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   426331篇
  免费   47937篇
  国内免费   164篇
  474432篇
  2018年   3918篇
  2016年   5303篇
  2015年   6943篇
  2014年   8205篇
  2013年   11281篇
  2012年   12861篇
  2011年   13313篇
  2010年   9133篇
  2009年   8484篇
  2008年   12186篇
  2007年   12700篇
  2006年   11899篇
  2005年   11359篇
  2004年   11407篇
  2003年   10688篇
  2002年   10509篇
  2001年   17467篇
  2000年   17475篇
  1999年   13952篇
  1998年   5046篇
  1997年   5290篇
  1996年   4930篇
  1995年   4625篇
  1994年   4495篇
  1993年   4544篇
  1992年   11627篇
  1991年   11564篇
  1990年   11321篇
  1989年   10936篇
  1988年   10512篇
  1987年   10104篇
  1986年   9382篇
  1985年   9256篇
  1984年   7769篇
  1983年   6726篇
  1982年   5173篇
  1981年   4629篇
  1980年   4475篇
  1979年   7430篇
  1978年   5877篇
  1977年   5410篇
  1976年   5207篇
  1975年   5606篇
  1974年   6309篇
  1973年   6174篇
  1972年   5773篇
  1971年   5229篇
  1970年   4639篇
  1969年   4576篇
  1968年   4424篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
131.
Everyone agrees that insurance for long-term care is inadequate in the United States. Disagreement exists, however, on whether such insurance should be provided through the private or public sector. Private insurance generally uses the experience-rating principle that persons with higher risk of illness are charged higher premiums. For private insurance for long-term care, this principle creates a dilemma. Most policies will be purchased by the elderly; yet, because the elderly have a high risk of needing long-term care, only about 20% of them can afford the cost of premiums. A public-private partnership by which the government partially subsidizes private long-term-care insurance is unlikely to resolve this dilemma. Only a social insurance program for long-term care can provide universal, affordable, and equitable coverage.  相似文献   
132.
The enzymatic nature of 5'-monodeiodination (5'-D) in avian liver homogenates was demonstrated by abolishment of activity by iopanoic acid (IOP). T3 production from T4 was dependent on enzyme and substrate concentrations, incubation time, incubation temperature, and pH. Two pathways of 5'-D activity were present in avian liver and exhibited characteristics similar to those described in mammalian tissues. Type II activity was identified as propylthiouracil (PTU)-insensitive activity. Type I (PTU-sensitive) was determined by difference between Total and Type II. Km values were 1.58 microM T4 for Total activity and 0.90 nM T4 for Type II, corresponding to the characteristics of the mammalian pathways. The effects of goitrogens on avian hepatic 5'-D were equivalent to those reported for the mammalian enzyme.  相似文献   
133.
134.
135.
Sullivantia species were found to produce quercetin 3-O-glycosides, several of which contain glucuronic acid, as well as pedalitin (6-hydroxy-7-O-methyl luteolin), pedalitin 6-O-glycosides, and small amounts of luteolin. Sullivantia has a unique combination of compounds that distinguishes it from other genera in the Saxifraginae for which flavonoid data are available. The nature of the flavonoid compounds is in accordance with a general trend within the Saxifragaceae of reduction and replacement of flavonols by flavones.  相似文献   
136.
Cytochrome b5 holoenzyme was bound asymmetrically in the tightly bound form to small unilamellar dimyristoylphosphatidylcholine vesicles. [3H]Taurine, a membrane-impermeant nucleophile, was added to the external medium and was then cross-linked to cytochrome carboxyl residues by the addition of a water-soluble carbodiimide. Nonpolar peptide was isolated after trypsin digestion of taurine-labeled apocytochrome b5 and contained 1.7-1.9 residues of taurine. The C-terminal tetrapeptide containing residues Thr130-Asn133 was generated by chymotryptic hydrolysis of radiolabeled nonpolar peptide and was purified by gel filtration and ion exchange chromatography. Amino acid analysis of the C-terminal tetrapeptide showed that about 1.6 mol of taurine was cross-linked per mol of peptide. When the experiment was performed with taurine trapped inside the vesicles, no cross-linking was observed. The results suggest that when cytochrome b5 holoenzyme is bound to vesicles in the tight binding form, the C terminus is located on the external surface of the vesicles.  相似文献   
137.
138.
139.
140.
Summary Eicosapentaenoic acid (EPA) volumetric productivity from an outdoor chemostat culture ofPhaeodactylum tricornutum UTEX 640 in a 50-l tubular photobioreactor varies with dilution rate, reaching a maximum of 47.8 mg l–1 d–1 at D=0.36 d–1. Continuous culture at high dilution rates' is proposed as the most adequate operating mode to maximize polyunsaturated fatty acid production.  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