全文获取类型
收费全文 | 507篇 |
免费 | 52篇 |
国内免费 | 1篇 |
出版年
2022年 | 3篇 |
2021年 | 13篇 |
2020年 | 8篇 |
2019年 | 10篇 |
2018年 | 10篇 |
2017年 | 8篇 |
2016年 | 14篇 |
2015年 | 26篇 |
2014年 | 15篇 |
2013年 | 20篇 |
2012年 | 27篇 |
2011年 | 34篇 |
2010年 | 17篇 |
2009年 | 20篇 |
2008年 | 26篇 |
2007年 | 23篇 |
2006年 | 19篇 |
2005年 | 16篇 |
2004年 | 23篇 |
2003年 | 23篇 |
2002年 | 9篇 |
2001年 | 20篇 |
2000年 | 13篇 |
1999年 | 11篇 |
1998年 | 9篇 |
1997年 | 5篇 |
1996年 | 4篇 |
1995年 | 4篇 |
1994年 | 11篇 |
1993年 | 7篇 |
1992年 | 13篇 |
1991年 | 10篇 |
1990年 | 5篇 |
1989年 | 9篇 |
1988年 | 5篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1984年 | 3篇 |
1981年 | 4篇 |
1979年 | 5篇 |
1976年 | 2篇 |
1975年 | 5篇 |
1974年 | 2篇 |
1973年 | 6篇 |
1972年 | 4篇 |
1971年 | 2篇 |
1970年 | 4篇 |
1969年 | 4篇 |
1968年 | 2篇 |
1967年 | 6篇 |
排序方式: 共有560条查询结果,搜索用时 921 毫秒
91.
Many physiological processes are controlled by a great diversity of Ca2+ signals that depend on Ca2+ entry into the cell and/or Ca2+ release from internal Ca2+ stores. Ca2+ mobilization from intracellular stores is gated by a family of messengers including inositol-1,4,5-trisphosphate (InsP3), cyclic ADP-ribose (cADPR), and nicotinic acid adenine dinucleotide phosphate (NAADP). There is increasing evidence for a novel intracellular Ca2+ release channel that may be targeted by NAADP and that displays properties distinctly different from the well-characterized InsP3 and ryanodine receptors. These channels appear to localize on a wider range of intracellular organelles, including the acidic Ca2+ stores. Activation of the NAADP-sensitive Ca2+ channels evokes complex changes in cytoplasmic Ca2+ levels by means of channel chatter with other intracellular Ca2+ channels. The recent demonstration of changes in intracellular NAADP levels in response to physiologically relevant extracellular stimuli highlights the significance of NAADP as an important regulator of intracellular Ca2+ signaling. 相似文献
92.
93.
P. R. Frade N. Englebert J. Faria P. M. Visser R. P. M. Bak 《Coral reefs (Online)》2008,27(4):913-925
The role of symbiont variation in the photobiology of reef corals was addressed by investigating the links among symbiont
genetic diversity, function and ecological distribution in a single host species, Madracis pharensis. Symbiont distribution was studied for two depths (10 and 25 m), two different light habitats (exposed and shaded) and three
host colour morphs (brown, purple and green). Two Symbiodinium genotypes were present, as defined by nuclear internal transcribed spacer 2 ribosomal DNA (ITS2-rDNA) variation. Symbiont
distribution was depth- and colour morph-dependent. Type B15 occurred predominantly on the deeper reef and in green and purple
colonies, while type B7 was present in shallow environments and brown colonies. Different light microhabitats at fixed depths
had no effect on symbiont presence. This ecological distribution suggests that symbiont presence is potentially driven by
light spectral niches. A reciprocal depth transplantation experiment indicated steady symbiont populations under environment
change. Functional parameters such as pigment composition, chlorophyll a fluorescence and cell densities were measured for 25 m and included in multivariate analyses. Most functional variation was
explained by two photobiological assemblages that relate to either symbiont identity or light microhabitat, suggesting adaptation
and acclimation, respectively. Type B15 occurs with lower cell densities and larger sizes, higher cellular pigment concentrations
and higher peridinin to chlorophyll a ratio than type B7. Type B7 relates to a larger xanthophyll-pool size. These unambiguous differences between symbionts can
explain their distributional patterns, with type B15 being potentially more adapted to darker or deeper environments than
B7. Symbiont cell size may play a central role in the adaptation of coral holobionts to the deeper reef. The existence of
functional differences between B-types shows that the clade classification does not necessarily correspond to functional identity.
