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   315篇
  免费   20篇
  335篇
  2023年   1篇
  2021年   1篇
  2020年   3篇
  2019年   3篇
  2018年   4篇
  2017年   4篇
  2016年   5篇
  2015年   14篇
  2014年   13篇
  2013年   22篇
  2012年   24篇
  2011年   22篇
  2010年   14篇
  2009年   12篇
  2008年   14篇
  2007年   26篇
  2006年   14篇
  2005年   27篇
  2004年   12篇
  2003年   20篇
  2002年   16篇
  2001年   9篇
  2000年   2篇
  1999年   4篇
  1998年   2篇
  1997年   4篇
  1996年   7篇
  1995年   5篇
  1994年   2篇
  1993年   4篇
  1991年   2篇
  1989年   2篇
  1988年   1篇
  1987年   2篇
  1985年   3篇
  1984年   1篇
  1983年   1篇
  1982年   1篇
  1981年   3篇
  1979年   2篇
  1978年   1篇
  1977年   1篇
  1976年   2篇
  1975年   2篇
  1973年   1篇
排序方式: 共有335条查询结果,搜索用时 0 毫秒
1.
How Can the Eco‐efficiency of a Region be Measured and Monitored?   总被引:2,自引:0,他引:2  
The concept of eco-efficiency is commonly referred to as a business link to sustainable development. In this article, ecoefficiency is examined at a regional level as an approach to promoting the competitiveness of economic activities in the Finnish Kymenlaakso region and mitigating their harmful impacts on the environment. The aim is to develop appropriate indicators for monitoring changes in the eco-efficiency of the region. A starting point is to produce indicators for the environmental and economic dimensions of regional development and use them for measuring regional eco-efficiency. The environmental impact indicators are based on a life-cycle assessment method, producing different types of environmental impact indicators: pressure indicators (e.g., emissions of CO2), impact category indicators (e.g., CO2 equivalents in the case of climate change), and a total impact indicator (aggregating different impact category indicator results into a single value). Environmental impact indicators based on direct material input, total material input, and total material requirement of the Kymenlaakso region are also assessed. The economic indicators used are the gross domestic product, the value added, and the output of the main economic sectors of Kymenlaakso. In the eco-efficiency assessment, the economic and environmental impact indicators are monitored in the same graph. In a few cases eco-efficiency ratios can also be calculated (the economic indicators are divided by the environmental indicators). Output (= value added + intermediate consumption) is used as an economic indicator related to the environmental impact indicators, which also cover the upstream processes of the region's activities. In the article, we also discuss the strengths and weaknesses of using the different environmental impact indicators.  相似文献   
2.
We selected a 2-difluoromethylornithine-resistant Ehrlich ascites-carcinoma cell line that grows in the presence of 20 mM-difluoromethylornithine. These cells contain 10-20 times the normal amount of hybridizable sequences for ornithine decarboxylase (EC 4.1.1.17) in their genomic DNA. We used these gene-amplified cells, their revertant counterparts (grown in the absence of the drug after an established gene amplification) and tumour cells grown in the presence of putrescine to investigate the changes of ornithine decarboxylase gene pattern and simultaneously occurring phenotypic changes, such as tumourigenicity and the expression of cell-surface glycoproteins. In the tumour cells reverted back to the normal gene frequency, not only did the amplified sequences disappear, but there were also signs of gene re-arrangements seen as a "gene jump', when a signal evidently moved to a heavier restriction fragment. Similar gene re-arrangement likewise occurred in cells exposed to putrescine. Although the wild-type tumour cells and the gene-amplified cells readily grew in the peritoneal cavity of mice, the revertant cells and the putrescine-treated cells had lost their tumourigenicity in mice. Gene-amplified tumour cells and the revertant cells showed distinct changes in their surface glycoprotein pattern in comparison with the parental cell line. These findings indicate that alterations of ornithine decarboxylase gene pattern/dosage may be associated with phenotypic changes possibly related to the tumourigenicity of these carcinoma cells.  相似文献   
3.
