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   1451篇
  免费   84篇
  国内免费   6篇
  2024年   3篇
  2023年   21篇
  2022年   47篇
  2021年   61篇
  2020年   56篇
  2019年   76篇
  2018年   66篇
  2017年   52篇
  2016年   65篇
  2015年   85篇
  2014年   92篇
  2013年   128篇
  2012年   92篇
  2011年   113篇
  2010年   65篇
  2009年   59篇
  2008年   48篇
  2007年   47篇
  2006年   40篇
  2005年   36篇
  2004年   37篇
  2003年   24篇
  2002年   22篇
  2001年   4篇
  2000年   11篇
  1999年   9篇
  1998年   3篇
  1997年   3篇
  1996年   3篇
  1995年   9篇
  1994年   7篇
  1993年   4篇
  1992年   9篇
  1991年   11篇
  1990年   15篇
  1989年   14篇
  1988年   22篇
  1987年   15篇
  1986年   5篇
  1985年   9篇
  1984年   14篇
  1983年   5篇
  1982年   5篇
  1981年   2篇
  1980年   4篇
  1979年   5篇
  1978年   4篇
  1975年   4篇
  1957年   1篇
  1956年   2篇
排序方式: 共有1541条查询结果,搜索用时 15 毫秒
991.

Synbiotic (probiotic bacteria and prebiotic) has beneficial effects on the gastrointestinal tract. This study was designed to investigate the effect of synbiotic supplementation on the growth of mild to moderate failure to thrive (FTT) children. A randomized, triple-blind, placebo-controlled trial was conducted involving 80 children aged 2–5 years with mild to moderate FTT, who were assigned at random to receive synbiotic supplementation (109 colony-forming units) or placebo for 30 days. The weights, height, and BMI were recorded in a structured diary, and the questionnaires were completed to monitor the numbers of infection episodes, gastrointestinal problems, admission to hospital, and appetite improvement during the study. Sixty-nine children completed the study. There were no differences in the demographic characteristic between the two groups. The mean weight was similar at baseline. After 30 days of intervention, the mean weight of the participants in the synbiotic group increased significantly than those in the placebo group (600?±?37 vs. 74?±?32 g/month P 0.000). BMI changes in synbiotic and placebo group were 0.44 and 0.07 kg/m2, and that the differences among the two groups were significant.(P 0.045) Furthermore, the height increment in synbiotic and placebo group was 0.41 and 0.37 cm respectively with no significant difference (P 0.761). Administration of 30-day synbiotic supplementation may significantly improve weight and BMI in Iranian children with mild to moderate FTT, but there is no effect on the height in this study. Further studies should be designed to found out the effect of synbiotic on growth parameters in undernourished and well-nourished children.

