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   1871篇
  免费   150篇
  2021篇
  2022年   16篇
  2021年   16篇
  2020年   8篇
  2019年   17篇
  2018年   15篇
  2017年   31篇
  2016年   32篇
  2015年   62篇
  2014年   83篇
  2013年   87篇
  2012年   95篇
  2011年   111篇
  2010年   60篇
  2009年   63篇
  2008年   81篇
  2007年   91篇
  2006年   75篇
  2005年   99篇
  2004年   81篇
  2003年   84篇
  2002年   83篇
  2001年   60篇
  2000年   65篇
  1999年   47篇
  1998年   40篇
  1997年   22篇
  1996年   32篇
  1995年   31篇
  1994年   28篇
  1993年   20篇
  1992年   37篇
  1991年   29篇
  1990年   26篇
  1989年   27篇
  1988年   21篇
  1987年   16篇
  1986年   15篇
  1985年   16篇
  1984年   15篇
  1983年   13篇
  1982年   13篇
  1981年   15篇
  1979年   15篇
  1978年   14篇
  1977年   8篇
  1976年   7篇
  1975年   21篇
  1974年   16篇
  1973年   9篇
  1968年   6篇
排序方式: 共有2021条查询结果,搜索用时 0 毫秒
991.
Objective: The aim of this study was to re‐evaluate data about oral status, mastication and nutrition in elderly men in Malmö, Sweden, recorded in 1985–1987, to assess associations between inadequate dietary habits, oral conditions and masticatory function. Materials and methods: Four hundred and eighty‐one men, aged 67–68, participated in a comprehensive health examination, including tooth and denture status and masticatory tests. A separate study of dietary habits and nutritional status was made. Ninety‐five men had inadequate dietary habits. The databases of dental/denture status, mastication, nutritional status and social network factors were re‐evaluated for assessment of associations. Results: No significant differences between those with adequate or inadequate nutrition were found with regard to the number of teeth, occlusal contacts or removable dentures. Also self‐assessed chewing did not show any differences. Conclusion: Inadequate dietary habits were independent of teeth and denture status. Some correlations to social network conditions could be identified. Overweight, obesity, low physical activity and high alcohol intake were more common among those with inadequate nutritional intake.  相似文献   
992.
A release of ammonium by non-nitrogen-fixing Anabaena cylindrica (grown on NH4Cl) in the presence of MSX (methionine sulfoximine) and absence of any external nitrogen source was found. In the light the release was maximal at 0.2 mM MSX, a concentration which did not affect net CO2 fixation nor the glycollate excretion, but inhibited the glutamine synthetase activity and the reassimilation of ammonium. It is suggested that the major source of the ammonium released is the photorespiratory conversion of glycine to serine as (1) the release was stimulated by increase in light intensity, (2) high CO2 (3%) lowered the release, if not given as a longer pretreatment (as CO2 or HCO 3 - ) when a stimulation was observed, (3) glyoxylate and glutamate stimulated the release, the latter compound particularly under nitrogen-deficient conditions and (4) isonicotinic acid hydrazide caused a reduced release of ammonium. Furthermore, a substantial part of the ammonium released by N2-fixing A. cylindrica in presence of MSX may thus originate from the glycollate pathway. The data show that in the light the glycine to serine conversion is active in cyanobacteria with a concomitant production of ammonium which is assimilated by glutamine synthetase.Abbreviations MSX L-methionine-Dl-sulfoximine - INH isonicotinic acid hydrazide - RuDP ribulose 1,5-diphosphate - Hepes N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid - GS glutamine synthetase - GOGAT glutamate synthase - DTT Dl-dithiothreitol  相似文献   
993.
994.
