全文获取类型
收费全文 | 17337篇 |
免费 | 45篇 |
国内免费 | 39篇 |
专业分类
系统科学 | 87篇 |
丛书文集 | 263篇 |
教育与普及 | 30篇 |
理论与方法论 | 94篇 |
现状及发展 | 8305篇 |
研究方法 | 721篇 |
综合类 | 7734篇 |
自然研究 | 187篇 |
出版年
2013年 | 85篇 |
2012年 | 202篇 |
2011年 | 402篇 |
2010年 | 86篇 |
2008年 | 240篇 |
2007年 | 250篇 |
2006年 | 296篇 |
2005年 | 291篇 |
2004年 | 389篇 |
2003年 | 274篇 |
2002年 | 267篇 |
2001年 | 490篇 |
2000年 | 452篇 |
1999年 | 330篇 |
1992年 | 288篇 |
1991年 | 230篇 |
1990年 | 243篇 |
1989年 | 239篇 |
1988年 | 204篇 |
1987年 | 238篇 |
1986年 | 281篇 |
1985年 | 338篇 |
1984年 | 236篇 |
1983年 | 229篇 |
1982年 | 213篇 |
1981年 | 230篇 |
1980年 | 233篇 |
1979年 | 573篇 |
1978年 | 491篇 |
1977年 | 443篇 |
1976年 | 357篇 |
1975年 | 409篇 |
1974年 | 568篇 |
1973年 | 463篇 |
1972年 | 477篇 |
1971年 | 562篇 |
1970年 | 703篇 |
1969年 | 547篇 |
1968年 | 550篇 |
1967年 | 541篇 |
1966年 | 435篇 |
1965年 | 386篇 |
1964年 | 126篇 |
1959年 | 193篇 |
1958年 | 331篇 |
1957年 | 247篇 |
1956年 | 190篇 |
1955年 | 192篇 |
1954年 | 195篇 |
1948年 | 131篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
951.
Electrochemistry. Making a potential difference 总被引:1,自引:0,他引:1
Blomgren GE 《Nature》2000,407(6805):681-682
952.
Synthetic single-helical conformations are quite common, but the formation of double helices based on recognition between the two constituent strands is relatively rare. Known examples include duplex formation through base-pair-specific hydrogen bonding and stacking, as found in nucleic acids and their analogues, and polypeptides composed of amino acids with alternating L and D configurations. Some synthetic polymers and self-assembled fibres have double-helical winding induced by van der Waals interactions. A third mode of non-covalent interaction, coordination of organic ligands to metal ions, can give rise to double, triple and quadruple helices, although in this case the assembly is driven by the coordination geometry of the metal and the structure of the ligands, rather than by direct inter-strand complementarity. Here we describe a family of oligomeric molecules with bent conformations, which exhibit dynamic exchange between single and double molecular helices in solution, through spiral sliding of the synthetic oligomer strands. The bent conformations leading to the helical shape of the molecules result from intramolecular hydrogen bonding within 2'-pyridyl-2-pyridinecarboxamide units, with extensive intermolecular aromatic stacking stabilizing the double-stranded helices that form through dimerization. 相似文献
953.
The role of microbes in accretion, lamination and early lithification of modern marine stromatolites 总被引:28,自引:0,他引:28
Reid RP Visscher PT Decho AW Stolz JF Bebout BM Dupraz C Macintyre IG Paerl HW Pinckney JL Prufert-Bebout L Steppe TF DesMarais DJ 《Nature》2000,406(6799):989-992
For three billion years, before the Cambrian diversification of life, laminated carbonate build-ups called stromatolites were widespread in shallow marine seas. These ancient structures are generally thought to be microbial in origin and potentially preserve evidence of the Earth's earliest biosphere. Despite their evolutionary significance, little is known about stromatolite formation, especially the relative roles of microbial and environmental factors in stromatolite accretion. Here we show that growth of modern marine stromatolites represents a dynamic balance between sedimentation and intermittent lithification of cyanobacterial mats. Periods of rapid sediment accretion, during which stromatolite surfaces are dominated by pioneer communities of gliding filamentous cyanobacteria, alternate with hiatal intervals. These discontinuities in sedimentation are characterized by development of surface films of exopolymer and subsequent heterotrophic bacterial decomposition, forming thin crusts of microcrystalline carbonate. During prolonged hiatal periods, climax communities develop, which include endolithic coccoid cyanobacteria. These coccoids modify the sediment, forming thicker lithified laminae. Preservation of lithified layers at depth creates millimetre-scale lamination. This simple model of modern marine stromatolite growth may be applicable to ancient stromatolites. 相似文献
954.
