全文获取类型
收费全文 | 863036篇 |
免费 | 95753篇 |
国内免费 | 494篇 |
专业分类
959283篇 |
出版年
2018年 | 8063篇 |
2016年 | 10848篇 |
2015年 | 14096篇 |
2014年 | 16830篇 |
2013年 | 24002篇 |
2012年 | 26764篇 |
2011年 | 27460篇 |
2010年 | 18698篇 |
2009年 | 17309篇 |
2008年 | 24549篇 |
2007年 | 25515篇 |
2006年 | 23823篇 |
2005年 | 22997篇 |
2004年 | 22722篇 |
2003年 | 21540篇 |
2002年 | 21041篇 |
2001年 | 39697篇 |
2000年 | 39664篇 |
1999年 | 31304篇 |
1998年 | 10650篇 |
1997年 | 11215篇 |
1996年 | 10442篇 |
1995年 | 9709篇 |
1994年 | 9362篇 |
1993年 | 9425篇 |
1992年 | 25466篇 |
1991年 | 24695篇 |
1990年 | 24067篇 |
1989年 | 23392篇 |
1988年 | 21740篇 |
1987年 | 20548篇 |
1986年 | 19239篇 |
1985年 | 18944篇 |
1984年 | 15666篇 |
1983年 | 13435篇 |
1982年 | 10167篇 |
1981年 | 9165篇 |
1980年 | 8756篇 |
1979年 | 14969篇 |
1978年 | 11637篇 |
1977年 | 10674篇 |
1976年 | 9973篇 |
1975年 | 10916篇 |
1974年 | 12040篇 |
1973年 | 11703篇 |
1972年 | 10820篇 |
1971年 | 9879篇 |
1970年 | 8591篇 |
1969年 | 8294篇 |
1968年 | 7919篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
Big, beautiful organisms are useful for biological education, increasing evolution literacy, and biodiversity conservation. But if educators gloss over the ubiquity of streamlined and miniaturized organisms, they unwittingly leave students and the public vulnerable to the idea that the primary evolutionary plot of every metazoan lineage is “progressive” and "favors" complexity. We show that simple, small, and intriguingly repulsive invertebrate animals provide a counterpoint to misconceptions about evolution. Our examples can be immediately deployed in biology courses and outreach. This context emphasizes that chordates are not the pinnacle of evolution. Rather, in the evolution of animals, miniaturization, trait loss, and lack of perfection are at least as frequent as their opposites. Teaching about invertebrate animals in a “tree thinking” context uproots evolution misconceptions (for students and the public alike), provides a mental scaffold for understanding all animals, and helps to cultivate future ambassadors and experts on these little‐known, weird, and fascinating taxa. 相似文献
982.
M.S. Blake K.H. Johnston G.J. Russell-Jones E.C. Gotschlich 《Analytical biochemistry》1984,136(1):175-179
A rapid, sensitive method has been developed to detect antibody-antigen complexes on “Western blots.” The methods of H. Towbin, T. Staehlin, and J. Gordon were used to separate and blot the antigens onto nitrocellulose. The remaining sites of attachment were blocked and the nitrocellulose was washed with polyoxyethylenesorbitan monolaurate (Tween 20). The blot was then reacted with the antiserum or hybridoma supernate to be tested. After the antigen-antibody reaction was completed, the blot was washed and treated with anti-antibody which has been conjugated to alkaline phosphatase. The alkaline phosphatase was detected by the reduction of the tetrazolium salt to diformazan by the hydrogen ions released in the formation of indigo by the reaction of the phosphatase on the indoxyl phosphate. The advantages of this method over previously described techniques are (1) use of Tween 20 allows the blot to be stained with Coomassie blue, (2) the substrates of the alkaline phosphatase reaction are stable for long periods of time, (3) the reaction products form an intense blue color which does not fade, (4) the resolution is extremely good with little to no band broadening, (5) the reaction is sensitive to picogram quantities of antigen, and (6) the reaction is quantitative. 相似文献
983.
984.
985.