  相似文献   
992.
Previous research demonstrated that aflatoxin contamination in corn is reduced by field application of wheat grains pre-inoculated with the non-aflatoxigenic Aspergillus flavus strain NRRL 30797. To facilitate field applications of this biocontrol isolate, a series of laboratory studies were conducted on the reliability and efficiency of replacing wheat grains with the novel bioplastic formulation Mater-Bi® to serve as a carrier matrix to formulate this fungus. Mater-Bi® granules were inoculated with a conidial suspension of NRRL 30797 to achieve a final cell density of approximately log 7 conidia/granule. Incubation of 20-g soil samples receiving a single Mater-Bi® granule for 60-days resulted in log 4.2–5.3 propagules of A. flavus/g soil in microbiologically active and sterilized soil, respectively. Increasing the number of granules had no effect on the degree of soil colonization by the biocontrol fungus. In addition to the maintenance of rapid vegetative growth and colonization of soil samples, the bioplastic formulation was highly stable, indicating that Mater-Bi® is a suitable substitute for biocontrol applications of A. flavus NRRL 30797.  相似文献   
993.
Metallothionein (MT) concentration in gills, liver, and kidney tissues of Persian sturgeon (Acipenser persicus) were determined following exposure to sublethal levels of waterborne cadmium (Cd) (50, 400, and 1,000 μg l−1) after 1, 2, 4, and 14 days. The increases of MT from background levels were 4.6-, 3-, and 2.8-fold for kidney, liver, and gills, respectively. The results showed that MT level change in the kidney is time and concentration dependent. Also, cortisol measurement revealed elevation at the day 1 of exposure and followed by MT increase in the liver. Cd concentrations in the cytosol of experimental tissues were measured, and the results indicated that Cd levels in the cytosol of liver, kidney, and gills increased 240.71-, 32.05-, and 40.16-fold, respectively, 14 days after exposure to 1,000 μg l−1 Cd. The accumulation of Cd in cytosol of tissues is in the order of liver > gills > kidney. Pearson correlation coefficients showed that the MT content in kidney is correlated with Cd concentration, the value of which is more than in liver and gills. Thus, kidney can be considered as a tissue indicator in A. persicus for waterborne Cd contamination.  相似文献   
994.
995.
地衣在自然界中是一种特殊的生物类群,具有独特的结构特征和较强的环境适应能力。它在环境污染评价,药物和香料开发利用方面具有较高的研究价值。目前对地衣的研究涉及地衣分类、地衣区系地理、地衣群落生态学、地衣化学、利用地衣评价环境质量以及地衣分子生物学等领域。回顾了新疆地衣研究近20年的发展概况,总结了新疆地衣研究的发展历程、特点,并展望了新疆地衣研究的方向。  相似文献   
996.
A comparison of the two most famous groups of calcium-regulated photoproteins, cnidarians and ctenophores, showed unexpectedly high degree of structural similarity regardless of their low sequence identity. It was suggested these photoproteins can play an important role in understanding the structural basis of bioluminescence activity. Based on this postulate, in this study the cDNA of mnemiopsin from luminous ctenophore Mnemiopsis leidyi was cloned, expressed, purified and sequenced. The purified cDNA, with 621 base pairs, coded a 206 residues protein. Sequence of mnemiopsin showed 93.5 and 51% similarity to other ctenophore proteins and cnidarians, respectively. The cDNA encoding apo-mnemiopsin of M. leidyi was expressed in Escherichia coli. The purified apo-protein showed a single band on SDS-PAGE (molecular weight ~27 kDa). A semi-synthetic mnemiopsin was prepared using coelenterazine and EDTA and its luminescence activity was measured in the presence of CaCl2. The results showed an optimum pH of 9.0 and lower calcium sensitivity compared to aequorin. Comparison of amino acid residues in substrate binding site indicated that binding pocket of ctenophores contains less aromatic residues than cnidarians. This can lead to a decline in the number of stacking interactions between substrate and protein which can affect the stability of coelenterazine in binding cavity. Structural comparison of photoproteins with low sequence identity and high 3D structural similarity, can present a new insight into the mechanism of light emission in photoproteins.  相似文献   
997.
The adsorption of helium and neon mixtures on single-walled carbon nanotubes (SWCNTs) was investigated at various temperatures (subcritical and supercritical) and pressures using canonical Monte Carlo (CMC) simulation. Adsorption isotherms were obtained at different temperatures (4, 40, 77 and 130 K) and pressures ranging from 1 to 16 MPa. Separation factors and isosteric enthalpies of adsorption were also calculated. Moreover, the adsorption isotherms were obtained at constant specific temperatures (4 and 40 K) and pressures (0.2 and 1.0 MPa) as a function of the amount adsorbed. All of the adsorption isotherms for an equimolar mixture of helium and neon have a Langmuir shape, indicating that no capillary condensation occurs. Both the helium and the neon adsorption isotherms exhibit similar behavior, and slightly more of the helium and neon mixture is adsorbed on the inner surfaces of the SWCNTs than on their outer surfaces. More neon is adsorbed than helium within the specified pressure range. The data obtained show that the isosteric enthalpies for the adsorption of neon are higher than those for helium under the same conditions, which means that adsorption of neon preferentially occurs by (15, 15) SWCNTs. Furthermore, the isosteric enthalpies of adsorption of both gases decrease with increasing temperature.  相似文献   
998.
Myostatin is a negative regulator of the growth and development of skeletal muscle mass. In fish, myostatin is expressed in several organs in addition to skeletal muscle. To understand the mechanisms regulating myostatin gene expression in the sea perch, Lateolabrax japonicus, we examined the methylation status of the myostatin gene promoter region in several tissues (liver, eye, kidney, brain, and heart) isolated from adult specimens. The frequency of methylated cytosines was very low in all tissues, regardless of the level of myostatin expression, suggesting that DNA methylation is not involved in the tissue-specific regulation of myostatin expression. Southern blot analysis of genomic DNA obtained from micrococcal nuclease-treated nuclei showed that chromatin digestion occurs in tissues where the myostatin gene is actively transcribed and that the myostatin gene is protected from micrococcal nuclease in tissues where myostatin is not expressed. The chromatin structure in the myostatin gene region appears to regulate its expression without DNA methylation.  相似文献   
999.
Brewer's spent grain (BSG) was used as a solid substrate for the production of α-amylase by Bacillus sp. KR-8104 in a submerged fermentation system. The production of α-amylase was maximized through statistical optimization of the BSG concentration and incubation time using the Doehlert experimental design. The highest tested amount of BSG (5%, w/v) in the optimization process resulted in a 5.1-fold enhancement of the response. Subsequently, we studied the role of the water-soluble and -insoluble fractions of BSG in the production of α-amylase. The results revealed that whole BSG had a greater effect on the production of α-amylase than each fraction had separately. Finally, when we examined the potential of BSG to replace the constituents of a medium formula, we observed that simultaneously adding BSG, omitting dextrin, and reducing the other ingredients concentration in the culture medium improved the production of α-amylase and made the production process more economical.  相似文献   
1000.

Background

Zoonotic infections pose a significant public health challenge for low- and middle-income countries and have traditionally been a neglected area of research. The Roadmap to Combat Zoonoses in India (RCZI) initiative conducted an exercise to systematically identify and prioritize research options needed to control zoonoses in India.

Methods and Findings

Priority setting methods developed by the Child Health and Nutrition Research Initiative were adapted for the diversity of sectors, disciplines, diseases and populations relevant for zoonoses in India. A multidisciplinary group of experts identified priority zoonotic diseases and knowledge gaps and proposed research options to address key knowledge gaps within the next five years. Each option was scored using predefined criteria by another group of experts. The scores were weighted using relative ranks among the criteria based upon the feedback of a larger reference group. We categorized each research option by type of research, disease targeted, factorials, and level of collaboration required. We analysed the research options by tabulating them along these categories. Seventeen experts generated four universal research themes and 103 specific research options, the majority of which required a high to medium level of collaboration across sectors. Research options designated as pertaining to ‘social, political and economic’ factorials predominated and scored higher than options focussing on ecological, genetic and biological, or environmental factors. Research options related to ‘health policy and systems’ scored highest while those related to ‘research for development of new interventions’ scored the lowest.

Conclusions

We methodically identified research themes and specific research options incorporating perspectives of a diverse group of stakeholders. These outputs reflect the diverse nature of challenges posed by zoonoses and should be acceptable across diseases, disciplines, and sectors. The identified research options capture the need for ‘actionable research’ for advancing the prevention and control of zoonoses in India.  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