全文获取类型
收费全文 | 4435篇 |
免费 | 21篇 |
国内免费 | 66篇 |
专业分类
系统科学 | 27篇 |
丛书文集 | 83篇 |
教育与普及 | 64篇 |
理论与方法论 | 5篇 |
现状及发展 | 1976篇 |
研究方法 | 202篇 |
综合类 | 2153篇 |
自然研究 | 12篇 |
出版年
2018年 | 40篇 |
2017年 | 30篇 |
2012年 | 58篇 |
2011年 | 86篇 |
2010年 | 38篇 |
2009年 | 104篇 |
2008年 | 90篇 |
2007年 | 118篇 |
2006年 | 72篇 |
2005年 | 120篇 |
2004年 | 132篇 |
2003年 | 84篇 |
2002年 | 85篇 |
2001年 | 219篇 |
2000年 | 195篇 |
1999年 | 141篇 |
1998年 | 32篇 |
1997年 | 28篇 |
1995年 | 30篇 |
1992年 | 100篇 |
1991年 | 90篇 |
1990年 | 93篇 |
1989年 | 71篇 |
1988年 | 78篇 |
1987年 | 80篇 |
1986年 | 59篇 |
1985年 | 98篇 |
1984年 | 81篇 |
1983年 | 62篇 |
1982年 | 65篇 |
1981年 | 59篇 |
1980年 | 75篇 |
1979年 | 146篇 |
1978年 | 134篇 |
1977年 | 107篇 |
1976年 | 79篇 |
1975年 | 73篇 |
1974年 | 98篇 |
1973年 | 103篇 |
1972年 | 89篇 |
1971年 | 111篇 |
1970年 | 100篇 |
1969年 | 96篇 |
1968年 | 95篇 |
1967年 | 91篇 |
1966年 | 86篇 |
1965年 | 58篇 |
1958年 | 41篇 |
1957年 | 32篇 |
1956年 | 29篇 |
排序方式: 共有4522条查询结果,搜索用时 15 毫秒
21.
Ross P Weinhouse H Aloni Y Michaeli D Weinberger-Ohana P Mayer R Braun S de Vroom E van der Marel GA van Boom JH Benziman M 《Nature》1987,325(6101):279-281
Cellulose is the most abundant renewable carbon resource on earth and is an indispensable raw material for the wood, paper, and textile industries. A model system to study the mechanism of cellulose biogenesis is the bacterium Acetobacter xylinum which produces pure cellulose as an extracellular product. It was from this organism that in vitro preparations which possessed high levels of cellulose synthase activity were first obtained in both membranous and soluble forms. We recently demonstrated that this activity is subject to a complex multi-component regulatory system, in which the synthase is directly affected by an unusual cyclic nucleotide activator enzymatically formed from GTP, and indirectly by a Ca (2+) -sensitive phosphodiesterase which degrades the activator. The cellulose synthase activator (CSA) has now been identified as bis-(3' 5')-cyclic diguanylic acid (5'G3'p5'G3'p) on the basis of mass spectroscopic data, nuclear magnetic resonance analysis and comparison with chemically synthesized material. We also report here on intermediary steps in the synthesis and degradation of this novel circular dinucleotide, which have been integrated into a model for the regulation of cellulose synthesis. 相似文献
22.
Mapping of behaviour in Drosophila mosaics 总被引:23,自引:0,他引:23
23.
A. Alkofahi J. K. Rupprecht Y. -M. Liu C. -J. Chang D. L. Smith J. L. McLaughlin 《Cellular and molecular life sciences : CMLS》1990,46(5):539-541
Summary Gigantecin (I), a novel tetrahydroxy-di-tetrahydrofuran fatty acid -lactone (acetogenin), was isolated from an ethanolic extract of the stem bark ofGoniothalamus giganteus Hook. f., Thomas (Annonaceae), by means of activity-directed fractionation (brine shrimp lethality test). This new compound is extremely cytotoxic to human tumor cells, inhibits crown gall tumors on potato discs, and is active in an assay designed to detect antimitotic agents (9 ASK). 相似文献
24.
Summary Metabolism of 26-hydroxycholesterol to 3-hydroxychol-5-en-24-oic acid and other C24-bile acids has been expected to occur by way of 3-hydroxycholest-5-en-26-oic acid in studies in vitro. 3-Hydroxycholest-5-en-26-oic acid was infused intravenously into bile fistula hamsters and the following C24-bile acids were identified: 3-hydroxychol-5-en-24-oic acid, lithocholic acid, chenodeoxycholic acid, and a small amount of cholic acid. 相似文献
25.
Y. Mamiya H. Sonobe K. Yoshida N. Hara Y. Fujimoto 《Cellular and molecular life sciences : CMLS》1995,51(4):363-367
Ecdysteroids in diapause eggs of the silkworm,Bombyx mori, were analyzed using high-performance liquid chromatography (HPLC) combined with radioimmunoassay (RIA). A relatively large amount of an unidentified free ecdysteroid and its phosphoric ester (conjugated form) were detected. These two compounds were isolated by a combination of column chromatography on silicic acid, thin-layer chromatography (TLC), and HPLC using a reverse-phase (RP) column. The purified compounds were identified as 3-epi-22-deoxy-20-hydroxyecdysone (22d20E) and 3-epi-22-deoxy-20-hydroxyecdysone 2-phosphate (22d20E2P) by means of mass spectrometry and nuclear magnetic resonance spectroscopy. to our knowledge, this is the first report of 22d20E and 22d20E2P. 相似文献
26.
27.
At ambient temperatures (Ta) of both 8 and 22 degrees C, intraventricular administration of TRH (10-80 microgram) produced a dose-dependent hypothermia in rats. The hypothermia was due to both decreased metabolic heat production and cutaneous vasodilatation. In contrast, at 30 degrees C Ta, TRH increased metabolic heat production (due to behavioral excitation) and led to hyperthermia. 相似文献
28.
Summary The C-terminal amino acid sequences of human and of porcine antithrombin III have been determined as Gly-Arg-Val-Ala-Asn-Pro-Cys-Val-Lys and Gly-Arg-Val-Ala-Asn-Pro-Cys, respectively. These sequences are highly homologous with the C-terminal sequence of human -1-proteinase inhibitor. 相似文献
29.
Summary 2-Halogeno-ethanols change the active site structure of -chymotrypsin more rapidly and effectively than ethanol, 1-propanol and urea, probably before producing an extensive conformation change. 相似文献
30.
D. Sabourault Françoise Bauché Y. Giudicelli J. Nordmann R. Nordmann 《Cellular and molecular life sciences : CMLS》1981,37(3):227-229
Summary After 3 weeks of continuous ethanol intoxication by inhalation, the maximal number and affinity of the -and -receptors of rat heart were unchanged. These data indicate that adrenergic receptor disturbances are not involved in the mechanism of chronic ethanol-induced triglyceride deposition in the heart.Acknowledgments. This work was supported by grants from the Institut National de la Santé et de la Recherche Médicale. We wish to thank Mr M. Clément for skilled technical assistance in performing chronic ethanol inhalation treatments. 相似文献