Subduction and collision processes in the Central Andes constrained by converted seismic phases 总被引:5,自引:0,他引:5
Yuan X Sobolev SV Kind R Oncken O Bock G Asch G Schurr B Graeber F Rudloff A Hanka W Wylegalla K Tibi R Haberland C Rietbrock A Giese P Wigger P Röwer P Zandt G Beck S Wallace T Pardo M Comte D 《Nature》2000,408(6815):958-961
The Central Andes are the Earth's highest mountain belt formed by ocean-continent collision. Most of this uplift is thought to have occurred in the past 20 Myr, owing mainly to thickening of the continental crust, dominated by tectonic shortening. Here we use P-to-S (compressional-to-shear) converted teleseismic waves observed on several temporary networks in the Central Andes to image the deep structure associated with these tectonic processes. We find that the Moho (the Mohorovici? discontinuity--generally thought to separate crust from mantle) ranges from a depth of 75 km under the Altiplano plateau to 50 km beneath the 4-km-high Puna plateau. This relatively thin crust below such a high-elevation region indicates that thinning of the lithospheric mantle may have contributed to the uplift of the Puna plateau. We have also imaged the subducted crust of the Nazca oceanic plate down to 120 km depth, where it becomes invisible to converted teleseismic waves, probably owing to completion of the gabbro-eclogite transformation; this is direct evidence for the presence of kinetically delayed metamorphic reactions in subducting plates. Most of the intermediate-depth seismicity in the subducting plate stops at 120 km depth as well, suggesting a relation with this transformation. We see an intracrustal low-velocity zone, 10-20 km thick, below the entire Altiplano and Puna plateaux, which we interpret as a zone of continuing metamorphism and partial melting that decouples upper-crustal imbrication from lower-crustal thickening. 相似文献
955.
A learning deficit related to age and beta-amyloid plaques in a mouse model of Alzheimer's disease 总被引:21,自引:0,他引:21
Chen G Chen KS Knox J Inglis J Bernard A Martin SJ Justice A McConlogue L Games D Freedman SB Morris RG 《Nature》2000,408(6815):975-979
Mice that overexpress the human mutant amyloid precursor protein (hAPP) show learning deficits, but the apparent lack of a relationship between these deficits and the progressive beta-amyloid plaque formation that the hAPP mice display is puzzling. In the water maze, hAPP mice are impaired before and after amyloid plaque deposition. Here we show, using a new water-maze training protocol, that PDAPP mice also exhibit a separate age-related deficit in learning a series of spatial locations. This impairment correlates with beta-amyloid plaque burden and is shown in both cross-sectional and longitudinal experimental designs. Cued navigation and object-recognition memory are normal. These findings indicate that A beta overexpression and/or A beta plaques are associated with disturbed cognitive function and, importantly, suggest that some but not all forms of learning and memory are suitable behavioural assays of the progressive cognitive deficits associated with Alzheimer's-disease-type pathologies. 相似文献
956.
Timing of the Martian dynamo 总被引:3,自引:0,他引:3
957.
Mu-opioid receptor desensitization by beta-arrestin-2 determines morphine tolerance but not dependence 总被引:19,自引:0,他引:19
Morphine is a powerful pain reliever, but also a potent inducer of tolerance and dependence. The development of opiate tolerance occurs on continued use of the drug such that the amount of drug required to elicit pain relief must be increased to compensate for diminished responsiveness. In many systems, decreased responsiveness to agonists has been correlated with the desensitization of G-protein-coupled receptors. In vitro evidence indicates that this process involves phosphorylation of G-protein-coupled receptors and subsequent binding of regulatory proteins called beta-arrestins. Using a knockout mouse lacking beta-arrestin-2 (beta arr2-/-), we have assessed the contribution of desensitization of the mu-opioid receptor to the development of morphine antinociceptive tolerance and the subsequent onset of physical dependence. Here we show that in mice lacking beta-arrestin-2, desensitization of the mu-opioid receptor does not occur after chronic morphine treatment, and that these animals fail to develop antinociceptive tolerance. However, the deletion of beta-arrestin-2 does not prevent the chronic morphine-induced up-regulation of adenylyl cyclase activity, a cellular marker of dependence, and the mutant mice still become physically dependent on the drug. 相似文献
958.
959.
Scarborough GA 《Cellular and molecular life sciences : CMLS》2000,57(6):871-883
Living cells require membranes and membrane transporters for the maintenance of life. After decades of biochemical scrutiny, the structures and molecular mechanisms by which membrane transporters catalyze transmembrane solute movements are beginning to be understood. The plasma membrane proton-translocating adenosine triphosphatase (ATPase) is an archetype of the P-type ATPase family of membrane transporters, which are important in a wide variety of cellular processes. The H+-ATPase has been crystallized and its structure determined to a resolution of 8 angstrom in the membrane plane. When considered together with the large body of biochemical information that has been accumulated for this transporter, and for enzymes in general, this new structural information is providing tantalizing insights regarding the molecular mechanism of active ion transport catalyzed by this enzyme. 相似文献
960.
Replication of linear genomes is incomplete and leaves terminal gaps. Solutions to this 'end replication' problem can be traced back to the prebiotic RNA world: 'fossils' of the presumptive archetypes of telomere structure and of the telomerase enzyme are retained in the terminal structures of some RNA viruses. Telomerase expression in mammals is ubiquitous in embryonic tissues but downregulated in somatic tissues of adults. Exceptions are regenerative tissues and, notably, tumor cells. Telomerase activation is controlled by cellular proliferation, and it is an early step in the development of many tumors. In contrast to mammals, indeterminately growing multicellular organisms, such as fish and crustaceae, maintain telomerase competence in all somatic tissues. In human tumor diagnostics, detection of proliferation markers with monoclonal antibodies is well established, and in this review, the significance of additional telomerase assays is evaluated. Telomerase inhibitors are attractive goals for application in tumor therapy, and telomerase knockout mice have proven that telomere erosion limits the lifespan of cells in vivo. In contrast, telomerase stimulation can be used to expand the potential of cellular proliferation in vitro, with possible applications for transplantation of in vitro expanded human cells, for immortalizing primary human cells as improved tissue models and for the isolation of otherwise intractable products, such as genuine human monoclonal antibodies. 相似文献