全文获取类型
收费全文 | 1151篇 |
免费 | 140篇 |
出版年
2022年 | 13篇 |
2021年 | 26篇 |
2020年 | 20篇 |
2019年 | 16篇 |
2018年 | 23篇 |
2017年 | 19篇 |
2016年 | 25篇 |
2015年 | 45篇 |
2014年 | 50篇 |
2013年 | 56篇 |
2012年 | 79篇 |
2011年 | 72篇 |
2010年 | 46篇 |
2009年 | 39篇 |
2008年 | 46篇 |
2007年 | 50篇 |
2006年 | 53篇 |
2005年 | 39篇 |
2004年 | 53篇 |
2003年 | 44篇 |
2002年 | 33篇 |
2001年 | 22篇 |
2000年 | 16篇 |
1999年 | 18篇 |
1998年 | 14篇 |
1997年 | 12篇 |
1996年 | 10篇 |
1992年 | 11篇 |
1991年 | 20篇 |
1990年 | 13篇 |
1989年 | 17篇 |
1988年 | 8篇 |
1987年 | 11篇 |
1986年 | 10篇 |
1985年 | 13篇 |
1984年 | 8篇 |
1983年 | 9篇 |
1982年 | 10篇 |
1981年 | 10篇 |
1980年 | 11篇 |
1979年 | 13篇 |
1978年 | 19篇 |
1977年 | 14篇 |
1976年 | 12篇 |
1975年 | 18篇 |
1974年 | 8篇 |
1973年 | 13篇 |
1969年 | 7篇 |
1968年 | 7篇 |
1965年 | 7篇 |
排序方式: 共有1291条查询结果,搜索用时 15 毫秒
961.
962.
We determine the kinetic parameters and of lactose transport in Escherichia coli cells as a function of the electrical potential difference (Δψ) at pH 7.3 and . We report that transport occurs simultaneously via two components: a component which exhibits a high (larger than 10 mM) and whose contribution is independent of Δψ, a component which exhibits a low independent of Δψ (0.5 mM) but whose increases drastically with increasing Δψ. We associate these components of lactose transport with facilitated diffusion and active transport, respectively. We analyze the dependence upon Δψ of and of the active transport component in terms of a mathematical kinetic model developed by Geck and Heinz (Geck, P. and Heinz, E. (1976) Biochim. Biophys. Acta 443, 49–63). We show that within the framework of this model, the analysis of our data indicates that active transport of lactose takes place with a H+/lactose stoichiometry greater than 1, and that the lac carrier in the absence of bound solutes (lactose and proton(s)) is electrically neutral. On the other hand, our data relative to facilitated diffusion tend to indicate that lactose transport via this mechanism is accompanied by a H+/lactose stoichiometry smaller than that of active transport. We discuss various implications which result from the existence of H+/lactose stoichiometry different for active transport and facilitated diffusion. 相似文献
963.
Roberts TJ Abbott EM Azizi E 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2011,366(1570):1488-1495
Muscles power movement, yet the conceptual link between muscle performance and locomotor performance is poorly developed. Frog jumping provides an ideal system to probe the relationship between muscle capacity and locomotor performance, because a jump is a single discrete event and mechanical power output is a critical determinant of jump distance. We tested the hypothesis that interspecific variation in jump performance could be explained by variability in available muscle power. We used force plate ergometry to measure power produced during jumping in Cuban tree frogs (Osteopilus septentrionalis), leopard frogs (Rana pipiens) and cane toads (Bufo marinus). We also measured peak isotonic power output in isolated plantaris muscles for each species. As expected, jump performance varied widely. Osteopilus septentrionalis developed peak power outputs of 1047.0 ± 119.7 W kg(-1) hindlimb muscle mass, about five times that of B. marinus (198.5 ± 54.5 W kg(-1)). Values for R. pipiens were intermediate (543.9 ± 96.2 W kg(-1)). These differences in jump power were not matched by differences in available muscle power, which were 312.7 ± 28.9, 321.8 ± 48.5 and 262.8 ± 23.2 W kg(-1) muscle mass for O. septentrionalis, R. pipiens and B. marinus, respectively. The lack of correlation between available muscle power and jump power suggests that non-muscular mechanisms (e.g. elastic energy storage) can obscure the link between muscle mechanical performance and locomotor performance. 相似文献
964.
Mowbray CE Bell AS Clarke NP Collins M Jones RM Lane CA Liu WL Newman SD Paradowski M Schenck EJ Selby MD Swain NA Williams DH 《Bioorganic & medicinal chemistry letters》2011,21(21):6596-6602
We describe the development of novel benzimidazoles as small molecule histamine H4 receptor (H4R) antagonists and their profiling in rat early toxicity studies. The discovery and optimisation of a second series of pyrimidine based antagonists is then described culminating in the identification of the clinical development candidate 13 (PF-3893787). The pre-clinical profile of 13 (PF-3893787) is presented including the development of a translatable biomarker. Our pragmatic approach to target selection, safety assessment, and testing for efficacy faced numerous challenges and we share a number of lessons which the team learned and which will assist us and others in future drug discovery projects. 相似文献
965.
966.
