全文获取类型
收费全文 | 604篇 |
免费 | 32篇 |
专业分类
636篇 |
出版年
2022年 | 4篇 |
2021年 | 9篇 |
2020年 | 10篇 |
2019年 | 5篇 |
2018年 | 6篇 |
2017年 | 8篇 |
2016年 | 22篇 |
2015年 | 18篇 |
2014年 | 28篇 |
2013年 | 25篇 |
2012年 | 35篇 |
2011年 | 40篇 |
2010年 | 30篇 |
2009年 | 21篇 |
2008年 | 35篇 |
2007年 | 34篇 |
2006年 | 25篇 |
2005年 | 30篇 |
2004年 | 21篇 |
2003年 | 29篇 |
2002年 | 21篇 |
2001年 | 7篇 |
2000年 | 5篇 |
1999年 | 8篇 |
1998年 | 8篇 |
1997年 | 5篇 |
1996年 | 7篇 |
1995年 | 6篇 |
1994年 | 8篇 |
1993年 | 13篇 |
1992年 | 5篇 |
1990年 | 3篇 |
1989年 | 8篇 |
1988年 | 6篇 |
1987年 | 4篇 |
1986年 | 10篇 |
1985年 | 5篇 |
1984年 | 10篇 |
1983年 | 5篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1978年 | 6篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1975年 | 3篇 |
1973年 | 3篇 |
1972年 | 3篇 |
1971年 | 3篇 |
排序方式: 共有636条查询结果,搜索用时 0 毫秒
111.
112.
Magne Refsnes Olav F. Dajani Dagny Sandnes G. Hege Thoresen John-Arne Rttingen Jens-Gustav Iversen Thoralf Christoffersen 《Journal of cellular physiology》1995,164(3):465-473
While many observations indicate that prostaglandins may act as positive regulators of hepatocyte proliferation, the underlying mechanisms are not known. We have examined some of the signal pathways in the growth response induced by prostaglandins in hepatocytes, with particular focus on adenylyl cyclase and phosphoinositide-specific phospholipase C. Adult rat hepatocytes were cultured as primary monolayers in serum-free medium in the presence of EGF and insulin. PGE2 or PGF2α (added 0-3 h after plating) enhanced the incorporation of [3H]-thymidine into DNA (measured at 50 h); at 100 γM the stimulation was about threefold. PGI2 and PGD2 also showed significant but smaller stimulatory effects. No significant increase in the level of cyclic AMP (cAMP) was detected in response to any of the prostaglandins. Low concentrations of glucagon (0.1-10 nM), a potent activator of hepatic adenylyl cyclase, or 8-bromo-cAMP (0.1-10 γM) enhanced the DNA synthesis. When 8-bromo-cAMP was used in maximally effective concentrations, no further stimulation was obtained by combining it with glucagon, whereas the effects of PGE2 and 8-bromo-cAMP were completely additive. All the prostaglandins also showed additivity with the effect of glucagon on the DNA synthesis. PGE2, PGF2α, PGI2, and PGD2 increased intracellular inositol-1,4,5-trisphosphate (InsP3), with a relative order of efficacy roughly corresponding to their activity as stimulators of DNA synthesis. Increases in cytosolic free Ca2+, as measured in single cells, were elicited in a majority of the hepatocytes by all these prostaglandins at 1 γM. Supramaximal concentrations of vasopressin, a strong activator of phospholipase C in hepatocytes, acted additively with PGE2 on the DNA synthesis. Pretreatment of the hepatocytes with a concentration of pertussis toxin that prevented the inhibitory effect of PGE2 on glucagon-induced cAMP accumulation did not abolish the ability of PGE2 to stimulate the DNA synthesis. The results do not support a role for adenylyl cyclase activation in the stimulatory effect of prostaglandins on hepatocyte growth. While the data are compatible with an involvement of phosphoinositide-specific phospholipase C in the growth-promoting effect of prostaglandins in cultured rat hepatocytes, they suggest this may not be the sole mechanism. © 1995 Wiley-Liss, Inc. 相似文献
113.
