全文获取类型
收费全文 | 6827篇 |
免费 | 465篇 |
国内免费 | 1篇 |
专业分类
7293篇 |
出版年
2024年 | 11篇 |
2023年 | 67篇 |
2022年 | 89篇 |
2021年 | 174篇 |
2020年 | 149篇 |
2019年 | 152篇 |
2018年 | 192篇 |
2017年 | 192篇 |
2016年 | 248篇 |
2015年 | 383篇 |
2014年 | 372篇 |
2013年 | 496篇 |
2012年 | 593篇 |
2011年 | 578篇 |
2010年 | 357篇 |
2009年 | 294篇 |
2008年 | 402篇 |
2007年 | 374篇 |
2006年 | 307篇 |
2005年 | 298篇 |
2004年 | 296篇 |
2003年 | 227篇 |
2002年 | 220篇 |
2001年 | 63篇 |
2000年 | 46篇 |
1999年 | 46篇 |
1998年 | 80篇 |
1997年 | 52篇 |
1996年 | 38篇 |
1995年 | 35篇 |
1994年 | 36篇 |
1993年 | 40篇 |
1992年 | 27篇 |
1991年 | 20篇 |
1990年 | 27篇 |
1989年 | 20篇 |
1988年 | 17篇 |
1987年 | 23篇 |
1986年 | 21篇 |
1985年 | 34篇 |
1984年 | 20篇 |
1983年 | 16篇 |
1982年 | 16篇 |
1981年 | 14篇 |
1980年 | 17篇 |
1979年 | 11篇 |
1973年 | 12篇 |
1972年 | 8篇 |
1968年 | 11篇 |
1961年 | 8篇 |
排序方式: 共有7293条查询结果,搜索用时 15 毫秒
61.
Eduardo Massad Annelies Wilder-Smith Raphael Ximenes Marcos Amaku Luis Fernandez Lopez Francisco Antonio Bezerra Coutinho Giovanini Evelim Coelho Jarbas Barbosa da Silva Jr Claudio José Struchiner Marcelo Nascimento Burattini 《Memórias do Instituto Oswaldo Cruz》2014,109(3):394-397
Brazil will host the FIFA World Cup™, the biggest single-event competition in the
world, from June 12-July 13 2014 in 12 cities. This event will draw an estimated
600,000 international visitors. Brazil is endemic for dengue. Hence, attendees of the
2014 event are theoretically at risk for dengue. We calculated the risk of dengue
acquisition to non-immune international travellers to Brazil, depending on the
football match schedules, considering locations and dates of such matches for June
and July 2014. We estimated the average per-capita risk and expected
number of dengue cases for each host-city and each game schedule chosen based on
reported dengue cases to the Brazilian Ministry of Health for the period between
2010-2013. On the average, the expected number of cases among the 600,000 foreigner
tourists during the World Cup is 33, varying from 3-59. Such risk estimates will not
only benefit individual travellers for adequate pre-travel preparations, but also
provide valuable information for public health professionals and policy makers
worldwide. Furthermore, estimates of dengue cases in international travellers during
the World Cup can help to anticipate the theoretical risk for exportation of dengue
into currently non-infected areas. 相似文献
62.
63.
