全文获取类型
收费全文 | 4696篇 |
免费 | 483篇 |
国内免费 | 2篇 |
专业分类
5181篇 |
出版年
2022年 | 30篇 |
2021年 | 65篇 |
2020年 | 43篇 |
2019年 | 51篇 |
2018年 | 39篇 |
2017年 | 38篇 |
2016年 | 91篇 |
2015年 | 162篇 |
2014年 | 165篇 |
2013年 | 207篇 |
2012年 | 307篇 |
2011年 | 280篇 |
2010年 | 212篇 |
2009年 | 163篇 |
2008年 | 252篇 |
2007年 | 299篇 |
2006年 | 253篇 |
2005年 | 245篇 |
2004年 | 213篇 |
2003年 | 196篇 |
2002年 | 193篇 |
2001年 | 64篇 |
2000年 | 54篇 |
1999年 | 63篇 |
1998年 | 55篇 |
1997年 | 45篇 |
1996年 | 48篇 |
1995年 | 36篇 |
1994年 | 57篇 |
1993年 | 41篇 |
1992年 | 52篇 |
1991年 | 53篇 |
1990年 | 50篇 |
1989年 | 38篇 |
1988年 | 36篇 |
1986年 | 50篇 |
1985年 | 48篇 |
1984年 | 54篇 |
1983年 | 43篇 |
1982年 | 53篇 |
1981年 | 50篇 |
1980年 | 47篇 |
1979年 | 44篇 |
1978年 | 40篇 |
1977年 | 37篇 |
1976年 | 31篇 |
1975年 | 40篇 |
1974年 | 40篇 |
1973年 | 35篇 |
1971年 | 28篇 |
排序方式: 共有5181条查询结果,搜索用时 9 毫秒
81.
Cameron EL 《Chemical senses》2007,32(8):775-782
Although considerable anecdotal evidence suggests that pregnancy affects olfactory sensitivity, scientific evidence is limited and inconclusive. Whereas hedonic ratings are affected by pregnancy, odor identification is not. The aim of the current study was to examine odor perception in women across pregnancy and in the postpartum period. One hundred nonsmoking women who were pregnant, postpartum, or had never been pregnant were tested on the University of Pennsylvania Smell Identification Test (UPSIT). Intensity ratings and scratch patterns were collected as potential indicators of sensitivity, and participants rated the odors' pleasantness. Participants also rated their own sense of smell. Mean UPSIT scores did not differ significantly across groups indicating no difference in odor identification. Trends in planned comparisons suggested that in the first trimester, odors were rated as more intense and less pleasant. In the first trimester, women scratched the odor strips significantly fewer times. Consistent with previous reports, 90% of pregnant women reported that specific odors smelled less pleasant and 60% reported that some odors smelled more pleasant. Although nearly two-thirds of pregnant women rated their olfactory sensitivity to be enhanced during pregnancy and overall pregnant women's self-rated olfactory sensitivity was higher than controls', self-ratings were not correlated with UPSIT scores nor odor intensity ratings. These results suggest that these and previous findings may reflect the fact that the effect of pregnancy on olfaction is small and inconsistent. 相似文献
82.
Saurer L McCullough KC Summerfield A 《Journal of immunology (Baltimore, Md. : 1950)》2007,179(6):3504-3514
Efficient induction of mucosal immunity usually employs nasal or oral vaccination while parenteral immunization generally is ineffective at generating mucosal immune responses. This relates to the unique ability of resident mucosal dendritic cells (DC) to induce IgA switching and to imprint mucosa-specific homing receptors on lymphocytes. Based on the well-established plasticity of the DC system, this study sought to investigate whether peripheral DC could be modulated toward "mucosa-type" DC by treatment with immunomodulatory, and therefore potentially adjuvant-like, factors. In this study, we show that monocyte-derived DCs pretreated with the vitamin A derivative all-trans retinoic acid (RA) indeed acquired several attributes characteristic of mucosal DC: secretion of TGF-beta and IL-6 and the capacity to augment mucosal homing receptor expression and IgA responses in cocultured lymphocytes. Addition of a TGF-beta-neutralizing Ab to cocultures significantly inhibited alpha4beta7 integrin, but not CCR9 mRNA expression by the lymphocytes. Both alpha4beta7 integrin and CCR9 mRNA expression, but not IgA production, were suppressed in the presence of a RA receptor antagonist. None of the observed effects on the lymphocytes were influenced by citral, a retinal dehydrogenase inhibitor, arguing against a role for de novo-synthesized RA. Collectively, our findings identified a novel role for RA as a mucosal immune modulator targeting DC. Our results further demonstrate that DC can act as efficient carriers of RA at least in vitro. Consequently, RA targeting of DC shows potential for promoting vaccine-induced mucosal immune responses via a parenteral route of immunization. 相似文献
83.
84.
