首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   6597篇
  免费   531篇
  2023年   53篇
  2022年   53篇
  2021年   201篇
  2020年   107篇
  2019年   137篇
  2018年   185篇
  2017年   187篇
  2016年   226篇
  2015年   394篇
  2014年   392篇
  2013年   525篇
  2012年   643篇
  2011年   556篇
  2010年   346篇
  2009年   296篇
  2008年   379篇
  2007年   377篇
  2006年   312篇
  2005年   293篇
  2004年   268篇
  2003年   247篇
  2002年   235篇
  2001年   53篇
  2000年   34篇
  1999年   48篇
  1998年   67篇
  1997年   41篇
  1996年   44篇
  1995年   37篇
  1994年   23篇
  1993年   20篇
  1992年   19篇
  1991年   17篇
  1990年   51篇
  1989年   12篇
  1988年   12篇
  1987年   16篇
  1986年   13篇
  1985年   13篇
  1984年   22篇
  1983年   20篇
  1982年   15篇
  1981年   22篇
  1980年   7篇
  1979年   10篇
  1978年   9篇
  1976年   6篇
  1975年   7篇
  1974年   7篇
  1973年   6篇
排序方式: 共有7128条查询结果,搜索用时 15 毫秒
31.
M orais , P.V. & D a C osta , M.S. 1990. Alterations in the major heterotrophic bacterial populations isolated from a. still bottled mineral water. Journal of Applied Bacteriology 69 , 750–757.
The heterotrophic bacterial population of a bottled mineral water stored in returnable glass bottles and in polyvinyl chloride (PVC) bottles at room temperature was studied over 9–12 months. The plate counts in R2A medium incubated at 22 and 37C were low initially, increasing to 104-105 cfu/ml within a few days of bottling. The number of bacteria recovered at 22C from PVC bottles was fairly constant during the storage period, but the population isolated at 37C decreased markedly after storage for 1 year. The major components of the population were Pseudomonas strains, one of which was identified as Pseudomonas vesicularis . Major changes took place during storage; two groups of bacteria (B and C) were dominant initially, but during the latter period of storage other groups (F, G and H) increased in number.  相似文献   
32.
Abstract: Arachidonic acid and oleoylacetylglycerol enhance depolarization-evoked glutamate release from hippocampal mossy fiber nerve endings. It was proposed this is a Ca2+-dependent effect and that protein kinase C is involved. Here we report that arachidonic acid and oleoylacetylglycerol synergistically potentiate the glutamate release induced by the Ca2+ ionophore ionomycin. The Ca2+ dependence of this effect was established, as removal of Ca2+ eliminated evoked release and the lipid-dependent potentiation. Also, Ca2+ channel blockers attenuated ionomycin- and KCI-evoked exocytosis, as well as the facilitating effects of the lipid mediators. Although facilitation required Ca2+, it may not involve an enhancement of evoked Ca2+ accumulation, because ionomycin-dependent glutamate release was potentiated under conditions that did not increase ionomycin-induced Ca2+ accumulation. Also, the facilitation may not depend on inhibition of K+ efflux, because enhanced release was observed in the presence of increasing concentrations of 4-aminopyridine and diazoxide did not reduce the lipid-dependent potentiation of exocytosis. In contrast, disruption of cytoskeleton organization with cytochalasin D occluded the lipid-dependent facilitations of both KCI- and ionomycin-evoked glutamate release. In addition, arachidonic acid plus glutamatergic or cholinergic agonists enhanced glutamate release, whereas a role for protein kinase C in the potentiation of exocytosis was substantiated using kinase inhibitors. It appears that the lipid-dependent facilitation of glutamate release from mossy fiber nerve endings requires Ca2+ and involves multiple presynaptic effects, some of which depend on protein kinase C.  相似文献   
33.
