全文获取类型
收费全文 | 281篇 |
免费 | 23篇 |
出版年
2021年 | 3篇 |
2020年 | 3篇 |
2019年 | 2篇 |
2018年 | 3篇 |
2017年 | 3篇 |
2016年 | 3篇 |
2015年 | 10篇 |
2014年 | 9篇 |
2013年 | 11篇 |
2012年 | 9篇 |
2011年 | 15篇 |
2010年 | 18篇 |
2009年 | 11篇 |
2008年 | 15篇 |
2007年 | 21篇 |
2006年 | 10篇 |
2005年 | 10篇 |
2004年 | 15篇 |
2003年 | 7篇 |
2002年 | 10篇 |
2001年 | 18篇 |
2000年 | 12篇 |
1999年 | 12篇 |
1998年 | 12篇 |
1997年 | 2篇 |
1996年 | 5篇 |
1995年 | 9篇 |
1994年 | 4篇 |
1993年 | 4篇 |
1992年 | 5篇 |
1990年 | 1篇 |
1989年 | 3篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1975年 | 4篇 |
1973年 | 2篇 |
1970年 | 1篇 |
1968年 | 1篇 |
1965年 | 2篇 |
1964年 | 1篇 |
排序方式: 共有304条查询结果,搜索用时 15 毫秒
131.
Despite their remoteness from sources of atmospheric pollutant emissions, the Afromontane tarns in the Maloti-Drakensberg region are perfect candidates to study the negative effects of acidifying atmospheric pollution, because mountain lakes are widely recognised as sentinel ecosystems, unimpacted by direct human disturbance within their catchments. Thirty-four tarns were sampled in the Maloti-Drakensberg region and most were found to be extremely sensitive to acidic deposition, as indicated by their low acid neutralising capacity. There are very few studies of freshwater critical loads for any region within South Africa. The steady-state water chemistry model (SSWC) was adapted and used to determine critical loads, whereas exceedance was estimated relative to modelled regional deposition data, in order to understand the risk of harmful effects to aquatic ecosystems. Seventy-six percent of sampled sites across the Maloti-Drakensberg would exceed critical loads even at the lowest modelled deposition levels, but there are no current measured deposition data for the region. The sensitivity of the Maloti-Drakensberg tarns needs to be considered in future policy formulation regarding acceptable levels of acidifying atmospheric pollution from South Africa’s energy sector and indicates the need for assessing aquatic ecosystem impacts in other regions of South Africa. 相似文献
132.
Melkaye G. Melka Michal Abrahamowicz Gabriel T. Leonard Michel Perron Louis Richer Suzanne Veillette Daniel Gaudet Tomá? Paus Zdenka Pausova 《PloS one》2013,8(12)
Visceral fat (VF) promotes the development of metabolic syndrome (MetS), which emerges as early as in adolescence. The clustering of MetS components suggests shared etiologies, but these are largely unknown and may vary between males and females. Here, we investigated the latent structure of pre-clinical MetS in a community-based sample of 286 male and 312 female adolescents, assessing their abdominal adiposity (VF) directly with magnetic resonance imaging. Principal component analysis of the five MetS-defining variables (VF, blood pressure [BP], fasting serum triglycerides, HDL-cholesterol and glucose) identified two independent components in both males and females. The first component was sex-similar; it explained >30% of variance and was loaded by all but BP variables. The second component explained >20% of variance; it was loaded by BP similarly in both sexes but additional loading by metabolic variables was sex-specific. This sex-specificity was not detected in analyses that used waist circumference instead of VF. In adolescence, MetS-defining variables cluster into at least two sub-syndromes: (1) sex-similar metabolic abnormalities of obesity-induced insulin resistance and (2) sex-specific metabolic abnormalities associated with BP elevation. These results suggest that the etiology of MetS may involve more than one pathway and that some of the pathways may differ between males and females. Further, the sex-specific metabolic abnormalities associated with BP elevation suggest the need for sex-specific prevention and treatment strategies of MetS. 相似文献
133.
Vincent Ranwez Frédéric Delsuc Sylvie Ranwez Khalid Belkhir Marie-Ka Tilak Emmanuel JP Douzery 《BMC evolutionary biology》2007,7(1):241
Background
Molecular sequence data have become the standard in modern day phylogenetics. In particular, several long-standing questions of mammalian evolutionary history have been recently resolved thanks to the use of molecular characters. Yet, most studies have focused on only a handful of standard markers. The availability of an ever increasing number of whole genome sequences is a golden mine for modern systematics. Genomic data now provide the opportunity to select new markers that are potentially relevant for further resolving branches of the mammalian phylogenetic tree at various taxonomic levels. 相似文献134.
