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   19725篇
  免费   1389篇
  国内免费   2篇
  21116篇
  2024年   8篇
  2023年   146篇
  2022年   298篇
  2021年   611篇
  2020年   392篇
  2019年   490篇
  2018年   583篇
  2017年   542篇
  2016年   827篇
  2015年   1240篇
  2014年   1300篇
  2013年   1592篇
  2012年   1884篇
  2011年   1740篇
  2010年   1059篇
  2009年   933篇
  2008年   1136篇
  2007年   1123篇
  2006年   1054篇
  2005年   874篇
  2004年   806篇
  2003年   710篇
  2002年   586篇
  2001年   130篇
  2000年   92篇
  1999年   132篇
  1998年   118篇
  1997年   81篇
  1996年   75篇
  1995年   55篇
  1994年   65篇
  1993年   43篇
  1992年   44篇
  1991年   39篇
  1990年   28篇
  1989年   29篇
  1988年   21篇
  1987年   25篇
  1986年   19篇
  1985年   16篇
  1984年   23篇
  1983年   17篇
  1982年   14篇
  1981年   14篇
  1980年   13篇
  1979年   11篇
  1977年   10篇
  1975年   12篇
  1974年   9篇
  1971年   8篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
Ana  Sabates 《Journal of Zoology》1994,234(1):89-103
The early stages of development of three blenniid species, Aidablennius sphynx (6.7–15.8 mm BL), Coryphoblennius galerita (4.3–13.9mm BL), and Lipophrys canevai (3.5–10.4 mm BL) are described from specimens collected in the western Mediterranean. The characteristics used for identification included meristic, developmental, morphological and pigmentation characters. Distinguishing characters of these species useful in differentiating them from other species of blenniids for which early stages are known in the Mediterranean are presented. Information on the occurrence of larvae and juveniles of these species off the Catalan coast (north-western Mediterranean) is also given.  相似文献   
12.
13.
14.
The genera Viburnum, Sambucus and Lonicera have been investigated for chromosome number and karyomorphology including Giemsa-C-banding, fluorochrome (DAPI/CMA) banding and cold treatment. Cold-induced undercontracted chromosome regions (CIRs) are found in Viburnum and Sambucus for the first time and are apparently identical with larger hc regions, shown by Giemsa C-banding. Certain narrow C-bands are not cold-sensitive. CIRs frequently react brightly CMA-positive in Viburnum and Sambucus, while DAPI fluorescence is virtually ineffective. The occurrence of CIRs within plants is possibly linked to certain nuclear characters such as large chromosomes and continuous condensation behaviour. Cold-induction has possibly also some influence on euchromatin condensation characteristics in prophasic chromosomes. Several karyological characters point to a closer relationship between Viburnum, Sambucus and Adoxa: Relatively large chromosomes, continuous condensation behaviour, reticulate to semireticulate interphase nuclei and presence of CIRs. These genera appear isolated from Lonicera and the Caprifoliaceae s.str., which differ remarkably in karyomorphology.  相似文献   
15.
Bioenergy could play a major role in decarbonizing energy systems in the context of the Paris Agreement. Large-scale bioenergy deployment could be related to sustainability issues and requires major infrastructure investments. It, therefore, needs to be studied carefully. The Bioenergy and Land Optimization Spatially Explicit Model (BLOEM) presented here allows for assessing different bioenergy pathways while encompassing various dimensions that influence their optimal deployment. In this study, BLOEM was applied to the Brazilian context by coupling it with the Brazilian Land Use and Energy Systems (BLUES) model. This allowed investigating the most cost-effective ways of attending future bioenergy supply projections and studying the role of recovered degraded pasture lands in improving land availability in a sustainable and competitive manner. The results show optimizing for limiting deforestation and minimizing logistics costs results in different outcomes. It also indicates that recovering degraded pasture lands is attractive from both logistics and climate perspectives. The systemic approach of BLOEM provides spatial results, highlighting the trade-offs between crop allocation, land use and the logistics dynamics between production, conversion, and demand, providing valuable insights for regional and national climate policy design. This makes it a useful tool for mapping sustainable bioenergy value chain pathways.  相似文献   
16.
