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181.
To study general stimulatory effects of 1,25-dihydroxyvitamin D3 on intestinal protein synthesis, slices of duodenal villi from 1,25-dihydroxyvitamin D3-treated and vitamin D-deficient rats were incubated in vitro for 90 min at the surface of medium containing [3H]leucine. Incorporation of the [3H]leucine into TCA-precipitated protein, which was shown to be linear for 12 h and 90% inhibited by cycloheximide, was increased by 50-60% at 26 h after a single injection of 125 ng of 1,25-dihydroxyvitamin D3 (three experiments, P less than 0.001). The increase, which was not due to circadian rhythm fluctuations of the intestine, was in synchrony with the second Ca2+ transport response observed by Halloran and DeLuca (Arch. Biochem. Biophys. 208, 477-486, 1981). However, no significant difference in [3H]leucine incorporation was observed before or during the initial Ca2+ transport response observed by Halloran and DeLuca, i.e., at 1.0, 3.0, and 6.5 h following an injection of 1,25-dihydroxyvitamin D3. The late onset of the 1,25-dihydroxyvitamin D3-induced increase in total protein synthesis implies that it is an indirect rather than a direct effect of the hormone. 相似文献
182.
Y. Tanaka J.K. Wichmann H.F. De Luca Y. Kobayashi N. Ikekawa 《Archives of biochemistry and biophysics》1983,225(2):649-655
To evaluate possible functional roles for 24,25-dihydroxyvitamin D3, 24,24-difluoro-25-hydroxyvitamin D3 has been synthesized and shown to be equally as active as 25-hydroxyvitamin D3 in all known functions of vitamin D. The use of the difluoro compound for this purpose is based on the assumption that the C-F bonds are stable in vivo and that the fluorine atom does not act as hydroxyl in biological systems. No 24,25-dihydroxyvitamin D3 was detected in the serum obtained from vitamin D-deficient rats that had been given 24,24-difluoro-25-hydroxyvitamin D3, while large amounts were found when 25-hydroxyvitamin D3 was given. Incubation of the 24,24-difluoro compound with kidney homogenate prepared from vitamin D-replete chickens failed to produce 24,25-dihydroxyvitamin D3, while the same preparations produced large amounts of 24,25-dihydroxyvitamin D3 from 25-hydroxyvitamin D3. Kidney homogenate prepared from vitamin D-deficient chickens produced 24,24-difluoro-1,25-dihydroxyvitamin D3 from 24,24-difluoro-25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 from 25-hydroxyvitamin D3. In binding to the plasma transport protein for vitamin D compounds, 24,24-difluoro-25-hydroxyvitamin D3 is less active than 25-hydroxyvitamin D3 and 24R,25-dihydroxyvitamin D3. In binding to the chick intestinal cytosol receptor, 24,24-difluoro-25-hydroxyvitamin D3 is more active than 25-hydroxyvitamin D3 which is itself more active than 24R,25-dihydroxyvitamin D3. The 24,24-difluoro-1,25-dihydroxyvitamin D3 is equal to 1,25-dihydroxyvitamin D3, and both are 10 times more active than 1,24R,25-trihydroxyvitamin D3 in this system. These results provide strong evidence that the C-24 carbon of 24,24-difluoro-25-hydroxyvitamin D3 cannot be hydroxylated in vivo, and, further, the 24-F substitution acts similar to H and not to OH in discriminating binding systems for vitamin D compounds. 相似文献
183.
In mechanistic studies by isotope-exchange tecniques of the conversion of D-fructose and D-glucose into 2-(hydroxyacetyl)furan, it was shown that both sugars are converted in acidified, tritiated water into the furan containing essentially no carbon-bound tritium. As the hydroxymethyl carbon atom of the furan corresponds to C-1 of the hexose, this result suggests that one of the hydrogen atoms in this group, when it is produced from D-glucose, must arise intramolecularly. This hypothesis was verified by synthesizing D-glucose-2-3H and converting it into the furan in acidified water. The 2-(hydroxyacetyl)furan obtained was labeled exclusively on the hydroxymethyl carbon atom, thus showing that intramolecular hydrogen-transfer occurs, during the conversion, from C-2 of D-glucose to the carbon atom corresponding to C-1. The specific activities of the product and reactant permitted calculation of the tritium isotope-effect (kh/kt4.4) for the reaction. The precise step for the transfer from C-2 of the aldose to the carbon atom corresponding to C-1 was found to be during the isomerization of D-glucose to D-fructose, as evidenced by the conversion of D-glucose-2-3H into D-fructose-1-3H in acidified water. 相似文献
184.
A screening method was developed for detection of bacterial mutants having active enzymes with altered electrophoretic mobility. The method is based on the use of a mixture of several clones, and examination of an extract of the mixture electrophoretically. A variant enzyme will thus be detectable by its position apart from the mixture of wild-type enzymes.Following exposure to a mutagenic agent, five mutants of E. coli K12 were detected and isolated. Two of these have variant MDH (malate dehydrogenates), the others have variant forms of 6-phosphogluconate dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase, and esterase.Preliminary mapping of the MDH locus has been performed. 相似文献
185.
186.
WASTEWATER TREATMENT WITH SUSPENDED AND NONSUSPENDED ALGAE 总被引:2,自引:0,他引:2
James P. Hoffmann 《Journal of phycology》1998,34(5):757-763
187.
免耕法对棉田生态系统能流功能的影响 总被引:4,自引:1,他引:3
比较了常规与免耕法对棉田生态系统中土壤微生物、害虫、天敌、棉株及其整个棉田生态系统能流功能的影响 .结果表明 ,免耕棉田土壤微生物量与年呼吸耗氧量均明显高于常规棉田 ,分别为常规棉田的 1 .32倍和 1 .63倍 ;免耕棉田的苗蚜与秋蚜、2代与 4代棉铃虫种群生产力下降 ,而伏蚜与 3代棉铃虫种群生产力增加 ;免耕棉田内天敌种群生产力及其捕食利用效率下降 ;免耕可提高棉田系统的光能利用率和总生产力 ,减少辅助能的投入 ,具有低耗、高效的特点 . 相似文献
188.
The theory of optimal foraging predicts abrupt changes in consumer behavior which lead to discontinuities in the functional
response. Therefore population dynamical models with optimal foraging behavior can be appropriately described by differential
equations with discontinuous right-hand sides. In this paper we analyze the behavior of three different Lotka–Volterra predator–prey
systems with optimal foraging behavior. We examine a predator–prey model with alternative food, a two-patch model with mobile
predators and resident prey, and a two-patch model with both predators and prey mobile. We show that in the studied examples,
optimal foraging behavior changes the neutral stability intrinsic to Lotka–Volterra systems to the existence of a bounded
global attractor. The analysis is based on the construction and use of appropriate Lyapunov functions for models described
by discontinuous differential equations.
Received: 23 March 1999 相似文献
189.
190.