全文获取类型
收费全文 | 510篇 |
免费 | 34篇 |
出版年
2023年 | 1篇 |
2022年 | 6篇 |
2021年 | 14篇 |
2020年 | 6篇 |
2019年 | 2篇 |
2018年 | 11篇 |
2017年 | 9篇 |
2016年 | 17篇 |
2015年 | 27篇 |
2014年 | 30篇 |
2013年 | 51篇 |
2012年 | 58篇 |
2011年 | 46篇 |
2010年 | 26篇 |
2009年 | 37篇 |
2008年 | 26篇 |
2007年 | 16篇 |
2006年 | 28篇 |
2005年 | 19篇 |
2004年 | 21篇 |
2003年 | 20篇 |
2002年 | 16篇 |
2001年 | 3篇 |
2000年 | 2篇 |
1999年 | 2篇 |
1998年 | 7篇 |
1997年 | 6篇 |
1996年 | 3篇 |
1995年 | 3篇 |
1994年 | 2篇 |
1993年 | 4篇 |
1992年 | 1篇 |
1989年 | 3篇 |
1988年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1983年 | 1篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1975年 | 1篇 |
1971年 | 1篇 |
1963年 | 1篇 |
1961年 | 1篇 |
1960年 | 1篇 |
1957年 | 1篇 |
1951年 | 1篇 |
排序方式: 共有544条查询结果,搜索用时 15 毫秒
1.
Rice fields are an alternative habitat for waterbirds and provide food and shelter for many avian species, but there is a lack of information about how the use of rice fields translates into population level effects. The aim of this study was to test the relationship between the use of rice fields by European waterbirds and trends in their populations. We tested this relationship during the autumn migration season and during the breeding season. Based on counts conducted over the last 23 years in natural marshes and areas of rice fields in Doñana (SW Spain), an index of rice field use was constructed for 76 bird species, which was then compared to these species’ European population trends obtained from the literature. A positive relationship was found between waterbird population trends and the use of rice fields during autumn migration season. Our study suggests that changes in the Common Agriculture Policy in Europe leading to reductions in areas of rice cultivation may have important effects on waterbirds. The restoration of former marsh areas and the maintenance of rice cultivation would seem to be more environmentally friendly approaches than the use of these areas to grow alternative crops or solar farms. 相似文献
2.
Rachel J. Primrose Toral Zaveri Alyssa J. Bakke Gregory R. Ziegler Howard R. Moskowitz John E. Hayes 《PloS one》2016,11(3)
Vaginal microbicides potentially empower women to protect themselves from HIV and other sexually transmitted infections (STIs), especially when culture, religion, or social status may prevent them from negotiating condom use. The open literature contains minimal information on factors that drive user acceptability of women’s health products or vaginal drug delivery systems. By understanding what women find to be most important with regard to sensory properties and product functionality, developers can iteratively formulate a more desirable product. Conjoint analysis is a technique widely used in market research to determine what combination of elements influence a consumer’s willingness to try or use a product. We applied conjoint analysis here to better understand what sexually-active woman want in a microbicide, toward our goal of formulating a product that is highly acceptable to women. Both sensory and non-sensory attributes were tested, including shape, color, wait time, partner awareness, messiness/leakage, duration of protection, and functionality. Heterosexually active women between 18 and 35 years of age in the United States (n = 302) completed an anonymous online conjoint survey using IdeaMap software. Attributes (product elements) were systematically presented in various combinations; women rated these combinations of a 9-point willingness-to-try scale. By coupling systematic combinations and regression modeling, we can estimate the unique appeal of each element. In this population, a multifunctional product (i.e., broad spectrum STI protection, coupled with conception) is far more desirable than a microbicide targeted solely for HIV protection; we also found partner awareness and leakage are potentially strong barriers to use. 相似文献
3.
Elisabet Johansson Lars Hedbys Per-Olof Larsson Klaus Mosbach Alf Gunnarsson Sigfrid Svensson 《Biotechnology letters》1986,8(6):421-424
Summary Three naturally occurring isomers of the disaccharideO--d-mannosyl-d-mannoside were synthesized by reversing the hydrolytic activity of jack bean -mannosidase at 75°C in a very high concentration of mannose. Higher oligosaccharides were also obtained at the later stages of the reaction. The maximum total yield of disaccharides was 37% (w/w) based on the total amount of saccharides. 相似文献
4.
Empis borealisfemales form swarms, and males carrying a nuptial gift come to swarms to mate. Males either mated with one of the females (accepted swarms) or left swarms without mating (refused swarms). Males mated with the younger (low wing-wear) and relatively larger females in accepted swarms. They seemed to be able to judge the relative size of the females but to ignore their absolute size. Visiting males stayed shorter in accepted swarms as female size variation increased. This probably reflects their greater ease in choosing a mate among females of relatively different sizes. Females in accepted swarms tended to be larger and to have less worn wings than females in rejected swarms. 相似文献
5.
