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   99560篇
  免费   9256篇
  国内免费   41篇
  2023年   335篇
  2021年   1508篇
  2020年   978篇
  2019年   1287篇
  2018年   1589篇
  2017年   1437篇
  2016年   2357篇
  2015年   4027篇
  2014年   4559篇
  2013年   5473篇
  2012年   7347篇
  2011年   7137篇
  2010年   4539篇
  2009年   4271篇
  2008年   6013篇
  2007年   6106篇
  2006年   5705篇
  2005年   5488篇
  2004年   5299篇
  2003年   5118篇
  2002年   4792篇
  2001年   1176篇
  2000年   897篇
  1999年   1118篇
  1998年   1288篇
  1997年   875篇
  1996年   808篇
  1995年   845篇
  1994年   726篇
  1993年   794篇
  1992年   775篇
  1991年   667篇
  1990年   677篇
  1989年   634篇
  1988年   609篇
  1987年   563篇
  1986年   502篇
  1985年   708篇
  1984年   752篇
  1983年   688篇
  1982年   770篇
  1981年   700篇
  1980年   712篇
  1979年   465篇
  1978年   544篇
  1977年   443篇
  1976年   459篇
  1975年   367篇
  1974年   412篇
  1973年   403篇
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
1.
Tourism accounts for 9% of global GDP and comprises 1.1 billion tourist arrivals per annum. Visits to wildlife tourist attractions (WTAs) may account for 20–40% of global tourism, but no studies have audited the diversity of WTAs and their impacts on the conservation status and welfare of subject animals. We scored these impacts for 24 types of WTA, visited by 3.6–6 million tourists per year, and compared our scores to tourists’ feedback on TripAdvisor. Six WTA types (impacting 1,500–13,000 individual animals) had net positive conservation/welfare impacts, but 14 (120,000–340,000 individuals) had negative conservation impacts and 18 (230,000–550,000 individuals) had negative welfare impacts. Despite these figures only 7.8% of all tourist feedback on these WTAs was negative due to conservation/welfare concerns. We demonstrate that WTAs have substantial negative effects that are unrecognised by the majority of tourists, suggesting an urgent need for tourist education and regulation of WTAs worldwide.  相似文献   
2.
3.
4.
5.
6.
7.
8.
A nonplanar distortion of the heme ofc-type cytochromes is conserved in the proteins isolated from diverse species based upon a comprehensive analysis of available highresolution X-ray crystal structures. This distortion is induced through the cysteine thioether linkages between the porphyrin pyrrole groups and the polypeptide and results in an asymmetric pyrrole distortion. This asymmetry in the heme distortion is also conserved. For other heme proteins which lack these covalent bonds, nearly planar porphyrins are observed. Resonance Raman evidence indicates that nonplanar distortion of porphyrins containing metals, like iron, with large core sizes (?2.00 Å) is energetically unfavorable and can occur only in the presence of significant environmental perturbations. Further, energy minimization and dynamics calculations on the ferric form of yeast iso-1-cytochromec, starting from the crystallographic coordinates and using a molecular mechanics force field which accurately reproduces nonplanar distortions in metalloporphyrins, suggest that this distortion is indeed maintained by the protein tertiary structure. It is proposed that this protein-linked heme distortion modulates electron transfer function through modification of redox potentials of the porphyrin ring and the protein binding properties ofc-type cytochromes.  相似文献   
9.
Abstract Competition experiments revealed that adenine and guanine were transported by a purine permease in both Candida glabrata 4 and a C. glabrata 4 cytosine permease negative mutant. The C. glabrata 4 cytosine permease negative mutant was isolated using 5-fluorocytosine selection. This mutant no longer transported cytosine, but transported adenine and guanine. A transport system for hypoxanthine was not detected. Hence, in addition to the cytosine permease, a purine permease exists in C. glabrata . This differs from the purine cytosine permeases in Saccharomyces cereuisiae and Candida albicans which transport adenine, cytosine, guanine and hypoxanthine.  相似文献   
10.
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