全文获取类型
收费全文 | 1414篇 |
免费 | 195篇 |
专业分类
1609篇 |
出版年
2022年 | 17篇 |
2021年 | 29篇 |
2020年 | 17篇 |
2019年 | 36篇 |
2018年 | 43篇 |
2017年 | 34篇 |
2016年 | 49篇 |
2015年 | 66篇 |
2014年 | 67篇 |
2013年 | 76篇 |
2012年 | 88篇 |
2011年 | 78篇 |
2010年 | 79篇 |
2009年 | 54篇 |
2008年 | 68篇 |
2007年 | 79篇 |
2006年 | 69篇 |
2005年 | 58篇 |
2004年 | 58篇 |
2003年 | 40篇 |
2002年 | 50篇 |
2001年 | 26篇 |
2000年 | 44篇 |
1999年 | 29篇 |
1998年 | 22篇 |
1997年 | 9篇 |
1996年 | 18篇 |
1995年 | 10篇 |
1994年 | 15篇 |
1993年 | 12篇 |
1992年 | 19篇 |
1991年 | 22篇 |
1990年 | 22篇 |
1989年 | 17篇 |
1988年 | 13篇 |
1987年 | 9篇 |
1986年 | 14篇 |
1985年 | 15篇 |
1984年 | 12篇 |
1983年 | 8篇 |
1982年 | 7篇 |
1980年 | 9篇 |
1977年 | 12篇 |
1976年 | 8篇 |
1975年 | 6篇 |
1973年 | 8篇 |
1972年 | 9篇 |
1971年 | 5篇 |
1969年 | 9篇 |
1967年 | 7篇 |
排序方式: 共有1609条查询结果,搜索用时 0 毫秒
1.
Cristian A. Acevedo Elizabeth Y. Sanchez Juan G. Reyes Manuel E. Young 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》2010,878(3-4):449-455
It is known that skin releases volatile organic compounds to the environment, and also that its emission pattern changes with aging of the skin. It could be considered, that these compounds are intermediaries in cell metabolism, since many intermediaries of metabolic pathways have a volatile potential. In this work, a simple and non-destructive method consisting of SPME sampling and GC/MS analysis was developed to identify volatile organic emanations from cell cultures. This technique, applied to skin cells culture, indicates that the cells or cell metabolism produce several skin emissions. Chemometric analysis was performed in order to explore the relationship between a volatile profile and the senescence of cell cultures. Volatile profiles were different for cell cultures in different degrees of senescence, indicating that volatile compound patterns could be used to provide information about the age of skin cells. 相似文献
2.
Amiloride fluxes across erythrocyte membranes 总被引:3,自引:0,他引:3
Amiloride is known to inhibit both the influx of Na+ and the activation of mitogenesis in many cultured cell lines. This paper describes experiments in which the permeability coefficient of amiloride was determined from measurements of tracer fluxes across human erythrocytes and resealed ghosts. From an analysis of these fluxes, a permeability coefficient of 10(-7) cm/s for the uncharged form of amiloride was deduced. Based upon this measured permeability value, we present calculations of intracellular accumulation times of amiloride in cells of differing surface-to-volume ratio. 相似文献
3.
Transcriptional regulatory network triggered by oxidative signals configures the early response mechanisms of japonica rice to chilling stress 总被引:3,自引:0,他引:3
4.
5.
6.
