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   2151篇
  免费   120篇
  国内免费   519篇
测绘学   63篇
大气科学   249篇
地球物理   1127篇
地质学   716篇
海洋学   297篇
天文学   80篇
综合类   100篇
自然地理   158篇
  2023年   8篇
  2022年   11篇
  2021年   34篇
  2020年   19篇
  2019年   23篇
  2018年   25篇
  2017年   8篇
  2016年   33篇
  2015年   25篇
  2014年   49篇
  2013年   46篇
  2012年   118篇
  2011年   164篇
  2010年   42篇
  2009年   60篇
  2008年   143篇
  2007年   142篇
  2006年   143篇
  2005年   147篇
  2004年   140篇
  2003年   90篇
  2002年   104篇
  2001年   82篇
  2000年   100篇
  1999年   47篇
  1998年   38篇
  1997年   44篇
  1996年   57篇
  1995年   65篇
  1994年   53篇
  1993年   89篇
  1992年   66篇
  1991年   88篇
  1990年   80篇
  1989年   57篇
  1988年   58篇
  1987年   42篇
  1986年   35篇
  1985年   30篇
  1984年   24篇
  1983年   15篇
  1982年   14篇
  1981年   13篇
  1980年   22篇
  1978年   5篇
  1976年   6篇
  1974年   8篇
  1948年   8篇
  1947年   8篇
  1943年   4篇
排序方式: 共有2790条查询结果,搜索用时 476 毫秒
21.
cDNA clones for glutathione S-transferases A (GST-A) and A1 (GST-A1) from plaice (Pleuronectes platessa) were expressed as N-terminally 6XHis tagged proteins in Escherichia coli and purified to homogeneity from Ni-NTA silica. GST-A was an efficient catalyst for conjugation of unsaturated alkenals derived from peroxidation of polyunsaturated fatty acids with the highest activity observed with trans-non-2-enal (8 micromol min(-1) mg(-1)). GST-A1 was a very efficient Se-independent glutathione peroxidase with an activity towards cumene hydroperoxide of 25 micromol min(-1) mg(-1). Although the enzymes exhibited moderately high activities towards the model substrate 1-chloro-2,4-dinitrobenzene (CDNB) they exhibited little or no activity towards other common prototypical xenobiotic substrates. Together with data for ontogeny, tissue distribution and inducibility of these enzymes, we contend that a primary function of these enzymes is protection from the harmful effects of lipid peroxidation products generated naturally or exacerbated by xenobiotic exposure.  相似文献   
22.
Gastropods and bivalves were collected at 15 sites at Vancouver and Victoria, Canada between 24 May and 7 June, 1999, to establish tissue concentrations of butyltin and phenyltin compounds, to record imposex symptoms in gastropods, and to assess the present status of organotin contamination around Vancouver. No neogastropods (such as Nucella lima) were found around Vancouver. Neogastropod populations could have been extirpated by severe TBT contamination in Vancouver, as relatively high concentrations of TBT were detected in tissues of Mytilus trossulus from Vancouver, and the neogastropods distributed in Vancouver might be sensitive to TBT. Recovery from imposex, however, was observed in neogastropod populations from three sites at Victoria and Mission Point. TBT contamination has continued around Vancouver, arising from continuous use of TBT in antifouling paints for vessels larger than 25 m in length; however, TBT has decreased around Victoria and Mission Point. Different patterns of TBT accumulation in tissue were observed among the bivalve species from Vancouver. The highest TBT concentration detected in Tresus capax suggested some possible adverse effects. TBT was the most predominant butyltin component in almost all bivalve specimens surveyed, suggesting a low rate of TBT metabolism. Phenyltin compounds were not detected in any molluscan specimens in this study.  相似文献   
23.
The role of endosymbiotic diatoms as pro-oxidant stressors in porifera has been investigated in the Antarctic sponge Haliclona dancoi in which the presence of diatoms is influenced by marked seasonal variations during the austral summer. Both chlorophaeopigments and frustules were absent in sponge tissues sampled in early November at the beginning of the summer and increased from the mid of December with slightly shifted temporal trends. The efficiency of antioxidant defenses in the sponge showed a marked response to symbionts with clearly enhanced values corresponding to the peak of diatoms.  相似文献   
24.
25.
M.S.Doty  M.Oguri 《海洋与湖沼》1964,6(3):297-308
本文从单位时间和单位体积内通过光合作用转变为有机碳的重量出发,介绍了一个测定有机物初级生产力的新的海洋学方法。为了使对此感兴趣的人都能够参照他们各自的需要来应用或改进这一同位素定量方法,我们简短地对操作方法和历史作了一般的评价之后,还作了相当详尽的叙述。  相似文献   
26.
27.
28.
29.
双刺镰状水蚤Drepanopus bispinosus为南极沿海伯尔顿盐湖(Burton lake)中的越冬种类之一,其雌性性成熟大约于每年的初冬季节,在随后8个多月的生活史中,经历了一个盐度从33(5月份)到39(11月份)的环境变化.本文报道了该水蚤雌性成体对环境盐度适应的季节差异,动物在冬季的耐低盐能力较夏季强,从冬季到夏季,动物对盐度的适应范围明显地向高盐度方向偏移,这与环境盐度对动物的驯化有关.在低盐水中,动物的呼吸率增高,且夏季大于冬季,因而进一步证实了环境盐度的变化对动物盐度适应性的影响.  相似文献   
30.
An integrate approach will be discussed for investigations on oxidative stress in xenobiotic toxicity. While the analysis of individual antioxidants is useful for their sensitivity and to understand the mode of action of a stressor, the integration with the analysis of the total antioxidant capacity provides a more holistic assessment of the overall biological significance of such variations. TOSC has a greater predictive value on the health condition of the organisms and allows to discriminate the different role of specific ROS in oxidative stress syndrome.  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