全文获取类型
收费全文 | 3212篇 |
免费 | 22篇 |
国内免费 | 4篇 |
专业分类
系统科学 | 54篇 |
丛书文集 | 1篇 |
理论与方法论 | 59篇 |
现状及发展 | 1882篇 |
研究方法 | 245篇 |
综合类 | 931篇 |
自然研究 | 66篇 |
出版年
2020年 | 17篇 |
2018年 | 53篇 |
2017年 | 41篇 |
2016年 | 54篇 |
2015年 | 39篇 |
2014年 | 42篇 |
2013年 | 45篇 |
2012年 | 132篇 |
2011年 | 182篇 |
2010年 | 72篇 |
2009年 | 25篇 |
2008年 | 113篇 |
2007年 | 97篇 |
2006年 | 112篇 |
2005年 | 114篇 |
2004年 | 105篇 |
2003年 | 89篇 |
2002年 | 82篇 |
2001年 | 53篇 |
2000年 | 39篇 |
1999年 | 36篇 |
1998年 | 13篇 |
1992年 | 28篇 |
1991年 | 22篇 |
1990年 | 29篇 |
1989年 | 21篇 |
1988年 | 18篇 |
1987年 | 32篇 |
1986年 | 24篇 |
1985年 | 28篇 |
1984年 | 30篇 |
1982年 | 29篇 |
1981年 | 18篇 |
1980年 | 37篇 |
1979年 | 86篇 |
1978年 | 74篇 |
1977年 | 96篇 |
1976年 | 24篇 |
1975年 | 40篇 |
1974年 | 98篇 |
1973年 | 86篇 |
1972年 | 106篇 |
1971年 | 78篇 |
1970年 | 94篇 |
1969年 | 100篇 |
1968年 | 73篇 |
1967年 | 94篇 |
1966年 | 82篇 |
1965年 | 62篇 |
1964年 | 18篇 |
排序方式: 共有3238条查询结果,搜索用时 15 毫秒
61.
Mechanotransduction refers to the transformation of physical forces into chemical signals. It generally involves stretch-sensitive channels or conformational change of cytoskeleton-associated proteins. Mechanotransduction is crucial for the physiology of several organs and for cell migration. The extent to which mechanical inputs contribute to development, and how they do this, remains poorly defined. Here we show that a mechanotransduction pathway operates between the body-wall muscles of Caenorhabditis elegans and the epidermis. This pathway involves, in addition to a Rac GTPase, three signalling proteins found at the hemidesmosome: p21-activated kinase (PAK-1), the adaptor GIT-1 and its partner PIX-1. The phosphorylation of intermediate filaments is one output of this pathway. Tension exerted by adjacent muscles or externally exerted mechanical pressure maintains GIT-1 at hemidesmosomes and stimulates PAK-1 activity through PIX-1 and Rac. This pathway promotes the maturation of a hemidesmosome into a junction that can resist mechanical stress and contributes to coordinating the morphogenesis of epidermal and muscle tissues. Our findings suggest that the C. elegans hemidesmosome is not only an attachment structure, but also a mechanosensor that responds to tension by triggering signalling processes. We suggest that similar pathways could promote epithelial morphogenesis or wound healing in other organisms in which epithelial cells adhere to tension-generating contractile cells. 相似文献
62.
Spin-orbit (SO) coupling--the interaction between a quantum particle's spin and its momentum--is ubiquitous in physical systems. In condensed matter systems, SO coupling is crucial for the spin-Hall effect and topological insulators; it contributes to the electronic properties of materials such as GaAs, and is important for spintronic devices. Quantum many-body systems of ultracold atoms can be precisely controlled experimentally, and would therefore seem to provide an ideal platform on which to study SO coupling. Although an atom's intrinsic SO coupling affects its electronic structure, it does not lead to coupling between the spin and the centre-of-mass motion of the atom. Here, we engineer SO coupling (with equal Rashba and Dresselhaus strengths) in a neutral atomic Bose-Einstein condensate by dressing two atomic spin states with a pair of lasers. Such coupling has not been realized previously for ultracold atomic gases, or indeed any bosonic system. Furthermore, in the presence of the laser coupling, the interactions between the two dressed atomic spin states are modified, driving a quantum phase transition from a spatially spin-mixed state (lasers off) to a phase-separated state (above a critical laser intensity). We develop a many-body theory that provides quantitative agreement with the observed location of the transition. The engineered SO coupling--equally applicable for bosons and fermions--sets the stage for the realization of topological insulators in fermionic neutral atom systems. 相似文献
63.
