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   1588篇
  免费   142篇
  1730篇
  2023年   7篇
  2022年   14篇
  2021年   23篇
  2020年   13篇
  2019年   20篇
  2018年   30篇
  2017年   31篇
  2016年   49篇
  2015年   95篇
  2014年   93篇
  2013年   97篇
  2012年   112篇
  2011年   107篇
  2010年   70篇
  2009年   60篇
  2008年   103篇
  2007年   67篇
  2006年   70篇
  2005年   68篇
  2004年   69篇
  2003年   72篇
  2002年   64篇
  2001年   42篇
  2000年   31篇
  1999年   31篇
  1998年   14篇
  1997年   16篇
  1996年   12篇
  1995年   10篇
  1994年   12篇
  1993年   13篇
  1992年   18篇
  1991年   15篇
  1990年   23篇
  1989年   15篇
  1987年   10篇
  1986年   14篇
  1985年   18篇
  1984年   10篇
  1983年   12篇
  1982年   5篇
  1981年   5篇
  1980年   10篇
  1979年   8篇
  1978年   4篇
  1976年   5篇
  1975年   4篇
  1974年   11篇
  1973年   8篇
  1972年   4篇
排序方式: 共有1730条查询结果,搜索用时 15 毫秒
31.
Goal, Scope and Background  Traditionally, comparative life cycle assessments (LCA) have not considered rebound effects, for instance in case of significant price differences among the compared products. No justifications have been made for this delimitation in scope. This article shows that price differences and the consequent effects of marginal consumer expenditure may influence the conclusions of comparative LCA significantly. We also show that considerations about rebound effects of price differences can be included in LCAs. Methods  The direct rebound effect of a price difference is marginal consumption. Based on statistical data on private consumption in different income groups (Statistics Denmark 2005a, 2005b), the present article provides an estimate of how an average Danish household will spend an additional 1 DKK for further consumer goods, when the household has gained money from choosing a cheaper product alternative. The approach is to use marginal income changes and the following changes in consumption patterns as an expression for marginal consumption. Secondly, the environmental impact potentials related to this marginal consumption are estimated by the use of environmental impact intensity data from an IO-LCA database (Weidema et al. 2005). Finally, it is discussed whether, and in which ways the conclusions of comparative LCAs can be affected by including the price difference between product alternatives. This is elucidated in a case study of a comparative LCA screening of two different kinds of Danish cheese products (Fricke et al. 2004). Results  Car purchase and driving, use and maintenance of dwelling, clothing purchase and insurance constitutes the largest percentages of the marginal consumption. In a case study of two cheeses, the including the impact potentials related to the price difference results in significant changes in the total impact potentials. Considering the relatively small price difference of the two products, it is likely also to have a significant influence on the results of comparative LCAs more generally. Discussion  The influence of marginal consumption in comparative LCAs is relevant to consider in situations with large differences in the price of the product alternatives being compared, and in situations with minor differences in the impact potentials related to the alternatives. However, different uncertainties are linked to determining the pattern for marginal consumption and the environmental impact potential related to this. These are first of all related to the method used, but also include inaccurate data of consumption in households, aggregation and weighting of income groups, aggregation of product groups, estimation and size of the price difference, and the general applicability of the results. Conclusion  Incorporating marginal consumption in consequential LCAs is possible in practice. In the case study used, including the rebound effects of the price difference has a significant influence on the result of the comparative LCA, as the result for the impact categories acidification and nutrient enrichment changes in favour of the expensive product. Recommendations and Perspectives  It is recommended that the rebound effects of price differences should be included more frequently in LCAs. In order to ensure this, further research in marginal consumption and investment patterns and IO data for different countries or regions is required. Furthermore, this study does not consider the economic distributional consequences of buying an expensive product instead of a cheaper product (e.g. related to how the profit is spent by those who provided the product). It should also be noted, that more expensive products not necessarily result in less consumption, as those who provided the product also will spend the money they have earned from the sale. Ideally, these consequences should also be further investigated. Likewise, the development of databases to include marginal consumption in PC-tools is needed. In general, considerations of marginal consumption would favour expensive product alternatives, depending, however, on the type of consumer. ESS-Submission Editor: Dr. David Hunkeler (david.hunkeler@aquaplustech.ch)  相似文献   
32.
Most biofilms in their natural environments are likely to consist of consortia of species that influence each other in synergistic and antagonistic manners. However, few reports specifically address interactions within multispecies biofilms. In this study, 17 epiphytic bacterial strains, isolated from the surface of the marine alga Ulva australis, were screened for synergistic interactions within biofilms when present together in different combinations. Four isolates, Microbacterium phyllosphaerae, Shewanella japonica, Dokdonia donghaensis, and Acinetobacter lwoffii, were found to interact synergistically in biofilms formed in 96-well microtiter plates: biofilm biomass was observed to increase by >167% in biofilms formed by the four strains compared to biofilms composed of single strains. When exposed to the antibacterial agent hydrogen peroxide or tetracycline, the relative activity (exposed versus nonexposed biofilms) of the four-species biofilm was markedly higher than that in any of the single-species biofilms. Moreover, in biofilms established on glass surfaces in flow cells and subjected to invasion by the antibacterial protein-producing Pseudoalteromonas tunicata, the four-species biofilms resisted invasion to a greater extent than did the biofilms formed by the single species. Replacement of each strain by its cell-free culture supernatant suggested that synergy was dependent both on species-specific physical interactions between cells and on extracellular secreted factors or less specific interactions. In summary, our data strongly indicate that synergistic effects promote biofilm biomass and resistance of the biofilm to antimicrobial agents and bacterial invasion in multispecies biofilms.  相似文献   
33.