Selective Cytotoxicity of 1,3,4-Thiadiazolium Mesoionic Derivatives on Hepatocarcinoma Cells (HepG2)
Gustavo Jabor Gozzi Amanda do Rocio Andrade Pires Glaucio Valdameri Maria Eliane Merlin Rocha Glaucia Regina Martinez Guilhermina Rodrigues Noleto Alexandra Acco Carlos Eduardo Alves de Souza Aurea Echevarria Camilla Moretto dos Reis Attilio Di Pietro Sílvia Maria Suter Correia Cadena 《PloS one》2015,10(6)
In this work, we evaluated the cytotoxicity of mesoionic 4-phenyl-5-(2-Y, 4-X or 4-X-cinnamoyl)-1,3,4-thiadiazolium-2-phenylamine chloride derivatives (MI-J: X=OH, Y=H; MI-D: X=NO2, Y=H; MI-4F: X=F, Y=H; MI-2,4diF: X=Y=F) on human hepatocellular carcinoma (HepG2), and non-tumor cells (rat hepatocytes) for comparison. MI-J, M-4F and MI-2,4diF reduced HepG2 viability by ~ 50% at 25 μM after 24-h treatment, whereas MI-D required a 50 μM concentration, as shown by 3-(4,5-dimethythiazol-2-yl)-2,5-diphenyltetrazolium bromide assays. The cytotoxicity was confirmed with lactate dehydrogenase assay, of which activity was increased by 55, 24 and 16% for MI-J, MI-4F and MI-2,4diF respectively (at 25 μM after 24 h). To identify the death pathway related to cytotoxicity, the HepG2 cells treated by mesoionic compounds were labeled with both annexin V and PI, and analyzed by flow cytometry. All compounds increased the number of doubly-stained cells at 25 μM after 24 h: by 76% for MI-J, 25% for MI-4F and MI-2,4diF, and 11% for MI-D. It was also verified that increased DNA fragmentation occurred upon MI-J, MI-4F and MI-2,4diF treatments (by 12%, 9% and 8%, respectively, at 25 μM after 24 h). These compounds were only weakly, or not at all, transported by the main multidrug transporters, P-glycoprotein, ABCG2 and MRP1, and were able to slightly inhibit their drug-transport activity. It may be concluded that 1,3,4-thiadiazolium compounds, especially the hydroxy derivative MI-J, constitute promising candidates for future investigations on in-vivo treatment of hepatocellular carcinoma. 相似文献
64.
65.
66.
Dung Beetle and Terrestrial Mammal Diversity in Forests, Indigenous Agroforestry Systems and Plantain Monocultures in Talamanca, Costa Rica 总被引:2,自引:0,他引:2
In order to explore the importance of indigenous agroforestry systems for biodiversity conservation, we compared the abundance,
species richness and diversity of dung beetles and terrestrial mammals across a gradient of different land use types from
agricultural monocultures (plantains) to agroforestry systems (cocoa and banana) and forests in the BriBri and Cabécar indigenous
reserves in Talamanca, Costa Rica. A total of 132,460 dung beetles of 52 species and 913 tracks of 27 terrestrial mammal species
were registered. Dung beetle species richness and diversity were greatest in the forests, intermediate in the agroforestry
systems and lowest in the plantain monocultures, while dung beetle abundance was greatest in the plantain monocultures. The
number of mammal tracks per plot was significantly higher in forests than in plantain monocultures, whereas mammal species
richness was higher in forests than in either cocoa agroforestry systems or plantain monocultures. Species composition of
both terrestrial mammals and dung beetles also varied across the different land use types. Our study indicates that indigenous
cocoa and banana agroforestry systems maintain an intermediate level of biodiversity (which is less than that of the original
forest but significantly greater than that of plantain monocultures) and provide suitable habitat for a number of forest-dependent
species. Although the agroforestry systems appear to serve as favorable habitats for many terrestrial mammal species, their
potential positive contribution to mammal conservation is being offset by heavy hunting pressure in the reserves. As in other
agricultural landscapes, the conservation of biodiversity in Talamanca will depend not only on maintaining the existing forest
patches and reducing the conversion of traditional agroforestry systems to monocultures, but also on reducing hunting pressure. 相似文献
67.