G R Newton P J Hansen F W Bazer M V Leslie D C Stephenson B G Low 《Biology of reproduction》1989,40(2):417-424
Allantoic and amniotic fluids were collected on Days 60 (n = 3), 100 (n = 4), and 140 (n = 3) of pregnancy. The presence of uterine milk proteins (UTM-proteins) in these samples was evaluated by Ouchterlony immunodiffusion and enzyme-linked immunoabsorbant assay (ELISA). Eight of ten samples of allantoic fluid and three of ten samples of amniotic fluid produced one or two immunoprecipitin bands against antiserum to UTM-proteins. Each band fused with immunoprecipitin bands from UTM-proteins purified from uterine fluid. Data from a semi-quantitative ELISA indicated that allantoic fluid from all ewes and amniotic fluid from six of ten ewes contained immunoreactive UTM-proteins. Concentrations of UTM-proteins in these fluids were not statistically affected by day of gestation (p greater than 0.10), but tended to decline as gestation advanced. Greater concentrations of UTM-proteins were detected in allantoic fluid than in amniotic fluid (p less than 0.05). The physical characteristics of the immunoreactive material in allantoic and amniotic fluids were examined by polyacrylamide gel electrophoresis and Western blotting. The immunoreactive material was found to possess pIs and molecular weights identical to UTM-proteins. These results indicate that fetal fluids contain material that reacts with antiserum to UTM-proteins and has physical properties similar to UTM-proteins. It is likely, therefore, that the UTM-proteins are transported across the placenta during gestation, perhaps to serve some function in the fetal compartment. 相似文献
85.
The structure of bovine F1-ATPase inhibited with ADP and beryllium fluoride at 2.0 angstroms resolution contains two ADP.BeF3- complexes mimicking ATP, bound in the catalytic sites of the beta(TP) and beta(DP) subunits. Except for a 1 angstrom shift in the guanidinium of alphaArg373, the conformations of catalytic side chains are very similar in both sites. However, the ordered water molecule that carries out nucleophilic attack on the gamma-phosphate of ATP during hydrolysis is 2.6 angstroms from the beryllium in the beta(DP) subunit and 3.8 angstroms away in the beta(TP) subunit, strongly indicating that the beta(DP) subunit is the catalytically active conformation. In the structure of F1-ATPase with five bound ADP molecules (three in alpha-subunits, one each in the beta(TP) and beta(DP) subunits), which has also been determined, the conformation of alphaArg373 suggests that it senses the presence (or absence) of the gamma-phosphate of ATP. Two catalytic schemes are discussed concerning the various structures of bovine F1-ATPase. 相似文献
86.
Cold-Induced Freezing Tolerance in Arabidopsis 总被引:34,自引:0,他引:34
Changes in the physiology of plant leaves are correlated with enhanced freezing tolerance and include accumulation of compatible solutes, changes in membrane composition and behavior, and altered gene expression. Some of these changes are required for enhanced freezing tolerance, whereas others are merely consequences of low temperature. In this study we demonstrated that a combination of cold and light is required for enhanced freezing tolerance in Arabidopsis leaves, and this combination is associated with the accumulation of soluble sugars and proline. Sugar accumulation was evident within 2 h after a shift to low temperature, which preceded measured changes in freezing tolerance. In contrast, significant freezing tolerance was attained before the accumulation of proline or major changes in the percentage of dry weight were detected. Many mRNAs also rapidly accumulated in response to low temperature. All of the cold-induced mRNAs that we examined accumulated at low temperature even in the absence of light, when there was no enhancement of freezing tolerance. Thus, the accumulation of these mRNAs is insufficient for cold-induced freezing tolerance. 相似文献
87.
Influence of White Clover Mosaic Potexvirus Infection on the Endogenous Cytokinin Content of Bean 总被引:1,自引:0,他引:1 下载免费PDF全文
Sean Francis Clarke Marian Jane McKenzie David John Burritt Paul Leslie Guy Paula Elizabeth Jameson 《Plant physiology》1999,120(2):547-552
The cytokinin content in the primary leaves of bean (Phaseolus vulgaris) was monitored for 10 d after inoculation with white clover mosaic potexvirus. The cytokinins were isolated, purified, separated by high-performance liquid chromatography, and quantified by radioimmunoassay. The cytokinins detected at the time of inoculation (d 0) were: (a) the free bases, zeatin (Z), dihydrozeatin (DZ), and isopentenyladenine; (b) the riboside, DZ riboside (DZR); (c) the O-glucosides of DZ, DZR, and Z riboside; (d) the nucleotides, Z riboside-5′-monophosphate and isopentenyladenosine-5′-monophosphate; and (e) trace amounts of Z-9-glucoside and DZ-9-glucoside. During the 10 d after inoculation with white clover mosaic potexvirus, marked quantitative changes in this cytokinin profile were observed. The concentration of the free bases and DZR decreased, accompanied by an increase in the 9-glucosides and the nucleotides. Virus titer increased rapidly 3 d after inoculation, attaining a maximum level at d 5. This increase coincided with the increases in the 9-glucosides and the nucleotides. We propose that the decline in the cytokinin free bases and riboside may allow the increase of virus titer in bean and lead to the senescence of infected leaves. 相似文献
88.