17.
Summary The feasibility of determining the relative populations of multi-conformer structures from NOE-derived distances alone is assessed. Without cross-validation of the NOE restraints, any population ratio can be refined to a similar quality of the fit. Complete cross-validation provides a less biased measure of fit and allows the estimation of the correct population ratio when used in conjunction with very tight distance restraints. With the qualitative distance restraints most commonly used in NMR structure determination, cross-validation is unsuccessful in providing the correct answer. Other experimental sources are therefore needed to determine relative populations of multi-conformer structures.To whom correspondence should be addressed.  相似文献   
18.
Abstract: A specific and sensitive radioimmunoassay procedure for Metenkephalin[Arg6,Phe7] which allows its measurement in regions of the rat brain is described. The antiserum was raised against the methionine sulphoxide derivative of the peptide, and all samples and standards were oxidized with hydrogen peroxide prior to use in the assay with chloramine T-oxidized 125I-labelled Met(O)-enkephalin[Arg6,Phe7]. The only significant cross-reactivity was 30% with the reduced heptapeptide Met-enkephalin[Arg6,Phe7]. The assay showed less than 0.15% cross-reactivity with fragments of the heptapeptide and with leucine-enkephalin-containing peptides. Acid acetone extraction of rat striatum followed by Sephadex G-50 chromatography and reverse-phase high pressure liquid chromatography showed that essentially all immunoreactivity co-chromatographed with Met-enkephalin[Arg6,Phe7]. This confirmed the specificity of the assay and showed that the striatum does not contain a high concentration of larger molecular weight forms with the heptapeptide at the COOH terminus. Distribution of the heptapeptide followed that of methionine enkephalin, with highest concentrations in the globus pallidus, intermediate levels in caudate-putamen and hypothalamus, and low levels in cortex and cerebellum.  相似文献   
19.
Atherosclerosis is an important cause of cardiovascular disorders worldwide. Natural botanical drugs have attracted attention due to their antioxidant, anti-inflammatory, and antiatherogenic properties in the treatment of atherosclerosis. Punicalagin is the major bioactive component of pomegranate peel, and has been shown to have antioxidant, anti-inflammatory, antiviral, anti proliferation, and anticancer properties. To explore its antiatherogenic properties at a molecular level, we investigated the genome-wide expression changes that occur in differentiated THP1 cells following treatment with a non-toxic dose of punicalagin. We also conducted a molecular docking simulation study to identify the molecular targets of punicalagin.  相似文献   
20.
Cyclic N6-threonylcarbamoyladenosine (‘cyclic t6A’, ct6A) is a non-thiolated hypermodification found in transfer RNAs (tRNAs) in bacteria, protists, fungi and plants. In bacteria and yeast cells ct6A has been shown to enhance translation fidelity and efficiency of ANN codons by improving the faithful discrimination of aminoacylated tRNAs by the ribosome. To further the understanding of ct6A biology we have determined the high-resolution crystal structures of CsdL/TcdA in complex with AMP and ATP, an E1-like activating enzyme from Escherichia coli, which catalyzes the ATP-dependent dehydration of t6A to form ct6A. CsdL/TcdA is a dimer whose structural integrity and dimer interface depend critically on strongly bound K+ and Na+ cations. By using biochemical assays and small-angle X-ray scattering we show that CsdL/TcdA can associate with tRNA with a 1:1 stoichiometry and with the proper position and orientation for the cyclization of t6A. Furthermore, we show by nuclear magnetic resonance that CsdL/TcdA engages in transient interactions with CsdA and CsdE, which, in the latter case, involve catalytically important residues. These short-lived interactions may underpin the precise channeling of sulfur atoms from cysteine to CsdL/TcdA as previously characterized. In summary, the combination of structural, biophysical and biochemical methods applied to CsdL/TcdA has afforded a more thorough understanding of how the structure of this E1-like enzyme has been fine tuned to accomplish ct6A synthesis on tRNAs while providing support for the notion that CsdA and CsdE are able to functionally interact with CsdL/TcdA.  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