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   17764篇
  免费   1522篇
  国内免费   9篇
  19295篇
  2023年   90篇
  2022年   189篇
  2021年   396篇
  2020年   225篇
  2019年   278篇
  2018年   356篇
  2017年   309篇
  2016年   489篇
  2015年   824篇
  2014年   911篇
  2013年   1076篇
  2012年   1440篇
  2011年   1315篇
  2010年   878篇
  2009年   837篇
  2008年   1093篇
  2007年   1109篇
  2006年   984篇
  2005年   1025篇
  2004年   930篇
  2003年   857篇
  2002年   828篇
  2001年   150篇
  2000年   130篇
  1999年   187篇
  1998年   239篇
  1997年   164篇
  1996年   142篇
  1995年   110篇
  1994年   121篇
  1993年   119篇
  1992年   103篇
  1991年   84篇
  1990年   122篇
  1989年   99篇
  1988年   84篇
  1987年   88篇
  1986年   49篇
  1985年   82篇
  1984年   91篇
  1983年   60篇
  1982年   79篇
  1981年   68篇
  1980年   50篇
  1979年   47篇
  1978年   37篇
  1977年   34篇
  1976年   30篇
  1975年   20篇
  1974年   28篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
As part of a detailed study, the syntheses, biological activities, and pharmacokinetic properties of hydroxylated analogues of the previously described broad spectrum antifungal agents, Sch 51048 (1), Sch 50001 (3), and Sch 50002 (4), are described. Based on an overall superior profile, one of the alcohols, Sch 56592 (2), was selected for clinical studies.  相似文献   
62.
Trioza eugeniae oviposited significantly more eggs on Syzygium paniculatum leaves free of eggs compared to leaves with pre-existing eggs in a choice experiment, suggesting that females modify oviposition based on cues associated with the presence of eggs. In separate experiments, females avoided ovipositing on parts of leaves where eggs were present, but readily oviposited on areas of the same leaf without eggs. Epideictic pheromones are apparently not used by this species because females readily oviposited on areas of a leaf from which eggs had previously been removed. Females laid 57% of all their eggs along the margins of normal leaves, but also oviposited on artificial margins produced by cutting the leaves. The preference for natural margins over artificial margins was eliminated when the entire margin was coated with an antitranspirant compound to mask the damaged edge. These studies show that ovipositing psyllids respond both to the presence of eggs and to the suitability of the substrate.  相似文献   
63.
To better understand how cathode performance and substrates affected communities that evolved in these reactors over long periods of time, microbial fuel cells were operated for more than 1 year with individual endproducts of lignocellulose fermentation (acetic acid, formic acid, lactic acid, succinic acid, or ethanol). Large variations in reactor performance were primarily due to the specific substrates, with power densities ranging from 835 ± 21 to 62 ± 1 mW/m3. Cathodes performance degraded over time, as shown by an increase in power of up to 26% when the cathode biofilm was removed, and 118% using new cathodes. Communities that developed on the anodes included exoelectrogenic families, such as Rhodobacteraceae, Geobacteraceae, and Peptococcaceae, with the Deltaproteobacteria dominating most reactors. Pelobacter propionicus was the predominant member in reactors fed acetic acid, and it was abundant in several other MFCs. These results provide valuable insights into the effects of long-term MFC operation on reactor performance.  相似文献   
64.
65.
66.
Archaeological evidence for prehistoric strategies of marine exploitation in Oceania may be profitably analyzed from an ecological perspective, in which individual sites and assemblages are viewed in the context of adaptation to local environmental constraints. This perspective is illustrated through the contrastive analysis of environment, technology, and faunal remains at three prehistoric Hawaiian sites. Differing strategies of marine exploitation evidenced for each site are shown to reflect local marine environmental conditions. An ecological approach shows greater promise for an understanding of prehistoric adaptation to marine environment than the typological analyses current in much archaeological work on fishing.  相似文献   
67.

Background

The incidence of acute coronary syndrome (ACS) in young people (≤65 years) is continuously rising. While prognostic factors in ACS are well-investigated less attention has been paid to their age-dependent prognostic value and their particular relevance in younger patients. The aim of our study was to assess the age-dependent prognostic impact of butyrylcholinesterase (BChE).