Background aimsNatural killer (NK) cell-based adoptive immunotherapy is a promising approach for the treatment of cancer. Ex vivo expansion and activation of NK cells under good manufacturing practice (GMP) conditions are crucial for facilitating large clinical trials. The goal of this study was to optimize a large-scale, feeder-free, closed system for efficient NK cell expansion.MethodsPeripheral blood mononuclear cells (PBMCs) from healthy donors and myeloma patients were cultured for 21 days using flasks, cell culture bags and bioreactors. Final products from different expansions were evaluated comparatively for phenotype and functionality.ResultsSignificant NK cell expansions were obtained in all systems. The bioreactor yielded a final product rich in NK cells (mean 38%) ensuring that a clinically relevant cell dose was reached (mean 9.8 × 109 NK cells). Moreover, we observed that NK cells expanded in the bioreactor displayed significantly higher cytotoxic capacity. It was possible to attribute this partially to a higher expression level of NKp44 compared with NK cells expanded in flasks.ConclusionsThese results demonstrate that large amounts of highly active NK cells for adoptive immunotherapy can be produced in a closed, automated, large-scale bioreactor under feeder-free current GMP conditions, facilitating clinical trials for the use of these cells.  相似文献   
995.
Abstract Nitrogenase (Fe-protein) was localized in the free-living cyanobacterium Anabaena cylindrica and in the cyanobionts of Cycas revoluta and Peltigera aphthosa , using colloidal gold as an immunocytochemical marker. The Fe-protein was found to be evenly distributed throughout the heterocyst cytoplasma in A. cylindrica and in both the cyanobionts, including multiple heterocysts of the C. revoluta cyanobiont. No label was observed in the vegetative cells of free-living A. cylindrica or of the cyanobionts, although the cyanobionts apparently live under microaerobic conditions.  相似文献   
996.
Objective: To investigate whether there is any association between obesity and sexual satisfaction and sexual abuse in a normal population. Research Methods and Procedures: A representative sample of 2810 subjects from a population study was interviewed about sexual satisfaction, sexual abuse, and life satisfaction. The answers from normal weight, overweight, and obese participants were compared. Univariate and multivariate analyses were performed. Results: Data were presented separately for two age groups, 18 to 49 and 50 to 74 years, and gender. The older group of obese men reported a greater decrease of sexual desire compared with 5 years prior than normal weight men [odds ratios (OR), 2.44; 95% confidence interval (CI), 1.4 to 4.3]. The older group of overweight men reported involuntary participation in sexual activities more often than normal weight men (OR, 2.06; 95% CI, 1.1 to 3.8). Although older overweight and obese women were diagnosed with a lingering disease (defined as >1 month) more often than normal weight women (overweight: OR, 2.41; 95% CI, 1.3 to 4.4; obese: OR, 4.45; 95% CI, 1.7 to 11.5), there was no difference between BMI groups in satisfaction with physical health. Discussion: Overweight and obese groups seem to be heterogeneous with respect to sexual satisfaction and experiences of sexual abuse. No significant differences were detected between BMI groups, which does not exclude the possibility of significant differences between BMI groups among patients seeking medical attention.  相似文献   
997.
The objective of this study is to investigate the influence of point mutations on the structural stability of coiled coil fragments of the human hair intermediate filament by molecular dynamics simulations and free energy calculations. Mutations in the helix termination motif of human hair keratin gene hHb6 seem to be connected to the hereditary hair dystrophy Monilethrix. The most common mutations reported are Glu413Lys and Glu413Asp, located at the C-terminal end of the coiled coil 2B rod domain of the IF. According to our simulations, significant conformational changes of the side chains at the mutation and neighboring sites occur due to the Glu413Lys mutation. Furthermore, the differences in electrostatic interactions cause a large change in free energy during transformation of Glu413 to Lys calculated by the thermodynamic integration approach. It is speculated that the structural rearrangement necessary to adapt the interactions in the mutated coiled coil leads to changes in the IF assembly or its stability. The second mutation, Glu413Asp, only leads to a small value of the calculated free energy difference that is within the error limits of the simulations. Thus, it has to be concluded that this mutation does not affect the coiled coil stability.  相似文献   
998.
