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161.
Résumé Nous présentons une étude du mécanisme de l'élastolyse par une élastase partiellement purifiée du pancréas. La méthode utilisée consiste dans le dosage spectrophotométrique de l'«azoelastine» dissoute. Cette protéine n'est que très lentement hydrolysée par la trypsine et chymotrypsine cristallisées. L'élastase possède une activité protéasique considérable. L'enzyme est très rapidement adsorbée sur l'élastine. Après une incubation à pH 4,5 avec l'enzyme adsorbée, le temps de latence de l'élastolyse disparaît et la rapidité de la réaction devient proportionnelle à la concentration en élastase. 相似文献
162.
L. Robert 《Cellular and molecular life sciences : CMLS》1955,11(8):316-317
Summary A method is proposed for dosage of serum procainesterase by differential spectrophotometry, based on the decrease of the optical density at 295 mµ in the course of the hydrolysis. 相似文献
163.
Williams TN Mwangi TW Wambua S Peto TE Weatherall DJ Gupta S Recker M Penman BS Uyoga S Macharia A Mwacharo JK Snow RW Marsh K 《Nature genetics》2005,37(11):1253-1257
The hemoglobinopathies, disorders of hemoglobin structure and production, protect against death from malaria. In sub-Saharan Africa, two such conditions occur at particularly high frequencies: presence of the structural variant hemoglobin S and alpha(+)-thalassemia, a condition characterized by reduced production of the normal alpha-globin component of hemoglobin. Individually, each is protective against severe Plasmodium falciparum malaria, but little is known about their malaria-protective effects when inherited in combination. We investigated this question by studying a population on the coast of Kenya and found that the protection afforded by each condition inherited alone was lost when the two conditions were inherited together, to such a degree that the incidence of both uncomplicated and severe P. falciparum malaria was close to baseline in children heterozygous with respect to the mutation underlying the hemoglobin S variant and homozygous with respect to the mutation underlying alpha(+)-thalassemia. Negative epistasis could explain the failure of alpha(+)-thalassemia to reach fixation in any population in sub-Saharan Africa. 相似文献
164.
Structure of the cross-beta spine of amyloid-like fibrils 总被引:1,自引:0,他引:1
Nelson R Sawaya MR Balbirnie M Madsen AØ Riekel C Grothe R Eisenberg D 《Nature》2005,435(7043):773-778
Numerous soluble proteins convert to insoluble amyloid-like fibrils that have common properties. Amyloid fibrils are associated with fatal diseases such as Alzheimer's, and amyloid-like fibrils can be formed in vitro. For the yeast protein Sup35, conversion to amyloid-like fibrils is associated with a transmissible infection akin to that caused by mammalian prions. A seven-residue peptide segment from Sup35 forms amyloid-like fibrils and closely related microcrystals, from which we have determined the atomic structure of the cross-beta spine. It is a double beta-sheet, with each sheet formed from parallel segments stacked in register. Side chains protruding from the two sheets form a dry, tightly self-complementing steric zipper, bonding the sheets. Within each sheet, every segment is bound to its two neighbouring segments through stacks of both backbone and side-chain hydrogen bonds. The structure illuminates the stability of amyloid fibrils, their self-seeding characteristic and their tendency to form polymorphic structures. 相似文献
165.
The frequencies of extended X-ray absorption fine-structure (EXAFS) measurements, which are oscillations occurring on the high-energy side of an X-ray absorption edge, can be used to identify interatomic distances in materials. We have used a dispersive X-ray spectrometer, which has no moving components, to make rapid measurements with minimal energy drift of the difference in EXAFS from the Fe K edge in an iron-cobalt thin film undergoing periodic strain through magnetostriction. We show that magnetostriction can be detected by differential X-ray absorption. The magnitude of the recorded signal relative to the noise shows a sensitivity to mean differential atomic motion of one femtometre: a factor of 100 times more sensitive than that normally available. 相似文献
166.