Abstract: The principal constituent of amyloid plaques found in the brains of individuals with Alzheimer's disease (AD) is a 39–42-amino-acid protein, amyloid β protein (Aβ). This study examined whether the measurement of Aβ levels in CSF has diagnostic value. There were 108 subjects enrolled in this prospective study: AD (n = 39), non-AD controls (dementing diseases/syndromes; n = 20), and other (n = 49). CSF was obtained by lumbar puncture, and Aβ concentrations were determined using a dual monoclonal antibody immunoradiometric sandwich assay. The mean Aβ value for the AD group (15.9 ± 6.8 ng/ml) was not significantly different from that for the non-AD control group (13.0 ± 7.1 ng/ml; p = 0.07), and substantial overlap in results were observed. Aβ values did not correlate with age ( r = −0.05, p = 0.59), severity of cognitive impairment ( r = 0.22, p = 0.21), or duration of AD symptoms ( r = 0.14, p = 0.45). These findings are in conflict with other reports in the literature; discrepant results could be due to the instability of Aβ in CSF. Aβ immunoreactivity decays rapidly under certain conditions, particularly multiple freeze/thaw cycles. Use of a stabilizing sample treatment buffer at the time of lumbar puncture allows storage of CSF without loss of Aβ reactivity. In conclusion, the total CSF Aβ level is not a useful marker for current diagnosis of AD.  相似文献   
34.
Machado-Joseph disease (MJD) is an autosomal dominant spinocerebellar degeneration originally described in families of Portuguese-Azorean ancestry. The hypothesis that its present world distribution could result from the spread of an original founder mutation has been raised. To test this possibility we have conducted a linkage disequilibrium study of markers segregating with the MJD1 locus in a total of 64 unrelated families of different geographical origins. Significant association was detected between the MJD1 locus and marker alleles at loci D14S280, D14S1050 and D14S81. All affected individuals, except one Chinese family, had allele 3 (237 bp) at D14S280. This finding is consistent with a founder effect in our MJD population. However, distinct haplotypes were observed in patients originating from the two Azorean islands showing the highest disease prevalence; therefore, the possible existence of more than one founder mutation can not be excluded with the markers currently available. Received: 27 February 1996 / Revised: 4 June 1996  相似文献   
35.
Summary Taxusbrevifolia is the source of paclitaxel (Taxol®), an anticancer drug. A method for regeneration ofTaxus brevifolia from immature zygotic embryos via somatic embryogenesis is described. Embryogenic callus tissues were obtained by culturing immature zygotic embryos on Lloyd and McCown medium (MCM) supplemented with 160 M 2,4-dichlorophenoxyacetic acid (2,4-D) + 5 M benzylaminopurine (BA) + 5 M naphthaleneacetic acid (NAA) for 4 weeks. Putative embryoids were obtained following transfer of cultures to MCM medium supplemented with 4 M BA + 5 M kinetin + 1 M NAA for 6 to 8 weeks. Conversion of embryos was obtained on MCM medium supplemented with 40 M abscisic acid (ABA) + 1% activated charcoal. Development of bipolar structures with recognizable shoot and root apices was observed in somatic embryos. Five percent of somatic embryos were regenerated into plantlets on half-strength growth regulator-free MCM medium.  相似文献   
36.
The cytokinin content in fruit tissue of the kiwifruit ( Actinidia deliciosa [A. Chev.] C. F. Liang et A. R. Ferguson var. deliciosa cv. Hayward) was monitored during fruit development to identify which cytokinins were present and if they were linked with specific stages of fruit growth. Cytokinins were isolated and purified by column chromatography and high-performance liquid chromatography and quantified by radioimmunoassay. A novel HPLC step utilising an amine column was successfully introduced as a preparative step in the separation of the O - and 9-glucosides from the free bases and ribosides. The radioimmunoassay results were validated, and the different cytokinins identified, by gas chromatography-mass spectrometry. Cytokinins detected in fruit included the cytokinin free bases, zeatin and isopentenyladenine, their ribosides, nucleotides and both O - and 9-glucosides. Both qualitative and quantitative changes of the cytokinins occurred during fruit development. A decrease in cytokinin concentration occurred after anthesis (from 342 pmol g−1 fresh weight at anthesis to 41 pmol g−1 fresh weight 27 days after anthesis). A large increase in cytokinin concentration and content per fruit occurred as the fruit reached commercial maturity (to 1900 pmol g−1 fresh weight). Individual cytokinins showed quite different patterns. Zeatin, in particular, showed a peak in concentration (13 pmol g−1 fresh weight) 11 days after anthesis that correlated with the beginning of the cell division phase of fruit growth. The accumulation of cytokinin (mostly zeatin riboside or zeatin nucleotide) in mature fruit may be of significance for the postharvest storage of kiwifruit fruit.  相似文献   
37.
