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   110633篇
  免费   1963篇
  国内免费   2184篇
  2023年   208篇
  2022年   249篇
  2021年   867篇
  2020年   602篇
  2019年   687篇
  2018年   12380篇
  2017年   11056篇
  2016年   8054篇
  2015年   1725篇
  2014年   1534篇
  2013年   1642篇
  2012年   5715篇
  2011年   14184篇
  2010年   12872篇
  2009年   8973篇
  2008年   10611篇
  2007年   12164篇
  2006年   1014篇
  2005年   1150篇
  2004年   1563篇
  2003年   1548篇
  2002年   1283篇
  2001年   646篇
  2000年   548篇
  1999年   383篇
  1998年   176篇
  1997年   182篇
  1996年   160篇
  1995年   118篇
  1994年   152篇
  1993年   119篇
  1992年   175篇
  1991年   184篇
  1990年   129篇
  1989年   105篇
  1988年   99篇
  1987年   109篇
  1986年   83篇
  1985年   92篇
  1984年   55篇
  1983年   65篇
  1982年   50篇
  1980年   38篇
  1979年   52篇
  1978年   41篇
  1977年   45篇
  1975年   40篇
  1974年   44篇
  1972年   270篇
  1971年   291篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
52.
Copper is an essential trace element that is extremely toxic to organisms and organs at high doses. We have investigated the histological and biochemical effects of a toxic dose of copper sulfate on the liver of term Ross broiler chicks. Fertilized eggs were divided into three groups: experimental, injected with 50 mcg/0.1 ml copper sulfate in the air chambers on day 1; sham, injected with 0.1 ml saline; and control, no injection. Term chicks were killed and their livers investigated histologically, with hematoxylin–eosin-stained sections examined under light microscopy, and biochemically, for malondialdehyde and glutathione levels. Histological examinations showed copper-treated samples with granular degeneration and necrosis of hepatocytes and impairment to the cell lining of the remark cords. The samples had a congestive appearance, with blood in the vena centralis and sinusoids, slight connective tissue increase, and lymphocyte infiltration. Control and sham group sections had normal appearances. As oxidative damage parameters, in the copper-treated group, malondialdehyde levels were increased and glutathione levels decreased. In the sham and control groups, there were no significant differences. At this toxic dose, copper sulfate shows oxidative damage according to the histology of term chick liver that are confirmed biochemically by the changes in malondialdehyde and glutathione levels.  相似文献   
53.
The purpose of this review is to provide a global perspective on Oncorhynchus salmonine introductions and put-and-take fisheries based on modern stocking programs, with special emphasis on freshwater ecosystems. We survey the global introductions of nine selected salmonines of the genus Oncorhynchus: golden trout, cutthroat trout, pink salmon, chum salmon, coho salmon, masu/cherry salmon, rainbow trout/steelhead, sockeye salmon/kokanee, and chinook salmon. The information is organized on a geographical basis by continent, and then by species and chronology. Two different objectives and associated definitions of ‘success’ for introductions are distinguished: (a) seed introduction: release of individuals with the purpose of creating a wild-reproducing, self-sustaining population; and (b) put-and-take introduction: release of individuals with the purpose of maintaining some level of wild population abundance, regardless of wild reproduction. We identify four major phenomena regarding global salmonine introductions: (1) general inadequacy of documentation regarding introductions; (2) a fundamental disconnect between management actions and ecological consequences of introductions; (3) the importance of global climate change on success of previous and future introductions; and (4) the significance of aquaculture as a key uncertainty in accidental introductions. We conclude this review with a recognition of the need to terminate ongoing stocking programs for introduced salmonines worldwide.  相似文献   
54.
55.
One of the major properties of the semi-synthetic minimal cell, as a model for early living cells, is the ability to self-reproduce itself, and the reproduction of the boundary layer or vesicle compartment is part of this process. A minimal bio-molecular mechanism based on the activity of one single enzyme, the FAS-B (Fatty Acid Synthase) Type I enzyme from Brevibacterium ammoniagenes, is encapsulated in 1-palmitoyl-2oleoyl-sn-glycero-3-phosphatidylcholine (POPC) liposomes to control lipid synthesis. Consequently molecules of palmitic acid released from the FAS catalysis, within the internal lumen, move toward the membrane compartment and become incorporated into the phospholipid bilayer. As a result the vesicle membranes change in lipid composition and liposome growth can be monitored. Here we report the first experiments showing vesicles growth by catalysis of one enzyme only that produces cell boundary from within. This is the prototype of the simplest autopoietic minimal cell.  相似文献   
56.

Background  

Identifying essential genes in bacteria supports to identify potential drug targets and an understanding of minimal requirements for a synthetic cell. However, experimentally assaying the essentiality of their coding genes is resource intensive and not feasible for all bacterial organisms, in particular if they are infective.  相似文献   
57.

Background  

Phylogenies capture the evolutionary ancestry linking extant species. Correlations and similarities among a set of species are mediated by and need to be understood in terms of the phylogenic tree. In a similar way it has been argued that biological networks also induce correlations among sets of interacting genes or their protein products.  相似文献   
58.
59.
60.
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