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   1891篇
  免费   154篇
  国内免费   1篇
  2023年   9篇
  2021年   41篇
  2020年   28篇
  2019年   30篇
  2018年   38篇
  2017年   29篇
  2016年   39篇
  2015年   75篇
  2014年   93篇
  2013年   102篇
  2012年   143篇
  2011年   127篇
  2010年   63篇
  2009年   74篇
  2008年   111篇
  2007年   116篇
  2006年   92篇
  2005年   85篇
  2004年   66篇
  2003年   48篇
  2002年   53篇
  2001年   42篇
  2000年   48篇
  1999年   38篇
  1998年   15篇
  1997年   11篇
  1996年   12篇
  1995年   16篇
  1994年   15篇
  1993年   13篇
  1992年   44篇
  1991年   24篇
  1990年   19篇
  1989年   29篇
  1988年   18篇
  1987年   20篇
  1986年   14篇
  1985年   16篇
  1984年   14篇
  1983年   14篇
  1981年   8篇
  1980年   9篇
  1979年   16篇
  1977年   13篇
  1976年   11篇
  1975年   12篇
  1972年   8篇
  1971年   8篇
  1970年   8篇
  1969年   10篇
排序方式: 共有2046条查询结果,搜索用时 187 毫秒
991.
992.
Herbs are the base used for treatment in Ayurveda. We describe a database named Phyto-Mellitus with information on plants traditionally used for diabetes with their chemical constituents. The active principles of these plants are antioxidant and free radical scavenging.

Availability

http://www.bicmlacw.org/bt/  相似文献   
993.

Background

While considerable scientific effort has been devoted to studying how birds navigate over long distances, relatively little is known about how targets are detected, obstacles are avoided and smooth landings are orchestrated. Here we examine how visual features in the environment, such as contrasting edges, determine where a bird will land.

Methodology/Principal Findings

Landing in budgerigars (Melopsittacus undulatus) was investigated by training them to fly from a perch to a feeder, and video-filming their landings. The feeder was placed on a grey disc that produced a contrasting edge against a uniformly blue background. We found that the birds tended to land primarily at the edge of the disc and walk to the feeder, even though the feeder was in the middle of the disc. This suggests that the birds were using the visual contrast at the boundary of the disc to target their landings. When the grey level of the disc was varied systematically, whilst keeping the blue background constant, there was one intermediate grey level at which the budgerigar''s preference for the disc boundary disappeared. The budgerigars then landed randomly all over the test surface. Even though this disc is (for humans) clearly distinguishable from the blue background, it offers very little contrast against the background, in the red and green regions of the spectrum.