Direct observations over the past four centuries show that the number of sunspots observed on the Sun's surface varies periodically, going through successive maxima and minima. Following sunspot cycle 23, the Sun went into a prolonged minimum characterized by a very weak polar magnetic field and an unusually large number of days without sunspots. Sunspots are strongly magnetized regions generated by a dynamo mechanism that recreates the solar polar field mediated through plasma flows. Here we report results from kinematic dynamo simulations which demonstrate that a fast meridional flow in the first half of a cycle, followed by a slower flow in the second half, reproduces both characteristics of the minimum of sunspot cycle 23. Our model predicts that, in general, very deep minima are associated with weak polar fields. Sunspots govern the solar radiative energy and radio flux, and, in conjunction with the polar field, modulate the solar wind, the heliospheric open flux and, consequently, the cosmic ray flux at Earth. 相似文献
64.
Suhre K Shin SY Petersen AK Mohney RP Meredith D Wägele B Altmaier E;CARDIoGRAM Deloukas P Erdmann J Grundberg E Hammond CJ de Angelis MH Kastenmüller G Köttgen A Kronenberg F Mangino M Meisinger C Meitinger T Mewes HW Milburn MV Prehn C Raffler J Ried JS Römisch-Margl W Samani NJ Small KS Wichmann HE Zhai G Illig T Spector TD Adamski J Soranzo N Gieger C 《Nature》2011,477(7362):54-60
Genome-wide association studies (GWAS) have identified many risk loci for complex diseases, but effect sizes are typically small and information on the underlying biological processes is often lacking. Associations with metabolic traits as functional intermediates can overcome these problems and potentially inform individualized therapy. Here we report a comprehensive analysis of genotype-dependent metabolic phenotypes using a GWAS with non-targeted metabolomics. We identified 37 genetic loci associated with blood metabolite concentrations, of which 25 show effect sizes that are unusually high for GWAS and account for 10-60% differences in metabolite levels per allele copy. Our associations provide new functional insights for many disease-related associations that have been reported in previous studies, including those for cardiovascular and kidney disorders, type 2 diabetes, cancer, gout, venous thromboembolism and Crohn's disease. The study advances our knowledge of the genetic basis of metabolic individuality in humans and generates many new hypotheses for biomedical and pharmaceutical research. 相似文献
65.
Sicardy B Ortiz JL Assafin M Jehin E Maury A Lellouch E Hutton RG Braga-Ribas F Colas F Hestroffer D Lecacheux J Roques F Santos-Sanz P Widemann T Morales N Duffard R Thirouin A Castro-Tirado AJ Jelínek M Kubánek P Sota A Sánchez-Ramírez R Andrei AH Camargo JI da Silva Neto DN Gomes AR Martins RV Gillon M Manfroid J Tozzi GP Harlingten C Saravia S Behrend R Mottola S Melendo EG Peris V Fabregat J Madiedo JM Cuesta L Eibe MT Ullán A Organero F Pastor S de Los Reyes JA Pedraz S Castro A 《Nature》2011,478(7370):493-496
The dwarf planet Eris is a trans-Neptunian object with an orbital eccentricity of 0.44, an inclination of 44 degrees and a surface composition very similar to that of Pluto. It resides at present at 95.7 astronomical units (1?AU is the Earth-Sun distance) from Earth, near its aphelion and more than three times farther than Pluto. Owing to this great distance, measuring its size or detecting a putative atmosphere is difficult. Here we report the observation of a multi-chord stellar occultation by Eris on 6 November 2010 UT. The event is consistent with a spherical shape for Eris, with radius 1,163?±?6?kilometres, density 2.52?±?0.05 grams per cm(3) and a high visible geometric albedo, Pv = 0.96(+0.09)(-0.04). No nitrogen, argon or methane atmospheres are detected with surface pressure larger than ~1?nanobar, about 10,000 times more tenuous than Pluto's present atmosphere. As Pluto's radius is estimated to be between 1,150 and 1,200 kilometres, Eris appears as a Pluto twin, with a bright surface possibly caused by a collapsed atmosphere, owing to its cold environment. We anticipate that this atmosphere may periodically sublimate as Eris approaches its perihelion, at 37.8 astronomical units from the Sun. 相似文献
66.
