全文获取类型
收费全文 | 21377篇 |
免费 | 1816篇 |
国内免费 | 12篇 |
出版年
2023年 | 113篇 |
2022年 | 310篇 |
2021年 | 603篇 |
2020年 | 331篇 |
2019年 | 412篇 |
2018年 | 511篇 |
2017年 | 395篇 |
2016年 | 669篇 |
2015年 | 1025篇 |
2014年 | 1175篇 |
2013年 | 1291篇 |
2012年 | 1793篇 |
2011年 | 1768篇 |
2010年 | 1105篇 |
2009年 | 955篇 |
2008年 | 1345篇 |
2007年 | 1285篇 |
2006年 | 1247篇 |
2005年 | 1048篇 |
2004年 | 996篇 |
2003年 | 940篇 |
2002年 | 819篇 |
2001年 | 264篇 |
2000年 | 195篇 |
1999年 | 248篇 |
1998年 | 217篇 |
1997年 | 149篇 |
1996年 | 134篇 |
1995年 | 96篇 |
1994年 | 110篇 |
1993年 | 113篇 |
1992年 | 100篇 |
1991年 | 96篇 |
1990年 | 85篇 |
1989年 | 110篇 |
1988年 | 62篇 |
1987年 | 81篇 |
1986年 | 71篇 |
1985年 | 106篇 |
1984年 | 88篇 |
1983年 | 75篇 |
1982年 | 58篇 |
1981年 | 59篇 |
1980年 | 68篇 |
1979年 | 70篇 |
1978年 | 30篇 |
1977年 | 38篇 |
1976年 | 39篇 |
1975年 | 38篇 |
1974年 | 43篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
1.
Minh Thac Nguyen Ryan Denniston Hien Thi Thu Nguyen Tuan Anh Hoang Hana Ross Anthony D. So 《PloS one》2014,9(1)
Illicit trade carries the potential to magnify existing tobacco-related health care costs through increased availability of untaxed and inexpensive cigarettes. What is known with respect to the magnitude of illicit trade for Vietnam is produced primarily by the industry, and methodologies are typically opaque. Independent assessment of the illicit cigarette trade in Vietnam is vital to tobacco control policy. This paper measures the magnitude of illicit cigarette trade for Vietnam between 1998 and 2010 using two methods, discrepancies between legitimate domestic cigarette sales and domestic tobacco consumption estimated from surveys, and trade discrepancies as recorded by Vietnam and trade partners. The results indicate that Vietnam likely experienced net smuggling in during the period studied. With the inclusion of adjustments for survey respondent under-reporting, inward illicit trade likely occurred in three of the four years for which surveys were available. Discrepancies in trade records indicate that the value of smuggled cigarettes into Vietnam ranges from $100 million to $300 million between 2000 and 2010 and that these cigarettes primarily originate in Singapore, Hong Kong, Macao, Malaysia, and Australia. Notable differences in trends over time exist between the two methods, but by comparison, the industry estimates consistently place the magnitude of illicit trade at the upper bounds of what this study shows. The unavailability of annual, survey-based estimates of consumption may obscure the true, annual trend over time. Second, as surveys changed over time, estimates relying on them may be inconsistent with one another. Finally, these two methods measure different components of illicit trade, specifically consumption of illicit cigarettes regardless of origin and smuggling of cigarettes into a particular market. However, absent a gold standard, comparisons of different approaches to illicit trade measurement serve efforts to refine and improve measurement approaches and estimates. 相似文献
2.
