全文获取类型
| 收费全文 | 139411篇 |
| 免费 | 7613篇 |
| 国内免费 | 41篇 |
专业分类
| 147065篇 |
出版年
| 2021年 | 1945篇 |
| 2020年 | 1231篇 |
| 2019年 | 1508篇 |
| 2018年 | 2201篇 |
| 2017年 | 1967篇 |
| 2016年 | 2827篇 |
| 2015年 | 3676篇 |
| 2014年 | 4269篇 |
| 2013年 | 5274篇 |
| 2012年 | 6488篇 |
| 2011年 | 6308篇 |
| 2010年 | 4398篇 |
| 2009年 | 3742篇 |
| 2008年 | 5396篇 |
| 2007年 | 5300篇 |
| 2006年 | 4917篇 |
| 2005年 | 4625篇 |
| 2004年 | 4471篇 |
| 2003年 | 4259篇 |
| 2002年 | 3967篇 |
| 2001年 | 4744篇 |
| 2000年 | 4399篇 |
| 1999年 | 3577篇 |
| 1998年 | 1449篇 |
| 1997年 | 1305篇 |
| 1996年 | 1163篇 |
| 1995年 | 1049篇 |
| 1994年 | 1035篇 |
| 1993年 | 1044篇 |
| 1992年 | 2651篇 |
| 1991年 | 2649篇 |
| 1990年 | 2643篇 |
| 1989年 | 2360篇 |
| 1988年 | 2238篇 |
| 1987年 | 2135篇 |
| 1986年 | 1949篇 |
| 1985年 | 1945篇 |
| 1984年 | 1660篇 |
| 1983年 | 1417篇 |
| 1982年 | 1084篇 |
| 1981年 | 1016篇 |
| 1979年 | 1539篇 |
| 1978年 | 1234篇 |
| 1977年 | 1111篇 |
| 1976年 | 1054篇 |
| 1975年 | 1224篇 |
| 1974年 | 1325篇 |
| 1973年 | 1417篇 |
| 1972年 | 1248篇 |
| 1971年 | 1098篇 |
排序方式： 共有10000条查询结果，搜索用时 15 毫秒
921.
922.
Laura E. Downing Daniel Edgar Patricia A. Ellison Marie‐Louise Ricketts 《Cell biochemistry and function》2017,35(1):12-32
Dietary procyanidins have emerged as important bioactive components that regulate various metabolic pathways to maintain homeostasis. Grape seed procyanidin extract (GSPE), in particular, has demonstrated regulatory effects on bile acid and lipid metabolism in vivo. While numerous studies in rodent models have shown the potent hypolipidemic action of grape seed extracts, human studies have shown inconsistent results. This review will focus on the molecular mechanisms underlying the hypolipidemic actions of GSPE identified to date, specifically highlighting the effects exerted via nuclear receptors. Such evidence may provide avenues for future research in human subjects with GSPE as a therapeutic treatment for the prevention and amelioration of the metabolic syndrome and cardiovascular disease. 相似文献
923.
Gustavo Souto de Sá e Souza Fábio Barbosa Rodrigues Adriano O. Andrade Marcus Fraga Vieira 《Computer methods in biomechanics and biomedical engineering》2017,20(8):901-904
Gait speed is an essential parameter of gait analysis. Our study proposed a simple and accurate method to extract a mean gait speed during walking on a treadmill using only kinematic data from markers placed on the heels of the participants’ feet. This method provided an attractive, simple method that remains resistant to errors in treadmill calibration. In addition, this method required only two markers, since heel markers are essential to gait analysis, and the proposed method is robust enough to differentiate among various gait speeds (mean error <1%). 相似文献
924.
925.
M. S. Ugelvik A. Skorping O. Moberg A. Mennerat 《Journal of evolutionary biology》2017,30(6):1136-1142
Parasites rely on resources from a host and are selected to achieve an optimal combination of transmission and virulence. Human‐induced changes in parasite ecology, such as intensive farming of hosts, might not only favour increased parasite abundances, but also alter the selection acting on parasites and lead to life‐history evolution. The trade‐off between transmission and virulence could be affected by intensive farming practices such as high host density and the use of antiparasitic drugs, which might lead to increased virulence in some host–parasite systems. To test this, we therefore infected Atlantic salmon (Salmo salar) smolts with salmon lice (Lepeophtheirus salmonis) sampled either from wild or farmed hosts in a laboratory experiment. We compared growth and skin damage (i.e. proxies for virulence) of hosts infected with either wild or farmed lice and found that, compared to lice sampled from wild hosts in unfarmed areas, those originating from farmed fish were more harmful; they inflicted more skin damage to their hosts and reduced relative host weight gain to a greater extent. We advocate that more evolutionary studies should be carried out using farmed animals as study species, given the current increase in intensive food production practices that might be compared to a global experiment in parasite evolution. 相似文献
926.
