全文获取类型
收费全文 | 3000篇 |
免费 | 326篇 |
国内免费 | 2篇 |
专业分类
3328篇 |
出版年
2021年 | 29篇 |
2019年 | 32篇 |
2018年 | 30篇 |
2017年 | 29篇 |
2016年 | 66篇 |
2015年 | 94篇 |
2014年 | 97篇 |
2013年 | 120篇 |
2012年 | 159篇 |
2011年 | 139篇 |
2010年 | 88篇 |
2009年 | 92篇 |
2008年 | 126篇 |
2007年 | 135篇 |
2006年 | 129篇 |
2005年 | 105篇 |
2004年 | 110篇 |
2003年 | 108篇 |
2002年 | 117篇 |
2001年 | 94篇 |
2000年 | 85篇 |
1999年 | 78篇 |
1998年 | 48篇 |
1997年 | 48篇 |
1996年 | 33篇 |
1995年 | 20篇 |
1994年 | 33篇 |
1993年 | 26篇 |
1992年 | 57篇 |
1991年 | 36篇 |
1990年 | 60篇 |
1989年 | 46篇 |
1988年 | 41篇 |
1987年 | 48篇 |
1986年 | 54篇 |
1985年 | 44篇 |
1984年 | 57篇 |
1983年 | 35篇 |
1982年 | 40篇 |
1981年 | 23篇 |
1980年 | 23篇 |
1979年 | 37篇 |
1978年 | 35篇 |
1977年 | 46篇 |
1976年 | 28篇 |
1975年 | 28篇 |
1974年 | 38篇 |
1973年 | 19篇 |
1972年 | 35篇 |
1970年 | 16篇 |
排序方式: 共有3328条查询结果,搜索用时 15 毫秒
1.
Sex ratios under asymmetrical local mate competition in the parasitoid wasp Nasonia vitripennis 总被引:1,自引:0,他引:1
Sex ratio theory has proved remarkably useful in testing theadaptive nature of animal behavior. A particularly productivearea in this respect is Hamilton's theory of local mate competition(LMC), which has been extended in numerous directions to includegreater biological realism, allowing more detailed tests inspecific organisms. We have presented one such extension, termedasymmetrical LMC, which occurs when egg laying by females ona patch is asynchronous, and emerging males do not disperse,resulting in the extent of LMC on a patch varying over time.Our aim here is to test whether the parasitoid wasp Nasoniavitripennis responds to variation in the degree of asymmetricalLMC. Specifically, we show that females adjust their offspringsex ratios in response to (1) variation in the amount of asynchronyin emergence between broods on a patch and (2) the number andproportion of previously parasitized hosts on the patch. Ourresults provide qualitative support for the predictions of theory,suggesting new levels of complexity in the sex ratio behaviorof this much-studied organism. However, our results do not alwaysprovide quantitative support for theory, suggesting furthercomplexities that must be clarified. 相似文献
2.
Jianxi Xiao Rayna M. Addabbo Janelle L. Lauer Gregg B. Fields Jean Baum 《The Journal of biological chemistry》2010,285(44):34181-34190
The mechanism by which enzymes recognize the “uniform” collagen triple helix is not well understood. Matrix metalloproteinases (MMPs) cleave collagen after the Gly residue of the triplet sequence Gly∼[Ile/Leu]-[Ala/Leu] at a single, unique, position along the peptide chain. Sequence analysis of types I-III collagen has revealed a 5-triplet sequence pattern in which the natural cleavage triplets are always flanked by a specific distribution of imino acids. NMR and MMP kinetic studies of a series of homotrimer peptides that model type III collagen have been performed to correlate conformation and dynamics at, and near, the cleavage site to collagenolytic activity. A peptide that models the natural cleavage site is significantly more active than a peptide that models a potential but non-cleavable site just 2-triplets away and NMR studies show clearly that the Ile in the leading chain of the cleavage peptide is more exposed to solvent and less locally stable than the Ile in the middle and lagging chains. We propose that the unique local instability of Ile at the cleavage site in part arises from the placement of the conserved Pro at the P3 subsite. NMR studies of peptides with Pro substitutions indicate that the local dynamics of the three chains are directly modulated by their proximity to Pro. Correlation of peptide activity to NMR data shows that a single locally unstable chain at the cleavage site, rather than two or three labile chains, is more favorable for cleavage by MMP-1 and may be the determining factor for collagen recognition. 相似文献
3.
4.
5.
Noni (Morinda citrifolia) is increasing in worldwide popularity as a food or dietary supplement with versatile health benefits. The aim of this study was to investigate the effects of Noni fruit on anxiety symptoms in vitro. To this end, a competitive GABAa receptor-binding assay was developed. Our preliminary study indicates that the methanol crude extract of Noni fruit showed significant affinity to the gamma-aminobutyric acid A (GABAa) inhibitory neurotransmitter receptors, and displayed 75% binding inhibition of the agonist radioligand [3H] muscimol at a concentration of 100 microg/ml. Further experiments demonstrated that the MeOH extract, and its BuOH and H2O partitions, exhibited IC50 values of 22.8, 27.2, and 17.1 microg/ml, respectively, in the GABAa-binding assay. Experimental results with Noni fruit indicate the presence of competitive ligand(s), which may bind to the GABAa receptor as an agonist, and thus induce its anxiolytic and sedative effects. The study provides an in vitro rationale for one of Noni's versatile and traditional uses. In addition, an HPLC fingerprint profile of the methanolic extract of Noni fruit has been established for quality control purpose. 相似文献
6.
Mutations in the conserved C-terminal sequence in thyroid hormone receptor dissociate hormone-dependent activation from interference with AP-1 activity. 总被引:4,自引:2,他引:2 下载免费PDF全文
7.
Alveolar function following surfactant deactivation 总被引:4,自引:0,他引:4
Nieman G. F.; Bredenberg C. E.; Clark W. R.; West N. R. 《Journal of applied physiology》1981,51(4):895-904
8.
9.
Tritiated calmodulin (T-CM) was bound to the EGTA-treated particulate fraction of cardiac muscle in a calcium-dependent manner with half-maximal binding occurring between 0.8 to 1.2 microM calcium. Binding exhibited high specificity at an optimum pH of 7.4-7.6. An excess of parvalbumin and other globular proteins did not displace T-CM. The Kd for the interaction was 2.5 +/- 0.83 microM. Binding was trypsin-sensitive, inhibited by high ionic strength and was heat inactivated at a midpoint of 48 - 50 degrees C. Competitive displacement of T-CM occurred with unlabeled troponin C and calmodulin over the same concentration range. The first-order rate constant of T-CM dissociation was 3.27 min-1. Calcium-dependent binding of T-CM was inhibited equally by both mepacrine and trifluoperazine with 50 percent inhibition occurring at 70 microM. 相似文献
10.
A comparative study of methods to enumerate sulphite-reducing Clostridium spores and Group D faecal streptococci in oysters demonstrated that pour plate solid agar techniques gave higher counts than liquid broth most probable number procedures. Reinforced clostridial broth with supplements to detect sulphite reduction was compared with pour plates of egg yolk-free tryptose sulphite cycloserine agar incubated at 37 degrees C for 24 h. Azide dextrose broth was compared with pour plates using Slanetz and Bartley (SB) agar or KF-streptococcus agar at 37 degrees C. Most probable number procedures used for both groups of organisms gave excessive numbers of improbable tube combinations. For enumeration of Group D faecal streptococci, a pour plate technique using SB agar incubated at 37 degrees C for 48 h is recommended. 相似文献