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   146867篇
  免费   5019篇
  国内免费   849篇
  2023年   164篇
  2022年   184篇
  2021年   1009篇
  2020年   565篇
  2019年   749篇
  2018年   12658篇
  2017年   11276篇
  2016年   8837篇
  2015年   3177篇
  2014年   3175篇
  2013年   3686篇
  2012年   8466篇
  2011年   16630篇
  2010年   14331篇
  2009年   10320篇
  2008年   12911篇
  2007年   14198篇
  2006年   3097篇
  2005年   3074篇
  2004年   3451篇
  2003年   3232篇
  2002年   2667篇
  2001年   1903篇
  2000年   1703篇
  1999年   1246篇
  1998年   540篇
  1997年   493篇
  1996年   415篇
  1995年   400篇
  1994年   313篇
  1993年   329篇
  1992年   662篇
  1991年   553篇
  1990年   482篇
  1989年   487篇
  1988年   424篇
  1987年   404篇
  1986年   319篇
  1985年   330篇
  1984年   278篇
  1983年   242篇
  1982年   191篇
  1981年   165篇
  1979年   220篇
  1978年   197篇
  1977年   180篇
  1976年   170篇
  1974年   196篇
  1972年   402篇
  1971年   397篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
11.
With DNA sequencing now getting cheaper more quickly than data storage or computation, the time may have come for genome informatics to migrate to the cloud.  相似文献   
12.
Rice fields are an alternative habitat for waterbirds and provide food and shelter for many avian species, but there is a lack of information about how the use of rice fields translates into population level effects. The aim of this study was to test the relationship between the use of rice fields by European waterbirds and trends in their populations. We tested this relationship during the autumn migration season and during the breeding season. Based on counts conducted over the last 23 years in natural marshes and areas of rice fields in Doñana (SW Spain), an index of rice field use was constructed for 76 bird species, which was then compared to these species’ European population trends obtained from the literature. A positive relationship was found between waterbird population trends and the use of rice fields during autumn migration season. Our study suggests that changes in the Common Agriculture Policy in Europe leading to reductions in areas of rice cultivation may have important effects on waterbirds. The restoration of former marsh areas and the maintenance of rice cultivation would seem to be more environmentally friendly approaches than the use of these areas to grow alternative crops or solar farms.  相似文献   
13.
The Asian Tree Toad genus Pedostibes, as currently understood, exhibits a conspicuously disjunct distribution, posing several immediate questions relating to the biogeography and taxonomy of this poorly known group. The type species, P. tuberculosus and P. kempi, are known only from India, whereas P. hosii, P. rugosus, and P. everetti are restricted to Southeast Asia. Several studies have shown that these allopatric groups are polyphyletic, with the Indian Pedostibes embedded within a primarily South Asian clade of toads, containing the genera Adenomus, Xanthophryne, and Duttaphrynus. Southeast Asian Pedostibes on the other hand, are nested within a Southeast Asian clade, which is the sister lineage to the Southeast Asian river toad genus Phrynoidis. We demonstrate that Indian and Southeast Asian Pedostibes are not only allopatric and polyphyletic, but also exhibit significant differences in morphology and reproductive mode, indicating that the Southeast Asian species’ are not congeneric with the true Pedostibes of India. As a taxonomic solution, we describe a new genus, Rentapia gen. nov. to accommodate the Southeast Asian species.  相似文献   
14.
15.
16.

Background  

Recent studies point to a great diversity of non-ribosomal peptide synthesis systems with major roles in amino acid and co-factor biosynthesis, secondary metabolism, and post-translational modifications of proteins by peptide tags. The least studied of these systems are those utilizing tRNAs or aminoacyl-tRNA synthetases (AAtRS) in non-ribosomal peptide ligation.  相似文献   
17.
The state of aggregation of the (Ca2+ + Mg2+)-ATPase in the membrane of sarcoplasmic reticulum and in reconstituted membrane systems has been studied using saturation-transfer electron spin resonance (ST-ESR). Saturation-transfer ESR spectra show that in the sarcoplasmic reticulum, the ATPase is relatively free to rotate, with an effective rotational correlation time of approx. 33 microseconds at 4 degrees C, consistent with a monomeric or dimeric structure. The rate of rotation is observed to decrease with decreasing molar ratio of lipid to protein. In reconstituted systems, rotational motion of the ATPase on the millisecond time scale ceases when the lipids are in the gel phase. Addition of decavanadate, which causes the formation of crystalline arrays in negatively stained electron micrographs, results in only a small reduction in rotation rate for the ATPase in the membrane. The experiments are interpreted in terms of a short-lived (on the millisecond time scale) protein-protein interaction, with the formation of crystalline clusters of ATPase molecules which form and melt rapidly.  相似文献   
18.
19.
20.

Background  

Substituted catechols are important precursors for large-scale synthesis of pharmaceuticals and other industrial products. Most of the reported chemical synthesis methods are expensive and insufficient at industrial level. However, biological processes for production of substituted catechols could be highly selective and suitable for industrial purposes.  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