Erik Olav Gracey Francesca Verones 《The International Journal of Life Cycle Assessment》2016,21(3):412-428
Purpose
Expanding renewable energy production is widely accepted as a promising strategy in climate change mitigation. However, even renewable energy production has some environmental impacts, some of which are not (yet) covered in life cycle impact assessment (LCIA). We aim to identify the most important cause-effect pathways related to hydropower production on biodiversity, as one of the most common renewable energy sources, and to provide recommendations for future characterization factor (CF) development.Methods
We start with a comprehensive review of cause-effect chains related to hydropower production for both aquatic and terrestrial biodiversity. Next, we explore contemporary coverage of impacts on biodiversity from hydropower production in LCA. Further, we select cause-effect pathways displaying some degree of consistency with existing LCA frameworks for method development recommendations. For this, we compare and contrast different hydrologic models and discuss how existing LCIA methodologies might be modified or combined to improve the assessment of biodiversity impacts from hydropower production.Results and discussion
Hydropower impacts were categorized into three overarching impact pathways: (1) freshwater habitat alteration, (2) water quality degradation, and (3) land use change. Impacts included within these pathways are flow alteration, geomorphological alteration to habitats, changes in water quality, habitat fragmentation, and land use transformation. For the majority of these impacts, no operational methodology exists currently. Furthermore, the seasonal nature of river dynamics requires a level of temporal resolution currently beyond LCIA modeling capabilities. State-of-the-art LCIA methods covering biodiversity impacts exist for land use and impacts from consumptive water use that can potentially be adapted to cases involving hydropower production, while other impact pathways need novel development.Conclusions
In the short term, coverage of biodiversity impacts from hydropower could be significantly improved by adding a time step representing seasonal ecological water demands to existing LCIA methods. In the long term, LCIA should focus on ecological response curves based on multiple hydrologic indices to capture the spatiotemporal aspects of river flow, by using models based on the “ecological limits to hydrologic alteration” (ELOHA) approach. This approach is based on hydrologic alteration-ecological response curves, including site-specific environmental impact data. Though data-intensive, ELOHA represents the potential to build a global impact assessment framework covering multiple ecological indicators from local impacts. Further, we recommend LCIA methods based on degree of regulation for geomorphologic alteration and a fragmentation index based on dam density for “freshwater habitat alteration,” which our review identified as significant unquantified threats to aquatic biodiversity.114.
115.
Marielle Ernst Levente Kriston Javier M. Romero Andreas M. Fr?lich Olav Jansen Jens Fiehler Jan-Hendrik Buhk 《PloS one》2016,11(2)
Purpose
We sought to develop a standardized curriculum capable of assessing key competencies in Interventional Neuroradiology by the use of models and simulators in an objective, quantitative, and efficient way. In this evaluation we analyzed the associations between the practical experience, theoretical knowledge, and the skills lab performance of interventionalists.Materials and Methods
We evaluated the endovascular skills of 26 participants of the Advanced Course in Endovascular Interventional Neuroradiology of the European Society of Neuroradiology with a set of three tasks (aneurysm coiling and thrombectomy in a virtual simulator and placement of an intra-aneurysmal flow disruptor in a flow model). Practical experience was assessed by a survey. Participants completed a written and oral examination to evaluate theoretical knowledge. Bivariate and multivariate analyses were performed.Results
In multivariate analysis knowledge of materials and techniques in Interventional Neuroradiology was moderately associated with skills in aneurysm coiling and thrombectomy. Experience in mechanical thrombectomy was moderately associated with thrombectomy skills, while age was negatively associated with thrombectomy skills. We found no significant association between age, sex, or work experience and skills in aneurysm coiling.Conclusion
Our study gives an example of how an integrated curriculum for reasonable and cost-effective assessment of key competences of an interventional neuroradiologist could look. In addition to traditional assessment of theoretical knowledge practical skills are measured by the use of endovascular simulators yielding objective, quantitative, and constructive data for the evaluation of the current performance status of participants as well as the evolution of their technical competency over time. 相似文献116.
117.
Alginate may be considered as a block co-polymer of D-mannuronic and L-guluronic acids, and consists of three types of blocks: homopolymeric blocks of mannuronic acid (MM) and of guluronic acid (GG), and blocks with an alternating sequence (MG). The block composition of alginates has been characterized by a simple chemical method involving partial hydrolysis with acid, followed by fractional precipitation of the acid-resistant part of the alginate. Alginates from eleven different species of brown algae have been examined and, for five species, alginates from different tissues have been compared. The results indicate that young tissue is rich in MM blocks, and that the difference between the alginates from different species is mainly due to the alginates from the older parts of the plants. Extracellular alginates from two types of bacteria have been examined. 相似文献
118.
Investigation of a toxic water-bloom of Microcystis aeruginosa (Cyanophyceae) in Lake Akersvatn,Norway 总被引:1,自引:1,他引:0
Kjetil Berg Wayne W. Carmichael Olav M. Skulberg Christel Benestad Bjarne Underdall 《Hydrobiologia》1987,144(2):97-103
During the summer and fall of 1984 and 1985, the eutrophic Lake Akersvatn in south-eastern Norway, used as reserve drinking
water reservoir, was found to produce heavy water-blooms of the colonial blue-green alga Microcystis aeruginosa. Samples of the water-bloom were found to be toxic using the mouse bioassay. No toxin was found free in the water as detected
by HPLC and mouse bioassay. The toxic cells (minimum lethal dose 8–15 mg/kg body weight in mice) and purified toxin (minimum
lethal dose 50 μg/kg body weight in mice) showed signs of poisoning in laboratory rats and mice identical to that of other
hepatotoxin-producing M. aeruginosa blooms and strains reported from other parts of the world. The toxin has chemical properties similar to the cyclic heptapeptide
reported for a South African M. aeruginosa toxin. The toxin from Lake Akersvatn bloom material has a molecular weight of 994. The toxic bloom of M. aeruginosa persisted from August to November in 1984 and reappeared in July of 1985. While water from Lake Akersvatn was not used for
municipal water supply during this period, the presence of toxic blue-green algae in a drinking water reservoir indicates
the need to develop monitoring and detection methods for toxic cells and toxin(s). 相似文献
119.