Characterization of adenine phosphoribosyltransferase from the human malaria parasite, Plasmodium falciparum 总被引:2,自引:0,他引:2
Because of their inability to synthesize purines de novo, malaria parasites rely on purine phosphoribosyltransferases (PRTases) to convert purine bases salvaged from the host cell (the erythrocyte) into the corresponding purine nucleoside monophosphates. Our studies with late trophozoites of the human malaria parasite, Plasmodium falciparum, showed that virtually all of the purine PRTase activity is accounted for by two distinct enzymes. One enzyme utilizes hypoxanthine, guanine and xanthine (Queen, S.A., Vander Jagt, D. and Reyes, P. (1988) Mol. Biochem. Parasitol. 30, 123-134). The second enzyme utilizes only adenine and is the subject of this paper. This latter enzyme exhibits a biphasic pH-activity profile and is moderately to weakly inhibited by several divalent metal ions. Several of the properties of the P. falciparum enzyme were found to differ significantly from those of human erythrocyte adenine PRTase. (1) The molecular weight (18,000) of the parasite enzyme is smaller than that of the host cell enzyme. (2) The parasite enzyme, unlike the erythrocyte enzyme, is not significantly inhibited by sulfhydryl reagents. (3) 6-Mercaptopurine and 2,6-diaminopurine proved to be competitive inhibitors of the parasite enzyme (Ki 0.70 and 1.0 mM, respectively); on the other hand, the human enzyme is not inhibited by these agents. (4) The Km for adenine (0.80 microM) and 5-phosphoribosyl-1-pyrophosphate (0.70 microM) displayed by the parasite enzyme are significantly smaller than the corresponding Km values shown by the erythrocyte enzyme. These distinctions between the parasite and host enzymes point to the possibility that adenine PRTase of P. falciparum may represent a potential target for chemotherapeutic attack. 相似文献
7.
Acid and alkaline phosphatase activities in culture liquid and mycelial extract during autolysis were studied in seven fungi of the general Ascomycotina, Basidiomycotina and Zygomycotina. High activities of extracellular and mycelial extract acid phosphatase and lower activities of alkaline phosphatase were found in Ascomycotina, and acid phosphatase was present in Basidiomycotina. In Zygomycotina only mycelial extract alkaline phosphatase activity was detected. A correlation between degree of autolysis, pH and acid phosphatase activity was demonstrated. 相似文献
8.
Pectic activity in autolyzed cultures of Botrytis cinerea in a medium with and without pectin was similar, but in the medium with pectin maximal activities occurred in younger cultures. The pectic activities found were polygalacturonase, polymethylgalacturonase, endo activity (pectin as substrate) and pectin lyase. The molecular weights of polygalacturonase, polymethylgalacturonase and endo activity (pectin as substrate) were 36000, 33000 and 30200 daltons respectively, and the molecular weight of pectin lyase was 18200 daltons. By gel electrophoresis four different pectic activities were detected, three in the top of the gel and one in the bottom. Two enzymes were characterized, the polygalacturonase activity (first band in the top) inhibited by Ca++ and the pectin lyase activity (in the bottom) which was not inhibited by Ca++. These enzymes are not induced by the presence of pectin in the medium during degradation of Botrytis cinerea. 相似文献
9.
Endoproteolytic cleavage of gp160 is required for the activation of human immunodeficiency virus 总被引:103,自引:0,他引:103
J M McCune L B Rabin M B Feinberg M Lieberman J C Kosek G R Reyes I L Weissman 《Cell》1988,53(1):55-67
The envelope protein of human immunodeficiency virus (HIV) is synthesized as a polyprotein (gp160) and cleaved intracellularly to a gp120-gp41 heterodimer. In this study, the tryptic-like endoproteolytic cleavage site was removed by site-directed mutagenesis and replaced with a chymotryptic-like site. The resultant mutant, RIP7/mut10, was found to be indistinguishable from wild-type HIV when analyzed at the level of proviral replication, RNA processing, protein expression, and viral assembly. However, the gp160 polyprotein was not cleaved and the mutated virions were biologically inactive, until and unless they were exposed to limiting concentrations of chymotrypsin. As is the case for other enveloped mammalian viruses, endoproteolytic cleavage of the HIV envelope protein and release of a unique hydrophobic domain appear to be necessary for the full expression of viral infectivity. 相似文献
10.
Tyrone M. Reyes 《The Western journal of medicine》1991,154(5):632-633
The international community has perspective and experience that will freshen our approaches to rehabilitation. Martin Grabois, MD*, editor of this special section, has gathered articles written by experts from other countries. The intention is to stimulate thought, discussion, and action—and to broaden horizons. 相似文献