首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   345篇
  免费   41篇
  2023年   2篇
  2021年   5篇
  2020年   9篇
  2019年   8篇
  2018年   8篇
  2017年   7篇
  2016年   9篇
  2015年   12篇
  2014年   13篇
  2013年   21篇
  2012年   16篇
  2011年   16篇
  2010年   11篇
  2009年   21篇
  2008年   14篇
  2007年   15篇
  2006年   18篇
  2005年   15篇
  2004年   18篇
  2003年   16篇
  2002年   13篇
  2001年   10篇
  2000年   11篇
  1999年   7篇
  1998年   4篇
  1997年   3篇
  1996年   4篇
  1995年   2篇
  1994年   6篇
  1993年   5篇
  1992年   2篇
  1990年   7篇
  1989年   4篇
  1988年   7篇
  1987年   3篇
  1986年   5篇
  1985年   5篇
  1984年   5篇
  1983年   2篇
  1982年   2篇
  1980年   1篇
  1979年   3篇
  1978年   5篇
  1977年   4篇
  1976年   4篇
  1972年   1篇
  1971年   2篇
  1969年   2篇
  1967年   1篇
  1961年   1篇
排序方式: 共有386条查询结果,搜索用时 177 毫秒
31.
Non-human primates could prove to be suitable models for the study of infectious diseases such as malaria, tuberculosis, and hepatitis; the molecules of their immune systems are in the process of being fully characterized. Due to the relevance of cytokines in the modulation of the immune response, a molecular analysis of these proteins in non-human primates from the Aotus genus was carried out. Peripheral blood mononuclear cells from four species of Aotusmonkey were obtained and their mRNAs for interleukin-2 (IL-2), IL-4, IL-6, IL-10, interferon-gamma (IFN), and tumor necrosis factor (TNF)-alpha were characterized. This study shows a high degree of conservation between nucleotide and amino acid sequences of cytokines from different Aotus species and those from humans. The TNF-alpha molecules were identical in amino acid sequences for both.  相似文献   
32.
33.
Patients with type 1 diabetes (T1D) present increased risk of cardiovascular disease (CVD). The aim of this study is to improve the assessment of lipoprotein profile in patients with T1D by using a robust developed method 1H nuclear magnetic resonance spectroscopy (1H NMR), for further correlation with clinical factors associated to CVD. Thirty patients with T1D and 30 non-diabetes control (CT) subjects, matched for gender, age, body composition (DXA, BMI, waist/hip ratio), regular physical activity levels and cardiorespiratory capacity (VO2peak), were analyzed. Dietary records and routine lipids were assessed. Serum lipoprotein particle subfractions, particle sizes, and cholesterol and triglycerides subfractions were analyzed by 1H NMR. It was evidenced that subjects with T1D presented lower concentrations of small LDL cholesterol, medium VLDL particles, large VLDL triglycerides, and total triglycerides as compared to CT subjects. Women with T1D presented a positive association with HDL size (p<0.005; R = 0.601) and large HDL triglycerides (p<0.005; R = 0.534) and negative (p<0.005; R = -0.586) to small HDL triglycerides. Body fat composition represented an important factor independently of normal BMI, with large LDL particles presenting a positive correlation to total body fat (p<0.005; R = 0.505), and total LDL cholesterol and small LDL cholesterol a positive correlation (p<0.005; R = 0.502 and R = 0.552, respectively) to abdominal fat in T1D subjects; meanwhile, in CT subjects, body fat composition was mainly associated to HDL subclasses. VO2peak was negatively associated (p<0.005; R = -0.520) to large LDL-particles only in the group of patients with T1D. In conclusion, patients with T1D with adequate glycemic control and BMI and without chronic complications presented a more favourable lipoprotein profile as compared to control counterparts. In addition, slight alterations in BMI and/or body fat composition showed to be relevant to provoking alterations in lipoproteins profiles. Finally, body fat composition appears to be a determinant for cardioprotector lipoprotein profile.  相似文献   
34.
