全文获取类型
收费全文 | 63848篇 |
免费 | 4777篇 |
国内免费 | 5515篇 |
出版年
2023年 | 1161篇 |
2022年 | 1403篇 |
2021年 | 2119篇 |
2020年 | 1908篇 |
2019年 | 2706篇 |
2018年 | 2420篇 |
2017年 | 1921篇 |
2016年 | 1886篇 |
2015年 | 1992篇 |
2014年 | 3423篇 |
2013年 | 4342篇 |
2012年 | 2558篇 |
2011年 | 3101篇 |
2010年 | 2319篇 |
2009年 | 2966篇 |
2008年 | 3028篇 |
2007年 | 3183篇 |
2006年 | 2887篇 |
2005年 | 2434篇 |
2004年 | 2125篇 |
2003年 | 1946篇 |
2002年 | 1758篇 |
2001年 | 1483篇 |
2000年 | 1213篇 |
1999年 | 1069篇 |
1998年 | 1035篇 |
1997年 | 971篇 |
1996年 | 903篇 |
1995年 | 875篇 |
1994年 | 911篇 |
1993年 | 764篇 |
1992年 | 765篇 |
1991年 | 706篇 |
1990年 | 569篇 |
1989年 | 550篇 |
1988年 | 502篇 |
1987年 | 479篇 |
1986年 | 456篇 |
1985年 | 698篇 |
1984年 | 917篇 |
1983年 | 752篇 |
1982年 | 781篇 |
1981年 | 628篇 |
1980年 | 625篇 |
1979年 | 558篇 |
1978年 | 443篇 |
1977年 | 409篇 |
1976年 | 380篇 |
1974年 | 253篇 |
1973年 | 260篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
81.
Abstract: The enzymatic hydrolysis of UDP-galactose in rat and calf brain was studied. The hydrolysis occurs in two steps: The first is the conversion of UDP-galactose to galactose-1-phosphate catalyzed by nucleotide pyrophosphatase (EC 3.6.1.9), and the second is the conversion of the latter to free galactose by alkaline phosphatase (EC 3.1.3.1). The overall conversion has a pH optimum of 9.0, but there is considerable activity at pH 7.4, which is the optimum for UDP-galactose:ceramide galactosyltransferase in the synthesis of cerebrosides. Preparations from cytosol from calf brain cerebellum or stem that were enriched in UDP-galactose hydrolytic activity inhibit cerebroside synthesis under conditions optimal for the synthesis. Microsome-rich and nuclear debris fractions contain the highest apparent specific activity among the subcellular fractions studied. Hydrolysis of UDP-galactose occurs in all areas of brain, brainstem having the highest activity. The apparent specific activity in jimpy mouse brain homogenate is nearly twice as high as in the control brain homogenate. 相似文献
82.
83.
G. Karrer 《Plant Ecology》1985,59(1-3):199-209
Five plant communities contrasting in successional status and human impact from the southern part of the ‘Wienerwald’ (Austria) are analyzed using vegetation relevés, spectra of area types and a newly proposed disjunction quotient. A climax community (Asperulo-Fagetum), a subclimax community (Querco-Carpinetum s.l.), an anthropogenous substitute community (Mesobromion) and two natural, non-climax permanent communities (Euphorbio saxatilis-Pinetum nigrae and Fumano-Stipetum eriocaulis) are recognized. The disjunction quotient is defined as the number of partial (discontinous) areas divided by the size of the total area of distribution of a species. In particular, the average disjunction quotients of the species in the first two communities reflect relatively table environments only slightly influenced by man, with many ancient, stable taxa. These communities are characterized by species with well-delimited, stable distribution areas. The species in the Mesobromion community have very low average disjunction quotients as its component species are widely and continuously distributed and are often promoted by man. In contrast to these communities, the species linked to the natural permanent, non-climax communities of extreme habitats, have high distribution quotients i.e. small, disconinuous areas; this illustrates the relic character of these plant communities and of the eastern edge of the Alps is a whole. Using the highly variable disjunction quotient of all species and communities examined, the concepts of climax and permanent communities (of different origin) are discussed with regard to European conditions. 相似文献
84.
Sequences related to the major subunit gene fedA of F107 fimbriae in porcine Escherichia coli strains that express adhesive fimbriae 总被引:1,自引:0,他引:1
Abstract Porcine Escherichia coli strains isolated from cases fo postweaning diarrhea or edema disease were analysed for the presence of fedA , the major subunit gene of F107 fimbriae. The E. coli isolates were known to contain colonisation factor '8813', or to express F107, 2134P or other fimbriae, different from F4, F5, F6, and F41. PCR with fedA -specific primers, restriction enzyme digestion of the PCR product, and nucleotide sequence analysis demonstrated that 2134P pili, colonisation factor '8813' and fimbriae identified on Australian strains of the O141 serotype belong to one family of F107 fimbrial antigens. 相似文献
85.
86.
87.
88.
《Journal of biological dynamics》2013,7(4):318-334
In this paper, we study a two-species competitive system where both the species produce toxin against each other at some cost to their growth rates. A much wider set of outcomes is possible for our system. These outcomes are important contrasts to competitive exclusion or bistable attractors that are often the outcomes for competitive systems. We show that toxin helps to gain an advantage in competition for toxic species whenever the cost of toxin production remains within some moderate value; otherwise it may result in the extinction of the species itself. 相似文献
89.
90.