Role of sulphuric acid, ammonia and galactic cosmic rays in atmospheric aerosol nucleation 总被引:1,自引:0,他引:1
Kirkby J Curtius J Almeida J Dunne E Duplissy J Ehrhart S Franchin A Gagné S Ickes L Kürten A Kupc A Metzger A Riccobono F Rondo L Schobesberger S Tsagkogeorgas G Wimmer D Amorim A Bianchi F Breitenlechner M David A Dommen J Downard A Ehn M Flagan RC Haider S Hansel A Hauser D Jud W Junninen H Kreissl F Kvashin A Laaksonen A Lehtipalo K Lima J Lovejoy ER Makhmutov V Mathot S Mikkilä J Minginette P Mogo S Nieminen T Onnela A Pereira P Petäjä T Schnitzhofer R Seinfeld JH Sipilä M Stozhkov Y 《Nature》2011,476(7361):429-433
Atmospheric aerosols exert an important influence on climate through their effects on stratiform cloud albedo and lifetime and the invigoration of convective storms. Model calculations suggest that almost half of the global cloud condensation nuclei in the atmospheric boundary layer may originate from the nucleation of aerosols from trace condensable vapours, although the sensitivity of the number of cloud condensation nuclei to changes of nucleation rate may be small. Despite extensive research, fundamental questions remain about the nucleation rate of sulphuric acid particles and the mechanisms responsible, including the roles of galactic cosmic rays and other chemical species such as ammonia. Here we present the first results from the CLOUD experiment at CERN. We find that atmospherically relevant ammonia mixing ratios of 100 parts per trillion by volume, or less, increase the nucleation rate of sulphuric acid particles more than 100-1,000-fold. Time-resolved molecular measurements reveal that nucleation proceeds by a base-stabilization mechanism involving the stepwise accretion of ammonia molecules. Ions increase the nucleation rate by an additional factor of between two and more than ten at ground-level galactic-cosmic-ray intensities, provided that the nucleation rate lies below the limiting ion-pair production rate. We find that ion-induced binary nucleation of H(2)SO(4)-H(2)O can occur in the mid-troposphere but is negligible in the boundary layer. However, even with the large enhancements in rate due to ammonia and ions, atmospheric concentrations of ammonia and sulphuric acid are insufficient to account for observed boundary-layer nucleation. 相似文献
67.
This paper presents an analysis of shift-contagion in energy markets, testing whether linkages between returns in energy markets increase during crisis periods. The research presented herein demonstrates how common movement between energy markets increases due to (i) shift-contagion across energy markets, reflected by structural transmission of shocks across markets and (ii) larger common shocks operating through standard cross-market interdependences. A regime-switching model was developed to detect shift-contagion across energy markets. In the approach adopted herein, the occurrence of shift-contagion is endogenously estimated rather than being exogenously assigned. The results show that shift-contagion has been a major feature of energy markets over the last decade. Evidence is presented which demonstrates that the linkages between energy markets do not appear to be stable. These results are remarkably accurate for forecasting Brent and natural gas for horizons for up to 50 days. Conversely, for WTI (West Texas Intermediate oil) and coal, the model performs well only for forecasting very short horizons (up to 20 days). For all products, the model shows significant biases for long horizons. 相似文献
68.
R. P. Massengo-Tiassé J. E. Cronan 《Cellular and molecular life sciences : CMLS》2009,66(9):1507-1517
The enoyl-acyl carrier protein reductase (ENR) is the last enzyme in the fatty acid elongation cycle. Unlike most enzymes
in this essential pathway, ENR displays an unusual diversity among organisms. The growing interest in ENRs is mainly due to
the fact that a variety of both synthetic and natural antibacterial compounds are shown to specifically target their activity.
The primary anti-tuberculosis drug, isoniazid, and the broadly used antibacterial compound, triclosan, both target this enzyme.
In this review, we discuss the diversity of ENRs, and their inhibitors in the light of current research progress.
Received 3 November 2008; received after revision 5 December 2008; accepted 8 December 2008 相似文献
69.
70.
Didier Vilette Josquin Courte Jean Michel Peyrin Laurent Coudert Laurent Schaeffer Olivier Andréoletti Pascal Leblanc 《Cellular and molecular life sciences : CMLS》2018,75(14):2557-2574
Prions are infectious agents that cause fatal neurodegenerative diseases. Current evidence indicates that they are essentially composed of an abnormally folded protein (PrPSc). These abnormal aggregated PrPSc species multiply in infected cells by recruiting and converting the host PrPC protein into new PrPSc. How prions move from cell to cell and progressively spread across the infected tissue is of crucial importance and may provide experimental opportunity to delay the progression of the disease. In infected cells, different mechanisms have been identified, including release of infectious extracellular vesicles and intercellular transfer of PrPSc-containing organelles through tunneling nanotubes. These findings should allow manipulation of the intracellular trafficking events targeting PrPSc in these particular subcellular compartments to experimentally address the relative contribution of these mechanisms to in vivo prion pathogenesis. In addition, such information may prompt further experimental strategies to decipher the causal roles of protein misfolding and aggregation in other human neurodegenerative diseases. 相似文献