Marguerite E. O’Haire 《应用发育科学》2017,21(3):200-216
Including animals in autism intervention is growing in both research and practice. A systematic literature review was conducted to collate and synthesize all empirical research on animal-assisted intervention (AAI) for autism published from 2012 to 2015. Findings from 28 included studies revealed that AAI programs generally include one animal per participant with a total contact time of approximately 10 hours over the course of 8 to 12 weeks. Research methodology is diverse and though limited in many cases, has improved over the last few years. The most commonly reported outcome was increased social interaction, which was unanimously significant across 22 studies. The need for further research is highlighted, calling for a focus on refining AAI techniques, identifying optimal circumstances for positive change as well as individuals who may not benefit, and independent replication of high quality studies to move AAI from an enrichment activity to an evidence-based practice for autism. 相似文献
927.
Chloroplast inheritance is a very important information to obtain when cpDNA is used to study phylogeography or reconstruct phylogenies. A procedure used for the analyses of chloroplast inheritance in the sea daffodil (Pancratium maritimum) is described using multi-facet approach (artificial cross, germination and end-point PCR amplification). 相似文献
928.
Thomas Gumbricht Rosa Maria Roman‐Cuesta Louis Verchot Martin Herold Florian Wittmann Ethan Householder Nadine Herold Daniel Murdiyarso 《Global Change Biology》2017,23(9):3581-3599
Wetlands are important providers of ecosystem services and key regulators of climate change. They positively contribute to global warming through their greenhouse gas emissions, and negatively through the accumulation of organic material in histosols, particularly in peatlands. Our understanding of wetlands’ services is currently constrained by limited knowledge on their distribution, extent, volume, interannual flood variability and disturbance levels. We present an expert system approach to estimate wetland and peatland areas, depths and volumes, which relies on three biophysical indices related to wetland and peat formation: (1) long‐term water supply exceeding atmospheric water demand; (2) annually or seasonally water‐logged soils; and (3) a geomorphological position where water is supplied and retained. Tropical and subtropical wetlands estimates reach 4.7 million km2 (Mkm2). In line with current understanding, the American continent is the major contributor (45%), and Brazil, with its Amazonian interfluvial region, contains the largest tropical wetland area (800,720 km2). Our model suggests, however, unprecedented extents and volumes of peatland in the tropics (1.7 Mkm2 and 7,268 (6,076–7,368) km3), which more than threefold current estimates. Unlike current understanding, our estimates suggest that South America and not Asia contributes the most to tropical peatland area and volume (ca. 44% for both) partly related to some yet unaccounted extended deep deposits but mainly to extended but shallow peat in the Amazon Basin. Brazil leads the peatland area and volume contribution. Asia hosts 38% of both tropical peat area and volume with Indonesia as the main regional contributor and still the holder of the deepest and most extended peat areas in the tropics. Africa hosts more peat than previously reported but climatic and topographic contexts leave it as the least peat‐forming continent. Our results suggest large biases in our current understanding of the distribution, area and volumes of tropical peat and their continental contributions. 相似文献
929.
Rivadeneyra Almudena Gonzalez-Martinez Alejandro Portela Gabriela R. Martin-Ramos Daniel J. Gonzalez-Lopez Jesús Rivadeneyra María A. 《Extremophiles : life under extreme conditions》2017,21(6):1049-1056
The ability of Halomonas maura to bioprecipitate carbonate and sulphate crystals in solid media at different manganese concentrations has been demonstrated in this study for the first time. The precipitated minerals were studied by X-ray diffraction, scanning and transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The precipitated minerals were different based on the manganese concentration present in the medium and the incubation time. In the absence of manganese, H. maura formed pseudokutnahorite crystals; in the presence of manganese, the concentration in the culture medium determined the precipitation carbonates, such as rhodochrosite and dolomites. However, in the presence of low concentrations of manganese chloride (MnCl2) (5 g/l), kutnohorite crystals were also formed. Finally, when H. maura was grown in the presence of manganese, small amounts of sulphate crystals (such as bassanite and gypsum) were detected. Our study of the precipitated minerals showed an active role of H. maura in the biomineralisation process, but the geochemical conditions, and the manganese concentrations in particular, were clearly influential.
相似文献930.