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   18615篇
  免费   2486篇
  国内免费   5487篇
  2024年   32篇
  2023年   429篇
  2022年   608篇
  2021年   953篇
  2020年   844篇
  2019年   1096篇
  2018年   894篇
  2017年   767篇
  2016年   893篇
  2015年   1206篇
  2014年   1595篇
  2013年   1542篇
  2012年   1926篇
  2011年   1789篇
  2010年   1330篇
  2009年   1285篇
  2008年   1420篇
  2007年   1279篇
  2006年   1164篇
  2005年   1015篇
  2004年   851篇
  2003年   676篇
  2002年   623篇
  2001年   455篇
  2000年   415篇
  1999年   299篇
  1998年   184篇
  1997年   143篇
  1996年   114篇
  1995年   81篇
  1994年   80篇
  1993年   41篇
  1992年   63篇
  1991年   64篇
  1990年   67篇
  1989年   54篇
  1988年   33篇
  1987年   31篇
  1986年   42篇
  1985年   33篇
  1984年   23篇
  1983年   28篇
  1982年   23篇
  1981年   21篇
  1980年   9篇
  1979年   17篇
  1978年   6篇
  1977年   9篇
  1976年   10篇
  1975年   7篇
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
1.
2.
<正>There is no experiencing without an experiencer,the self.Whether the experiencer knows it is"the self"that is experiencing is a different and tricky philosophical issue.Dating back to Ivan Pavlov’s dogs,the"experience"of animals has  相似文献   
3.
The incidence of breast cancer has increased in Asian countries and rates of hormone receptor (HR) negative breast cancer exceed those of Western countries. Epidemiologic data suggest that the association between body size and BC risk may vary by HR status, and could differ geographically. While body size may influence BC risk by moderating the synthesis and metabolism of circulating sex-steroid hormones, insulin-like growth factor (IGF)-1 and related binding proteins, there is a dearth of literature among Asian women. We aimed to examine these specific associations in a sample of Chinese women. In Sichuan Province 143 women aged ≥40 years were recruited through outpatient services (2011–2012). Questionnaires, anthropometric measurements, and blood samples were utilized for data collection and linear regression was applied in data analyses. Among women <50 years we observed a non-monotonic positive association between body mass index (BMI) and 17β-estradiol, and a reversed J-shaped association between BMI and IGF-1 (p ≤0.05). We observed similar associations between waist-to-hip ratio and these markers. Our finding of augmented IGF-1 among women with low body mass may have implications for understanding breast tumor heterogeneity in diverse populations and should be evaluated in larger prospective studies with cancer outcomes.  相似文献   
4.
贝壳历来是生物工程和材料学研究的重要对象。贝壳中的贝壳基质蛋白质在贝壳的形成与发育过程中具有重要的调控作用。Whirlin类蛋白质(Whirlin-like protein,WLP)是一种从厚壳贻贝(Mytilus coruscus)中鉴定的新型贝壳基质蛋白质。序列分析结果显示,该蛋白质含有PDZ(postsynaptic density/Discs large/Zonula occludens)结构域,而该结构域对贝壳生物矿化的影响目前尚无报道。为深入了解WLP在贝壳形成中对碳酸钙晶体的影响,在序列分析基础上,采用密码子优化结合原核重组表达,获得其重组表达产物后,开展了重组WLP对碳酸钙晶体形貌及晶型的影响研究,结晶速度抑制以及碳酸钙晶体结合分析。分析结果表明,重组WLP能诱导文石型碳酸钙晶体的形貌和方解石型碳酸钙晶体的晶型发生改变;同时重组WLP对碳酸钙晶体具有结合作用,且能抑制碳酸钙晶体的结晶速度。上述结果表明,WLP对贝壳的形成及发育具有重要影响,并可能在贝壳肌棱柱层的形成中发挥了重要作用。  相似文献   
5.
Yang  Su-Rong  Sun  Huan-Xin  Hu  Zhen-Zhen  Wang  Si-Heng  Sun  Hui  Xue  Yin-Jia  Ye  Chen-Bo 《Sleep and biological rhythms》2017,15(1):57-65
Sleep and Biological Rhythms - Chronic sleep deprivation (SD) is an overwhelming problem in young students. Firstly, we investigated whether different levels of pre-training SD had effects on...  相似文献   
6.
7.
Hydroxyacid dehydrogenases are responsible for the conversion of 2-keto acids to 2-hydroxyacids and have a wide range of biotechnological applications. In this study, a D-lactate dehydrogenase (D-LDH) from a Sporolactobacillus inulinus strain was experimentally verified to have both the D-LDH and glutamate dehydrogenase (GDH) activities (reversible deamination). The catalytic mechanism was demonstrated by identification of key residues from the crystal structure analysis and site-directed mutagenesis. The Arg234 and Gly79 residues of this enzyme play a significant role in both D-LDH and GDH activities. His295 and Phe298 in DLDH744 were identified to be key residues for lactate dehydrogenase (LDH) activity only whereas Tyr101 is a unique residue that is critical for GDH activity. Characterization of the biochemical properties contributes to understanding of the catalytic mechanism of this novel D-lactate dehydrogenase enzyme.  相似文献   
8.
9.
10.
In this study, we investigated two selection biases that may affect the obesity-mortality link over the life course: mortality selection and healthy participant effects. If these selection mechanisms are stronger among obese adults than among non-obese adults, they may contribute to the weakening obesity-mortality link over the life course. We used data from the National Health and Nutrition Examination Survey 1988–2010 with linked mortality files from 1988–2011. We employed weighted Cox models to test and adjust for these two selection biases. We also used complementary log-log models, adjusted for a normal distribution of frailty, to test for mortality selection effects; accelerated failure-time models to mitigate the mortality selection effect; and ordinary least squares regression to test for healthy participant effects. The link between class II/III obesity and mortality weakens at older ages. We did not find evidence for significant mortality selection or healthy participant effects. Also, even if the healthy participant effects were stronger among obese adults, they are not strong enough to produce a weakening association between obesity and morbidity at higher ages at the time of the survey. Therefore, neither of these selection biases explains the diminishing effect of class II/III obesity on mortality over the life course.  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