The nucleotide sequence data reported in this paper have been submitted to the GenBank nucleotide sequence database and have been assigned the accession number U17107. The nameB*3509 was officially assigned by the WHO Nomenclature Committee in December 1994  相似文献   
38.
Machado-Joseph disease (MJD) is a late-onset, progressive, neurodegenerative disorder caused by the expansion of an unstable trinucleotide (CAG) repeat sequence in a novel gene (MJD1) on chromosome 14. Previous studies showed that age at onset is negatively correlated with the number of CAG repeat units, but only part of the variation in onset age is explained by CAG repeat length. Ages at onset and CAG repeat lengths of 136 MJD patients from 23 kindreds of Portuguese descent were analyzed, to determine whether familial factors independent of CAG repeat length modulate age at onset of MJD. Correlation among sibs for onset age adjusted for CAG repeat length was .43, which indicates that an environmental or genetic factor common to sibs influences onset age. Positive correlations were also observed for avuncular (r = .22) and first-cousin pairs (r = .28), which supports the hypothesis that a genetic factor is influencing age at onset. Commingling analysis of onset ages adjusted for CAG repeat length identified three distributions in this population of affected individuals. Further studies of a much larger sample are needed to determine whether these distributions represent the influence of a genetic or environmental factor.  相似文献   
39.
The effect of nitrogen and fatty-acid-rich substrates on the production of 1-octen-3-ol by the edible fungus Pleurotus pulmonarius, during growth in both shaken flask and fermentor cultures, and in-vitro, in post-harvested mycelium, was studied. Addition of soybean flour and soybean oil to the growth medium enhanced 1-octen-3-ol production about sevenfold and doubled the fungal biomass, as compared to that obtained from P. pulmonarius cultured on a defined synthetic medium. A clear relationship between the production of 1-octen-3-ol and lipoxygenase activity was found during the growth of mushroom pellets. The highest in-vitro generation of 1-octen-3-ol was obtained upon addition of exogenous linoleic acid and pure O2 to pellets grown with soybean fluor and soybean oil. This generation was even higher than that of fruiting bodies exposed to the same conditions. These results suggest that lipoxygenase activity and, subsequently, 1-octen-3-ol biosynthesis in P. pulmonarius are enhanced by the presence of substrates containing fatty acids in the growth medium. Correspondence to: D. Levanon  相似文献   
40.
Lactoside-binding lectins (galectins) with molecular weights of about 14.5 kDa (galectin-1) and 29–35 kDa (galectin-3) bind preferentially to polylactosaminoglycan-containing glycoconjugates and have been found on the surface of tumour cells and implicated in cell-cell and cell-extracellular matrix adhesion and metastasis. We have demonstrated by immunoblotting that both galectin-1 and galectin-3 are present in extracts of endothelial cells cultured from bovine aorta, rat lung, mouse lung and mouse brain microvessels, whereas mouse hepatic sinusoidal endothelial cells expressed primarily galectin-1. These galectins were also localized by indirect immunofluorescent labelling on the surface of the different endothelial cells in culture and by immunohistochemical staining in human tissuesin vivo. Anti-galectin-1 antibodies inhibited the adhesion of liver-preferring murine RAW117-H10 large-cell lymphoma cells to hepatic sinusoidal endothelial cells or lung microvessel endothelial cellsin vitro. The data indicate that galectin-1 is expressed on the extracellular surface of endothelial cells and can mediate in part the adhesion of RAW117-H10 cells to liver microvessel endothelial cells.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号