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   4563篇
  免费   455篇
  5018篇
  2024年   3篇
  2023年   32篇
  2022年   72篇
  2021年   157篇
  2020年   78篇
  2019年   92篇
  2018年   110篇
  2017年   116篇
  2016年   161篇
  2015年   284篇
  2014年   330篇
  2013年   321篇
  2012年   428篇
  2011年   392篇
  2010年   246篇
  2009年   207篇
  2008年   312篇
  2007年   260篇
  2006年   268篇
  2005年   242篇
  2004年   236篇
  2003年   181篇
  2002年   206篇
  2001年   43篇
  2000年   18篇
  1999年   28篇
  1998年   25篇
  1997年   23篇
  1996年   26篇
  1995年   14篇
  1994年   15篇
  1993年   19篇
  1992年   7篇
  1991年   12篇
  1990年   9篇
  1989年   6篇
  1988年   3篇
  1987年   4篇
  1986年   3篇
  1985年   6篇
  1983年   3篇
  1982年   3篇
  1981年   3篇
  1980年   2篇
  1979年   2篇
  1967年   1篇
  1965年   2篇
  1964年   1篇
  1963年   2篇
  1954年   1篇
排序方式: 共有5018条查询结果,搜索用时 15 毫秒
21.
Genetically and phenotypically identical immune cell populations can be highly heterogenous in terms of their immune functions and protein secretion profiles. The microfluidic chip-based single-cell highly multiplexed secretome proteomics enables characterization of cellular heterogeneity of immune responses at different cellular and molecular layers. Increasing evidence has demonstrated that polyfunctional T cells that simultaneously produce 2+ proteins per cell at the single-cell level are key effector cells that contribute to the development of potent and durable cellular immunity against pathogens and cancers. The functional proteomic technology offers a wide spectrum of cellular function assessment and can uniquely define highly polyfunctional cell subsets with cytokine signatures from live individual cells. This high-dimensional single-cell analysis provides deep dissection into functional heterogeneity and helps identify predictive biomarkers and potential correlates that are crucial for immunotherapeutic product design optimization and personalized immunotherapy development to achieve better clinical outcomes.  相似文献   
22.
Hickey, Matthew S., Charles J. Tanner, D. Sean O'Neill,Lydia J. Morgan, G. Lynis Dohm, and Joseph A. Houmard. Insulin activation of phosphatidylinositol 3-kinase in human skeletal muscle invivo. J. Appl. Physiol. 83(3):718-722, 1997.The purpose of this investigation was to determinewhether insulin-stimulated phosphatidylinositol 3-kinase (PI3-kinase)activity is detectable in needle biopsies of human skeletal muscle.Sixteen healthy nonobese males matched for age, percent fat, fastinginsulin, and fasting glucose participated in one of two experimentalprotocols. During an intravenous glucose tolerance test (IVGTT)protocol, insulin-stimulated PI3-kinase activity was determined frompercutaneous needle biopsies at 2, 5, and 15 min post-insulinadministration (0.025 U/kg). In the second group, a 2-h, 100 mU · m2 · min1euglycemic hyperinsulinemic clamp was performed, and biopsies wereobtained at 15, 60, and 120 min after insulin infusion was begun.Insulin stimulated PI3-kinase activity by 1.6 ± 0.2-, 2.2 ± 0.3-, and 2.2 ± 0.4-fold at 2, 5, and 15 min, respectively, duringthe IVGTT. During the clamp protocol, PI3-kinase was elevated by 5.3 ± 1.3-, 8.0 ± 2.6-, and 2.7 ± 1.4-fold abovebasal at 15, 60, and 120 min, respectively. Insulin-stimulatedPI3-kinase activity at 15 min post-insulin administration wassignificantly greater during the clamp protocol vs. the IVGTT(P < 0.05). These observations suggest that insulin-stimulated PI3-kinase activity is detectable inneedle biopsies of human skeletal muscle, and furthermore, that theeuglycemic, hyperinsulinemic clamp protocol may be a useful tool toassess insulin signaling in vivo.

  相似文献   
23.
24.
Summary A method of staging late third instar larvae on the basis of salivary gland morphology is described. Using this technique, we investigated stage related amino acid requirements forDrosophila leg disc evagination in vitro. It was found that the requirement for glutamine lasted longer than that of proline. The staging technique should help in the detailed exploration of the late 3rd instar time period in order to bridge the gap between biochemistry and morphogenesis in the initiation of disc evagination.  相似文献   
25.
