全文获取类型
收费全文 | 112013篇 |
免费 | 2255篇 |
国内免费 | 891篇 |
专业分类
115159篇 |
出版年
2023年 | 79篇 |
2022年 | 159篇 |
2021年 | 338篇 |
2020年 | 223篇 |
2019年 | 248篇 |
2018年 | 12082篇 |
2017年 | 10941篇 |
2016年 | 7854篇 |
2015年 | 1372篇 |
2014年 | 1145篇 |
2013年 | 1251篇 |
2012年 | 5543篇 |
2011年 | 14037篇 |
2010年 | 12797篇 |
2009年 | 8977篇 |
2008年 | 10810篇 |
2007年 | 12459篇 |
2006年 | 1211篇 |
2005年 | 1513篇 |
2004年 | 1974篇 |
2003年 | 1976篇 |
2002年 | 1688篇 |
2001年 | 575篇 |
2000年 | 478篇 |
1999年 | 308篇 |
1998年 | 270篇 |
1997年 | 193篇 |
1996年 | 142篇 |
1995年 | 129篇 |
1994年 | 156篇 |
1993年 | 156篇 |
1992年 | 221篇 |
1991年 | 235篇 |
1990年 | 165篇 |
1989年 | 171篇 |
1988年 | 174篇 |
1987年 | 171篇 |
1986年 | 144篇 |
1985年 | 165篇 |
1984年 | 167篇 |
1983年 | 137篇 |
1982年 | 95篇 |
1981年 | 110篇 |
1980年 | 93篇 |
1979年 | 95篇 |
1978年 | 99篇 |
1976年 | 84篇 |
1974年 | 90篇 |
1972年 | 315篇 |
1971年 | 348篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Cytochrome P450 2E1-derived reactive oxygen species mediate paracrine stimulation of collagen I protein synthesis by hepatic stellate cells. 总被引:3,自引:0,他引:3
Natalia Nieto Scott L Friedman Arthur I Cederbaum 《The Journal of biological chemistry》2002,277(12):9853-9864
To evaluate possible fibrogenic effects of CYP2E1-dependent generation of reactive oxygen species, a model was developed using co-cultures of HepG2 cells, which do (E47 cells) or do not (C34 cells) express cytochrome P450 2E1 (CYP2E1) with stellate cells. There was an increase in intra- and extracellular H(2)O(2), lipid peroxidation, and collagen type I protein in stellate cells co-cultured with E47 cells compared with stellate cells alone or co-cultured with C34 cells. The increase in collagen was prevented by antioxidants and a CYP2E1 inhibitor. CYP3A4 did not mimic the stimulatory effects found with CYP2E1. Collagen mRNA levels remained unchanged, and pulse-chase analysis indicated similar half-lives of collagen I protein between both co-cultures. However, collagen protein synthesis was increased in E47 co-culture. Hepatocytes from pyrazole-treated rats (with high levels of CYP2E1) induced collagen protein in primary stellate cells, and antioxidants and CYP2E1 inhibitors blocked this effect. These results suggest that increased translation of collagen mRNA by CYP2E1-derived reactive oxygen species is responsible for the increase in collagen protein produced by the E47 co-culture. These co-culture models may be useful for understanding the impact of CYP2E1-derived ROS on stellate cell function and activation. 相似文献
2.
Hostile intercommunity relations, including attacking and killing extra-community infants of both sexes have occurred at most
wild chimpanzee sites. We describe three recent cases of intercommunity attacks on infants committed by members of the Ngogo
chimpanzee community in Kibale National Park, Uganda. Two of the attacks resulted in confirmed infanticides while a third
attack probably resulted in the infant's death. In common with previous accounts of chimpanzee infanticides, the attacks described
here occurred during boundary patrols outside the Ngogo community's usual range, adult and adolescent males were the main
participants, one infant was cannibalized after being killed, and the victims’ mothers did not accompany the attacking party
back to the Ngogo range. However, the patrol parties during each infanticide were larger than before and included females
from the Ngogo community. Our observations indirectly support both the range expansion and imbalance of power hypotheses,
which address why and under which conditions chimpanzee intercommunity encounters lead to aggression. These cases of intercommunity
infanticide add to the growing database of the phenomenon in wild chimpanzees. 相似文献
3.
4.
5.
6.
An endopeptidase which digests denatured collagen to small, dialysable fragments was purified 2675-fold from medium that had been conditioned by the culture of fibroblasts grown from explants of human gingiva. This enzyme was inhibited by chelating agents, but not by phenylmethylsulphonyl fluoride nor by N-ethylmaleimide, and is therefore probably a metalloproteinase. It showed no demonstrable activity against native collagen or ovalbumin, while alpha-casein was digested slowly, if at all. It therefore belongs to the group of enzymes which have been called tissue gelatinases. This gelatinase was secreted in a latent form or forms and could be activated by proteolysis with trypsin. The active enzyme had an apparent molecular weight of 69 000 (gel chromatography) or 72 000 (gel electrophoresis in sodium dodecyl sulphate) and an apparent isoelectric point of 4.15. 相似文献
7.
8.
Steven J. Tucker David McClelland Kristina Sep?i? Roderick H. Scott 《生物化学与生物物理学报:生物膜》2003,1614(2):171-181
The ability of two alkyl pyridinium sponge toxin preparations (poly-APS and halitoxin) to form transient pores/lesions in cell membranes and allow transfection of plasmid cDNA have been investigated using HEK 293 cells. Poly-APS and halitoxin preparations caused a collapse in membrane potential, reductions in input resistance and increased Ca2+ permeability. At least partial recovery was observed after poly-APS application but recovery was more rarely seen with halitoxin. The transfection with plasmid cDNAs for an enhanced green fluorescent protein (EGFP) and human tumour necrosis factor receptor 2 (TNFR2) was assessed for both toxin preparations and compared with lipofectamine. Stable transfection was achieved with poly-APS although it was less efficient than lipofectamine. These results show that viable cells transfected with alien cDNA can be obtained using novel transient pore-forming alkyl pyridinium sponge toxins and a simple pre-incubation protocol. This provides the first proof of principle that pore-forming alkyl pyridinium compounds can be used to deliver cDNA to the intracellular environment without permanently compromising the plasma membrane. 相似文献
9.
IN addition to its well known antiviral activity, interferon has recently been shown to inhibit the multiplication of tumour and mammalian cells in cell culture1–6. We report here the inhibition by interferon of DNA synthesis induced in mouse spleen lymphocytes by the non-viral stimuli phytohaemagglutinin (PHA) and allogeneic lymphocytes. These findings are in accord with our contention that interferon affects cell function and, furthermore, they suggest that by acting on lymphocytes, interferon plays a role in the immunological response of the host. 相似文献
10.
CHLOROPLASTS are more permeable to amino-acids1,2 than are cell membranes3 and also discriminate between their optical isomers2. This has led to the suggestion that the passage of amino-acids through the limiting membranes of chloroplasts is facilitated by carriers2. Using the additional criteria of saturation and competition, we support this suggestion and show that at least two carriers are probably involved. 相似文献