Kevin Karbstein Kathleen Prinz Frank Hellwig Christine Rmermann 《Ecology and evolution》2020,10(11):5015-5033
Intraspecific trait variation (ITV), based on available genetic diversity, is one of the major means plant populations can respond to environmental variability. The study of functional trait variation and diversity has become popular in ecological research, for example, as a proxy for plant performance influencing fitness. Up to now, it is unclear which aspects of intraspecific functional trait variation (iFDCV) can be attributed to the environment or genetics under natural conditions. Here, we examined 260 individuals from 13 locations of the rare (semi‐)dry calcareous grassland species Trifolium montanum L. in terms of iFDCV, within‐habitat heterogeneity, and genetic diversity. The iFDCV was assessed by measuring functional traits (releasing height, biomass, leaf area, specific leaf area, leaf dry matter content, Fv/Fm, performance index, stomatal pore surface, and stomatal pore area index). Abiotic within‐habitat heterogeneity was derived from altitude, slope exposure, slope, leaf area index, soil depth, and further soil factors. Based on microsatellites, we calculated expected heterozygosity (He) because it best‐explained, among other indices, iFDCV. We performed multiple linear regression models quantifying relationships among iFDCV, abiotic within‐habitat heterogeneity and genetic diversity, and also between separate functional traits and abiotic within‐habitat heterogeneity or genetic diversity. We found that abiotic within‐habitat heterogeneity influenced iFDCV twice as strong compared to genetic diversity. Both aspects together explained 77% of variation in iFDCV ( = .77, F2, 10 = 21.66, p < .001). The majority of functional traits (releasing height, biomass, specific leaf area, leaf dry matter content, Fv/Fm, and performance index) were related to abiotic habitat conditions indicating responses to environmental heterogeneity. In contrast, only morphology‐related functional traits (releasing height, biomass, and leaf area) were related to genetics. Our results suggest that both within‐habitat heterogeneity and genetic diversity affect iFDCV and are thus crucial to consider when aiming to understand or predict changes of plant species performance under changing environmental conditions. 相似文献
3.
Benjamin C. Blum Weiwei Lin Matthew L. Lawton Qian Liu Julian Kwan Isabella Turcinovic Ryan Hekman Pingzhao Hu Andrew Emili 《Molecular & cellular proteomics : MCP》2022,21(1):100189
Metabolism is recognized as an important driver of cancer progression and other complex diseases, but global metabolite profiling remains a challenge. Protein expression profiling is often a poor proxy since existing pathway enrichment models provide an incomplete mapping between the proteome and metabolism. To overcome these gaps, we introduce multiomic metabolic enrichment network analysis (MOMENTA), an integrative multiomic data analysis framework for more accurately deducing metabolic pathway changes from proteomics data alone in a gene set analysis context by leveraging protein interaction networks to extend annotated metabolic models. We apply MOMENTA to proteomic data from diverse cancer cell lines and human tumors to demonstrate its utility at revealing variation in metabolic pathway activity across cancer types, which we verify using independent metabolomics measurements. The novel metabolic networks we uncover in breast cancer and other tumors are linked to clinical outcomes, underscoring the pathophysiological relevance of the findings. 相似文献
4.
A particularly vexing phenomenon within invasion ecology is the occurrence of spontaneous collapses within seemingly well-established exotic populations. Here, we assess the frequency of collapses among 68 exotic bird populations established in Hawaii, Puerto Rico, Los Angeles and Miami. Following other published definitions, we define a ‘collapse’ as a decline in abundance of ≥90 % within ≤10 years that lasts for at least 3 years. We show that 44 of the 68 exotic bird populations have exhibited declines at some point within their time series. Sixteen of the populations declined sufficiently to be defined as collapsed. It took on average 3.8 ± 1.8 years for populations to decline into a collapsed state, and this state persisted on average for 7.1 ± 6.3 years across (collapsed) populations. We compared the severity and duration of declines across all 44 declining populations according to taxonomic Order and geographic region. Neither variable explained substantial variation in the metrics of collapse. Our results indicate that severe, rapid, and persistent population declines may be common among exotic populations. We suggest that incorporating the probability and persistence of collapses into management decisions can inform efforts to enact control or eradication measures. We also suggest that applying our approach to other taxa and locations is crucial for improving our understanding of when and where collapses are likely to occur. 相似文献
5.