Shortly prior to death, many species of Lepidoptera infected with nucleopolyhedrovirus climb upwards on the host plant. This results in improved dissemination of viral occlusion bodies over plant foliage and an increased probability of transmission to healthy conspecific larvae. Following applications of Spodoptera exigua multiple nucleopolyhedrovirus for control of Spodoptera exigua on greenhouse-grown sweet pepper crops, necrophagy was observed by healthy S. exigua larvae that fed on virus-killed conspecifics. We examined whether this risky behavior was induced by olfactory or phagostimulant compounds associated with infected cadavers. Laboratory choice tests and olfactometer studies, involving infected and non-infected cadavers placed on spinach leaf discs, revealed no evidence for greater attraction of healthy larvae to virus-killed over non-infected cadavers. Physical contact or feeding on infected cadavers resulted in a very high incidence of transmission (82–93% lethal disease). Observations on the behavior of S. exigua larvae on pepper plants revealed that infected insects died on the uppermost 10% of foliage and closer to the plant stem than healthy conspecifics of the same stage, which we considered clear evidence of baculovirus-induced climbing behavior. Healthy larvae that subsequently foraged on the plant were more frequently observed closer to the infected than the non-infected cadaver. Healthy larvae also encountered and fed on infected cadavers significantly more frequently and more rapidly than larvae that fed on non-infected cadavers. Intraspecific necrophagy on infected cadavers invariably resulted in virus transmission and death of the necrophagous insect. We conclude that, in addition to improving the dissemination of virus particles over plant foliage, baculovirus-induced climbing behavior increases the incidence of intraspecific necrophagy in S. exigua, which is the most efficient mechanism of transmission of this lethal pathogen.  相似文献   
35.
36.
The pPT23A plasmid family of Pseudomonas syringae contains members that contribute to the ecological and pathogenic fitness of their P. syringae hosts. In an effort to understand the evolution of these plasmids and their hosts, we undertook a comparative analysis of the phylogeny of plasmid genes and that of conserved chromosomal genes from P. syringae. In total, comparative sequence and phylogenetic analyses were done utilizing 47 pPT23A family plasmids (PFPs) from 16 pathovars belonging to six genomospecies. Our results showed that the plasmid replication gene (repA), the only gene currently known to be distributed among all the PFPs, had a phylogeny that was distinct from that of the P. syringae hosts of these plasmids and from those of other individual genes on PFPs. The phylogenies of two housekeeping chromosomal genes, those for DNA gyrase B subunit (gyrB) and primary sigma factor (rpoD), however, were strongly associated with genomospecies of P. syringae. Based on the results from this study, we conclude that the pPT23A plasmid family represents a dynamic genome that is mobile among P. syringae pathovars.  相似文献   
37.
Ethanol ingestion is known to interfere with folate absorption and metabolism. A fostering/crossfostering analysis of maternal ethanol exposure effects on jejunum and ileum kinetic parameters in vivo of offspring rat folic acid absorption at 21 days postpartum was carried out. The rats were divided into four groups: CP, control pups; GP, pups exposed to ethanol only during gestation; LP, pups exposed to ethanol only during lactation; GLP, pups exposed to ethanol during gestation and lactation. Jejunal and ileal loop transport studies were performed using in vivo perfusion at a flow rate of 3 ml/min for 5 min. Folic acid concentrations of 0.25, 0.5, 1, 1.5 and 2.5 μm were used. Jejunal and ileal absorption values were determined by the difference between the initial and the final amounts of substrate in the perfusate and expressed as picomoles per square centimeter of intestinal surface every 5 min. The results indicated that ethanol consumption by the dams during gestation and/or lactation led to significant changes in V max, with no significant changes in apparent K m. These findings suggest that exposure to ethanol during gestational and suckling periods leads to a general delay in postnatal body weight and that intestinal folate absorption appears to be upregulated in suckling rats, this effect being higher in the LP group.  相似文献   
38.
