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   1052篇
  免费   53篇
  2022年   8篇
  2021年   12篇
  2020年   6篇
  2019年   4篇
  2018年   14篇
  2017年   15篇
  2016年   20篇
  2015年   30篇
  2014年   35篇
  2013年   82篇
  2012年   55篇
  2011年   57篇
  2010年   38篇
  2009年   23篇
  2008年   51篇
  2007年   63篇
  2006年   47篇
  2005年   48篇
  2004年   46篇
  2003年   46篇
  2002年   48篇
  2001年   34篇
  2000年   34篇
  1999年   33篇
  1998年   25篇
  1997年   11篇
  1996年   12篇
  1995年   14篇
  1994年   15篇
  1993年   10篇
  1992年   14篇
  1991年   7篇
  1990年   17篇
  1989年   16篇
  1988年   20篇
  1987年   12篇
  1986年   15篇
  1985年   7篇
  1984年   6篇
  1983年   7篇
  1982年   5篇
  1981年   8篇
  1980年   6篇
  1979年   4篇
  1977年   3篇
  1974年   2篇
  1973年   5篇
  1972年   5篇
  1971年   2篇
  1923年   1篇
排序方式: 共有1105条查询结果,搜索用时 15 毫秒
51.
Animals are assumed to obtain/conserve energy effectively to maximise their fitness, which manifests itself in a variety of behavioral strategies. For marine animals, however, these behavioral strategies are generally unknown due to the lack of high-resolution monitoring techniques in marine habitats. As large marine herbivores, immature green turtles do not need to allocate energy to reproduction but are at risk of shark predation, although it is a rare occurrence. They are therefore assumed to select/use feeding and resting sites that maximise their fitness in terms of somatic growth, while avoiding predation. We investigated fine-scale behavioral patterns (feeding, resting and other behaviors), microhabitat use and time spent on each behavior for eight immature green turtles using data loggers including: depth, global positioning system, head acceleration, speed and video sensors. Immature green turtles at Iriomote Island, Japan, spent an average of 4.8 h feeding on seagrass each day, with two peaks, between 5∶00 and 9∶00, and between 17∶00 and 20∶00. This feeding pattern appeared to be restricted by gut capacity, and thus maximised energy acquisition. Meanwhile, most of the remaining time was spent resting at locations close to feeding grounds, which allowed turtles to conserve energy spent travelling and reduced the duration of periods exposed to predation. These behavioral patterns and time allocations allow immature green turtles to effectively obtain/conserve energy for growth, thus maximising their fitness.  相似文献   
52.

Background/Aims

The Japanese National Hospital Organization evidence-based medicine (EBM) Study group for Adverse effects of Corticosteroid therapy (J-NHOSAC) is a Japanese hospital-based cohort study investigating the safety of the initial use of glucocorticoids (GCs) in patients with newly diagnosed autoimmune diseases. Using the J-NHOSAC registry, the purpose of this observational study is to analyse the rates, characteristics and associated risk factors of intracellular infections in patients with newly diagnosed autoimmune diseases who were initially treated with GCs.

Methodology/Principal Findings

A total 604 patients with newly diagnosed autoimmune diseases treated with GCs were enrolled in this registry between April 2007 and March 2009. Cox proportional-hazards regression was used to determine independent risk factors for serious intracellular infections with covariates including sex, age, co-morbidity, laboratory data, use of immunosuppressants and dose of GCs. Survival was analysed according to the Kaplan-Meier method and was assessed by the log-rank test. There were 127 serious infections, including 43 intracellular infections, during 1105.8 patient-years of follow-up. The 43 serious intracellular infections resulted in 8 deaths. After adjustment for covariates, diabetes (Odds ratio [OR]: 2.5, 95% confidence interval [95% CI] 1.1–5.9), lymphocytopenia (≦1000/μl, OR: 2.5, 95% CI 1.2–5.2) and use of high-dose (≧30 mg/day) GCs (OR: 2.4, 95% CI 1.1–5.3) increased the risk of intracellular infections. Survival curves showed lower intracellular infection-free survival rate in patients with diabetes, lymphocytopaenia and high-dose GCs treatments.