A new medium supplement mixture, PPRF92, has been developed to enable the serial subculture of human diploid fibroblasts (MRC-5 cells) on microcarriers. Furthermore, the PPRF92 supplements enable cell growth at serum levels as low as 1%. Through an optimization programme, the PPRF92 supplements have evolved into a simple mixture with the concentrations of key components at a level that makes the overall cost very competitive with medium containing 10% foetal bovine serum (FBS). Furthermore, the PPRF92 supplement mixture is most efficacious when FBS is replaced with the cheaper, and more widely available, adult bovine serum (ABS). Although medium exchange with serum is necessary in order to achieve confluence on microcarriers, the PPRF92 mixture is only necessary at the initiation of each passage. Using the medium replinishment protocol that has been developed in our laboratory, MRC-5 cells were successfully serially passaged through 13 bead-to-bead transfers on microcarriers in DMEM/F12 medium enriched with the PPRF92 supplement mixture reported here, and 1% ABS.Correspondence to: L. A. Behie  相似文献   
26.
In secondary leaves from spinach plants pretreated in vermiculite for 24 h with 300 mM NaCl, glycinebetaine accumulated at a rate of circa 0.16 mol 100 g-1 Chl d-1 (2 mol g-1 FW d-1), about three times the rate of control plants. The soluble carbohydrate and free amino acid contents did not increase significantly following salinisation until after 4 d when the relative growth rate also decreased. Leaf proline levels remained very low throughout the experimental period. K+ on a tissue water basis remained constant at 200 mM while Cl- and Na+ levels increased linearly to reach 175 and 100 mM respectively after 5 d of saline treatment. The osmotic pressure of leaf tissue also increased from 300 to 500 mosmol kg-1. These experimental conditions were considered suitable to study glycinebetaine biosynthesis and its induction by salinity in the absence of marked growth inhibition or metabolic disturbance. Radioactive labelled [14C]serine, ethanolamine and choline (all 1 mol, 13.3 MBq in 10 l) were fed to detached secondary leaves via the petiole 24 h after the exposure of plants to salt. The rate of isotope incorporation into water soluble products, lipids and residue was measured over a further 24 h. The major metabolic fate of exogenous [14C]choline and [14C]ethanolamine was incorporation into glycinebetaine while less 14C-label was found in phosphatidyl choline and phosphatidyl ethanolamine. Incorporation rates were identical in control and salinised leaves and were adequate to account for observed values of glycinebetaine accumulation previously reported in spinach. In contrast the labelling of glycinebetaine from [14C]serine was twice as great in salinated plants as in the controls. These results, together with short term labelling experiment with [14C]ethanolamine using leaf slices, were consistent with the formation of glycinebetaine via serine, ethanolamine and its methylated derivatives to choline with some control being exerted at the serine level. However a flux through the phosphorylated intermediates is not excluded.From a consideration of these results and the published data on barley subjected to water stress (Hanson and Scott, 1980 Plant Physiol. 66, 342–348) there appear to be significant differences in the biosynthetic pathways in spinach and barley.Abbreviations BHT butylated hydroxytoluerte (2,6-di-tert-butyl-4-methylphenol) - C1 one-carbon fragment - 1,2DG diglyceride moiety - DW day weight - MCW methanol-chloroform-water (12:5:1, by vol.) - PA phosphatidic acid - PC phosphatidyl choline - PMME phosphatidyl monomethylethanolamine - PDME phosphatidyl dimethylethanolamine - PE phosphatidyl ethanolamine - PPO 2,5-diphenyloxazole - POPOP 1,4-bis(5-phenyloxazoyl) benzene  相似文献   
27.
Four patients developed massive pulmonary embolism after jejuno-ileal bypass for morbid obesity. All patients were in Greenfield's Class IV and were in shock. Severe hypoxia was evidenced in their blood gases. The patients were managed with digitalis, diuretics, Solu-Medrol (methylprednisolone sodium succinate), oxygen, and heparin therapy. Each patient underwent partial vena cava interruption with Mobin Uddin's umbrella, and all four survived without residual complications.  相似文献   
28.