Mody RK Greene SA Gaul L Sever A Pichette S Zambrana I Dang T Gass A Wood R Herman K Cantwell LB Falkenhorst G Wannemuehler K Hoekstra RM McCullum I Cone A Franklin L Austin J Delea K Behravesh CB Sodha SV Yee JC Emanuel B Al-Khaldi SF Jefferson V Williams IT Griffin PM Swerdlow DL 《PloS one》2011,6(2):e16579
Background
In May 2008, PulseNet detected a multistate outbreak of Salmonella enterica serotype Saintpaul infections. Initial investigations identified an epidemiologic association between illness and consumption of raw tomatoes, yet cases continued. In mid-June, we investigated two clusters of outbreak strain infections in Texas among patrons of Restaurant A and two establishments of Restaurant Chain B to determine the outbreak''s source.Methodology/Principal Findings
We conducted independent case-control studies of Restaurant A and B patrons. Patients were matched to well controls by meal date. We conducted restaurant environmental investigations and traced the origin of implicated products. Forty-seven case-patients and 40 controls were enrolled in the Restaurant A study. Thirty case-patients and 31 controls were enrolled in the Restaurant Chain B study. In both studies, illness was independently associated with only one menu item, fresh salsa (Restaurant A: matched odds ratio [mOR], 37; 95% confidence interval [CI], 7.2–386; Restaurant B: mOR, 13; 95% CI 1.3–infinity). The only ingredient in common between the two salsas was raw jalapeño peppers. Cultures of jalapeño peppers collected from an importer that supplied Restaurant Chain B and serrano peppers and irrigation water from a Mexican farm that supplied that importer with jalapeño and serrano peppers grew the outbreak strain.Conclusions/Significance
Jalapeño peppers, contaminated before arrival at the restaurants and served in uncooked fresh salsas, were the source of these infections. Our investigations, critical in understanding the broader multistate outbreak, exemplify an effective approach to investigating large foodborne outbreaks. Additional measures are needed to reduce produce contamination. 相似文献967.
Feronato Sofia Galvão Razzolini Emanuel Morey Germán Augusto Murrieta Boeger Walter A. 《Systematic parasitology》2022,99(3):341-346
Systematic Parasitology - Megalodoras uranoscopus (Eigenmann & Eigenmann) (Siluriformes, Doradidae) (the giant-talking catfish or the giant-raphael catfish), from the... 相似文献
968.
The ribonucleoprotein Male Specific Lethal (MSL) complex is required for X chromosome dosage compensation in Drosophila melanogaster males. Beginning at 3 h of development the MSL complex binds transcribed X-linked genes and modifies chromatin. A subset of MSL complex proteins, including MSL1 and MSL3, is also necessary for full expression of autosomal heterochromatic genes in males, but not females. Loss of the non-coding roX RNAs, essential components of the MSL complex, lowers the expression of heterochromatic genes and suppresses position effect variegation (PEV) only in males, revealing a sex-limited disruption of heterochromatin. To explore the molecular basis of this observation we examined additional proteins that participate in compensation and found that MLE, but not Jil-1 kinase, contributes to heterochromatic gene expression. To determine if identical regions of roX RNA are required for dosage compensation and heterochromatic silencing, we tested a panel of roX1 transgenes and deletions and find that the X chromosome and heterochromatin functions are separable by some mutations. Chromatin immunoprecipitation of staged embryos revealed widespread autosomal binding of MSL3 before and after localization of the MSL complex to the X chromosome at 3 h AEL. Autosomal MSL3 binding was dependent on MSL1, supporting the idea that a subset of MSL proteins associates with chromatin throughout the genome during early development. The broad localization of these proteins early in embryogenesis supports the idea of direct action at autosomal sites. We postulate that this may contribute to the sex-specific differences in heterochromatin that we, and others, have noted. 相似文献
969.
Mitochondrial protein-coding genes (mt genes) encode subunits forming complexes of crucial cellular pathways, including those involved in the vital process of oxidative phosphorylation (OXPHOS). Despite the vital role of the mitochondrial genome (mt genome) in the survival of organisms, little is known with respect to its adaptive implications within marine invertebrates. The molluscan Class Cephalopoda is represented by a marine group of species known to occupy contrasting environments ranging from the intertidal to the deep sea, having distinct metabolic requirements, varied body shapes and highly advanced visual and nervous systems that make them highly competitive and successful worldwide predators. Thus, cephalopods are valuable models for testing natural selection acting on their mitochondrial subunits (mt subunits). Here, we used concatenated mt genes from 17 fully sequenced mt genomes of diverse cephalopod species to generate a robust mitochondrial phylogeny for the Class Cephalopoda. We followed an integrative approach considering several branches of interest–covering cephalopods with distinct morphologies, metabolic rates and habitats–to identify sites under positive selection and localize them in the respective protein alignment and/or tridimensional structure of the mt subunits. Our results revealed significant adaptive variation in several mt subunits involved in the energy production pathway of cephalopods: ND5 and ND6 from Complex I, CYTB from Complex III, COX2 and COX3 from Complex IV, and in ATP8 from Complex V. Furthermore, we identified relevant sites involved in protein-interactions, lining proton translocation channels, as well as disease/deficiencies related sites in the aforementioned complexes. A particular case, revealed by this study, is the involvement of some positively selected sites, found in Octopoda lineage in lining proton translocation channels (site 74 from ND5) and in interactions between subunits (site 507 from ND5) of Complex I. 相似文献
970.
Elisabetta Poluzzi Emanuel Raschi Brian Godman Ariola Koci Ugo Moretti Marija Kalaba Bjorn Wettermark Miriam Sturkenboom Fabrizio De Ponti 《PloS one》2015,10(3)