全文获取类型
收费全文 | 5086篇 |
免费 | 429篇 |
专业分类
5515篇 |
出版年
2023年 | 58篇 |
2022年 | 95篇 |
2021年 | 162篇 |
2020年 | 100篇 |
2019年 | 128篇 |
2018年 | 140篇 |
2017年 | 127篇 |
2016年 | 212篇 |
2015年 | 284篇 |
2014年 | 300篇 |
2013年 | 317篇 |
2012年 | 421篇 |
2011年 | 380篇 |
2010年 | 270篇 |
2009年 | 192篇 |
2008年 | 289篇 |
2007年 | 253篇 |
2006年 | 248篇 |
2005年 | 173篇 |
2004年 | 173篇 |
2003年 | 176篇 |
2002年 | 137篇 |
2001年 | 54篇 |
2000年 | 28篇 |
1999年 | 57篇 |
1998年 | 41篇 |
1997年 | 16篇 |
1996年 | 34篇 |
1995年 | 22篇 |
1994年 | 25篇 |
1993年 | 24篇 |
1992年 | 32篇 |
1991年 | 40篇 |
1990年 | 19篇 |
1989年 | 18篇 |
1988年 | 18篇 |
1987年 | 19篇 |
1986年 | 15篇 |
1985年 | 18篇 |
1984年 | 28篇 |
1983年 | 21篇 |
1982年 | 18篇 |
1981年 | 18篇 |
1980年 | 18篇 |
1978年 | 13篇 |
1977年 | 19篇 |
1976年 | 16篇 |
1884年 | 13篇 |
1882年 | 12篇 |
1881年 | 12篇 |
排序方式: 共有5515条查询结果,搜索用时 15 毫秒
981.
982.
The species Perna viridis is a highly consumed species, which fast growth makes it an interesting aquaculture alternative for Venezuelan and Trinidad coasts. With the aim to contribute with its nutritional value information, this study analyzed lipid and fatty acid contents from samples taken in five locations from Eastern Venezuela and three from Trinidad West Coast. Total lipids were extracted and quantified, from a pooled sample of 100 organisms per location, by standard gravimetric methods, and their identification and quantification was done by TLC/FID (Iatroscan system). Furthermore, the esterified fatty acids of total lipid, phospholipids and triacylglycerols were identified and quantified by gas chromatography. Eastern Venezuela samples from Los Cedros, La Brea and Chaguaramas showed the highest total lipid values of 7.92, 7.74 and 7.53, respectively, and the minimum values were obtained for La Restinga (6.08%). Among lipid composition, Chacopata samples showed the lowest phospholipid concentration (48.86%) and the maximum values for cholesterol (38.87%) and triacylglycerols (12.26%); besides, La Esmeralda and Rio Caribe samples exhibited maximum phospholipids (88.71 and 84.93 respectively) and minimum cholesterol (6.50 and 4.42%) concentrations. Saturated fatty acids represented between 15.04% and 65.55% within total lipid extracts, with maximum and minimum values for La Esmeralda and Chacopata, respectively. Polyunsaturated results resulted between 7.80 and 37.18%, with higher values in La Brea and lower values in La Esmeralda. For phospholipids, saturated fatty acids concentrations varied between 38.81 and 48.68% for Chaguaramas and Chacopata samples, respectively. In the case of polyunsaturated fatty acids, these varied between non detected and 34.51%, with high concentrations in Los Cedros (27.97%) and Chaguaramas (34.51%) samples. For the triacylglycerols, the saturated fatty acids composition oscillated between 14.27 and 53.80% with low concentrations for Chacopata and high concentration for La Restinga; the polyunsaturated fatty acids were between 4.66 and 35.55% with lower values for Chacopata and higher values for Chaguaramas samples. P. viridis is recommended for human being consumption, according to the high content of unsaturated fatty acids found for this species. 相似文献
983.
Rivera J Sarmiento L Parra E Toro G Neira M Méndez J Barbosa J Caldas ML 《Biomédica : revista del Instituto Nacional de Salud》2011,31(3):372-380
Introduction. Influenza is an acute respiratory infection that may be seasonal or pandemic. In 2009 The World Health Organization (WHO) declared an influenza pandemia; 3,876 cases and 239 deaths were reported in Colombia. Objective. The morphological changes in lung tissues associated with virus infection H1N1/v09 were described from autopsied victims. Materials and methods. Seventy-five cases were diagnosed by RT-PCR for influenza A H1N1/v09, of which the lungs of 20 were selected for morphological study by light microscopy, optical microscopy, high-resolution transmission electron microscopy and immunohistochemistry. Results. Of the 75 cases, 83% had viral pneumonitis and 17% alveolitis. Complications included intra-alveolar hemorrhage (66%), edema (89%), diffuse alveolar damage (2%), and bacterial co-infection (32%). Morphological changes were as follows: destruction of the alveolar epithelium and interstitium, edema, macrophages with vacuolated cytoplasm,and infiltration of polymorphonuclear leukocytes in the alveolar lumen and interstitium, vacuolization cytoplasmic type I pneumocytes and electronedense bodies in cellular debris in the alveolar lumen, and immunoreactivity of viral antigens in bronchiolar epithelial cells and alveolar infiltrate. Conclusion. The low percentage of bacterial co-infection observed in these cases was a prominent feature, and suggested that the fatal result was probably not associated with secondary bacterial disease (Indicated by previous reports). The tissue lesions were attributed to tissue damage due to viral lesion, as well as the cellular and humoral inflammatory response associated with infiltration by polymorphonucleocytes and macrophages in the interstitium and alveolar lumen. 相似文献
984.
