全文获取类型
收费全文 | 14580篇 |
免费 | 41篇 |
国内免费 | 46篇 |
专业分类
系统科学 | 154篇 |
丛书文集 | 53篇 |
教育与普及 | 45篇 |
理论与方法论 | 88篇 |
现状及发展 | 5775篇 |
研究方法 | 659篇 |
综合类 | 7569篇 |
自然研究 | 324篇 |
出版年
2013年 | 101篇 |
2012年 | 243篇 |
2011年 | 600篇 |
2010年 | 97篇 |
2008年 | 259篇 |
2007年 | 293篇 |
2006年 | 297篇 |
2005年 | 305篇 |
2004年 | 366篇 |
2003年 | 298篇 |
2002年 | 294篇 |
2001年 | 387篇 |
2000年 | 410篇 |
1999年 | 273篇 |
1992年 | 280篇 |
1991年 | 202篇 |
1990年 | 209篇 |
1989年 | 211篇 |
1988年 | 203篇 |
1987年 | 228篇 |
1986年 | 213篇 |
1985年 | 298篇 |
1984年 | 236篇 |
1983年 | 166篇 |
1982年 | 144篇 |
1981年 | 141篇 |
1980年 | 175篇 |
1979年 | 416篇 |
1978年 | 312篇 |
1977年 | 311篇 |
1976年 | 274篇 |
1975年 | 285篇 |
1974年 | 398篇 |
1973年 | 354篇 |
1972年 | 308篇 |
1971年 | 411篇 |
1970年 | 581篇 |
1969年 | 421篇 |
1968年 | 415篇 |
1967年 | 413篇 |
1966年 | 444篇 |
1965年 | 292篇 |
1964年 | 123篇 |
1959年 | 134篇 |
1958年 | 250篇 |
1957年 | 152篇 |
1956年 | 170篇 |
1955年 | 128篇 |
1954年 | 109篇 |
1948年 | 153篇 |
排序方式: 共有10000条查询结果,搜索用时 422 毫秒
971.
Iron-mediated inhibition of H+-ATPase in plasma membrane vesicles isolated from wheat roots 总被引:4,自引:0,他引:4
Yang YL Zhang F He WL Wang XM Zhang LX 《Cellular and molecular life sciences : CMLS》2003,60(6):1249-1257
The mechanisms of iron-mediated inhibition of the H(+)-ATPase activity of plasma membrane (PM) vesicles isolated from wheat roots were investigated. Both FeSO(4) and FeCl(3) significantly inhibited PM H(+)-ATPase activity, and the inhibition could be reversed by the addition of the metal ion chelator EDTA-Na(2) or a specific Fe(2+) chelator, indicating that the inhibitory effect was due to specific action of Fe(2+) or Fe(3+). Measurement of the extent of lipid peroxidation showed that oxidative damage on the PM caused by Fe(2+) or Fe(3+) seemed to be correlated with the inhibition of PM H(+)-ATPase activity. However, prevention of lipid peroxidation with butylated hydroxytoluene did not affect iron-mediated inhibition in the PM H(+)-ATPase, suggesting that the inhibition of the PM H(+)-ATPase was not a consequence of lipid peroxidation caused by iron. Investigation of the effects of various reactive oxygen species scavengers on the iron-mediated inhibition of H(+)-ATPase activity indicated that hydroxyl radicals (*OH) and hydrogen peroxide (H(2)O(2)) might be involved in the Fe(2+)-mediated decrease in PM H(+)-ATPase activity. Moreover, iron caused a decrease in plasma protein thiol (P-SH), and Fe(3+) brought a higher degree of oxidation in thiol groups than Fe(2+) at the same concentration. Modification of the thiol redox state in the PM suggested that reducing thiol groups were essential to maintain PM H(+)-ATPase activity. Incubation of the specific thiol modification reagent 5,5-dithio-bis(2-nitrobenzoic acid) with the rightside-out and inside-out PM revealed that thiol oxidation occurred at the apoplast side of the PM. Western blotting analysis revealed a decrease in H(+)-ATPase content caused by iron. Taken together, these results suggested that thiol oxidation might account for the inhibition of PM H(+)-ATPase caused by iron, and that *OH and H(2)O(2) were also involved in Fe(2+)-mediated inhibition. 相似文献
972.