Conclusions

We conclude that budgerigars use visual edges to target and guide landings. Calculations of photoreceptor excitation reveal that edge detection in landing budgerigars is performed by a color-blind luminance channel that sums the signals from the red and green photoreceptors, or, alternatively, receives input from the red double-cones. This finding has close parallels to vision in honeybees and primates, where edge detection and motion perception are also largely color-blind.  相似文献   
994.
Objective: To determine whether the U.S. Centers for Disease Control and Prevention (CDC; CDC Reference) or International Obesity Task Force (IOTF; IOTF Reference) BMI cut‐off points for classifying adiposity status in children are more effective at predicting future health risk. Research Methods and Procedures: The sample (N = 1709) included 4‐ to 15‐year‐old (at baseline) boys and girls from the Bogalusa Heart Study. Overweight and obesity status were determined using both the CDC Reference and IOTF Reference BMI cut‐off points at baseline. The ability of childhood overweight and obesity, determined from the two BMI classification systems, to predict obesity and metabolic disorders in young adulthood (after a 13‐ to 24‐year follow‐up) was then compared. Results: Independently of the classification system employed to determine adiposity based on childhood BMI, the odds of being obese and having all of the metabolic disorders in young adulthood were significantly (p < 0.05) higher in the overweight and obese groups by comparison with the nonoverweight groups. Childhood overweight and obesity, determined by both the CDC Reference and IOTF Reference, had a low sensitivity and a high specificity for predicting obesity and metabolic disorders in young adulthood. Overweight and obesity as determined by the CDC Reference were slightly more sensitive and slightly less specific than the corresponding values based on the IOTF Reference. Discussion: Overweight and obesity during childhood, as determined by both the CDC and IOTF BMI cut‐off points, are strong predictors of obesity and coronary heart disease risk factors in young adulthood. The differences in the predictive capacity of the CDC Reference and IOTF Reference are, however, minimal.  相似文献   
995.
996.
A Gram-positive, non-motile, aerobic, coccus-shaped bacterium, designated strain LNB-140T, was isolated from a sewage treatment plant in the Republic of Korea and was characterised using a polyphasic taxonomic approach. Comparative 16S rRNA gene sequence analysis showed that strain LNB-140T belongs to genus Tessaracoccus in the family Propionibacteriaceae of the phylum Actinobacteria. The 16S rRNA gene sequence similarities between strain LNB-140T and type strains of the genus, Tessaracoccus flavescens SST-39T and Tessaracoccus rhinocerotis YIM101269T are 97.8 and 97.4 %, respectively. The chemotaxonomic properties of strain LNB-140T are consistent with those of members of the genus Tessaracoccus: a quinone system with MK-9(H4) as the predominant menaquinone; anteiso-C15:0 and iso C15:0 as the predominant cellular fatty acids; and ll-2,6-diaminopimelic acid as the diagnostic peptidoglycan diamino acid. The major polar lipids were identified as diphosphatidylglycerol and phosphatidylethanolamine. The G+C content of the genomic DNA was determined to be 67.1 mol%. Differential phenotypic properties along with low DNA–DNA relatedness (<30 ± 3.2 %) with closely related type strains show that strain LNB-140T is distinct from previously described members of the genus Tessaracoccus and represents a novel species in this genus, for which the name Tessaracoccus defluvii sp. nov. is proposed. The type strain is LNB-140T (=KEMB 5401-076T = JCM 17540T).  相似文献   
997.
Development of high performance lithium‐ion (Li‐ion) power packs is a topic receiving significant attention in research today. Future development of the Li‐ion power packs relies on the development of high capacity and high rate anodes. More specifically, materials undergo either conversion or an alloying mechanism with Li. However, irreversible capacity loss (ICL) is one of the prime issues for this type of negative electrode. Traditional insertion‐type materials also experience ICL, but it is considered negligible. Therefore, eliminating ICL is crucial before the fabrication of practical Li‐ion cells with conventional cathodes such as LiFePO4, LiMn2O4, etc. There are numerous methods for eliminating ICL such as pre‐treating the electrode, usage of stabilized Li metal powder, chemical and electrochemical lithiation, sacrificial salts for both anode and cathode, etc. The research strategies that have been explored are reviewed here in regards to the elimination of ICL from the high capacity anodes as described. Additionally, mitigating ICL observed from the carbonaceous anodes is discussed and compared.  相似文献   
998.
999.
Increased myostatin expression, resulting in muscle loss, has been associated with hyperammonemia in mammalian models of cirrhosis. However, there is evidence that hyperammonemia in avian embryos results in a reduction of myostatin expression, suggesting a proliferative myogenic environment. The present in vitro study examines species differences in myotube and liver cell response to ammonia using avian and murine-derived cells. Primary myoblasts and liver cells were isolated from embryonic day 15 and 17 chick embryos to be compared with mouse myoblasts (C2C12) and liver (AML12) cells. Cells were exposed to varying concentrations of ammonium acetate (AA; 2.5, 5, or 10 mM) to determine the effects of ammonia on the cells. Relative expression of myostatin mRNA, determined by quantitative real-time PCR, was significantly increased in AA (10 mM) treated C2C12 myotubes compared to both ages of chick embryonic myotube cultures after 48 h (P < 0.02). Western blot analysis of myostatin protein confirmed an increase in myostatin expression in AA-treated C2C12 myotubes compared to the sodium acetate (SA) controls, while myostatin expression was decreased in the chick embryonic myotube cultures when treated with AA. Myotube diameter was significantly decreased in AA-treated C2C12 myotubes compared to controls, while avian myotube diameter increased with AA treatment (P < 0.001). There were no significant differences between avian and murine liver cell viability, assessed using 2′, 7′- bis-(2-carboxyethyl)-5-(and-6-)-carboxyfluorescein, acetoxymethyl ester, when treated with AA. However, after 24 h, AA-treated avian myotubes showed a significant increase in cell viability compared to the C2C12 myotubes (P < 0.05). Overall, it appears that there is a positive myogenic response to hyperammonemia in avian myotubes compared to murine myotubes, which supports a proliferative myogenic environment.  相似文献   
1000.
Functional annotation is seldom straightforward with complexities arising due to functional divergence in protein families or functional convergence between non‐homologous protein families, leading to mis‐annotations. An enzyme may contain multiple domains and not all domains may be involved in a given function, adding to the complexity in function annotation. To address this, we use binding site information from bound cognate ligands and catalytic residues, since it can help in resolving fold‐function relationships at a finer level and with higher confidence. A comprehensive database of 2,020 fold‐function‐binding site relationships has been systematically generated. A network‐based approach is employed to capture the complexity in these relationships, from which different types of associations are deciphered, that identify versatile protein folds performing diverse functions, same function associated with multiple folds and one‐to‐one relationships. Binding site similarity networks integrated with fold, function, and ligand similarity information are generated to understand the depth of these relationships. Apart from the observed continuity in the functional site space, network properties of these revealed versatile families with topologically different or dissimilar binding sites and structural families that perform very similar functions. As a case study, subtle changes in the active site of a set of evolutionarily related superfamilies are studied using these networks. Tracing of such similarities in evolutionarily related proteins provide clues into the transition and evolution of protein functions. Insights from this study will be helpful in accurate and reliable functional annotations of uncharacterized proteins, poly‐pharmacology, and designing enzymes with new functional capabilities. Proteins 2017; 85:1319–1335. © 2017 Wiley Periodicals, Inc.  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