Questions: Is post‐dispersal seed predation a factor contributing to a positive feedback mechanism for dominance of tall‐tussock grasslands? Do seeds dispersed into neighbouring microhabitats of contrasting dominance differ in their probabilities of being predated? Does predation rate vary among predator groups? Do seed eaters selectively forage among the available seeds? Location: The southern and flooding sub‐regions of the Argentinian Pampas. Methods: We examined seed predation by vertebrate and invertebrate predators within two microhabitats of grassland mosaics (highly dominated tall‐grass patches vs. scarcely dominated short‐grass matrix) for different seed species in semi‐natural grasslands. Proportion of seeds eaten by different predator groups was estimated through exclusion experiments and analysed using ANOVA for split‐split‐plot designs. Experiments were performed during the autumn of two consecutive years in both Pampa sub‐regions. Results: Removal of seeds after a five day trial was two to four times higher in the tall‐grass patches than in the short‐grass matrix. During the same period, vertebrate predation was six times higher than invertebrate predation in the tall‐grass phase of the Southern Pampa, but it did not differ in the short‐grass matrix. Relative predation among phases showed the same pattern in the Hooding Pampa, where preferences by seed species also varied according to phase. Conclusions: The highest predation intensity shown by vertebrates in the tall‐grass patches indicates that they are the main seed predators in these systems, possibly because this microhabitat grants them refuge against carnivorous predators. This could significantly reduce the available seeds for recruitment of subordinate species after different disturbance events (fires, trampling by large herbivores, burrowing), representing an active filter to the floristic composition of the patch and contributing with other mechanisms to the dominance of tall‐grass species.  相似文献   
39.
Introduction – The cobalt is an essential element for leguminous plants but may be harmful for other species; for that reason determination of Co(II) is very important for the management of polluted areas and for discover plants with capacity for the hyperaccumulation of heavy metals, which has produced a growing necessity of fast, sensitive and selective analytical techniques. Objective – To develop an analytical procedure for the determination of cobalt in plant tissue by coupling the ionic chromatography to the luminol‐based chemiluminescence detection. Methodology – The sample was digested in a mixture of concentrated nitric acid and hydrogen peroxide, using an microwave oven to dissolve the Co(II). The solution containing Co(II) ions was injected to an ionic chromatograph using oxalic acid as the eluent. The detection was based on the catalytic effect of Co(II) on the luminol chemiluminescence using perborate or percarbonate as oxidants. Experimental variables, such as concentrations, pH, flow rates and acid digestion conditions were optimised. Results – Well‐resolved chromatographic peaks were obtained. The height and area showed linear dependences with the Co(II) concentration, which were used to quantify the heavy metal, with recoveries up to 95%. The microwave irradiation (60 s) was sufficient for the complete mineralisation of 200 mg of sample, employing 2 mL of the acid mixture. The method was free from the interferences, requiring less than 12 minutes to complete the analysis. Conclusion – The method was simple and rapid for the determination of cobalt in plant tissue with detection limits comparable to those obtained with more sophisticated and expensive analytical equipments. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   
40.
Phytochemical investigations of the n-hexane extract from the roots of Peltodon longipes (Lamiaceae) resulted in the isolation of 12 known abietane diterpenes (1-12). Structures were established on the basis of one and two dimensional nuclear magnetic resonance spectroscopic data ((1)H and (13)C, COSY, HSQC and HMBC), electron ionization mass spectrometric analysis (EIMS) as well as comparison with data from literature. These compounds, as well as eight known diterpenes (13-19) from Salvia miltiorrhiza, and two from Salvia sahendica (20 and 21) were evaluated for their cytotoxic effects in human pancreatic (MIAPaCa-2) and melanoma (MV-3) tumor cell lines using the MTT assay. Tanshinone IIa (13), 7α-acetoxyroyleanone (1), 1,2-dihydrotanshinone (16) and cryptotanshinone (14) had the highest cytotoxic effects in MIAPaCa-2, displaying IC(50) of 1.9, 4.7, 5.6, and 5.8 μM, respectively. Structure-activity relationships of abietane diterpenoid quinones are discussed.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号