This study concerns inter- and intraspecific differences between yeasts at assimilation of different nitrogen sources. Alterations in the content of free amino acids in cells and media as well as in the related enzyme activities during growth were studied. The hydroxylamine (HA)-tolerant Endomycopsis lipolytica was examined and compared with the nitrate-reducing Cryptococcus albidus, and Saccharomyces cerevisiae, requiring fully reduced nitrogen for growth. Special attention was paid to alanine, aspartic acid, and glutamic acid, the amino acids closely related to the Krebs cycle keto acids. The amino acids were analyzed as their n-propyl N-acetyl esters by gas-liquid chromatography (GLC).The composition of the amino acid pool was similar for the three yeasts. Glutamic acid was predominant; in early log-phase cells of E. lipolytica contents of 200–234 mol·g-1 dry weight were found. A positive correlation between the specific growth rate and the size of the amino acid pool was observed.The assimilation of ammonia was mediated by glutamate dehydrogenase (GDH). The NADP-GDH was the dominating enzyme in all three yeasts showing the highest specific activity in Cr. albidus grown on nitrate (6980 nmol· (min-1)·(mg protein-1). Glutamine synthetase (GS) displayed a high specific activity in S. cerevisiae, which also had a high amount of glutamine. The assimilation of HA did not differ greatly from the assimilation of ammonium in E. lipolytica. The existing differences could rather be explained as provoked by the concentration of available nitrogen.  相似文献   
999.
Mammalian alcohol dehydrogenase (ADH) constitutes a complex system with different forms and extensive multiplicity (ADH1–ADH6) that catalyze the oxidation and reduction of a wide variety of alcohols and aldehydes. The ADH1 enzymes, the classical liver forms, are involved in several metabolic pathways beside the oxidation of ethanol, e.g. norepinephrine, dopamine, serotonin and bile acid metabolism. This class is also able to further oxidize aldehydes into the corresponding carboxylic acids, i.e. dismutation. ADH2, can be divided into two subgroups, one group consisting of the human enzyme together with a rabbit form and another consisting of the rodent forms. The rodent enzymes almost lack ethanol-oxidizing capacity in contrast to the human form, indicating that rodents are poor model systems for human ethanol metabolism. ADH3 (identical to glutathione-dependent formaldehyde dehydrogenase) is clearly the ancestral ADH form and S-hydroxymethylglutathione is the main physiological substrate, but the enzyme can still oxidize ethanol at high concentrations. ADH4 is solely extrahepatically expressed and is probably involved in first pass metabolism of ethanol beside its role in retinol metabolism. The higher classes, ADH5 and ADH6, have been poorly investigated and their substrate repertoire is unknown. The entire ADH system can be seen as a general detoxifying system for alcohols and aldehydes without generating toxic radicals in contrast to the cytochrome P450 system.  相似文献   
1000.
Oligonucleotide catalogues from 16S rRNA have been a major source of information for phylogenetic reconstruction among procaryotes. Several large procaryote groups have been analyzed and phylogenies presented. Catalogues are also available for many chloroplasts. The hypotheses of phylogeny are derived mainly from similarity (phenetic) comparisons of the catalogues and the extent of the homoplasy (parallelisms and reversals) involved has not been estimated properly. Although catalogues are currently being superseded by complete sequence data, an evaluation of the strength of catalogue data, and hence of the strength of the extensive phylogenetic hypotheses derived from them, is in order. Cladistic analysis of 16S rRNA oligonucleotide catalogues from three blue-green procaryotes, Prochloron, and chloroplasts of a red alga, Euglena, a green alga and two flowering plants shows that there is extensive homoplasy in the catalogues and several phylogenetic trees are possible. The corresponding consensus trees indicate that little or nothing can be said about interrelationships and chloroplast origin on the basis of these particular catalogues, except that Prochloron may be more closely related to the blue-greens than to chloroplasts.  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