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   537篇
  免费   47篇
  2023年   3篇
  2022年   13篇
  2021年   24篇
  2020年   11篇
  2019年   13篇
  2018年   11篇
  2017年   18篇
  2016年   20篇
  2015年   19篇
  2014年   23篇
  2013年   45篇
  2012年   42篇
  2011年   33篇
  2010年   15篇
  2009年   21篇
  2008年   18篇
  2007年   25篇
  2006年   26篇
  2005年   29篇
  2004年   21篇
  2003年   18篇
  2002年   10篇
  2001年   13篇
  2000年   16篇
  1999年   9篇
  1998年   5篇
  1997年   4篇
  1996年   2篇
  1995年   2篇
  1992年   10篇
  1991年   5篇
  1990年   3篇
  1988年   3篇
  1987年   5篇
  1986年   2篇
  1985年   2篇
  1983年   4篇
  1982年   2篇
  1981年   2篇
  1979年   4篇
  1977年   3篇
  1971年   3篇
  1969年   3篇
  1968年   3篇
  1967年   4篇
  1963年   2篇
  1959年   2篇
  1955年   1篇
  1952年   1篇
  1924年   1篇
排序方式: 共有584条查询结果,搜索用时 15 毫秒
541.
542.
The occurrence of the fish Cephalopholis taeniops (Serranidae: Epinephelinae) has been recorded for the first time in the Canary Islands (eastern subtropical Atlantic). Repeated observations of adult stages of the species (??30?cm total length) inside oil platforms coming from the Gulf of Guinea greatly suggest an introduction related to the transport of ballast water in these structures, and show their importance in the dispersal of adult stages of large-sized species of littoral fish. Oil platforms constitute one special case in the introduction of marine species in the context of ballast water transport. This poorly known phenomenon may greatly favour marine species invasion, and might have significant repercussions in many regions -especially in the current climate change context- as well as explaining some surprising cases of occurrence of expatriated adult individuals of certain species.  相似文献   
543.
544.
Proteoglycan breakdown was studied in a coculture model which mimics the confrontation between synovium and cartilage that occurs in rheumatoid arthritis. Bovine nasal-septum cartilage discs radioactively labeled (35SO42? with or without [3H]glucosamine) and ‘chased’ in non-radioactive medium were cultured in contact with minced rheumatoid synovial membranes for intervals up to 8 days. Synovium-stimulated (2–3 fold) cartilage breakdown was unaffected by ascorbate supplementation. Labeled products (small molecules plus proteoglycan complexes) in culture media were characterized by chromatographic, sedimentation and enzymic digestion methods. Breakdown was dominated by the release of a range of proteoglycan products, fully disaggregated and incapable of reaggregation with added hyaluronate. Because constituent glycosaminoglycans were of uniform size, proteoglycan polydispersity was attributed to differences in core protein length. Hydrocortisone inhibited degradation and partially prevented the shift of proteoglycans to lower average molecular weight. An additional breakdown pattern occasionally noted during the initial 48 h of coculture was characterized by release of a subpopulation of low charge-density proteoglycan bearing shortened glycosaminoglycan chains, consistent with glycosidase action. We conclude that rheumatoid synovia exhibit two distinct cartilage degradative potencies in vitro that may be important in vivo: (a) A variable hyaluronidase-like activity at early culture times, and (b) a dominant proteolytic activity generating an array of disaggregated proteoglycann products that differ largely on the basis of their core lengths. The response to hydrocortisone is consistent with inhibition of proteolysis through the stabilization of cellular membranes.  相似文献   
545.
The study of any intervention able to counteract SARS-CoV-2 pandemic is considerably envisaged. It was previously shown, in in vitro models of infections, that the LED blue light is able to decrease the viral load of HSV-1 and ZIKV. In our study, LED photobiomodulation therapy (PBMT) at blue wavelengths (450, 454 and 470 nm) was tested in an in vitro model of SARS-CoV-2 infection, employing three experimental settings: SARS-CoV-2 was irradiated and then transferred to cells; already infected cells were irradiated; cells were irradiated prior to infection. A decrement of the viral load was observed when previously infected cells were irradiated with all three tested wavelengths and relevant effects were registered especially at 48 hours post-infection, possibly suggesting that the blue light could interfere with the intracellular viral replication machinery. Our in vitro findings could represent the starting point for translational applications of PBMT as a supportive approach to fight SARS-CoV-2.  相似文献   
546.