This study supports the use of ITS2 as an ecological and functionally meaningful marker in Symbiodinium. 相似文献
94.
95.
The plant UDP-dependent glucosyltransferase (UGT) BpUGT94B1 catalyzes the synthesis of a glucuronosylated cyanidin-derived flavonoid in red daisy (Bellis perennis). The functional properties of BpUGT94B1 were investigated using protein modeling, site-directed mutagenesis, and analysis of the substrate specificity of isolated wild-type and mutated forms of BpUGT94B1. A single unique arginine residue (R25) positioned outside the conserved plant secondary product glycosyltransferase region was identified as crucial for the activity with UDP-glucuronic acid. The mutants R25S, R25G, and R25K all exhibited only 0.5% to 2.5% of wild-type activity with UDP-glucuronic acid, but showed a 3-fold increase in activity with UDP-glucose. The model of BpUGT94B1 also enabled identification of key residues in the acceptor pocket. The mutations N123A and D152A decreased the activity with cyanidin 3-O-glucoside to less than 15% of wild type. The wild-type enzyme activity toward delphinidin-3-O-glucoside was only 5% to 10% of the activity with cyanidin 3-O-glucoside. Independent point mutations of three residues positioned near the acceptor B ring were introduced to increase the activity toward delphinidin-3-O-glucoside. In all three mutant enzymes, the enzymatic activity toward both acceptors was reduced to less than 15% of wild type. The model of BpUGT94B1 allowed for correct identification of catalytically important residues, within as well as outside the plant secondary product glycosyltransferase motif, determining sugar donor and acceptor specificity. 相似文献
96.
Ja Kyong Ko Jin Seop Bak Min Woo Jung Hee Jin Lee In-Geol Choi Tae Hyun Kim Kyoung Heon Kim 《Bioresource technology》2009,100(19):4374-4380
Rice straw was pretreated using aqueous-ammonia solution at moderate temperatures to enable production of the maximum amount of fermentable sugars from enzymatic hydrolysis. The effects of various operating variables including pretreatment temperature, pretreatment time, the concentration of ammonia and the solid-to-liquid ratio on the degree of lignin removal and the enzymatic digestibility were optimized using response surface methodology. The optimal reaction conditions, which resulted in an enzymatic digestibility of 71.1%, were found to be 69 °C, 10 h and an ammonia concentration of 21% (w/w). The effects of different commercial cellulases and the additional effect of a non-cellulolytic enzyme, xylanase, were also evaluated. Additionally, simultaneous saccharification and fermentation was conducted with rice straw to assess the ethanol production yield and productivity. 相似文献
97.
Michael CW Chan Renee WY Chan Wendy CL Yu Carol CC Ho WH Chui CK Lo Kit M Yuen Yi Guan John M Nicholls JS Malik Peiris 《Respiratory research》2009,10(1):102
Background
Highly pathogenic avian influenza (HPAI) H5N1 virus is entrenched in poultry in Asia and Africa and continues to infect humans zoonotically causing acute respiratory disease syndrome and death. There is evidence that the virus may sometimes spread beyond respiratory tract to cause disseminated infection. The primary target cell for HPAI H5N1 virus in human lung is the alveolar epithelial cell. Alveolar epithelium and its adjacent lung microvascular endothelium form host barriers to the initiation of infection and dissemination of influenza H5N1 infection in humans. These are polarized cells and the polarity of influenza virus entry and egress as well as the secretion of cytokines and chemokines from the virus infected cells are likely to be central to the pathogenesis of human H5N1 disease.Aim
To study influenza A (H5N1) virus replication and host innate immune responses in polarized primary human alveolar epithelial cells and lung microvascular endothelial cells and its relevance to the pathogenesis of human H5N1 disease.Methods
We use an in vitro model of polarized primary human alveolar epithelial cells and lung microvascular endothelial cells grown in transwell culture inserts to compare infection with influenza A subtype H1N1 and H5N1 viruses via the apical or basolateral surfaces.Results
We demonstrate that both influenza H1N1 and H5N1 viruses efficiently infect alveolar epithelial cells from both apical and basolateral surface of the epithelium but release of newly formed virus is mainly from the apical side of the epithelium. In contrast, influenza H5N1 virus, but not H1N1 virus, efficiently infected polarized microvascular endothelial cells from both apical and basolateral aspects. This provides a mechanistic explanation for how H5N1 virus may infect the lung from systemic circulation. Epidemiological evidence has implicated ingestion of virus-contaminated foods as the source of infection in some instances and our data suggests that viremia, secondary to, for example, gastro-intestinal infection, can potentially lead to infection of the lung. HPAI H5N1 virus was a more potent inducer of cytokines (e.g. IP-10, RANTES, IL-6) in comparison to H1N1 virus in alveolar epithelial cells, and these virus-induced chemokines were secreted onto both the apical and basolateral aspects of the polarized alveolar epithelium.Conclusion
The predilection of viruses for different routes of entry and egress from the infected cell is important in understanding the pathogenesis of influenza H5N1 infection and may help unravel the pathogenesis of human H5N1 disease. 相似文献98.