973.
Inada N Oguri M Pindor B Hennawi JF Chiu K Zheng W Ichikawa S Gregg MD Becker RH Suto Y Strauss MA Turner EL Keeton CR Annis J Castander FJ Eisenstein DJ Frieman JA Fukugita M Gunn JE Johnston DE Kent SM Nichol RC Richards GT Rix HW Sheldon ES Bahcall NA Brinkmann J Ivezić Z Lamb DQ McKay TA Schneider DP York DG 《Nature》2003,426(6968):810-812
Gravitational lensing is a powerful tool for the study of the distribution of dark matter in the Universe. The cold-dark-matter model of the formation of large-scale structures (that is, clusters of galaxies and even larger assemblies) predicts the existence of quasars gravitationally lensed by concentrations of dark matter so massive that the quasar images would be split by over 7 arcsec. Numerous searches for large-separation lensed quasars have, however, been unsuccessful. All of the roughly 70 lensed quasars known, including the first lensed quasar discovered, have smaller separations that can be explained in terms of galaxy-scale concentrations of baryonic matter. Although gravitationally lensed galaxies with large separations are known, quasars are more useful cosmological probes because of the simplicity of the resulting lens systems. Here we report the discovery of a lensed quasar, SDSS J1004 + 4112, which has a maximum separation between the components of 14.62 arcsec. Such a large separation means that the lensing object must be dominated by dark matter. Our results are fully consistent with theoretical expectations based on the cold-dark-matter model. 相似文献
974.
Recent ice ages on Mars 总被引:1,自引:0,他引:1
A key pacemaker of ice ages on the Earth is climatic forcing due to variations in planetary orbital parameters. Recent Mars exploration has revealed dusty, water-ice-rich mantling deposits that are layered, metres thick and latitude dependent, occurring in both hemispheres from mid-latitudes to the poles. Here we show evidence that these deposits formed during a geologically recent ice age that occurred from about 2.1 to 0.4 Myr ago. The deposits were emplaced symmetrically down to latitudes of approximately 30 degrees--equivalent to Saudi Arabia and the southern United States on the Earth--in response to the changing stability of water ice and dust during variations in obliquity (the angle between Mars' pole of rotation and the ecliptic plane) reaching 30-35 degrees. Mars is at present in an 'interglacial' period, and the ice-rich deposits are undergoing reworking, degradation and retreat in response to the current instability of near-surface ice. Unlike the Earth, martian ice ages are characterized by warmer polar climates and enhanced equatorward transport of atmospheric water and dust to produce widespread smooth deposits down to mid-latitudes. 相似文献
975.
Price PA Fox DW Kulkarni SR Peterson BA Schmidt BP Soderberg AM Yost SA Berger E Djorgovski SG Frail DA Harrison FA Sari R Blain AW Chapman SC 《Nature》2003,423(6942):844-847
Past studies of cosmological gamma-ray bursts (GRBs) have been hampered by their extreme distances, resulting in faint afterglows. A nearby GRB could potentially shed much light on the origin of these events, but GRBs with a redshift z 相似文献
976.
In bats and technological sonars, the gain of the receiver is progressively increased with time after the transmission of a signal to compensate for acoustic propagation loss. The current understanding of dolphin echolocation indicates that automatic gain control is not a part of their sonar system. In order to test this understanding, we have performed field measurements of free-ranging echolocating dolphins. Here we show that dolphins do possess an automatic gain control mechanism, but that it is implemented in the transmission phase rather than the receiving phase of a sonar cycle. We find that the amplitude of the dolphins' echolocation signals are highly range dependent; this amplitude increases with increasing target range, R, in a 20 log(R) fashion to compensate for propagation loss. If the echolocation target is a fish school with many sound scatterers, the echoes from the school will remain nearly constant with range as the dolphin closes in on it. This characteristic has the same effect as time-varying gain in bats and technological sonar when considered from a sonar system perspective. 相似文献
977.