Loricifera is a phylum of minute animals that live exclusively in marine sediments. A total of 33 species have been described so far in this phylum; however, several more are already known from preliminary observations. Loriciferans are characterised by a complex life cycle, which involves a succession of several adult and larval stages. Here, we describe a new type of loriciferan larval stage: the Shira larva. The gross morphology of this larva is generally similar to that of the most prominent larval type of Loricifera, the so-called Higgins larva. However, the Shira larva possesses a number of unique features, namely (1) a single pair of anteroventral setae is present in the most anterior region of the abdomen, (2) the bases of the anteroventral setae are very large and swollen, (3) the thorax and abdomen are thinner than the introvert and (4) the abdominal region is divided into five sub-regions. Accordingly, we described the new species, Tenuiloricus shirayamai gen. nov. et sp. nov. (incertae sedis). The new findings are discussed from a comparative perspective with the Higgins larva as well as with the fossil of a putative loriciferan larval stage from the Middle Cambrian.  相似文献   
34.
Genetic susceptibility and environmental estrogen-like compounds   总被引:4,自引:0,他引:4  
Environmental chemicals with estrogenic activities have been suggested to be able to interact with the endocrine system. Endogenous estrogen is synthesized in the ovarian theca cells of premenopausal women or in the stromal adipose cells of the breast of postmenopausal women and minor quantities in peripheral tissue. These cells, as well as breast tissue, express all the necessary enzymes for this synthesis, CYP17, CYP11a, CYP19, 17-beta-hydroxysteroid hydrogenase, steroid sulfatase as well as enzymes further hydroxylating estradiol, such as CYP1A1, CYP3A4, CYP1B1, catechol-o-methyltransferase (COMT). Polymorphisms in these enzymes may have a possible role in the link between environmental estrogens and hormone-like substances and the interindividual risk of breast cancer.  相似文献   
35.
The crystal structure of Thermus thermophilus elongation factor G (EF-G) carrying the point mutation His573Ala was determined at a resolution of 2.8 A. The mutant has a more closed structure than that previously reported for wild-type EF-G. This is obtained by a 10 degrees rigid rotation of domains III, IV and V with regard to domains I and II. This rotation results in a displacement of the tip of domain IV by approximately 9 A. The structure of domain III is now fully visible and reveals the double split beta-alpha-beta motif also observed for EF-G domain V and for several ribosomal proteins. A large number of fusidic acid resistant mutations found in domain III have now been possible to locate. Possible locations for the effector loop and a possible binding site for fusidic acid are discussed in relation to some of the fusidic acid resistant mutations.  相似文献   
36.
MicroRNA-122 (miR-122) is an abundant liver-specific miRNA, implicated in fatty acid and cholesterol metabolism as well as hepatitis C viral replication. Here, we report that a systemically administered 16-nt, unconjugated LNA (locked nucleic acid)-antimiR oligonucleotide complementary to the 5′ end of miR-122 leads to specific, dose-dependent silencing of miR-122 and shows no hepatotoxicity in mice. Antagonism of miR-122 is due to formation of stable heteroduplexes between the LNA-antimiR and miR-122 as detected by northern analysis. Fluorescence in situ hybridization demonstrated uptake of the LNA-antimiR in mouse liver cells, which was accompanied by markedly reduced hybridization signals for mature miR-122 in treated mice. Functional antagonism of miR-122 was inferred from a low cholesterol phenotype and de-repression within 24 h of 199 liver mRNAs showing significant enrichment for miR-122 seed matches in their 3′ UTRs. Expression profiling extended to 3 weeks after the last LNA-antimiR dose revealed that most of the changes in liver gene expression were normalized to saline control levels coinciding with normalized miR-122 and plasma cholesterol levels. Combined, these data suggest that miRNA antagonists comprised of LNA are valuable tools for identifying miRNA targets in vivo and for studying the biological role of miRNAs and miRNA-associated gene-regulatory networks in a physiological context.  相似文献   
37.