Leslie Milk Robert Daber Mitchell Lewis 《Protein science : a publication of the Protein Society》2010,19(6):1162-1172
The Lac repressor has been used as a tool to understand protein–DNA recognition for many years. Recent experiments have demonstrated the ability of the Lac repressor to control gene expression in various eukaryotic systems, making the quest for an arsenal of protein–DNA binding partners desirable for potential therapeutic applications. Here, we present the results of the most exhaustive screen of Lac repressor‐DNA binding partners to date, resulting in the elucidation of functional rules for Lac–DNA binding. Even within the confines of a single protein–DNA scaffold, modes of binding of different protein–DNA partners are sufficiently diverse so as to prevent elucidation of generalized rules for recognition for a single protein, much less an entire protein family. 相似文献
89.
Anushi E Rajapaksa Jenny J Ho Aisha Qi Rob Bischof Tri-Hung Nguyen Michelle Tate David Piedrafita Michelle P McIntosh Leslie Y Yeo Els Meeusen Ross L Coppel James R Friend 《Respiratory research》2014,15(1):60
Background
Pulmonary-delivered gene therapy promises to mitigate vaccine safety issues and reduce the need for needles and skilled personnel to use them. While plasmid DNA (pDNA) offers a rapid route to vaccine production without side effects or reliance on cold chain storage, its delivery to the lung has proved challenging. Conventional methods, including jet and ultrasonic nebulizers, fail to deliver large biomolecules like pDNA intact due to the shear and cavitational stresses present during nebulization.Methods
In vitro structural analysis followed by in vivo protein expression studies served in assessing the integrity of the pDNA subjected to surface acoustic wave (SAW) nebulisation. In vivo immunization trials were then carried out in rats using SAW nebulized pDNA (influenza A, human hemagglutinin H1N1) condensate delivered via intratracheal instillation. Finally, in vivo pulmonary vaccinations using pDNA for influenza was nebulized and delivered via a respirator to sheep.Results
The SAW nebulizer was effective at generating pDNA aerosols with sizes optimal for deep lung delivery. Successful gene expression was observed in mouse lung epithelial cells, when SAW-nebulized pDNA was delivered to male Swiss mice via intratracheal instillation. Effective systemic and mucosal antibody responses were found in rats via post-nebulized, condensed fluid instillation. Significantly, we demonstrated the suitability of the SAW nebulizer to administer unprotected pDNA encoding an influenza A virus surface glycoprotein to respirated sheep via aerosolized inhalation.Conclusion
Given the difficulty of inducing functional antibody responses for DNA vaccination in large animals, we report here the first instance of successful aerosolized inhalation delivery of a pDNA vaccine in a large animal model relevant to human lung development, structure, physiology, and disease, using a novel, low-power (<1 W) surface acoustic wave (SAW) hand-held nebulizer to produce droplets of pDNA with a size range suitable for delivery to the lower respiratory airways. 相似文献90.
John S. Brimacombe Farouk Hunedy Agnes M. Mather Leslie C.N. Tucker 《Carbohydrate research》1979,68(2):231-238
Reduction of 1,6-anhydro-3,4-dideoxy-β-D-glycero-hex-3-enopyranos-2-ulose (levoglucosenone) with lithium aluminium hydride afforded principally 1,6-anhydro-3,4-dideoxy-β-D-threo-hex-3-enopyranose (3), which was converted into 3,4-dihydro-2(S)-hydroxymethyl-2H-pyran (8) following acid-catalysed methanolysis and reductive rearrangement of the resulting α-glycoside 4 with lithium aluminium hydride. 1,6-Anhydro-3,4-dideoxy-2-O-toluene-p-sulphonyl-β-D-threo-hexopyranose, prepared from 3, reacted slowly with sodium azide in hot dimethyl sulphoxide to give 1,6-anhydro-2-azido-2,3,4-trideoxy-β-D-erythro-hexopyranose, which was transformed into a mixture of methyl 2-acetamido-6-O-acetyl-2,3,4-trideoxy-α-D-erythro-hexopyranoside (10) and the corresponding β anomer following acid-catalysed methanolysis, catalytic reduction, and acetylation. Acid treatment of methyl 4,6-O-benzylidene-3-deoxy-α-D-erythro-hexopyranosid-2-ulose yielded the enone 15, which was readily transformed into methyl 6-O-acetyl-3,4-dideoxy-α-D-glycero-hexopyranosid-2-ulose (19). Procedures for the conversions of DL-8, 10, and 19 into methyl 2,6-diacetamido-2,3,4,6-tetradeoxy-α-D-erythro-hexopyranoside (methyl N,N′-di-acetyl-α-purpurosaminide C) have already been described. 相似文献