Stephanie C. Penley Cynthia M. Gaudet Steven W. Threlkeld 《Journal of visualized experiments : JoVE》2013,(82)
Working and reference memory are commonly assessed using the land based radial arm maze. However, this paradigm requires pretraining, food deprivation, and may introduce scent cue confounds. The eight-arm radial water maze is designed to evaluate reference and working memory performance simultaneously by requiring subjects to use extra-maze cues to locate escape platforms and remedies the limitations observed in land based radial arm maze designs. Specifically, subjects are required to avoid the arms previously used for escape during each testing day (working memory) as well as avoid the fixed arms, which never contain escape platforms (reference memory). Re-entries into arms that have already been used for escape during a testing session (and thus the escape platform has been removed) and re-entries into reference memory arms are indicative of working memory deficits. Alternatively, first entries into reference memory arms are indicative of reference memory deficits. We used this maze to compare performance of rats with neonatal brain injury and sham controls following induction of hypoxia-ischemia and show significant deficits in both working and reference memory after eleven days of testing. This protocol could be easily modified to examine many other models of learning impairment. 相似文献
135.
Katerina Artavanis-Tsakonas Wilhelm A. Weihofen John M. Antos Bradley I. Coleman Christy A. Comeaux Manoj T. Duraisingh Rachelle Gaudet Hidde L. Ploegh 《The Journal of biological chemistry》2010,285(9):6857-6866
Like their human hosts, Plasmodium falciparum parasites rely on the ubiquitin-proteasome system for survival. We previously identified PfUCHL3, a deubiquitinating enzyme, and here we characterize its activity and changes in active site architecture upon binding to ubiquitin. We find strong evidence that PfUCHL3 is essential to parasite survival. The crystal structures of both PfUCHL3 alone and in complex with the ubiquitin-based suicide substrate UbVME suggest a rather rigid active site crossover loop that likely plays a role in restricting the size of ubiquitin adduct substrates. Molecular dynamics simulations of the structures and a model of the PfUCHL3-PfNedd8 complex allowed the identification of shared key interactions of ubiquitin and PfNedd8 with PfUCHL3, explaining the dual specificity of this enzyme. Distinct differences observed in ubiquitin binding between PfUCHL3 and its human counterpart make it likely that the parasitic DUB can be selectively targeted while leaving the human enzyme unaffected. 相似文献
136.
An analysis of the celluar constituents of the leaves of an aquatic fern, Marsilea vestita, was done after and during the formation of different leaf forms. Land leaves of Marsilea produced almost twice as much dry material, soluble carbohydrate, insoluble protein and soluble peptides compared to submerged leaves on a fresh weight basis, but on a dry weight basis there is tittle difference between the two types of leaves. the submerged leaf soluble nitrogen fraction consisted mostly of ammonia and free amino acids, while that of the land leaf probably con tained mostly peptides. Free arginine was present in large quantities in megaspores and as the plant matured, at the expense of the megaspore, the amount of arginine left in the pool diminished. A correlation was found between soluble sugar increase and leaf elongation, Just prior to the production of submerged leaves the developing leaves contained low levels of protein and soluble sugar, but high levels of soluble nitrogen and an increased amount of starch. A theory which involves metabolism was proposed to account for the leaf form changes in Marsilea. 相似文献
137.
138.
Alexander Kotrschal Eva JP Lievens Josefin Dahlbom Andreas Bundsen Svetlana Semenova Maria Sundvik Alexei A Maklakov Svante Winberg Pertti Panula Niclas Kolm 《Evolution; international journal of organic evolution》2014,68(4):1139-1149
Animal personalities range from individuals that are shy, cautious, and easily stressed (a “reactive” personality type) to individuals that are bold, innovative, and quick to learn novel tasks, but also prone to routine formation (a “proactive” personality type). Although personality differences should have important consequences for fitness, their underlying mechanisms remain poorly understood. Here, we investigated how genetic variation in brain size affects personality. We put selection lines of large‐ and small‐brained guppies (Poecilia reticulata), with known differences in cognitive ability, through three standard personality assays. First, we found that large‐brained animals were faster to habituate to, and more exploratory in, open field tests. Large‐brained females were also bolder. Second, large‐brained animals excreted less cortisol in a stressful situation (confinement). Third, large‐brained animals were slower to feed from a novel food source, which we interpret as being caused by reduced behavioral flexibility rather than lack of innovation in the large‐brained lines. Overall, the results point toward a more proactive personality type in large‐brained animals. Thus, this study provides the first experimental evidence linking brain size and personality, an interaction that may affect important fitness‐related aspects of ecology such as dispersal and niche exploration. 相似文献
139.
Solveig Tangedal Marianne Aanerud Louise JP Persson Karl A Brokstad Per S Bakke Tomas M Eagan 《Respiratory research》2014,15(1)