As part of the Russian-Finnish research studies on Lake Ladoga, joint expeditions were organized in 1992 and 1993. Water samples were collected for intercalibration of chemical analysis methods and to monitor the chemical quality of the lake water.In August of 1992 water samples were taken from northern Lake Ladoga for intercalibration of Russian and Finnish analysis methods. In August 1993 water samples were collected from 23 sampling stations in all parts of the lake; some of these were also used for intercalibration purposes.The oxygen, colour and CODMn results were at the same level in the intercalibration. In 1993, the Ptot results obtained were acceptable. In Ntot, Fe and Mn analysis there seemed to be systematic and random errors between some results.The Secchi depth ranged from 1.5 m to 3.3 m. The average concentrations for the total phosphorus ranged from 15 µg 1–1 to 29 µg 1–1. The total nitrogen values were from 620 µg 1–1 to 690 µg 1–1. The N:P ratio varied from 24 to 40. The concentration of phosphorus indicated mesotrophic or even eutrophic conditions in the lake. Phosphorus seemed to be the limiting nutrient to bacteria and algae.  相似文献   
4.
Transgenic plant cell cultures have a potential for production and secretion of important proteins and peptides. To assess the possibilities of using a stable barley suspension culture for secretion of heterologous proteins in active form, we expressed the cDNA of the thermostable-glucanase (EGI) ofTrichoderma reesei in barley suspension cells. The cDNA coding for EGI and its signal sequence was placed under the control of the CaMV 35S promoter and the construction was transferred to the cells by particle bombardment. Stably transformed lines were obtained by selecting for a cotransformed antibiotic resistance marker. The expression of EGI cDNA led to accumulation of EGI in the culture medium, as shown by analysis with EGI-specific antibodies. Enzymatic assays confirmed that the EGI secreted by the suspension cells retained its activity and thermostable character. Furthermore, it was shown that the enzyme produced by the transgenic suspension culture could be used for degradation of soluble-glucans during mashing.  相似文献   
5.
Ollikainen  Minna  Simola  Heikki  Niinioja  Riitta 《Hydrobiologia》1993,269(1):405-413
Sedimentary diatom assemblages in two large oligotrophic clear-water lakes were analysed, to assess their present ecological state and possible eutrophication due to diffuse nutrient loading. The lakes Pyhäjärvi and Puruvesi (Finnish lake district) are proportionally large for their catchment areas which accounts for their long retention times (ca 7 and 11 yr) and oligohumic character. Pyhäjärvi was studied by pairwise comparison of surface sediment diatom assemblages collected in 1985 and 1990 at 12 sites from different parts of the lake. In Puruvesi, the stratigraphy of diatoms was analysed in two short cores from 8 m and 32 m depths.The diatom assemblages of the two lakes are rather similar, and quite distinct from the assemblages of the mesohumic large lakes of the area. Cyclotella kuetzingiana is the most common planktonic dia- tom, but Aulacoseira ambigua abounds in Pyhäjärvi at sites with local sources of eutrophication. A diverse assemblage of benthic forms, especially Fragilaria and Achnanthes spp. characterizes the shallow bottoms in both lakes.There was little change within the short-core diatom profiles of Puruvesi, but the floral composition of the 8-m and 32-m sites differed markedly. The 8-m site, with 60–70% of benthic forms, represents illuminated bottom, on which much of the buried algae have lived in situ, while the deeper site is true profundal, dominated by sedimented planktonic algae.In Pyhäjärvi there was a slight increase in the benthic diatoms from 1985 to 1990, coinciding with increased phosphorus and chlorophyll concentrations as well as Secchi depth lowering. We interprete this observation as a very early step of eutrophication, of which first the sessile algal communities of the illuminated bottom areas have benefited.  相似文献   
6.
The effect of nitrogen fertilization on the phytomass production, shoot length and leaf secondary phenolics in nine Salix myrsinifolia clones was investigated. Cuttings taken from 1-year-old and 2-year-old shoot parts of field cultivated clones were grown at three concentrations of nitrogen (7, 150 and 300 ppm) in a greenhouse for one growing season. The willow clones differed significantly in phytomass yield and secondary phenolics content. Nitrogen fertilization affected significantly the growth and secondary metabolism of willow clones. In most clones, the addition of nitrogen from a sub-optimum concentration (7 ppm) to an optimum concentration (150 ppm) appeared to reduce the amounts of salicortin, chlorogenic acid and unknown salicylate and increased shoot phytomass, but a supraoptimum nitrogen concentration (300 ppm) resulted in highly variable growth and secondary phenolic responses. A significantly negative correlation between leaf phytomass and amount of total phenolics at sub-optimum and optimum N-treatments indicates trade-off between growth and secondary metabolism in willow clones at these treatments. However, the leaf phytomass:total amount of phenolics ratio varied significantly among clones, and in all clones it was not significantly lower at sub-optimum N-treatment than at optimum N-treatment.  相似文献   
7.
