全文获取类型
收费全文 | 2634篇 |
免费 | 277篇 |
专业分类
2911篇 |
出版年
2021年 | 33篇 |
2019年 | 17篇 |
2018年 | 35篇 |
2017年 | 31篇 |
2016年 | 47篇 |
2015年 | 65篇 |
2014年 | 94篇 |
2013年 | 92篇 |
2012年 | 129篇 |
2011年 | 104篇 |
2010年 | 72篇 |
2009年 | 61篇 |
2008年 | 101篇 |
2007年 | 87篇 |
2006年 | 100篇 |
2005年 | 94篇 |
2004年 | 80篇 |
2003年 | 77篇 |
2002年 | 79篇 |
2001年 | 86篇 |
2000年 | 86篇 |
1999年 | 57篇 |
1998年 | 30篇 |
1997年 | 28篇 |
1996年 | 25篇 |
1995年 | 22篇 |
1994年 | 28篇 |
1993年 | 27篇 |
1992年 | 55篇 |
1991年 | 57篇 |
1990年 | 55篇 |
1989年 | 71篇 |
1988年 | 65篇 |
1987年 | 69篇 |
1986年 | 47篇 |
1985年 | 45篇 |
1984年 | 43篇 |
1983年 | 26篇 |
1982年 | 38篇 |
1981年 | 31篇 |
1980年 | 28篇 |
1979年 | 37篇 |
1978年 | 40篇 |
1977年 | 28篇 |
1976年 | 29篇 |
1975年 | 29篇 |
1974年 | 36篇 |
1973年 | 30篇 |
1971年 | 21篇 |
1966年 | 16篇 |
排序方式: 共有2911条查询结果,搜索用时 0 毫秒
41.
42.
Chemokinetic accumulation of human neutrophils on immune complex-coated substrata: analysis at a boundary 总被引:6,自引:1,他引:5 下载免费PDF全文
The locomotory behavior of human blood neutrophil leukocytes was studied at a boundary between two surfaces with different chemokinetic properties. This was achieved by time-lapse cinematography of neutrophils moving on coverslips coated with BSA, then part-coated with immune complexes by adding anti-BSA IgG with a straight-line boundary between the BSA and the immune complexes. Cell locomotion was filmed in microscopic fields bisected by the boundary, and kinetic behavior was assessed by comparing speed (orthokinesis), turning behavior (klinokinesis), and the rate of diffusion of the cells on each side of the boundary, using a recently described mathematical analysis of kinesis. In the absence of serum or complement, the proportion of motile cells and their speed and rate of diffusion were greater on BSA than on antiBSA, but there was no consistent difference in turning behavior between cells on the two surfaces. The immune complexes were therefore negatively chemokinetic in comparison with BSA, and this resulted from a negative orthokinesis with little or no contribution from klinokinesis. As would be predicted theoretically, this resulted in gradual accumulation of cells on the immune complexes even in the absence of a chemotactic factor. In further studies, a parallel plate flow chamber was used to show that, under conditions of flow, neutrophils accumulated much more rapidly on a surface coated with BSA- anti-BSA than on BSA alone. Moreover, neutrophils on immune complex- coated surfaces lost their ability to form rosettes with IgG-coated erythrocytes. This suggests that neutrophils on immune complex-coated surfaces redistribute their Fc receptors (RFc gamma) to the under surface, and that the lowered speed of locomotion is due to tethering of neutrophils by substratum-bound IgG-Fc. 相似文献
43.
44.
Summary The effects of 25 kHz ultrasound on murine testes was studied, as mimicking possible World WAR II SONAR exposure to swimming maintenance personnel. Very few specimens were found to exhibit morphological tissue alterations, depending upon length of exposure time and proximity of the source to the tissue. Thermal processes seem to be eliminated, but microstreaming may be implicated, as the physical mechanism(s) of interaction. 相似文献
45.
46.