Massive iceberg discharges from the Northern Hemisphere ice sheets, 'Heinrich events', coincided with the coldest periods of the last ice age. There is widespread evidence for Heinrich events and their profound impact on the climate and circulation of the North Atlantic Ocean, but their influence beyond that region remains uncertain. Here we use a combination of molecular fingerprints of algal productivity and radioisotope tracers of sedimentation to document eight periods of increased productivity in the subpolar Southern Ocean during the past 70,000 years that occurred within 1,000-2,000 years of a Northern Hemisphere Heinrich event. We discuss possible causes for such a link, including increased supply of iron from upwelling and increased stratification during the growing season, which imply an alteration of the global ocean circulation during Heinrich events. The mechanisms linking North Atlantic iceberg discharges with subantarctic productivity remain unclear at this point. We suggest that understanding how the Southern Ocean was altered during these extreme climate perturbations is critical to understanding the role of the ocean in climate change. 相似文献
167.
Hillier LW Graves TA Fulton RS Fulton LA Pepin KH Minx P Wagner-McPherson C Layman D Wylie K Sekhon M Becker MC Fewell GA Delehaunty KD Miner TL Nash WE Kremitzki C Oddy L Du H Sun H Bradshaw-Cordum H Ali J Carter J Cordes M Harris A Isak A van Brunt A Nguyen C Du F Courtney L Kalicki J Ozersky P Abbott S Armstrong J Belter EA Caruso L Cedroni M Cotton M Davidson T Desai A Elliott G Erb T Fronick C Gaige T Haakenson W Haglund K Holmes A Harkins R Kim K Kruchowski SS Strong CM Grewal N Goyea E 《Nature》2005,434(7034):724-731
Human chromosome 2 is unique to the human lineage in being the product of a head-to-head fusion of two intermediate-sized ancestral chromosomes. Chromosome 4 has received attention primarily related to the search for the Huntington's disease gene, but also for genes associated with Wolf-Hirschhorn syndrome, polycystic kidney disease and a form of muscular dystrophy. Here we present approximately 237 million base pairs of sequence for chromosome 2, and 186 million base pairs for chromosome 4, representing more than 99.6% of their euchromatic sequences. Our initial analyses have identified 1,346 protein-coding genes and 1,239 pseudogenes on chromosome 2, and 796 protein-coding genes and 778 pseudogenes on chromosome 4. Extensive analyses confirm the underlying construction of the sequence, and expand our understanding of the structure and evolution of mammalian chromosomes, including gene deserts, segmental duplications and highly variant regions. 相似文献
168.
Time-resolved optical spectroscopy is widely used to study vibrational and electronic dynamics by monitoring transient changes in excited state populations on a femtosecond timescale. Yet the fundamental cause of electronic and vibrational dynamics--the coupling between the different energy levels involved--is usually inferred only indirectly. Two-dimensional femtosecond infrared spectroscopy based on the heterodyne detection of three-pulse photon echoes has recently allowed the direct mapping of vibrational couplings, yielding transient structural information. Here we extend the approach to the visible range and directly measure electronic couplings in a molecular complex, the Fenna-Matthews-Olson photosynthetic light-harvesting protein. As in all photosynthetic systems, the conversion of light into chemical energy is driven by electronic couplings that ensure the efficient transport of energy from light-capturing antenna pigments to the reaction centre. We monitor this process as a function of time and frequency and show that excitation energy does not simply cascade stepwise down the energy ladder. We find instead distinct energy transport pathways that depend sensitively on the detailed spatial properties of the delocalized excited-state wavefunctions of the whole pigment-protein complex. 相似文献
169.
170.
Understanding the flow of water through the body of a glacier is important, because the spatial distribution of water and the rate of infiltration to the glacier bottom is one control on water storage and pressure, glacier sliding and surging, and the release of glacial outburst floods. According to the prevailing hypothesis, this water flow takes place in a network of tubular conduits. Here we analyse video images from 48 boreholes drilled into the small Swedish glacier Storglaci?ren, showing that the glacier's hydrological system is instead dominated by fractures that convey water at slow speeds. We detected hydraulically connected fractures at all depths, including near the glacier bottom. Our observations indicate that fractures provide the main pathways for surface water to reach deep within the glacier, whereas tubular conduits probably form only in special circumstances. A network of hydraulically linked fractures offers a simple explanation for the origin and evolution of the englacial water flow system and its seasonal regeneration. Such a fracture network also explains radar observations that reveal a complex pattern of echoes rather than a system of conduits. Our findings may be important in understanding the catastrophic collapse of ice shelves and rapid hydraulic connection between the surface and bed of an ice sheet. 相似文献