Conclusions/Significance

Patients with newly diagnosed autoimmune diseases were at high risk of developing intracellular infection during initial treatment with GCs. Our findings provide background data on the risk of intracellular infections of patients with autoimmune diseases. Clinicians showed remain vigilant for intracellular infections in patients with autoimmune diseases who are treated with GCs.  相似文献   
53.
54.
In order to understand the ecological adaptations of primates to survive in temperate forests, we need to know the general patterns of plant phenology in temperate and tropical forests. Comparative analyses have been employed to investigate general trends in the seasonality and abundance of fruit and young leaves in tropical and temperate forests. Previous studies have shown that (1) fruit fall biomass in temperate forest is lower than in tropical forest, (2) non-fleshy species, in particular acorns, comprise the majority of the fruit biomass in temperate forest, (3) the duration of the fruiting season is shorter in temperate forest, and (4) the fruiting peak occurs in autumn in most temperate forests. Through our comparative analyses of the fruiting and flushing phenology between Asian temperate and tropical forests, we revealed that (1) fruiting is more annually periodic (the pattern in one year is similar to that seen in the next year) in temperate forest in terms of the number of fruiting species or trees, (2) there is no consistent difference in interannual variations in fruiting between temperate and tropical forests, although some oak-dominated temperate forests exhibit extremely large interannual variations in fruiting, (3) the timing of the flushing peak is predictable (in spring and early summer), and (4) the duration of the flushing season is shorter. The flushing season in temperate forests (17–28 % of that in tropical forests) was quite limited, even compared to the fruiting season (68 %). These results imply that temperate primates need to survive a long period of scarcity of young leaves and fruits, but the timing is predictable. Therefore, a dependence on low-quality foods, such as mature leaves, buds, bark, and lichens, would be indispensable for temperate primates. Due to the high predictability of the timing of fruiting and flushing in temperate forests, fat accumulation during the fruit-abundant period and fat metabolization during the subsequent fruit-scarce period can be an effective strategy to survive the lean period (winter).  相似文献   
55.
56.
Four components of ovomucoid were digested exhaustively and four kinds of glycopeptide corresponding to the four components were separated by gel filtration. Each glycopeptide was shown to be homogenious by paper chromatography and paper electrophoresis. Molar ratios of carbohydrate components of these glycopeptides varied to some extent but the amino acid compositions of these glycopeptides were essentially identical with each other with the exception of alanine. Aspartic acid and threonine were predominant amino acids in the all glycopeptides. It is most likely that the modes of linkages between polysaccharide and protein in individual ovomucoid I, II, III and IV are essentially the same, and that the carbohydrate moiety is linked to the protein via asparaginyl residue or the hydroxyl group of threonine, although the possibility of the linkages to glutamine and serine can not be excluded.  相似文献   
57.
The flavonoid constituents of defatted soybeans were studied. Two kinds of new natural isoflavones and four kinds of isoflavones were isolated by rechromatography on silica gel and Sephadex LH–20 columns of ethyl acetate extracts.

One of the new isoflavones was identified as 6″-O-acetyl daidzin, C23H22O10, by UV, IR, PMR and CMR, and the four kinds of isoflavones were also identified as daidzein, daidzin, genistein and genistin.

High performance liquid chromatography of the ethyl acetate and acetone extracts were carried out, and six main peaks corresponding to the six isoflavones were recognized.

Glycitein and glycitein-7-O-glucoside were not detected in the soybeans.  相似文献   
58.
Sulfite reductase using reduced methyl viologen as an electron donor was purified about 94-fold from a red alga, Porphyra yezoensis. The enzyme was ultracentrifugically homogenous and could reduce sulfite to sulfide quantitatively with an uptake of six electrons. The enzyme had a pH optimum in the vicinity of 7.5. The Km for sulfite was determined to be 6.5×l0?4m. The purified preparation of the algal reductase showed its absorption maximum at 385 mμ and slight shoulders at 408, 456, 485, 600 and 664 mμ in addition to an intense peak at 278 mμ. Metal analysis of the purified enzyme suggested the presence of iron and copper in the molecule. NADPH, NADH or the reduced form of spinach ferredoxin could not be a direct electron donor for the purified algal sulfite reductase.  相似文献   
59.
Ferredoxin-sulfite reductase (Fd-SiR) [hydrogen-sulfide: ferredoxin oxidoreductase, EC 1.8.7.1] from spinach leaves has been purified to homogeneity by a new procedure. Subunit analysis by sodium dodecyl sulfate gel electrophoresis yielded a single protein band with a molecular weight of 71,000. Gel electrophoresis in non-denaturing media at different acrylamide concentrations gave a molecular weight of 270,000, suggesting that the native enzyme was composed of four identical subunits. In the presence of 0.2 m sodium chloride, however, gel filtration produced a value of 136,000, indicating the presence of dimer in this ionic environment. A plot of substrate (sulfite) concentration versus enzymatic (Fd-SiR) activity yielded a sigmoidal curve, giving a Hill coefficient (n?) of 2.1. Purified Fd-SiR, in the oxidized form, had absorption maxima at 279, 385, 588 and 714nm. Thus the enzyme has the property of a siroheme-containing protein.  相似文献   
60.
Soybean 7S and 11S globulins were stored at relative humidities (RHs) of 11% and 96% at 50°C. The redispersibility of the proteins at RH 96% decreased in a short time. However, it did not decrease, when stored for 45 days at RH 11%. Gel filtration showed that the proteins polymerized during storage. The effects of urea, sodium dodecyl sulfate (SDS) and 2-mercaptoethanol (2-ME) on the redispersibilities of the proteins at RH 96% showed that the hydrogen, hydrophobic and disulfide bonds participate in the polymerization of 7S globulin, and that the disulfide bond is strongly related to the polymerization of 11S globulin. Redispersibility was restored with 2-ME in both the 7S and 11S globulins and some of the proteins in the supernatant redispersed with 2-ME were observed to be similar to the native ones with respect to the gel filtration, electrophoretic behavior and circular dichroism spectrum.  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