全文获取类型
收费全文 | 25786篇 |
免费 | 89篇 |
国内免费 | 183篇 |
专业分类
系统科学 | 161篇 |
丛书文集 | 401篇 |
教育与普及 | 77篇 |
理论与方法论 | 139篇 |
现状及发展 | 10871篇 |
研究方法 | 1100篇 |
综合类 | 12904篇 |
自然研究 | 405篇 |
出版年
2013年 | 214篇 |
2012年 | 409篇 |
2011年 | 817篇 |
2010年 | 192篇 |
2008年 | 497篇 |
2007年 | 504篇 |
2006年 | 523篇 |
2005年 | 542篇 |
2004年 | 538篇 |
2003年 | 582篇 |
2002年 | 715篇 |
2001年 | 849篇 |
2000年 | 764篇 |
1999年 | 519篇 |
1992年 | 416篇 |
1991年 | 324篇 |
1990年 | 348篇 |
1989年 | 349篇 |
1988年 | 355篇 |
1987年 | 347篇 |
1986年 | 346篇 |
1985年 | 476篇 |
1984年 | 351篇 |
1983年 | 270篇 |
1982年 | 250篇 |
1981年 | 260篇 |
1980年 | 310篇 |
1979年 | 736篇 |
1978年 | 562篇 |
1977年 | 576篇 |
1976年 | 492篇 |
1975年 | 512篇 |
1974年 | 697篇 |
1973年 | 602篇 |
1972年 | 651篇 |
1971年 | 714篇 |
1970年 | 925篇 |
1969年 | 781篇 |
1968年 | 787篇 |
1967年 | 711篇 |
1966年 | 656篇 |
1965年 | 486篇 |
1964年 | 155篇 |
1959年 | 269篇 |
1958年 | 483篇 |
1957年 | 356篇 |
1956年 | 329篇 |
1955年 | 268篇 |
1954年 | 295篇 |
1948年 | 234篇 |
排序方式: 共有10000条查询结果,搜索用时 484 毫秒
171.
Summary Direct incorporation and cold trap experiments suggest that cholest-4-en-3-one is an intermediate in the conversion of cholesterol into 3-hydroxymethyl-A-nor-cholestane in the marine spongeAxinella verrucosa. Cholest-4-en-3-one is further transformed by the sponge into cholest-4-en-3-ol, 5-cholestan-3-one and 5-cholestan-3-ol; these compounds arise from side reactions, which are not part of the major metabolic route leading to 3-hydroxymethyl-A-norsteranes. 相似文献
172.
L. de Napoli E. Fattorusso Silvana Magno L. Mayol 《Cellular and molecular life sciences : CMLS》1981,37(11):1132-1132
Summary Furocaulerpin, an acetylenic sesquiterpenoid possessing a furan ring, has been isolated from the marine algaCaulerpa prolifera, and its stereostructure elucidated mainly on the basis of physico-chemical data.This work was carried out in the frame work of the Progetto Finalizzato per l'Oceanografia e i Fondi Marini, CNR, Rome. Thanks are due to the Centro di Metodologie Chimico-Fisiche of the University of Naples (Mr I. Giudicianni) for the determination of PMR-spectra. 相似文献
173.
Summary The concentrations of testosterone (T) and 5-dihydrotestosterone were measured in fluid collected from the rete testis of immature and adult rats. The results indicate that adult levels of T are attained in the seminiferous tubules much earlier than in the peripheral circulation.This work was performed at the Worcester Foundation for Experimental Biology, Shrewsbury, Massachusetts, USA and supported by the National Institutes of Health through grants HD12641 and HD12642. We thank Dr B.V. Caldwell for antiserum to testosterone. 相似文献
174.
E. Knecht J. Renau-Piqueras J. Hernández-Yago S. Grisolía 《Cellular and molecular life sciences : CMLS》1981,37(5):456-457
Summary Protein from resting or phytohemagglutinin-stimulated human peripheral blood T-lymphocytes, pulse-labeled in vitro for 1 h with3H-leucine, had a half-life of 30 h. 相似文献
175.
176.
Summary In reserpinized mice the occurrence of goldthioglucose hypothalamic lesions was significantly lower than in control mice. Some protection was also conferred by serotonin-receptor blockers and by treatment with nialamide+DL--methyldopa, but the protective effect of reserpine was not reversed by serotonergic and dopaminergic agonists, alone or in combination, nor by insulin. 相似文献
177.
Summary -Galactosidase activity in intact cells of 21 species ofStreptomyces was measured using ONPG hydrolysis, without addition of a permeabilizing agent. Differences in the induction efficiency of ONPG-hydrolytic activity by lactose or galactose, which could have taxonomic implications, were observed among the species. 相似文献
178.
F. Bonneaux P. Labrude E. Dellacherie 《Cellular and molecular life sciences : CMLS》1981,37(8):884-886
Summary Stroma-free hemoglobin solutions present some drawbacks when used as blood substitutes, essentially because the hemoprotein has a low vascular retention, due to its small hydrodynamic volume. Covalent coupling of the protein with dextran derivatives artificially increases its size and affords polymeric conjugates whose oxygen-binding properties (Barcroft's curve, Hill coefficient) depend on the molecular weight.The authors wish to thank Prof. J. Neel and Prof. C. Vigneron for useful discussions and criticism. 相似文献
179.
G. Cimino A. Crispino S. De Rosa S. De Stefano G. Sodano 《Cellular and molecular life sciences : CMLS》1981,37(9):924-926
Summary Several high molecular weight polyacetylenes have been isolated from the spongePetrosia ficiformis found in dark caves. These compounds are related to, but different from, the polyacetylenes isolated from the same sponge living in its usual habitat.This work is a part of the Progetto finalizzato per l'Oceanografia e i Fondi Marini, CNR, Roma.The authors thank Mr G. Scognamiglio for the HPLC work, Mr C. Di Pinto for the NMR spectra and Mr G. Villani for the collection of the sponge. 相似文献
180.
Incorporation of 32P into adrenal mitochondrial phospholipids (PL) incrased in ACTH-treated rats, but it decreased in diabetics, inspite of the fact that these animals showed adrenal overacity. Since diabetics did not show increased 11 beta-hydroxylation. as opposed to ACTH-treated rats, it is suggested that the stimulation of this enzyme activity by exogenous ACTH is related to an increased turnover of PL at the mitochondrial membrane. The process is impaired in diabetics and prevents the stimulation of 11 beta-hydroxylation. 相似文献