Methods

Retrospective cohort study including 624 patients with ACS. Patients were stratified by age into equal groups (n = 208) corresponding to “young patients” (45–64 years), "middle-aged patients” (65–84 years) and “old patients” (85–100 years). Cox regression hazard analysis was used to assess the influence of BChE on survival.

Results

After a mean follow-up time of 4.0 (interquartile range [IQR] 2.0–6.4) years, 154 patients (24.7%) died due to a cardiac cause. In the overall cohort, BChE was indirectly associated with cardiac mortality-free survival (adjusted hazard ratio (HR): 0.70 (95% confidence interval [CI] 0.53–0.93, p = 0.01). The primary-analysis of BChE by age strata showed the strongest effect in the age group 45–64 years with an adjusted HR per 1-SD of 0.28 (95% CI 0.12–0.64, p = 0.003), a weaker association with mortality in middle aged (65–84 years: adjusted HR per 1-SD 0.66 [95% CI: 0.41–1.06], p = 0.087), and no association in older patients (85–100 years: adjusted HR per 1-SD 0.89 [95% CI: 0.58–1.38], p = 0.613).

Conclusion

BChE is a strong predictor for cardiac mortality specifically in younger patients with ACS aged between 45 and 64 years. No significant association of BChE with cardiac-mortality was detected in other age classes.  相似文献   
68.
The discovery, synthesis and in vitro activity of a novel series of rhodanine based phosphodiesterase-4 (PDE4) inhibitors is described. Structure-activity relationship studies directed toward improving potency led to the development of submicromolar inhibitors 2n and 3i (IC(50)=0.89 & 0.74 microM). The replacement of rhodanine with structurally related heterocycles was also investigated and led to the synthesis of pseudothiohydantoin 7 (IC(50)=0.31 microM).  相似文献   
69.
Cells lining the developing seed coats of legumes efflux photosynthates (mostly sucrose) and salts (mostly of potassium) into the apoplast for uptake by the developing embryo. These effluxes increase transiently in response to an increase in turgor in the effluxing cells. Detached coats of developing seed of P. haseolus vulgaris and Vicia faba were used to study the effects of turgor on the rates of efflux, on the membrane potential difference and on the membrane pH difference, using a number of inhibitors and agents which might affect signal cascades involving cytoplasmic calcium concentration. Effluxes were measured by measuring the concentrations of solutes of interest in solution samples placed in halves of detached seed coats, the paired halves serving as control and treated sample where appropriate. It is shown that a number of substances affect sucrose and potassium effluxes differently, and that hypo-osmotic shock depolarizes the efflux cells and acidifies the cytoplasm (in P. vulgaris). It is concluded that sucrose and potassium effluxes, although both are increased by an increase in turgor, are affected by different signal pathways. Further, it is also concluded that the signal that increases the rates of both sucrose efflux (via sucrose-proton antiport) and proton pump acts directly on the antiporter rather than on the pump. There are interesting parallels and contrasts between these processes and those in plants such as the charophyte Lamprothamnium after hypo-osmotic shock.  相似文献   
70.
To what extent does glutamine turnover keep pace with oxidative metabolism in the rat heart? To address this question, the following groups of substrates were presented to the isolated, working rat heart: 1) glucose (5 mM), insulin (40 microU/ml), and [2-13C]acetate (5 mM; high workload, n = 5); 2) pyruvate (2.5 mM) and [2-13C]acetate (5 mM; normal workload, n = 5); or 3) propionate (1 mM) and [2-13C]acetate (2.5 mM; normal workload, n = 3). In a subset of these experiments, the exchange of glutamate and glutamine was quantified by separation with ion exchange chromatography and analysis by GC-MS. There was an apparent equilibration of mass isotopomers of glutamate and glutamine after 50 min of perfusion, although the extent of equilibration was not determined. The fractional enrichment in glutamine was 31% of the enrichment of glutamate with the three different perfusates. From high-resolution nuclear magnetic resonance spectra, we found a ratio of glutamine to glutamate content of 94.1, 53.4, and 96.9%, respectively, for each experimental group. In experiments for which l-[1-13C]glutamine (5 mM) was included in the perfusate of group 2, [1-13C]glutamine was detected in the heart, but transfer of 13C from glutamine to glutamate was not detected (n = 4). We conclude that, in the perfused working heart, production of glutamine by amidation of glutamate takes place and can be detected, whereas the reverse process, generation of glutamate from glutamine, remains undetected.  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