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   738篇
  免费   61篇
  2024年   1篇
  2023年   7篇
  2022年   11篇
  2021年   17篇
  2020年   7篇
  2019年   13篇
  2018年   23篇
  2017年   27篇
  2016年   36篇
  2015年   47篇
  2014年   51篇
  2013年   65篇
  2012年   83篇
  2011年   76篇
  2010年   49篇
  2009年   43篇
  2008年   57篇
  2007年   50篇
  2006年   41篇
  2005年   24篇
  2004年   24篇
  2003年   16篇
  2002年   9篇
  2001年   5篇
  2000年   3篇
  1999年   4篇
  1998年   1篇
  1997年   3篇
  1996年   2篇
  1990年   2篇
  1989年   1篇
  1971年   1篇
排序方式: 共有799条查询结果,搜索用时 421 毫秒
751.
The fan worm Bispira brunnea is one of the most attractive sabellid polychaetes from Caribbean coral reef areas and it is exploited for ornamental purposes. An understanding of the structure of its aggregations, morphology and reproductive biology will provide information required to facilitate artificial propagation of this species. Ten aggregations were collected in October 2013, February and March 2014 in the Majahual reef lagoon, Mexican Caribbean. Whole aggregations were examined under light microscopy and scanning electron microscopy and the histology of oogenesis was determined. Aggregations were composed of 24–56 individuals and included juveniles and adults. The adults (5–20?mm) did not display any noticeable form of sexual dimorphism. In B. brunnea both sexual (hermaphroditism or gonochorism) and asexual reproduction occurred at the same time within the population: 92.71% reproduced sexually and 52% by architomy. The buds produced by architomy were inside the parental tube, at three regenerative stages. The sex ratio was 36.75% males, 33.11% females, 22.84% hermaphrodites and 7.28% unknown (gametes not seen). Gametes were distributed in the last thoracic segments and throughout the abdomen. Oogenesis was extra-ovarian, development followed four discrete stages and the oocytes were small and asynchronous (60.97 μm in diameter). Sperm morphology was adapted to external fertilization in the water column. Sequential (protandrous) hermaphroditism is suggested to occur in B. brunnea. The pyramidellid ectosymbiont mollusc Odostomia (Eulimastoma) caniculatum is recorded here for the first time as being associated with a sabellid worm.  相似文献   
752.
753.
A genetic polymorphism of beta-chain rabbit (Oryctolagus cuniculus) hemoglobin is demonstrated by means of acid starch gel electrophoresis. The biochemical evidence presented suggests that a previously reported substitution of a neutral amino acid for a histidine is responsible for the detected genetic variation. Segregation analysis was performed in a sample of 15 matings with 49 offspring and confirmed the genetic hypothesis: two common alleles at an autosomal locus. The calculated gene frequencies in a random sample of 125 individuals are HBB*1 = 0.48 and HBB*2 = 0.52.  相似文献   
754.
Purpose

In support of the sustainable development of our societies, future engineers should have elementary knowledge in sustainability assessment and use of life cycle assessment. Publications on pedagogical experience with teaching life cycle assessment (LCA) in high-level education are however scarce. Here, we describe and discuss 20 years of experience in teaching LCA at MSc level in an engineering university with the ambition to share our insights and inspire teaching of LCA as part of a university curriculum.

Methods

We detail the design of an LCA course taught at the Technical University of Denmark since 1997. The course structure relies on (i) a structured combination of theoretical teaching, practical assignments and hands-on practice on LCA case studies, and (ii) the conduct of real-life LCA case studies in collaboration with companies or other organisations. Through the semester-long duration of the course, students from different engineering backgrounds perform full-fledged LCA studies in groups, passing through two iterations—a screening LCA supporting a more targeted LCA.

Results and discussion

The course design, which relies on a learning-by-doing principle, is transparently described to inspire LCA teachers among the readers. Historical evolution and statistics about the course, including its 192 case studies run in collaboration with 105 companies and institutions, are analysed and serve as basis to discuss the benefits and challenges of its different components, such as the theory acquisition, the assignment work, the LCA software learning, the conduct of case studies, the merits of industrial collaborations and grading approaches.

Conclusions

We demonstrate the win-win situation created by the setting of the course, in which the students are actively engaged and learn efficiently how to perform an LCA while the collaborating companies often get useful insights into their analysed case studies. The course can also be an eye opener for companies unfamiliar with LCA, who get introduced to life cycle thinking and the potential benefits of LCA. We have no hesitation in recommending industries and LCA teachers to engage into such collaborations even in the fundamental teaching of LCA techniques.