New mutation affecting the synthesis of some membrane proteins and sporulation in Bacillus subtilis. 总被引:2,自引:1,他引:1 下载免费PDF全文
A new mutation, mpo, which affects the synthesis of some membrane proteins and sporulation in Bacillus subtilis was identified. The mpo mutation was tightly linked to the overproduction of membrane proteins MP32 and MP18 (molecular weights of 32,000 and 18,000, respectively) and the temperature-sensitive sporulation phenotype. Genetic analysis showed that the mpo mutation maps between the spoIIIB and lys loci. 相似文献
986.
Studies on larval population densities and adult emergence rates of the Brugian filariasis vectors Mansonia bonneae Edwards and Ma. dives Schiner were conducted in freshwater swamp forest bordering the Sadong River, Serian District, Sarawak, East Malaysia, during 1984-85. Three species of aquatic host-plants in the Family Araceae were identified as supporting immature stages of the Ma. bonneae/dives complex. Proportions of positive plants were 4.7%, 6.5% and 3.4% with 6.4 +/- 2.6, 7.3 +/- 2.8 and 10.1 +/- 1.1 larvae per positive plant, respectively, for the plant species Homalomena cordata Schott, H. rostrata Griffiths and Hydrostemma motleyi (Hook. f.) Mabberley. These data indicate no significant preferences between the three types of host-plant. Detailed monitoring of the host-plant H. cordata revealed no significant monthly fluctuations in larval density per plant nor the proportion of positive plants. 11.6% of larvae were Ma. dives and 88.4% were Ma. bonneae. Mean daily yields of Ma. bonneae/dives adults per square metre of H. cordata vegetated water surface were 0.45 males plus 0.57 females during the wet season (December-February) compared with 0.2 males plus 0.31 females during the dry season (June-August). Thus output of adults per plant was approximately halved, and suitable breeding areas were further reduced, during the dry season. By extrapolation from these rates, a crude mean estimate for productivity of Ma. bonneae/dives females is 1.6 million per hectare per annum in swamp forest habitats vegetated with any of the host-plants studied. 相似文献
987.
988.
In vivo cellular responses to electrophoretically applied dynorphin in the rat hippocampus 总被引:1,自引:0,他引:1
Recent immunohistochemical and radioimmunochemical observations have demonstrated a differential distribution of immunoreactive dynorphin (DYN) in rat brain. The presence of DYN immunoreactivity in a major intrinsic fiber pathway within the rat hippocampus (the mossy fiber system) has led us to evaluate the possible role of DYN and other closely related peptides in this structure. Single cell activity and hippocampal field potentials have been recorded from the CA1-CA3 cellular fields in halothane or urethane anesthetized rats. DYN, DYN1-13, DYN1-8, and alpha-neo-endorphin had an excitatory effect on most CA1-CA3 neurons encountered as has been previously observed for opiates and other opioid peptides. This response could be blocked by naloxone or by co-administration of Mg++ ion suggesting an indirect (synaptic) mechanism of excitation similar to that hypothetized for enkephalin. A significant number of CA3 neurons, however, exhibited a non-naloxone sensitive inhibitory response to DYN, related opioid peptides, and the kappa agonist WIN 35-197 (ethylketocyclazocine). Field potential analysis of CA1-CA3 neuronal responses to mossy fiber activation also indicated an excitatory, Mg++ reversible, action of iontophoretically applied DYN. These observations support our cytochemical and assay studies indicating diverse opioid systems within the rat hippocampus. In addition, these functional studies are congruent with other evidence suggesting multiple opioid mechanisms in this structure. 相似文献
989.
The visible and proton NMR spectral responses of imidazole methemoglobin by the binding of inositol hexaphosphate were examined in the 2-40 degrees C range. The magnitude of the +/- (inositol hexaphosphate) visible difference spectrum increased and the intensity of the 33 ppm NMR peak decreased with lowering of the temperature. The NMR results were quantitatively analyzed with a simple two-state allosteric model. The results show that the T conformer fraction is 0.6 at 20 degrees C and that the equilibrium shifts toward the T state at lower temperature. The large changes in delta H and delta S associated with the equilibrium suggest participation of numerous factors in the determination of the equilibrium position. The increase in the T conformer population of imidazole methemoglobin, which is pure low-spin, suggests that the appearance of the T state with decreasing temperature is not directly coupled to an increase in spin of the heme iron. 相似文献
990.