Modeling of experiments on biofilm penetration effects in a fluidized bed nitrification reactor 总被引:2,自引:0,他引:2
A four-component, diffusion-reaction model with double Michaelis-Menten kinetics was used to describe the experimental data obtained from a laboratory biofilm, fluidized-bed nitrification reactor. Theory and experiment demonstrated that the stoichiometric ratio (3.5 mg O(2)/mg NH(4) (+)-N) can be employed as a criterion to determine whether the limiting substrate is oxygen or ammonia. For the present work, in the range of concentrations where limitation occurred, 4 mg/L NH(4) (+)-N and 14 mg/L O(2), the ratio of oxygen to ammonia in the bulk liquid determined which substrate was penetration-limiting-O(2) if <3.5 and NH(4) (+) if > 3.5. Halforder kinetics with respect to the limiting substrate described the apparent overall rates. Simulations provided biofilm concentration profiles which demonstrated the role of the oxygen-ammonia ratio. Experiments indicated that, generally, high NO(2) (-) concentrations can be expected. These depend on the residence time, biofilm area, and oxygen concentration. This dependency was investigated with the model, as was the parametric sensitivity with respect to the saturation constants. Particularly important for the NO(2) (-) levels were the ratios of the saturation constants for oxygen. 相似文献
47.
Baker's yeast, Saccharomyces cerevisiae, was investigated for the combined influence of dissolved oxygen and glucose concentration in continuous culture. A reactor was operated at a range of dilution rates (0.1, 0.2, 0.25, 0.27, and 3.0 h(-1)), above and below the critical value that separates the oxidative and fermentation regions. For each dilution rate (D), steady states were established at each of five to ten different dissolved oxygen concentrations (DO) in the range of 0.01-5 mg/L. The use of on-line mass spectrometry facilitated the measurement of gaseous and dissolved O(2), CO(2), and ethanol. Intracellular carbohydrate, protein, RNA, DNA, lipid, and cytochrome concentrations were measured. Cell size measurements were reduced to specific surface areas. Cytochrome content showed up to 100% variation during a 20-day period of adaptation at D = 0.2 h(-1) to low DO. Eventually, the culture behaved the same at DO = 0.05 mg/L as it did initially at 3 mg/L. At D = 0.2, 0.25, and 0.27 h(-1), the transition between oxidation and fermentation was characterized by a critical DO which decreased with decreasing D. The X-D curves were shifted such that the critical D value was reduced with decreasing DO. Specific oxygen update rates varied with DO according to the saturation kinetics. Specific cell surface areas increased with decreasing DO. Cytochrome content generally decreased with decreasing DO, and Q(O(2) ) could be linearly related to the total cytochrome content, which exhibited a maximum at D = 0.27 h(-1). 相似文献
48.
l-Glutamate uptake, thiourea uptake, and methylammonium uptake and the intracellular ammonium concentration were measured in wild-type and mutant cells of Aspergillus nidulans held in various concentrations of ammonium and urea. The levels of l-glutamate uptake, thiourea uptake, nitrate reductase, and hypoxanthine dehydrogenase activity are determined by the extracellular ammonium concentration. The level of methylammonium uptake is determined by the intracellular ammonium concentration. The uptake and enzyme characteristics of the ammonium-derepressed mutants, meaA8, meaB6, DER3, amrA1, xprD1, and gdhA1, are described. The gdhA mutants lack normal nicotinamide adenine dinucleotide phosphate-glutamate dehydrogenase (NADP-GDH) activity and are derepressed with respect to both external and internal ammonium. The other mutant classes are derepressed only with respect to external ammonium. The mutants meaA8, DER3, amrA1, and xprD1 have low levels of one or more of the l-glutamate, thiourea, and methylammonium uptake systems. A model for ammonium regulation in A. nidulans is put forward which suggests: (i) NADP-GDH located in the cell membrane complexes with extracellular ammonium. This first regulatory complex determines the level of l-glutamate uptake, thiourea uptake, nitrate reductase, and xanthine dehydrogenase by repression or inhibition, or both. (ii) NADP-GDH also complexes with intracellular ammonium. This second and different form of regulatory complex determines the level of methylammonium uptake by repression or inhibition, or both. 相似文献
49.
Glucose-2-t as a tracer for glucose metabolism 总被引:23,自引:0,他引:23
50.