  相似文献   
755.
Relatively little is known about the biology and ecology of the world’s largest (heaviest) bony fish, the ocean sunfish Mola mola, despite its worldwide occurrence in temperate and tropical seas. Studies are now emerging that require many common perceptions about sunfish behaviour and ecology to be re-examined. Indeed, the long-held view that ocean sunfish are an inactive, passively drifting species seems to be entirely misplaced. Technological advances in marine telemetry are revealing distinct behavioural patterns and protracted seasonal movements. Extensive forays by ocean sunfish into the deep ocean have been documented and broad-scale surveys, together with molecular and laboratory based techniques, are addressing the connectivity and trophic role of these animals. These emerging molecular and movement studies suggest that local distinct populations may be prone to depletion through bycatch in commercial fisheries. Rising interest in ocean sunfish, highlighted by the increase in recent publications, warrants a thorough review of the biology and ecology of this species. Here we review the taxonomy, morphology, geography, diet, locomotion, vision, movements, foraging ecology, reproduction and species interactions of M. mola. We present a summary of current conservation issues and suggest methods for addressing fundamental gaps in our knowledge.  相似文献   
756.
A fast uptake of the tri-cationic 5-(4-carboxyphenyl)-10,15,20-tris(4-methylpyridinium-4-yl)porphyrin tri-iodide (P-H), independent of the presence or absence of proteins in the culture medium, occurs during incubation of NCTC 2544 human keratinocytes with this porphyrin. By contrast, the uptake of the poly-S-lysine conjugate (P-(Lys)(n)) is faster in serum-free medium than in the supplemented medium suggesting that P-(Lys)(n) interacts with serum proteins. The P-(Lys)(n) uptake is almost an order of magnitude greater than that of P-H in serum-free or supplemented culture medium. With histidine as a specific probe of type II photodynamic reactions, the relative photosensitizing effectiveness of the conjugate is only one fourth that of P-H. Nevertheless, the photocytotoxicity of the conjugate is strongly enhanced as compared to that of P-H as a result of its larger uptake. Thus, the doses achieving 50% of photocytotoxicity after incubation with 5 microM of the conjugate and its parent cationic porphyrin are about 20 min and 1 h, respectively. Similarly, the initial rate of the cell lipid peroxidation induced by photosensitization with P-(Lys)(n) is about 8 times higher than that obtained with P-H. Fluorescence microscopy reveals that P-H is more diffusely located in the cytoplasm than P-(Lys)(n) which seems to accumulate in lysosome-like structures. Little if any staining of the nucleus is observed with both photosensitizers.  相似文献   
757.
doi: 10.1111/j.1741‐2358.2012.00638.x Effects of the head lift exercise on the swallow function: a systematic review Objective: Ageing is associated with modifications in upper oesophageal sphincter function that may be deleterious to deglutition. The head lift exercise (Shaker exercise) is a head‐raising work out aiming to improve the opening of this segment, and ultimately to reduce aspiration. We aimed to review critically the evidence on the effects of this structured intervention programme and to identify gaps to be filled by future research. Material and Methods: We identified published reports addressing the effect of this exercise in increasing the anteroposterior diameter and cross‐sectional area of the deglutitive upper oesophageal sphincter opening or dysphagic symptoms. Results: Nine methodologically heterogeneous studies were included in the review, and the reported effects of the head lift exercise were an increase in the anterior excursion of the larynx and in the anteroposterior diameter of the upper oesophageal sphincter opening, and the elimination of dysphagic symptoms. Conclusion: Further studies including control groups and accounting for potential confounders are needed for a sound assessment of the effectiveness of this technique. The data on the functional results are, however, promising for dysphagia interventions.  相似文献   
758.
New tetracyclic benzofurocoumarin (benzopsoralen) analogues were synthesized and their inhibitory effect on the growth of tumor cell lines was evaluated. The human tumor cell lines used were MDA MB231 (breast adenocarcinoma), HeLa (cervix adenocarcinoma) and TCC-SUP (bladder transitional cell carcinoma). The in vitro antitumor activity of the new benzopsoralens was discussed in terms of structure–activity relationship. Molecular docking studies with human-CYP2A6 enzymes were also carried out with the synthesized compounds in order to evaluate the potential of these compounds to interact with the heme group of the enzymes. The results have demonstrated that the linear compounds have the most pronounced activity against tumor cell lines and this might be related to the better accessibility that these compounds have to the active site in relation to the angular ones that have shown in the majority of the cases multiple binding poses in the active site of CYP2A6.  相似文献   
759.
Before the 90s, data on Paleolithic human occupation of southern Portugal was very scarce. During the last decade, the knowledge of the Upper Paleolithic of Algarve increased substantially due to the work of a research team based at the University of Algarve. The present paper is a report on the recent results from Algarve, focusing specially on the site of Vale Boi. It will present the chronology and stratigraphy of different human occupations from the early Upper Paleolithic up to the early Neolithic. It will focus on aspects of zooarchaeology and the exploitation of large and medium mammals as well as on marine fauna. In addition, we will present new data on stone and bone tools. Finally, we will also refer to the social and symbolic aspects present at the site, base on shell and teeth pendants and to an engraved plaquette with animal motifs.  相似文献   
760.
The molecular details underlying the time-dependent assembly of protein complexes in cellular networks, such as those that occur during differentiation, are largely unexplored. Focusing on the calcium-induced differentiation of primary human keratinocytes as a model system for a major cellular reorganization process, we look at the expression of genes whose products are involved in manually-annotated protein complexes. Clustering analyses revealed only moderate co-expression of functionally related proteins during differentiation. However, when we looked at protein complexes, we found that the majority (55%) are composed of non-dynamic and dynamic gene products (‘di-chromatic’), 19% are non-dynamic, and 26% only dynamic. Considering three-dimensional protein structures to predict steric interactions, we found that proteins encoded by dynamic genes frequently interact with a common non-dynamic protein in a mutually exclusive fashion. This suggests that during differentiation, complex assemblies may also change through variation in the abundance of proteins that compete for binding to common proteins as found in some cases for paralogous proteins. Considering the example of the TNF-α/NFκB signaling complex, we suggest that the same core complex can guide signals into diverse context-specific outputs by addition of time specific expressed subunits, while keeping other cellular functions constant. Thus, our analysis provides evidence that complex assembly with stable core components and competition could contribute to cell differentiation.  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