BackgroundThe US Centers for Disease Control and Prevention has repeatedly called for Coronavirus Disease 2019 (COVID-19) vaccine equity. The objective our study was to measure equity in the early distribution of COVID-19 vaccines to healthcare facilities across the US. Specifically, we tested whether the likelihood of a healthcare facility administering COVID-19 vaccines in May 2021 differed by county-level racial composition and degree of urbanicity.Methods and findingsThe outcome was whether an eligible vaccination facility actually administered COVID-19 vaccines as of May 2021, and was defined by spatially matching locations of eligible and actual COVID-19 vaccine administration locations. The outcome was regressed against county-level measures for racial/ethnic composition, urbanicity, income, social vulnerability index, COVID-19 mortality, 2020 election results, and availability of nontraditional vaccination locations using generalized estimating equations.Across the US, 61.4% of eligible healthcare facilities and 76.0% of eligible pharmacies provided COVID-19 vaccinations as of May 2021. Facilities in counties with >42.2% non-Hispanic Black population (i.e., > 95th county percentile of Black race composition) were less likely to serve as COVID-19 vaccine administration locations compared to facilities in counties with <12.5% non-Hispanic Black population (i.e., lower than US average), with OR 0.83; 95% CI, 0.70 to 0.98, p = 0.030. Location of a facility in a rural county (OR 0.82; 95% CI, 0.75 to 0.90, p < 0.001, versus metropolitan county) or in a county in the top quintile of COVID-19 mortality (OR 0.83; 95% CI, 0.75 to 0.93, p = 0.001, versus bottom 4 quintiles) was associated with decreased odds of serving as a COVID-19 vaccine administration location.There was a significant interaction of urbanicity and racial/ethnic composition: In metropolitan counties, facilities in counties with >42.2% non-Hispanic Black population (i.e., >95th county percentile of Black race composition) had 32% (95% CI 14% to 47%, p = 0.001) lower odds of serving as COVID administration facility compared to facilities in counties with below US average Black population. This association between Black composition and odds of a facility serving as vaccine administration facility was not observed in rural or suburban counties. In rural counties, facilities in counties with above US average Hispanic population had 26% (95% CI 11% to 38%, p = 0.002) lower odds of serving as vaccine administration facility compared to facilities in counties with below US average Hispanic population. This association between Hispanic ethnicity and odds of a facility serving as vaccine administration facility was not observed in metropolitan or suburban counties.Our analyses did not include nontraditional vaccination sites and are based on data as of May 2021, thus they represent the early distribution of COVID-19 vaccines. Our results based on this cross-sectional analysis may not be generalizable to later phases of the COVID-19 vaccine distribution process.ConclusionsHealthcare facilities in counties with higher Black composition, in rural areas, and in hardest-hit communities were less likely to serve as COVID-19 vaccine administration locations in May 2021. The lower uptake of COVID-19 vaccinations among minority populations and rural areas has been attributed to vaccine hesitancy; however, decreased access to vaccination sites may be an additional overlooked barrier.

Inmaculada Hernandez and colleagues investigate the disparities in early-phase distribution of COVID-19 Vaccines across U.S. Counties.  相似文献   
29.
We tested for correlations in the degree of spatial similarity between algal and terrestrial plants communities along 5500 km of temperate Australian coastline and whether the strength of correlation weakens with increasing distance from the coast. We identified strong correlations between macroalgal and terrestrial plant communities within the first 100 km from shore, where the strength of these marine–terrestrial correlations indeed weakens with increasing distance inland. As such, our results suggest that marine‐driven community homogenization processes decompose with increasing distance from the shore toward inland. We speculate that the proximity to the marine environment produces lower levels of community turnover on land, and this effect decreases progressively farther inland. Our analysis suggests underlying ecological and evolutionary processes that give rise to continental‐scale biogeographic influence from sea to land.  相似文献   
30.
Plant phenology—the timing of cyclic or recurrent biological events in plants—offers insight into the ecology, evolution, and seasonality of plant‐mediated ecosystem processes. Traditionally studied phenologies are readily apparent, such as flowering events, germination timing, and season‐initiating budbreak. However, a broad range of phenologies that are fundamental to the ecology and evolution of plants, and to global biogeochemical cycles and climate change predictions, have been neglected because they are “cryptic”—that is, hidden from view (e.g., root production) or difficult to distinguish and interpret based on common measurements at typical scales of examination (e.g., leaf turnover in evergreen forests). We illustrate how capturing cryptic phenology can advance scientific understanding with two case studies: wood phenology in a deciduous forest of the northeastern USA and leaf phenology in tropical evergreen forests of Amazonia. Drawing on these case studies and other literature, we argue that conceptualizing and characterizing cryptic plant phenology is needed for understanding and accurate prediction at many scales from organisms to ecosystems. We recommend avenues of empirical and modeling research to accelerate discovery of cryptic phenological patterns, to understand their causes and consequences, and to represent these processes in terrestrial biosphere models.  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