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   5958篇
  免费   711篇
  国内免费   3篇
  2021年   84篇
  2020年   48篇
  2019年   60篇
  2018年   82篇
  2017年   85篇
  2016年   104篇
  2015年   164篇
  2014年   208篇
  2013年   290篇
  2012年   294篇
  2011年   300篇
  2010年   213篇
  2009年   170篇
  2008年   226篇
  2007年   255篇
  2006年   234篇
  2005年   222篇
  2004年   207篇
  2003年   203篇
  2002年   204篇
  2001年   138篇
  2000年   162篇
  1999年   133篇
  1998年   94篇
  1997年   71篇
  1996年   76篇
  1995年   70篇
  1994年   57篇
  1993年   59篇
  1992年   109篇
  1991年   103篇
  1990年   84篇
  1989年   88篇
  1988年   108篇
  1987年   107篇
  1986年   81篇
  1985年   82篇
  1984年   97篇
  1983年   88篇
  1982年   57篇
  1981年   70篇
  1980年   58篇
  1979年   79篇
  1978年   50篇
  1977年   45篇
  1976年   55篇
  1975年   41篇
  1974年   55篇
  1973年   43篇
  1969年   44篇
排序方式: 共有6672条查询结果,搜索用时 31 毫秒
71.
Summary A general observer-based estimator method is developed and applied for process modelling and monitoring. This parameter estimation technique was successfully applied to a L-lysine fermentation process. It was a useful tool to detect the effect of major culture conditions on cell growth and product synthesis. It can also be used for the development of adaptive optimal control schemes.  相似文献   
72.
73.
74.
A novel series of substituted piperidine-2-ones has been identified as antagonists of angiotensin II. These compounds showed high affinity for the receptor in bovine adrenal cortex binding assays with IC50's as low as 20nM. They are potent inhibitors of angiotensin II induced contractions in rabbit aortic rings, with pA2 values as high as 9. A number of these compounds are also orally active as antihypertensives in spontaneously hypertensive rat preparations.  相似文献   
75.
76.
Summary The relationship between granule density, protein content, and Ca and S contents were studied in two secretory granule fractions, from parotid glands of the rat, previously shown to constitute different stages in granule maturation. The density of the lighter fraction was between 1.133 and 1.142 g/ml, while that of the heavier fraction was greater than 1.142 g/ml. The mean protein content of the denser granules was 12% greater than that of the lighter granules (P<0.03), while the dry-mass elemental concentrations in the two granule fractions were unchanged. These results indicate that protein is added to granules during the maturation process (presumably by vesicular traffic), and that the resulting increase in granule density is not driven simply by decrease in water content and/or increased concentrations of inorganic Ca or S in the granules. The elemental concentration values also indicate that the diffusible elements permeate the granule membrane during the fractionation procedures.  相似文献   
77.
Conclusion Anthropologists have too often supported — in their words, in their presences, and in their deeds — the paternalistic claims of rulers suppressing the very cultures they want to study. Much anthropological work legitimates substitution of the language of Beijing for Hakka and Holo, justifies ignoring Taiwanese culture, and subordinates consideration of its specific features to writing about Chinese civilization, just as Taiwanese are economically and politically subordinated to the ethnic minority dictatorship which still rules Taiwan under the fiction of being the government of China. The China that Arthur Wolf and others serve in return for support of their research from what purports to be the Republic of China is an egregious, but unfortunately not unique, example.Stephen O. Murray is with El Instituto Obregón, San Francisco, California. Keelung Hong is with the Cancer Research Institute, University of California, San Francisco.As is normal in anthropology, except for Taiwan, native terms are romanized in the native language (in the first author's recording of the second author's Daidiong idiolect, without indication of the extremely intricate tone differentia), not in the official language of the ethnic oligarchy ruling Taiwan (Beijinghua). Mainland place names are rendered in pinyin. We have followed the convention of treating Chinese and Taiwanese personal names as two words, because books are catalogued with such names. If personal names are considered as two words (an imposition of the hoary dogma that words in Sino-Tibetan languages are monosyllabic), it seems to us that both words should be capitalized. For Taiwanese placenames, we have not separated syllables by dashes or by spaces.  相似文献   
78.
Synopsis Male beaugregory damselfish,Stegastes leucostictus, were provided with three types of artificial breeding structures to determine if they change habitats based on past or future reproductive payoffs. All three site types quickly lured males away from their natural sites. In comparison to those living on natural sites, those using artificial sites were less likely to move to different areas and had a higher reproductive success. When given no choice, male reproductive success was correlated to structural type. A second experiment provided males with an additional structure after using the intermediate quality type for 2 months. Males would often initially use both sites but would eventually shift their spawning activity to the new site if it was of the same quality or better than the old one. However, males would not move if the new site was of inferior quality. When given a new site identical to the initial one, approximately half of the males shifted to the new site. There was no evidence that reproductive performance influenced a male's decision to use a new site.  相似文献   
79.
The tumor-promoting agent 12-0-tetradecanoyl-phorbol-13-acetate (TPA) caused a time- and dose-dependent morphological change in Y-1 adrenocortical tumor cells. The morphological alteration was apparent 2 hr following addition of 1 microgram/ml TPA to cell cultures and became more striking with longer treatment times. Smaller doses of TPA took a longer time to produce an effect. Cultures grown in the presence of TPA exhibited more rounding and piling up of cells than similar cultures maintained in medium lacking TPA. These TPA-stimulated morphological changes were reversible, and after 24 hr in TPA-free media, the cultured cells began to flatten. After 96 hr in TPA-free media they resembled the control cultures. The reversibility of the morphological change was also dose dependent: cells treated with 1 microgram/ml TPA took a longer time to resume the typical control morphology than did cultures treated with 0.01 microgram/ml TPA. In addition, TPA treatment resulted in a decrease in cell growth rate, an increase in steroid production, and an increase in the localization of free catalytic units of cAMP-dependent protein kinase in the cytoplasm. The steroidogenic effect of ACTH on the cell population was inhibited in cultures maintained in TPA. The results of this study indicate that TPA induces morphological changes in the Y-1 adrenocortical tumor cell population while increasing steroidogenesis and the activation of cAMP-dependent protein kinase and decreasing cell growth rate.  相似文献   
80.
Negative staining revealed a tetragonal surface array (S layer) on all the members of a serogroup of Aeromonas hydrophila which possess high virulence for fish. The S layers were similar on all the strains examined, with unit cell dimensions of approximately 12 nm. A single representative strain, strain TF7, was selected for further analysis. Freeze-cleaved and etched preparations and sections for electron microscopy showed that the S layer was the outermost component of the cell envelope. This was confirmed by observation of thin sections. Computer-generated enhancements of the negatively stained micrographs showed the subunit organization to a resolution of less than 4 nm. Two structural units of identical lattice constants alternated in the array in both axes, and one of them was apparently dominant as the center of mass. The lesser unit was rotated 20 degrees from the dominant axes of symmetry and was formed by the junction of linker projections from a corner of the four components of the dominant unit. This interpretation was supported by finding that the array consists of a single polypeptide (molecular weight, 52,000). The unit cell as defined showed p4 symmetry, and a = b = 12.2 nm.  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