全文获取类型
收费全文 | 83110篇 |
免费 | 143303篇 |
国内免费 | 29718篇 |
出版年
2019年 | 2929篇 |
2018年 | 2581篇 |
2017年 | 2347篇 |
2016年 | 2616篇 |
2015年 | 3111篇 |
2014年 | 3291篇 |
2013年 | 3299篇 |
2012年 | 3976篇 |
2011年 | 4492篇 |
2010年 | 5424篇 |
2009年 | 10430篇 |
2008年 | 4869篇 |
2007年 | 4707篇 |
2006年 | 3585篇 |
2005年 | 3597篇 |
2004年 | 3523篇 |
2003年 | 3028篇 |
2002年 | 3774篇 |
2001年 | 4351篇 |
1999年 | 6833篇 |
1998年 | 9054篇 |
1997年 | 9209篇 |
1996年 | 8548篇 |
1995年 | 8832篇 |
1994年 | 8248篇 |
1993年 | 7899篇 |
1992年 | 7869篇 |
1991年 | 7870篇 |
1990年 | 8705篇 |
1989年 | 7938篇 |
1988年 | 7222篇 |
1987年 | 6326篇 |
1986年 | 5855篇 |
1985年 | 5314篇 |
1984年 | 4178篇 |
1983年 | 3335篇 |
1982年 | 3681篇 |
1981年 | 3329篇 |
1980年 | 3257篇 |
1979年 | 3344篇 |
1978年 | 3066篇 |
1977年 | 2993篇 |
1976年 | 2807篇 |
1975年 | 2373篇 |
1974年 | 2520篇 |
1973年 | 2546篇 |
1972年 | 2854篇 |
1971年 | 2635篇 |
1970年 | 2391篇 |
1969年 | 2421篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
81.
Transduction and Genetic Homology Between Pseudomonas Species putida and aeruginosa 总被引:4,自引:0,他引:4 下载免费PDF全文
Genetic crosses occur by transduction between the species Pseudomonas aeruginosa and P. putida. The frequency relative to intraspecific transfer is reduced and varies among markers, suggesting that these genomes contain discrete regions of homology and nonhomology. 相似文献
82.
83.
84.
A. P. Farrell P. L. Lutz 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1975,102(2):159-166
Summary On adapting brackish waterAnguilla anguilla to fresh water it was noted that, while the plasma sodium, magnesium,pCO2 and pH were held reasonably constant, there was a substantial fall in chloride concentration (–33 mEq). The gradient of the linear correlations between plasma sodium and chloride also fell (brackish water gradient=0.92, fresh water gradient=0.21) indicating that a new pattern of plasma ion interrelationships was being established. Comparison with plasma Na/Cl ion ratios from other fishes suggested that this phenomenon was peculiar toA. anguilla. Corresponding with the very low plasma chloride levels plasma bicarbonate was four to five times that found in other fishes, and this was thought related to the finding that the haematocrit value almost doubled during adaptation to fresh water. In fresh water adapted fish a fall in plasma chloride was associated with a rise in plasma bicarbonate, however the charge compensation effect of this response was only partial as summing the common plasma cations and anions left an anion deficit of about 34 mEq to be accounted for. 相似文献
85.
86.
In a study of the effect of glycerin in transport media on Vibrio parahaemolyticus and Salmonella, it was found that a concentration of 30% glycerin was highly inhibitory for V. parahaemolyticus and to a lesser degree for Salmonella. The incorporation of peptone or human feces in media did not reduce the inhibitory effect of glycerin. In media with 15% glycerin, viable counts of V. parahaemolyticus and Salmonella increased after 24 hr of incubation both in the presence and absence of feces. Due to the concurrent increase in the total bacterial count in the media containing feces, no enrichment effect was noted. 相似文献
87.
88.
Luke L. M. Heaton Eduardo López Philip K. Maini Mark D. Fricker Nick S. Jones 《Proceedings. Biological sciences / The Royal Society》2010,277(1698):3265-3274
Cord-forming fungi form extensive networks that continuously adapt to maintain an efficient transport system. As osmotically driven water uptake is often distal from the tips, and aqueous fluids are incompressible, we propose that growth induces mass flows across the mycelium, whether or not there are intrahyphal concentration gradients. We imaged the temporal evolution of networks formed by Phanerochaete velutina, and at each stage calculated the unique set of currents that account for the observed changes in cord volume, while minimizing the work required to overcome viscous drag. Predicted speeds were in reasonable agreement with experimental data, and the pressure gradients needed to produce these flows are small. Furthermore, cords that were predicted to carry fast-moving or large currents were significantly more likely to increase in size than cords with slow-moving or small currents. The incompressibility of the fluids within fungi means there is a rapid global response to local fluid movements. Hence velocity of fluid flow is a local signal that conveys quasi-global information about the role of a cord within the mycelium. We suggest that fluid incompressibility and the coupling of growth and mass flow are critical physical features that enable the development of efficient, adaptive biological transport networks. 相似文献
89.
Free and polymerized tubulin in cultured bone cells and Chinese hamster ovary cells: the influence of cold and hormones 总被引:1,自引:1,他引:0
A low pH method of liposome-membrane fusion (Schneider et al., 1980, Proc. Natl. Acad. Sci. U. S. A. 77:442) was used to enrich the mitochondrial inner membrane lipid bilayer 30-700% with exogenous phospholipid and cholesterol. By varying the phospholipid-to- cholesterol ratio of the liposomes it was possible to incorporate specific amounts of cholesterol (up to 44 mol %) into the inner membrane bilayer in a controlled fashion. The membrane surface area increased proportionally to the increase in total membrane bilayer lipid. Inner membrane enriched with phospholipid only, or with phospholipid plus cholesterol up to 20 mol %, showed randomly distributed intramembrane particles (integral proteins) in the membrane plane, and the average distance between intramembrane particles increased proportionally to the amount of newly incorporated lipid. Membranes containing between 20 and 27 mol % cholesterol exhibited small clusters of intramembrane particles while cholesterol contents above 27 mol % resulted in larger aggregations of intramembrane particles. In phospholipid-enriched membranes with randomly dispersed intramembrane particles, electron transfer activities from NADH- and succinate-dehydrogenase to cytochrome c decreased proportionally to the increase in distance between the particles. In contrast, these electron- transfer activities increased with decreasing distances between intramembrane particles brought about by cholesterol incorporation. These results indicate that (a) catalytically interacting redox components in the mitochondrial inner membrane such as the dehydrogenase complexes, ubiquinone, and heme proteins are independent, laterally diffusible components; (b) the average distance between these redox components is effected by the available surface area of the membrane lipid bilayer; and (c) the distance over which redox components diffuse before collision and electron transfer mediates the rate of such transfer. 相似文献
90.
Bacteriocin-resistant mutants of Erwinia chrysanthemi: possible involvement of iron acquisition in phytopathogenicity. 总被引:12,自引:9,他引:3 下载免费PDF全文
A series of bacteriocin-resistant mutants of Erwinia chrysanthemi 3937JRH were unable to elicit soft-rot symptoms on saintpaulia plants. The loss of pathogenicity was correlated with the disappearance of one to three outer membrane polypeptides (molecular weights, about 80,000 to 90,000) whose production in wild-type strains was greatly enhanced under iron-limited growth conditions. The mutants did not exhibit altered extracellular pectinolytic or cellulolytic activities. 相似文献