Mario H.J. Vogt Elisabet H. Stet Ronney A. De Abreu Jos P.M. Bökkerink Lambert H.J. Lambooy Frans J.M. Trijbels 《生物化学与生物物理学报:疾病的分子基础》1993,1181(2):189-194
The importance of methyl-thioIMP (Me-tIMP) formation for methylmercaptopurine ribonucleoside (Me-MPR) cytotoxicity was studied in Molt F4 cells. Cytotoxicity of Me-MPR is caused by Me-tIMP formation with concomitant inhibition of purine de novo synthesis. Inhibition of purine de novo synthesis resulted in decreased purine nucleotide levels and enhanced 5-phosphoribosyl-1-pyrophosphate (PRPP) levels, with concurrent increased pyrimidine nucleotide levels. The Me-tIMP concentration increased proportionally with the concentration of Me-MPR. High Me-tIMP concentration also caused inhibition of PRPP synthesis. Maximal accumulation of PRPP thus occurred at low Me-MPR concentrations. As little as 0.2 μM Me-MPR resulted already after 2 h in maximal inhibition of formation of adenine and guanine nucleotides, caused by inhibition of purine de novo synthesis by Me-tIMP. Under these circumstances increased intracellular PRPP concentrations could be demonstrated, resulting in increased levels of pyrimidine nucleotides. So, in Molt F4 cells, formation of Me-tIMP form Me-MPR results in cytotoxicity by inhibition of purine de novo synthesis. 相似文献
6.
Abstract: Different neurotransmitter receptor agonists [carbachol, serotonin, noradrenaline, histamine, endothelin-1, and trans -(1 S ,3 R )-aminocyclopentyl-1,3-dicarboxylic acid ( trans -ACPD)], known as stimuli of phospholipase C in brain tissue, were tested for phospholipase D stimulation in [32 P]Pi -prelabeled rat brain cortical and hippocampal slices. The accumulation of [32 P]phosphatidylethanol was measured as an index of phospholipase D-catalyzed transphosphatidylation in the presence of ethanol. Among the six neurotransmitter receptor agonists tested, only noradrenaline, histamine, endothelin-1, and trans -ACPD stimulated phospholipase D in hippocampus and cortex, an effect that was strictly dependent of the presence of millimolar extracellular calcium concentrations. The effect of histamine (EC50 18 µ M ) was inhibited by the H1 receptor antagonist mepyramine with a K i constant of 0.7 n M and was resistant to H2 and H3 receptor antagonists (ranitidine and tioperamide, respectively). Endothelin-1-stimulated phospholipase D (EC50 44 n M ) was not blocked by BQ-123, a specific antagonist of the ETA receptor. Endothelin-3 and the specific ETB receptor agonist safarotoxin 6c were also able to stimulate phospholipase D with efficacies similar to that of endothelin-1, and EC50 values of 16 and 3 n M , respectively. These results show that histamine and endothelin-1 stimulate phospholipase D in rat brain through H1 and ETB receptors, respectively. 相似文献
7.
Elisabet Font Mercedes Sitges Fausto G. Hegardt 《Biochemical and biophysical research communications》1982,105(2):705-710
Rat liver homogeneous 32P-labeled hydroxy methylglutaryl coenzyme A reductase, was treated independently with CNBr and trypsin and the resulting [32P]phosphopeptides were analyzed by disc gel electrophoresis. CNBr treatment produced only one 32P-fragment of Mr 18,000. The time course of trypsin hydrolysis initially showed the appearance of some phosphopeptides, which were lately converted in two phosphopeptides of low Mr. These results provide direct support for the concept that hydroxy methyl glutaryl coenzyme A reductase kinase solubilized from microsomes phosphorylates only two sites or set of sites in the reductase molecule. 相似文献
8.
Previous autoradiographic studies aimed at showing neurones using GABA as their neurotransmitter have been hampered by the fact that the substance is a ubiquitous metabolite and therefore accumulated by a large variety of cells, including glia. Consequently, GABA uptake markers without this widespread uptake are desirable, and one, [3H]isoguvacine, has shown promising results in autoradiographic experiments. Its uptake has now been further studied with quantitative radiochemical techniques.
The uptake of the drug was slow compared to GABA uptake and reached a tissue/medium ratio of about 3 after 120 min. The uptake could be inhibited by GABA, beta-alanine or ouabain, and by incubating the retinas at 0°C. The uptake kinetics were complex but suggested a high affinity uptake system (Km about 10−8 M) and perhaps one or several systems with lower affinities.
The results indicate that [3H]isoguvacine and [3H]GABA are accumulated and retained by the same neurones, which most likely use GABA as their neurotransmitter. 相似文献
9.
In general, the rate of nitrogen fixation decreased when the lichen Peltigera canina (L.) Willd. was exposed to sulfur dioxide gas at levels from 0.1 to 500 ppm; at 30 ppm, however, nitrogen fixation was stimulated. Chlorophyll content decreased as level of sulfur dioxide increased. 相似文献
10.