The probability that protected areas will deliver their potential for maintaining or enhancing biodiversity is likely to be
maximised if they are appropriately and effectively managed. As a result, governments and conservation agencies are devoting
much attention to the management of protected areas. In the U.K., the demand for performance accountability has resulted in
Public Service Agreements (PSA) that set out targets for government departments to deliver results in return for investments
being made. One such target for England is to ensure that all nationally important wildlife sites are in favourable condition
by 2010. Here, we tested the hypothesis, of potential strategic importance, that the ecological condition of these sites is
predictable from relationships with a range of physical, environmental and demographic variables. We used binary logistic
regression to investigate these relationships, using the results of English Nature’s 1997–2003 condition assessment exercise.
Generally, sites in unfavourable condition tend to be larger in area, located at higher elevations, but with higher human
population density and are more spatially isolated from units of the same habitat. However, despite the range of different
parameters included in our models, the extent to which the condition of any given site could be predicted was low. Our results
have implications for the delivery of PSA targets, funding allocation, and the location of new protected areas. 相似文献
6.
James B Munro Roger B Altman Chang‐Shung Tung Kevin Y Sanbonmatsu Scott C Blanchard 《The EMBO journal》2010,29(4):770-781
A key intermediate in translocation is an ‘unlocked state’ of the pre‐translocation ribosome in which the P‐site tRNA adopts the P/E hybrid state, the L1 stalk domain closes and ribosomal subunits adopt a ratcheted configuration. Here, through two‐ and three‐colour smFRET imaging from multiple structural perspectives, EF‐G is shown to accelerate structural and kinetic pathways in the ribosome, leading to this transition. The EF‐G‐bound ribosome remains highly dynamic in nature, wherein, the unlocked state is transiently and reversibly formed. The P/E hybrid state is energetically favoured, but exchange with the classical P/P configuration persists; the L1 stalk adopts a fast dynamic mode characterized by rapid cycles of closure and opening. These data support a model in which P/E hybrid state formation, L1 stalk closure and subunit ratcheting are loosely coupled, independent processes that must converge to achieve the unlocked state. The highly dynamic nature of these motions, and their sensitivity to conformational and compositional changes in the ribosome, suggests that regulating the formation of this intermediate may present an effective avenue for translational control. 相似文献
7.
8.
Petronela Weisová Ujval Anilkumar Caitriona Ryan Caoimhín G. Concannon Jochen H.M. Prehn Manus W. Ward 《BBA》2012,1817(5):744-753
The preconditioning response conferred by a mild uncoupling of the mitochondrial membrane potential (Δψm) has been attributed to altered reactive oxygen species (ROS) production and mitochondrial Ca2 + uptake within the cells. Here we have explored if altered cellular energetics in response to a mild mitochondrial uncoupling stimulus may also contribute to the protection. The addition of 100 nM FCCP for 30 min to cerebellar granule neurons (CGNs) induced a transient depolarization of the Δψm, that was sufficient to significantly reduce CGN vulnerability to the excitotoxic stimulus, glutamate. On investigation, the mild mitochondrial ‘uncoupling’ stimulus resulted in a significant increase in the plasma membrane levels of the glucose transporter isoform 3, with a hyperpolarisation of Δψm and increased cellular ATP levels also evident following the washout of FCCP. Furthermore, the phosphorylation state of AMP-activated protein kinase (AMPK) (Thr 172) was increased within 5 min of the uncoupling stimulus and elevated up to 1 h after washout. Significantly, the physiological changes and protection evident after the mild uncoupling stimulus were lost in CGNs when AMPK activity was inhibited. This study identifies an additional mechanism through which protection is mediated upon mild mitochondrial uncoupling: it implicates increased AMPK signalling and an adaptive shift in energy metabolism as mediators of the preconditioning response associated with FCCP-induced mild mitochondrial uncoupling. 相似文献
9.
10.