99.
The glutamate/GABA-glutamine cycle: aspects of transport, neurotransmitter homeostasis and ammonia transfer 总被引:1,自引:0,他引:1
Neurons are metabolically handicapped in the sense that they are not able to perform de novo synthesis of neurotransmitter glutamate and gamma-aminobutyric acid (GABA) from glucose. A metabolite shuttle known as the glutamate/GABA-glutamine cycle describes the release of neurotransmitter glutamate or GABA from neurons and subsequent uptake into astrocytes. In return, astrocytes release glutamine to be taken up into neurons for use as neurotransmitter precursor. In this review, the basic properties of the glutamate/GABA-glutamine cycle will be discussed, including aspects of transport and metabolism. Discussions of stoichiometry, the relative role of glutamate vs. GABA and pathological conditions affecting the glutamate/GABA-glutamine cycling are presented. Furthermore, a section is devoted to the accompanying ammonia homeostasis of the glutamate/GABA-glutamine cycle, examining the possible means of intercellular transfer of ammonia produced in neurons (when glutamine is deamidated to glutamate) and utilized in astrocytes (for amidation of glutamate) when the glutamate/GABA-glutamine cycle is operating. A main objective of this review is to endorse the view that the glutamate/GABA-glutamine cycle must be seen as a bi-directional transfer of not only carbon units but also nitrogen units. 相似文献
100.
Tamás Juhász Csaba Matta Zoltán Mészár Georgina Nagy Zsolt Szíjgyártó Zsanett Molnár Bernadett Kolozsvári Éva Bakó Róza Zákány 《Central European Journal of Biology》2010,5(5):572-584
We aimed to find a transfection method which provides high efficiency with minimal cytotoxic and/or apoptotic effects for
gene transfer into multilayer primary chondrogenic cell cultures. The pEGFP-C1 plasmid was introduced into the cell culture
and the efficiency of transformation quantified by GFP fluorescence; the resulting nucleofection was effective but resulted
in severe apoptosis. Two liposomal reagents designed to allow transfection into adherent cells did not deliver the plasmids
sufficiently and cartilage formation did not occur. In addition, a third liposomal compound, recommended for transfection
into either adherent or suspension cell cultures, lead to acceptable transfection efficiency but no cartilage formation. When
an amphiphilic reagent was used however, there was acceptable transfection efficiency as well as cartilage formation. The
viability of the cells which were transfected using the amphiphilic reagent remained unaffected but proliferation was severely
diminished, particularly in the presence of GFP. In addition, the amount of cartilage decreased when GFP was expressed, despite
unchanged levels of mRNAs of sox9 and aggrecan core protein, factors reflecting on the efficiency of chondrogenesis. Overexpression of both the constitutively
active delta and gamma isoforms of catalytic subunit of calcineurin, a protein phosphatase described as a positive regulator
of chondrogenesis, decreased protein level of Sox9 and subsequent cartilage formation. In conclusion, we found that amphiphilic
reagent applied prior to the adhesion of cells provides a useful means to transfer plasmids to primary differentiating chondrogenic
cells. 相似文献