Living organisms have some common and unique strategies to response to thermal stress. However, the amount of data on thermal stress response of certain organism is still lacking, especially psychrophilic yeast from the extreme habitat. Therefore, it is not known whether psychrophilic yeast shares the common responses of other organisms when exposed to thermal stresses. In this work, the cold shock and heat shock responses in Antarctic psychrophilic yeast Glaciozyma antarctica PI12 which had an optimal growth temperature of 12 °C were determined. The expression levels of 14 thermal stress-related genes were measured using real-time quantitative PCR (qPCR) when the yeast cells were exposed to cold shock (0 °C), mild cold shock (5 °C), and heat shock (22 °C) conditions. The expression profiles of the 14 genes at these three temperatures varied indicating that these genes had their specific roles to ensure the survival of the yeast. Under cold shock condition, the afp4 and fad genes were over-expressed possibly as a way for the G. antarctica PI12 to avoid ice crystallization in the cell and to maintain the membrane fluidity. Under the heat shock condition, hsp70 was significantly up-regulated possibly to ensure the proteins fold properly. Among the six oxidative stress-related genes, MnSOD and prx were up-regulated under cold shock and heat shock, respectively, possibly to reduce the negative effects caused by oxidative stress. Interestingly, it was found that the trehalase gene, nth1 that plays a role in degrading excess trehalose, was down-regulated under the heat shock condition possibly as an alternative way to accumulate trehalose in the cells to protecting them from being damaged.  相似文献   
547.
Phenotypic characterization of soybean event 335-13, which possesses oil with an increased oleic acid content (> 85%) and reduced palmitic acid content (< 5%), was conducted across multiple environments during 2004 and 2005. Under these conditions, the stability of the novel fatty acid profile of the oil was not influenced by environment. Importantly, the novel soybean event 335-13 was not compromised in yield in both irrigated and non-irrigated production schemes. Moreover, seed characteristics, including total oil and protein, as well as amino acid profile, were not altered as a result of the large shift in the fatty acid profile. The novel oil trait was inherited in a simple Mendelian fashion. The event 335-13 was also evaluated as a feedstock for biodiesel. Extruded oil from event 335-13 produced a biodiesel with improved cold flow and enhanced oxidative stability, two critical fuel parameters that can limit the utility of this renewable transportation fuel.  相似文献   
548.
The ongoing Covid-19 is a contagious disease, and it is characterised by different symptoms such as fever, cough, and shortness of breath. Rising concerns about Covid-19 have severely affected the healthcare system in all countries as the Covid-19 outbreak has developed at a rapid rate all around the globe. Intriguing, a clinically used drug, acetazolamide (a specific inhibitor of carbonic anhydrase, CA, EC 4.2.1.1), is used to treat high-altitude pulmonary oedema (HAPE), showing a high degree of clinical similarities with the pulmonary disease caused by Covid-19. In this context, this preliminary study aims to provide insights into some factors affecting the Covid-19 patients, such as hypoxaemia, hypoxia as well as the blood CA activity. We hypothesise that patients with Covid-19 problems could show a dysregulated acid–base status influenced by CA activity. These preliminary results suggest that the use of CA inhibitors as a pharmacological treatment for Covid-19 may be beneficial.  相似文献   
549.
In this work it was hypothesized that secondary succession on sites that have been managed by single planting of mangrove species is compromised by residual stressors, which could reduce the ecosystem??s structural development and lower its functions. Forest structure and environmental characteristics of three planted mangrove stands are compared with reference sites. Structural attributes showed significant differences in the comparison of planted and reference stands. Avicennia schaueriana was the dominant species within both natural regeneration and old-growth stands in terms of basal area (99.2 and 99.4?%, 69.6 and 84.5?%, and 59.0 and 87.1?% for Itacorubi, Saco Grande, and Ratones, respectively). Restoration stands were dominated by Laguncularia racemosa (80.6 and 94.2?% for Saco Grande and Ratones, respectively), except at one site (Itacorubi), where A. schaueriana prevailed (99.7?%). Even though restoration and regeneration stands at Itacorubi showed similar species composition and dominance, cohort sorting revealed an inferior regeneration potential in the restoration stand. Multiple correlation analysis indicated that variables related to elevation disruptions (p w ?=?0.521) were the environmental drivers responsible for the differences observed in forest structure. At restoration sites an impaired pattern of secondary succession was observed, indicating that single species plantings may be ineffective if characteristics of the site, as well as of the area surrounding it, are not considered. The inadequate management of restoration sites can therefore have implications for both immediate and long-term large-scale ecosystem services.  相似文献   
550.
Geometric scaling predicts a major challenge for legged, terrestrial locomotion. Locomotor support requirements scale identically with body mass (α M1), while force-generation capacity should scale α M2/3 as it depends on muscle cross-sectional area. Mammals compensate with more upright limb postures at larger sizes, but it remains unknown how sprawling tetrapods deal with this challenge. Varanid lizards are an ideal group to address this question because they cover an enormous body size range while maintaining a similar bent-limb posture and body proportions. This study reports the scaling of ground reaction forces and duty factor for varanid lizards ranging from 7 g to 37 kg. Impulses (force×time) (α M0.99−1.34) and peak forces (α M0.73−1.00) scaled higher than expected. Duty factor scaled α M0.04 and was higher for the hindlimb than the forelimb. The proportion of vertical impulse to total impulse increased with body size, and impulses decreased while peak forces increased with speed.  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