全文获取类型
收费全文 | 41924篇 |
免费 | 3540篇 |
国内免费 | 2447篇 |
出版年
2024年 | 84篇 |
2023年 | 470篇 |
2022年 | 1107篇 |
2021年 | 1803篇 |
2020年 | 1198篇 |
2019年 | 1412篇 |
2018年 | 1565篇 |
2017年 | 1104篇 |
2016年 | 1609篇 |
2015年 | 2478篇 |
2014年 | 2742篇 |
2013年 | 3060篇 |
2012年 | 3657篇 |
2011年 | 3299篇 |
2010年 | 2058篇 |
2009年 | 1787篇 |
2008年 | 2169篇 |
2007年 | 2023篇 |
2006年 | 1791篇 |
2005年 | 1568篇 |
2004年 | 1384篇 |
2003年 | 1248篇 |
2002年 | 1127篇 |
2001年 | 888篇 |
2000年 | 817篇 |
1999年 | 744篇 |
1998年 | 396篇 |
1997年 | 336篇 |
1996年 | 300篇 |
1995年 | 259篇 |
1994年 | 250篇 |
1993年 | 190篇 |
1992年 | 319篇 |
1991年 | 310篇 |
1990年 | 264篇 |
1989年 | 225篇 |
1988年 | 194篇 |
1987年 | 207篇 |
1986年 | 159篇 |
1985年 | 153篇 |
1984年 | 102篇 |
1983年 | 93篇 |
1982年 | 83篇 |
1981年 | 65篇 |
1979年 | 97篇 |
1978年 | 80篇 |
1977年 | 78篇 |
1976年 | 65篇 |
1975年 | 67篇 |
1974年 | 75篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
2.
Plant somatic cells have the capability to switch their cell fates from differentiated to undifferentiated status under proper
culture conditions, which is designated as totipotency. As a result, plant cells can easily regenerate new tissues or organs
from a wide variety of explants. However, the mechanism by which plant cells have such remarkable regeneration ability is
still largely unknown. In this study, we used a set of meristem-specific marker genes to analyze the patterns of stem cell
differentiation in the processes of somatic embryogenesis as well as shoot or root organogenesis in vitro. Our studies furnish preliminary and important information on the patterns of the de novo stem cell differentiation during various types of in vitro organogenesis. 相似文献
3.
4.
Wen-Yeh Hsieh Tang-Ching Kuan Kun-Shan Cheng Yan-Chiou Liao Mu-Yuan Chen Pei-Heng Lin Yuan-Chang Hsu Chen-Yi Huang Wei-Hua Hsu Sheng-Yao Yu Chih-Sheng Lin 《International journal of biological sciences》2012,8(8):1197-1205
Objective: Pleural effusion is common problem, but the rapid and reliable diagnosis for specific pathogenic effusions are lacking. This study aimed to identify the diagnosis based on clinical variables to differentiate pleural tuberculous exudates from other pleural effusions. We also investigated the role of renin-angiotensin system (RAS) and matrix metalloproteinase (MMPs) in the pathogenesis of pleural exudates.Experimental design: The major components in RAS and extracellular matrix metabolism, including angiotensin converting enzyme (ACE), ACE2, MMP-2 and MMP-9 activities, were measured and compared in the patients with transudative (n = 45) and exudative (n = 80) effusions. The exudative effusions were come from the patients with tuberculosis (n = 20), pneumonia (n = 32), and adenocarcinoma (n = 28).Results: Increased ACE and equivalent ACE2 activities, resulting in a significantly increased ACE/ACE2 ratio in exudates, were detected compared to these values in transudates. MMP-9 activity in exudates was significantly higher than that in transudates. The significant correlation between ACE and ACE2 activity that was found in transudates was not found in exudates. Advanced analyses showed significantly increased ACE and MMP-9 activities, and decreased ACE2 activity in tuberculous pleural effusions compared with those in pneumonia and adenocarcinoma effusions. The results indicate that increased ACE and MMP-9 activities found in the exudates were mainly contributed from a higher level of both enzyme activities in the tuberculous pleural effusions.Conclusion: Interplay between ACE and ACE2, essential functions in the RAS, and abnormal regulation of MMP-9 probably play a pivotal role in the development of exudative effusions. Moreover, the ACE/ACE2 ratio combined with MMP-9 activity in pleural fluid may be potential biomarkers for diagnosing tuberculous pleurisy. 相似文献
5.
6.
7.
以半矮秆育种为代表的“绿色革命”极大地提高了作物产量,但也带来氮营养利用效率降低的严重问题。“绿色革命”主要基于调控赤霉素的代谢和信号转导而实现。前期的研究发现,赤霉素信号转导关键因子DELLA蛋白通过调控GRF4而负调控氮素的吸收利用,为半矮秆品系氮利用效率低的问题提供了解决方案。最近的一项研究进一步揭示了GA信号途径与氮响应交叉互作的新机制。该研究发现水稻(Oryza sativa)NGR5是氮素调控分蘖数目的一个关键基因,其表达受氮诱导。通过招募PRC2,NGR5对D14和OsSPL14等分蘖抑制基因所在位点进行H3K27me3甲基化修饰,从而抑制其表达。而在半矮秆背景下超表达NGR5可以提高低氮水平下的水稻产量。NGR5同时也被发现为赤霉素受体GID1的一个新靶标,受到其负调控。该研究发现了调控赤霉素信号通路的新机制,并对高产高效的新一代“绿色革命”育种实践具有重要启示。 相似文献
8.
9.
Correlation between bilayer destabilization and activity enhancement by diacylglycerols in reconstituted Ca-ATPase vesicles 总被引:1,自引:0,他引:1
Using the reconstituted Ca-ATPase vesicles as a model system, we demonstrated that the presence of 1,2-dioleoyl-sn-glycerol (diolein) in the membrane introduces a pronounced enhancement in the Ca-transport function of Ca-ATPase, while the 1,2-dipalmitoyl-sn-glycerol (dipalmitin) does not. We also found by both 31P NMR and freeze-fraction electron microscopy that diolein destabilized lipid bilayers to a greater extent than did dipalmitin. We conclude that the tendency of diacylglycerols to destabilize the phospholipid bilayer is related to their capacity to enhance the activity of the membrane calcium pump. 相似文献
10.
Shashank Hambarde Chi-Lin Tsai Raj K. Pandita Albino Bacolla Anirban Maitra Vijay Charaka Clayton R. Hunt Rakesh Kumar Oliver Limbo Remy Le Meur Walter J. Chazin Susan E. Tsutakawa Paul Russell Katharina Schlacher Tej K. Pandita John A. Tainer 《Molecular cell》2021,81(14):2989-3006.e9
- Download : Download high-res image (222KB)
- Download : Download full-size image