全文获取类型
收费全文 | 43558篇 |
免费 | 94篇 |
国内免费 | 146篇 |
专业分类
系统科学 | 212篇 |
丛书文集 | 826篇 |
教育与普及 | 115篇 |
理论与方法论 | 231篇 |
现状及发展 | 19353篇 |
研究方法 | 1698篇 |
综合类 | 20824篇 |
自然研究 | 539篇 |
出版年
2013年 | 301篇 |
2012年 | 580篇 |
2011年 | 1133篇 |
2010年 | 259篇 |
2008年 | 759篇 |
2007年 | 787篇 |
2006年 | 827篇 |
2005年 | 834篇 |
2004年 | 737篇 |
2003年 | 798篇 |
2002年 | 775篇 |
2001年 | 1342篇 |
2000年 | 1226篇 |
1999年 | 809篇 |
1994年 | 373篇 |
1992年 | 786篇 |
1991年 | 619篇 |
1990年 | 685篇 |
1989年 | 673篇 |
1988年 | 643篇 |
1987年 | 689篇 |
1986年 | 684篇 |
1985年 | 861篇 |
1984年 | 674篇 |
1983年 | 570篇 |
1982年 | 527篇 |
1981年 | 500篇 |
1980年 | 603篇 |
1979年 | 1401篇 |
1978年 | 1186篇 |
1977年 | 1114篇 |
1976年 | 837篇 |
1975年 | 910篇 |
1974年 | 1247篇 |
1973年 | 1080篇 |
1972年 | 1076篇 |
1971年 | 1334篇 |
1970年 | 1762篇 |
1969年 | 1334篇 |
1968年 | 1287篇 |
1967年 | 1278篇 |
1966年 | 1128篇 |
1965年 | 823篇 |
1959年 | 484篇 |
1958年 | 729篇 |
1957年 | 565篇 |
1956年 | 496篇 |
1955年 | 433篇 |
1954年 | 470篇 |
1948年 | 298篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
12.
13.
14.
15.
16.
17.
18.
P pili in uropathogenic E. coli are composite fibres with distinct fibrillar adhesive tips. 总被引:25,自引:0,他引:25
Escherichia coli is a frequent cause of several common bacterial infections in humans and animals, including urinary tract infections, bacteraemia and bacteria-related diarrhoea and is also the main cause of neonatal meningitis. Microbial attachment to surfaces is a key event in colonization and infection and results mainly from a stereochemical fit between microbial adhesins and complementary receptors on host cells. Bacterial adhesins required for extracellular colonization by Gram-negative bacteria are often minor components of heteropolymeric fibres called pili which must be oriented in an accessible manner in these structures to be able to bind to specific receptor architectures. P pili mediate the binding of uropathogenic E. coli to a digalactoside receptor determinant present in the urinary tract epithelium. We report here that the adhesin is a component of distinct fibrillar structures present at the tips of the pili. These virulence-associated tip fibrillae are thin, flexible polymers composed mostly of repeating subunits of PapE that frequently terminate with the alpha-D-galactopyranosyl-(1-4)-beta-D-galactopyranose or Gal alpha (1-4)Gal binding PapG adhesin. 相似文献
19.
Although theoretical studies show that overcompensatory density-dependent mechanisms can potentially generate regular or chaotic fluctuations in animal numbers, the majority of realistic single-species models of invertebrate populations are not overcompensatory enough to cause sustained population cycles. The possibility that overcompensation may generate cycles or chaos in vertebrate populations has seldom been considered. Here we show that highly overcompensatng density-dependent mortality can generate recurrent population crashes consistent with those observed in a naturally limited population of Soay sheep. The observed interval of three or more years between crashes points to sharp 'focusing' of mortality over a narrow range of population density. 相似文献
20.
Humoral immune reactions to heat shock proteins (hsp) from microorganisms are one aspect of microbial infections in humans. The production of antibodies which are specific to epitopes present on procaryotic hsp leads also to the appearance of cross-reactive serum antibodies in the host organism that react with human hsp. This article discusses the consequences of such autoreactive antibodies for the host in context with the development of immune tolerance and autoimmune diseases, especially rheumatoid arthritis (RA), and in experimental animal models for arthritis such as adjuvant arthritis in rats. On the basis of epitope cross-reactivity between hsp and other host proteins, a hypothesis is presented for the development of autoimmune disease following the production of hsp-specific antibodies. 相似文献