María Lorena Falcone Ferreyra Eduardo Rodriguez María Isabel Casas Guillermo Labadie Erich Grotewold Paula Casati 《The Journal of biological chemistry》2013,288(44):31678-31688
Flavonoids accumulate in plant vacuoles usually as O-glycosylated derivatives, but several species can also synthesize flavonoid C-glycosides. Recently, we demonstrated that a flavanone 2-hydroxylase (ZmF2H1, CYP93G5) converts flavanones to the corresponding 2-hydroxy derivatives, which are expected to serve as substrates for C-glycosylation. Here, we isolated a cDNA encoding a UDP-dependent glycosyltransferase (UGT708A6), and its activity was characterized by in vitro and in vivo bioconversion assays. In vitro assays using 2-hydroxyflavanones as substrates and in vivo activity assays in yeast co-expressing ZmF2H1 and UGT708A6 show the formation of the flavones C-glycosides. UGT708A6 can also O-glycosylate flavanones in bioconversion assays in Escherichia coli as well as by in vitro assays with the purified recombinant protein. Thus, UGT708A6 is a bifunctional glycosyltransferase that can produce both C- and O-glycosidated flavonoids, a property not previously described for any other glycosyltransferase. 相似文献
68.
69.
Virgínia?Teles?Dohanik Wagner?Gonzaga?Gon?alves Leandro?Licursi?Oliveira José?Cola?Zanuncio José?Eduardo?Serr?oEmail author 《Protoplasma》2018,255(6):1703-1712
Vitellogenin receptor (VgR) is a low-density lipoprotein receptor responsible for the mediated endocytosis of vitellogenin (Vg) during egg formation in insects. The maturing oocyte is enveloped by a follicular epithelium, which has large intercellular spaces during Vg accumulation (patency). However, Vg has been reported in the cytoplasm of follicular cells, indicating that there may be a transcellular route for its transport. This study verified the presence of VgR in the follicular cells of the ovaries of the honeybee Apis mellifera and the wasp Polistes simillimus in order to evaluate if Vg is transported via transcytosis in these insects. Antibodies specific for vitellogenin receptor (anti-VgR), vitellogenin (anti-Vg), and clathrin (anti-Clt) were used for immunolocalization. The results showed the presence of VgR on the apical and basal plasma membranes of follicular cells of the vitellogenic follicles in both species, indicating that VgR may have been transported from the basal to the apical cell domain, followed by its release into the perivitelline space, evidenced by the presence of apical plasma membrane projections containing VgR. Co-localization proved that Vg bind to VgR and that the transport of this protein is mediated by clathrin. These data suggest that, in these social insects, Vg is transported via clathrin-mediated VgR transcytosis in follicular cells. 相似文献
70.
Eduardo Molina Josep Maria Espelta Joan Pino Guillem Bagaria Dolors Armenteras 《Biotropica》2018,50(5):740-749
The spatial heterogeneity of resource availability is a major driver of biodiversity patterns. Some environmental conditions and resources are characterized by large‐scale patterns of variation within the landscape. Clumped local discontinuities or discrete elements also increase spatial heterogeneity, promoting local ‘biodiversity hot spots’ by modifying habitat characteristics and promoting plant–animal interactions. Clay licks are faunal attractors owing to their role in the nutritional ecology of the user species; nevertheless, the effect of their presence on the surrounding vegetation has been poorly quantified. Here, we use data from 100 × 10 m transects and evaluate the effects of the presence of clay licks on forest diversity and structure at local and landscape scales. In clay lick areas, there was a higher abundance of certain species, which helps to homogenize species composition between localities counteracting the natural distance‐decay of compositional similarity between transects without clay lick influence (controls). Compared to control sites, clay lick′s forests had higher palm densities, shorter but more variable individuals in the canopy and understory, a thinner canopy layer, and denser herbaceous and ground level covers. These differences were found along the whole length of transects in both sampled areas types. These results reveal that the presence of discrete elements (i.e., clay licks) may help to explain the compositional and structural heterogeneity of Amazonian forests influencing ecological processes such as seed dispersal and trampling. These considerations may be relevant for other biomes where clay licks are present and give weight to their inclusion in conservation initiatives in tropical forests. 相似文献