains in aqueous solution. Interestingly, the native beta-glucan did not show antitumor activity, whereas the sulfated and carboxymethylated derivatives exhibit significant antitumor activities against S-180 and gastric carcinoma tumor cells. This work showed that good water solubility, relatively high chain stiffness, and moderate molecular mass of the derivatives in aqueous solution contribute beneficial to enhancement of antitumor activity. 相似文献
55.
Hermjakob H Montecchi-Palazzi L Bader G Wojcik J Salwinski L Ceol A Moore S Orchard S Sarkans U von Mering C Roechert B Poux S Jung E Mersch H Kersey P Lappe M Li Y Zeng R Rana D Nikolski M Husi H Brun C Shanker K Grant SG Sander C Bork P Zhu W Pandey A Brazma A Jacq B Vidal M Sherman D Legrain P Cesareni G Xenarios I Eisenberg D Steipe B Hogue C Apweiler R 《Nature biotechnology》2004,22(2):177-183
56.
57.
Despite the importance of stem cells in plant and animal development, the common mechanisms of stem cell maintenance in both systems have remained elusive. Recently, the importance of hydrogen peroxide (H2O2) signaling in priming stem cell differentiation has been extensively studied in animals. Here, we show that different forms of reactive oxygen species (ROS) have antagonistic roles in plant stem cell regulation, which were established by distinct spatiotemporal patterns of ROS‐metabolizing enzymes. The superoxide anion () is markedly enriched in stem cells to activate WUSCHEL and maintain stemness, whereas H2O2 is more abundant in the differentiating peripheral zone to promote stem cell differentiation. Moreover, H2O2 negatively regulates biosynthesis in stem cells, and increasing H2O2 levels or scavenging leads to the termination of stem cells. Our results provide a mechanistic framework for ROS‐mediated control of plant stem cell fate and demonstrate that the balance between and H2O2 is key to stem cell maintenance and differentiation. 相似文献
58.
The rhizospheric bacterium JW-SD2 was identified as Pseudomonas frederiksbergensis based on phenotypic features, the Biolog Identification System and 16S rRNA sequence analysis. The phosphate-solubilizing activity, acidification in culture media, growth rate and organic acid secretion of JW-SD2 were investigated during 192 h of cultivation. The phosphate solubilized by JW-SD2 reached 7.75 mM. The decrease of pH and increase of titratable acidity were closely correlated (Pearson’s r?=??0.953 and 0.969, respectively) with the phosphate-solubilizing activity. High concentrations of gluconic, 2-ketogluconic, pyruvic, maleic and malic acids were detected before 96 h of culture, when the strain displayed a high level of phosphate-solubilizing activity, indicating that these organic acids were efficient components in phosphate solubilization. However, acetic acid did not affect phosphate solubilization as shown by a remarkable increase at 144 h of culture when the phosphate-solubilizing activity decreased. The phosphate-solubilizing ability of JW-SD2 was significantly (P?<?0.01) affected by environmental factors. Over a broad ranges of temperature (20?35 °C), pH (4?9), salinity (0?3.0 %), and volume of medium (1/5?3/5 of flask volume), the phosphate solubilized by JW-SD2 remained above 4.00 mM, demonstrating good potential in adapting to a changing environment. The inoculation experiments indicated that JW-SD2 could significantly (P?<?0.05) promote growth of poplar (Populus euramericana cv. NL-895) in both sterilized and non-sterilized soils. The effects of plant growth promotion were greater in non-sterilized than in sterilized soil. During the 150 days of the trial, the effects of plant growth promotion by JW-SD2 first increased then decreased over time, suggesting that, in field applications, the periodic supplementation of the strain into the rhizosphere should be considered. 相似文献
59.
Weak cell cycle dependency but strong distortive effects of transfection with Lipofectamine 2000 in near‐physiologically synchronized cell culture 下载免费PDF全文
Previously, we reported a method to generate and validate cell cycle‐synchronized cultures of multiple mammalian suspension cell lines under near‐physiological conditions. This method was applied to elucidate the putative interdependencies of the cell cycle and recombinant protein expression in the human producer cell line HEK293s using Lipofectamine 2000 and the reporter plasmid pcDNA3.3 enhanced green fluorescent protein, destabilized using PEST sequence. A population‐resolved modeling approach was applied to quantitatively assess putative variations of cell cycle dependent expression rates based on the obtained experimental data. We could not confirm results published earlier by other groups, based on nonphysiological synchronization attempts, reporting transfection efficiency being strongly dependent on the cell cycle phase at transfection time point. On the other hand, it is demonstrated that transfection and protein expression distort the progression of the cell cycle. 相似文献
60.
Ouyang G Chen Z Cai XJ Song BA Bhadury PS Yang S Jin LH Xue W Hu DY Zeng S 《Bioorganic & medicinal chemistry》2008,16(22):9699-9707
Fourteen title compounds, 1-substituted-5-substitutedphenylthio-4-pyrazolaldoxime ester derivatives 4a-4n, were synthesized from the starting material 1-substitutedphenyl-3-methyl-5-substitutedphenylthio-4-pyrazolaldoximes 3 by treatment with acyl chloride. The synthesized compounds were characterized by physical constants, and the structures of the title compounds were further confirmed by IR, 1H NMR, 13C NMR and elemental analysis. The bioassay results showed that title compounds possessed weak to good anti-TMV bioactivity with 4l showing significant enhancement of disease resistance in tobacco leaves with high affinity for TMV CP. 相似文献