The newly discovered fungal species Aspergillus saccharolyticus was found to produce a culture broth rich in β-glucosidase activity. In this present work, the main β-glucosidase of A.?saccharolyticus responsible for the efficient hydrolytic activity was identified, isolated, and characterized. Ion exchange chromatography was used to fractionate the culture broth, yielding fractions with high β-glucosidase activity and only 1 visible band on an SDS-PAGE gel. Mass spectrometry analysis of this band gave peptide matches to β-glucosidases from aspergilli. Through a polymerase chain reaction approach using degenerate primers and genome walking, a 2919 bp sequence encoding the 860 amino acid BGL1 polypeptide was determined. BGL1 of A.?saccharolyticus has 91% and 82% identity with BGL1 from Aspergillus aculeatus and BGL1 from Aspergillus niger , respectively, both belonging to Glycoside Hydrolase family 3. Homology modeling studies suggested β-glucosidase activity with preserved retaining mechanism and a wider catalytic pocket compared with other β-glucosidases. The bgl1 gene was heterologously expressed in Trichoderma reesei QM6a, purified, and characterized by enzyme kinetics studies. The enzyme can hydrolyze cellobiose, p-nitrophenyl-β-d-glucoside, and cellodextrins. The enzyme showed good thermostability, was stable at 50?°C, and at 60?°C it had a half-life of approximately 6?h.  相似文献   
38.
BackgroundPay-for-performance (P4P) programmes to incentivise health providers to improve quality of care have been widely implemented globally. Despite intuitive appeal, evidence on the effectiveness of P4P is mixed, potentially due to differences in how schemes are designed. We exploited municipality variation in the design features of Brazil’s National Programme for Improving Primary Care Access and Quality (PMAQ) to examine whether performance bonuses given to family health team workers were associated with changes in the quality of care and whether the size of bonus mattered.Methods and findingsFor this quasi-experimental study, we used a difference-in-differences approach combined with matching. We compared changes over time in the quality of care delivered by family health teams between (bonus) municipalities that chose to use some or all of the PMAQ money to provide performance-related bonuses to team workers with (nonbonus) municipalities that invested the funds using traditional input-based budgets. The primary outcome was the PMAQ score, a quality of care index on a scale of 0 to 100, based on several hundred indicators (ranging from 598 to 660) of health care delivery. We did one-to-one matching of bonus municipalities to nonbonus municipalities based on baseline demographic and economic characteristics. On the matched sample, we used ordinary least squares regression to estimate the association of any bonus and size of bonus with the prepost change over time (between November 2011 and October 2015) in the PMAQ score. We performed subgroup analyses with respect to the local area income of the family health team. The matched analytical sample comprised 2,346 municipalities (1,173 nonbonus municipalities; 1,173 bonus municipalities), containing 10,275 family health teams that participated in PMAQ from the outset. Bonus municipalities were associated with a 4.6 (95% CI: 2.7 to 6.4; p < 0.001) percentage point increase in the PMAQ score compared with nonbonus municipalities. The association with quality of care increased with the size of bonus: the largest bonus group saw an improvement of 8.2 percentage points (95% CI: 6.2 to 10.2; p < 0.001) compared with the control. The subgroup analysis showed that the observed improvement in performance was most pronounced in the poorest two-fifths of localities. The limitations of the study include the potential for bias from unmeasured time-varying confounding and the fact that the PMAQ score has not been validated as a measure of quality of care.ConclusionsPerformance bonuses to family health team workers compared with traditional input-based budgets were associated with an improvement in the quality of care.

Nasser Fardousi and colleagues investigate the association between performance bonuses and the quality of primary health care delivered by family health teams in Brazil.  相似文献   
39.
Following several experimental investigations, an improved method of decalcification has been devised. The principle of this decalcification method is to obtain complete decalcification by a mixture of as high pH as possible without diminishing the stainability of the Nissl-granules (with Einarson's progressive staining method by means of gallocyanin). This is accomplished by the help of a buffer solution of equal parts of 8 N formic acid and 1 N sodium formate (pH 2.2). After-treatment consists only in rinsing in flowing water for 24 hours. Dehydration is in alcohol (70%, 96%, 100%); cedar oil; ligroin. Embedding in paraffin follows.  相似文献   
40.
Hydatidiform moles (HMs) are abnormal human pregnancies with vesicular chorionic villi, imposing two clinical challenges; miscarriage and a risk of gestational trophoblastic neoplasia (GTN). The parental type of most HMs are either diandric diploid (PP) or diandric triploid (PPM). We consecutively collected 154 triploid or near-triploid samples from conceptuses with vesicular chorionic villi. We used analysis of DNA markers and/or methylation sensitive-MLPA and collected data from registries and patients records. We performed whole genome SNP analysis of one case of twinning (PP+PM).In all 154 triploids or near-triploids we found two different paternal contributions to the genome (P1P2M). The ratios between the sex chromosomal constitutions XXX, XXY, and XYY were 5.7: 6.9: 1.0. No cases of GTN were observed. Our results corroborate that all triploid human conceptuses with vesicular chorionic villi have the parental type P1P2M. The sex chromosomal ratios suggest approximately equal frequencies of meiosis I and meiosis II errors with selection against the XYY conceptuses or a combination of dispermy, non-disjunction in meiosis I and meiosis II and selection against XYY conceptuses. Although single cases of GTN after a triploid HM have been reported, the results of this study combined with data from previous prospective studies estimate the risk of GTN after a triploid mole to 0% (95% CI: 0–1,4%).  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