Summary Clinical and cytogenetic findings in an infant girl with multiple congenital anomalies, principally anophthalmia, are presented. The patient's karyotype was 47,XX, +del(4)(pterq21), the largest partial trisomy of chromosome 4 reported. The possible mechanism of the origin of this abnormality is discussed.To whom offprint requests should be sent  相似文献   
8.
We have generated transgenic mice carrying human ornithine decarboxylase gene. Two different transgene constructs were used: (i) a 5'-truncated human ornithine decarboxylase gene and (ii) an intact human ornithine decarboxylase gene. Transgenic mice carrying the 5'-truncated gene did not express human ornithine decarboxylase-specific mRNA. Transgenic mice carrying the intact human ornithine decarboxylase gene expressed human-specific ornithine decarboxylase mRNA in all tissues studied. However, as indicated by actual enzyme assays, the expression pattern was highly unusual. In comparison with their wild-type littermates, the transgenic mice exhibited greatly elevated enzyme activity in almost every tissue studied. Ornithine decarboxylase activity was moderately elevated in parenchymal organs such as liver, kidney, and spleen. Tissues like heart, muscle, lung, thymus, testis, and brain displayed an enzyme activity that was 20 to 80 times higher than that in the respective tissues of nontransgenic animals. The offspring of the first transgenic male founder animal did not show any overt abnormalities, yet their reproductive performance was reduced. The second transgenic founder animal, showing similar aberrant expression of ornithine decarboxylase in all tissues studied, including an extremely high activity in testis, was found to be infertile. Histological examination of the tissues of the latter animal revealed marked changes in testicular morphology. The germinal epithelium was hypoplastic, and the spermatogenesis was virtually totally shut off. Similar examination of male members of the first transgenic mouse line revealed comparable, yet less severe, histological changes in testis.  相似文献   
9.
Hepatic enzymes connected with the formation and metabolism of free D-glucuronic acid were affected in rats after treatment with disulfiram or diethyldithiocarbamate (300 mg/kg, intragastrically, per day, 4 X). The activities of UDPglucose dehydrogenase, UDPglucuronic acid pyrophosphatase, UDPglucuronosyltransferase and L-gulonate dehydrogenase were enhanced, while those of glucose-6-phosphate dehydrogenase, beta-glucuronidase and D-glucuronolactone dehydrogenase were inhibited. These changes were more pronounced with disulfiram than diethyldithiocarbamate. Treatment with phenobarbital (80 mg/kg, i.p., per day, 4 X) enhanced UDP glucuronosyl-transferase, but brought about different effects on the other enzymes. Concurrent administration of phenobarbital with disulfiram or diethyldithiocarbamate led to potentiation or antagonism of the primary effects of each compound when given alone. The results suggest that activation of the D-glucuronic acid pathway may proceed in various ways, and that it is not necessarily followed by a simultaneous induction of the microsomal mixed-function oxygenase activity.  相似文献   
10.
Macrophages are essential for the development of innate immune responses against a variety of infectious factors. They detect invading pathogens via their pattern recognition receptors such as toll-like receptors (TLRs). TLR7/8 recognizes ssRNA from various viruses. In the present study, we have used 2-DE gel-based proteomics to find novel TLR7/8 target proteins in human monocyte-derived macrophages in order to improve our understanding of the virus recognition by this TLR. A total of 27 protein spots were found to be reproducibly differentially expressed between control and TLR7/8 activated 2-DE gel pairs, 18 spots being more than two-fold upregulated and nine spots being at least two-fold downregulated. Several proteins involved in defense against toxic superoxide (O2-) and other reactive oxygen species, such as manganese-containing superoxide dismutase (SOD2), glutathione peroxidase, and peroxiredoxins 1 and 6 were highly upregulated after TLR7/8 activation. Western blot analysis showed that activation of macrophages with TLR2, TLR3, TLR4, and TLR7/8 ligands also strongly upregulated SOD2 protein expression. In conclusion, our results show that the activation of pattern recognition receptors of the innate immune system results in strong upregulation of SOD2 gene expression suggesting that SOD2 protects macrophages from oxidative stress during microbial infection.  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