Reticular synthesis and the design of new materials 总被引:18,自引:0,他引:18
The long-standing challenge of designing and constructing new crystalline solid-state materials from molecular building blocks is just beginning to be addressed with success. A conceptual approach that requires the use of secondary building units to direct the assembly of ordered frameworks epitomizes this process: we call this approach reticular synthesis. This chemistry has yielded materials designed to have predetermined structures, compositions and properties. In particular, highly porous frameworks held together by strong metal-oxygen-carbon bonds and with exceptionally large surface area and capacity for gas storage have been prepared and their pore metrics systematically varied and functionalized. 相似文献
978.
Synapses recycle their spent vesicles in order to keep up with on-going neurotransmitter release. To investigate vesicle recycling in the small synapses of hippocampal neurons, we have used an optical recording method that permits us to resolve single-vesicle events. Here we show that an exocytic event can terminate with three modes of vesicle retrieval: a fast (400-860 ms) 'kiss-and-run' mode that has a selective fusion pore; a slow (8-21 s) 'compensatory' mode; and a 'stranded' mode of recycling, in which a vesicle is left on the cell surface until a nerve impulse triggers its retrieval. We have also observed that, in response to a nerve impulse, synapses with low release probability primarily use the kiss-and-run mode, whereas high release probability terminals predominantly use the compensatory mode of vesicle retrieval. 相似文献
979.
Hf isotope composition of zircons and implication for the petrogenesis of Yajiangqiao granite,Hunan Province,China 总被引:8,自引:1,他引:7
WANGXiang W.L.Griffin WANGZhicheng ZHOUXinmin 《科学通报(英文版)》2003,48(10):995-998
In the peralumineous granite of Yajiangqiao zircon population can be divided into two groups (i.e. Zircon Ⅰ and Zircon Ⅱ), which were formed in magmatic chamber and in emplacement place, respectively. The Hf isotope compositions of two stages of zircon show that the host magma was essentially derived from crustal material by melting. However, some higher 176Hf/177Hf ratios indicate that the granitic zircons should contain fine crystal of zircon formed in mantle-derived magma. In fact, the backscattered electron imaging and the electron microprobe analysis reveal that there is another type of zircon included within Zircon Ⅰ, which is quite different from Zircon Ⅰ and Zircon Ⅱ in morphology and chemistry. They are considered to be the product of the mantle-derived magma intruded into the granitic magma chamber at the beginning of anatexis. Thus,it is suggested that the formation of Yajiangqiao granite is related to the underplating of mantle magma. 相似文献
980.
GEChendong O.Slaymaker T.F.Pedersen 《科学通报(英文版)》2003,48(21):2357-2361
Pb geochronology and sediment core profiles of organic carbon, total sulfur and organic carbon isotope (d 13C) values were used to reconstruct the local environ-mental history of the Shamei Lagoon, located in the Wan-quan River Estuary, eastern Hainan Island, China. Totalsulfur and d 13C values decreased upwards in the top 30 cm of a sediment core that spanned the last 200 years of deposi-tion. Total sulfur concentration and d 13C values respectively decreased upward from 1.92% to 0.36%, and -20.63?to -23.64? The C/S ratio in the 19th century and earlier was relatively stable in the range of 0.47—0.80, and there was a positive correlation between organic carbon and total sulfur. Since around 1900 AD, the C/S ratio increased rapidly to a maximum of 3.94, but no simple correlation was found be-tween organic carbon and total sulfur during this more re-cent period. These results indicated that before 1800 AD, the lagoon had a fully marine character, and the location of to-day抯 Wanquan River Estuary was an open embayment.From 1800 to 1900, the salinity of Shamei Lagoon decreased noticeably. The amount of seawater which could enter thelagoon decreased gradually as the Yudai spit grew. Today,seawater scarcely affects the lagoon; it is essentially a fresh-water basin. 相似文献