The innate immune system is the first line of defense against pathogens, and it plays a fundamental role in coordinating a protective immune response in birds. Although many studies have evaluated avian immune responses in the laboratory, many fewer studies to date have done so in a field setting. To gain insight into interspecific differences in immune function in wild birds, we used a field-deployed in vitro microbicidal assay to measure constitutive innate immunity of whole blood collected from three common passerines in suburban Chicago, Illinois, in 2009. Data from one microbe, Escherichia coli 8739, revealed that American Robins (Turdus migratorius) had significantly lower bactericidal capacity than House Sparrows (Passer domesticus) or Gray Catbirds (Dumetella carolinensis). Bactericidal capacity for E. coli 8739 tended to be lower for birds infested with chewing lice than those without chewing lice, and male birds had lower microbicidal capacity than females in the case of Staphylococcus aureus. This study demonstrates the potential for field-deployable eco-immunologic tools to inform infectious disease ecology research. 相似文献
985.
Cutting edge: reactive oxygen species inhibitors block priming, but not activation, of the NLRP3 inflammasome 总被引:1,自引:0,他引:1
Bauernfeind F Bartok E Rieger A Franchi L Núñez G Hornung V 《Journal of immunology (Baltimore, Md. : 1950)》2011,187(2):613-617
A common denominator among the multiple damage-inducing agents that ultimately lead to activation of NLRP3 has not yet been identified. Recently, production of reactive oxygen species (ROS) has been suggested to act as a common event upstream of the NLRP3 inflammasome machinery. Because de novo translation of NLRP3 is an essential step in the activation of NLRP3, we investigated the role of substances that inhibit either ROS production or its oxidative activity. Although we observe that NLRP3 inflammasome activation is unique among other known inflammasomes in its sensitivity to ROS inhibition, we have found that this phenomenon is attributable to the fact that NLRP3 strictly requires priming by a proinflammatory signal, a step that is blocked by ROS inhibitors. Although these data do not exclude a general role for ROS production in the process of NLRP3-triggered inflammation, they would put ROS upstream of NLRP3 induction, but not activation. 相似文献
986.
Nitrate transceptor(s) in plants 总被引:1,自引:0,他引:1
The availability of mineral nutrients in the soil dramatically fluctuates in both time and space. In order to optimize their nutrition, plants need efficient sensing systems that rapidly signal the local external concentrations of the individual nutrients. Until recently, the most upstream actors of the nutrient signalling pathways, i.e. the sensors/receptors that perceive the extracellular nutrients, were unknown. In Arabidopsis, increasing evidence suggests that, for nitrate, the main nitrogen source for most plant species, a major sensor is the NRT1.1 nitrate transporter, also contributing to nitrate uptake by the roots. Membrane proteins that fulfil a dual nutrient transport/signalling function have been described in yeast and animals, and are called 'transceptors'. This review aims to illustrate the nutrient transceptor concept in plants by presenting the current evidence indicating that NRT1.1 is a representative of this class of protein. The various facets, as well as the mechanisms of nitrate sensing by NRT1.1 are considered, and the possible occurrence of other nitrate transceptors is discussed. 相似文献
987.
Sheridan JT Worthington EN Yu K Gabriel SE Hartzell HC Tarran R 《The Journal of biological chemistry》2011,286(2):1381-1388
Members of the Anoctamin (Ano)/TMEM16A family have recently been identified as essential subunits of the Ca2+-activated chloride channel (CaCC). For example, Ano1 is highly expressed in multiple tissues including airway epithelia, where it acts as an apical conduit for transepithelial Cl− secretion and helps regulate lung liquid homeostasis and mucus clearance. However, little is known about the oligomerization of this protein in the plasma membrane. Thus, utilizing mCherry- and eGFP-tagged Ano1 constructs, we conducted biochemical and Förster resonance energy transfer (FRET)-based experiments to determine the quaternary structure of Ano1. FRET and co-immunoprecipitation studies revealed that tagged Ano1 subunits directly associated before they reached the plasma membrane. This association was not altered by changes in cytosolic Ca2+, suggesting that this is a fixed interaction. To determine the oligomeric structure of Ano1, we performed chemical cross-linking, non-denaturing PAGE, and electromobility shift assays, which revealed that Ano1 exists as a dimer. These data are the first to probe the quaternary structure of Ano1. Understanding the oligomeric nature of Ano1 is an essential step in the development of therapeutic drugs that could be useful in the treatment of cystic fibrosis. 相似文献
988.
989.
Synthesis of [difluoro-(3-alkenylphenyl)-methyl]-phosphonic acids on non-crosslinked polystyrene and their evaluation as inhibitors of PTP1B 总被引:1,自引:0,他引:1
A series of [difluoro-(3-alkenylphenyl)-methyl]-phosphonates were prepared on non-crosslinked polystyrene, a soluble polymer support. After cleavage from the support, the resulting phosphonic acids were examined for inhibition with protein tyrosine phosphatase 1B. Compound 20, bearing an alpha,beta-unsaturated allyl ester moiety, was the most potent of this series of compounds, being a reversible, competitive inhibitor with a K(i) of 8.0+/-1.4 microM. 相似文献
990.
Lauren C. Bresee Merril L. Knudtson Jianguo Zhang Lynden Crowshoe Sofia B. Ahmed Marcello Tonelli William A. Ghali Hude Quan Braden Manns Gabriel Fabreau Brenda R. Hemmelgarn 《CMAJ》2014,186(10):E372-E380