全文获取类型
收费全文 | 3837篇 |
免费 | 323篇 |
国内免费 | 5篇 |
专业分类
4165篇 |
出版年
2023年 | 27篇 |
2022年 | 51篇 |
2021年 | 113篇 |
2020年 | 58篇 |
2019年 | 84篇 |
2018年 | 102篇 |
2017年 | 78篇 |
2016年 | 117篇 |
2015年 | 206篇 |
2014年 | 239篇 |
2013年 | 245篇 |
2012年 | 319篇 |
2011年 | 276篇 |
2010年 | 161篇 |
2009年 | 170篇 |
2008年 | 195篇 |
2007年 | 170篇 |
2006年 | 149篇 |
2005年 | 137篇 |
2004年 | 138篇 |
2003年 | 129篇 |
2002年 | 127篇 |
2001年 | 58篇 |
2000年 | 42篇 |
1999年 | 48篇 |
1998年 | 29篇 |
1997年 | 22篇 |
1996年 | 26篇 |
1995年 | 26篇 |
1994年 | 16篇 |
1993年 | 24篇 |
1992年 | 21篇 |
1991年 | 25篇 |
1990年 | 23篇 |
1989年 | 17篇 |
1988年 | 32篇 |
1987年 | 18篇 |
1986年 | 13篇 |
1985年 | 11篇 |
1984年 | 11篇 |
1983年 | 18篇 |
1982年 | 17篇 |
1981年 | 18篇 |
1980年 | 15篇 |
1979年 | 21篇 |
1978年 | 22篇 |
1976年 | 16篇 |
1975年 | 10篇 |
1974年 | 13篇 |
1973年 | 20篇 |
排序方式: 共有4165条查询结果,搜索用时 10 毫秒
21.
W Korytowski C C Felix B Kalyanaraman 《Biochemical and biophysical research communications》1988,154(2):781-788
Oxidase electrode measurements as well as optical and electron spin resonance spectroscopic data have shown that synthetic neuromelanin oxidizes the neurotoxin metabolite 1-methyl-4-phenyl-2,3-dihydropyridinium in a dose-dependent manner forming 1-methyl-4-phenylpyridinium and hydrogen peroxide. Hydroxyl radicals are formed in this reaction which is promoted by iron chelates. In contrast, neither 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine nor 1-methyl-4-phenylpyridinium reacts with synthetic neuromelanin in a similar fashion. The mechanism of selective toxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in pigmented neuronal cells is discussed in the light of these findings. 相似文献
22.
23.
Dibutyryl cyclic AMP triggers Ca2+ influx and Ca2+-dependent electrical activity in pancreatic B cells 总被引:2,自引:0,他引:2
In the presence of 7 mM glucose, dibutyryl cyclic AMP induced electrical activity in otherwise silent mouse pancreatic B cells. This activity was blocked by cobalt or D600, two inhibitors of Ca2+ influx. Under similar conditions, dibutyryl cyclic AMP stimulated 45Ca2+ influx (5-min uptake) in islet cells; this effect was abolished by cobalt and partially inhibited by D600. The nucleotide also accelerated 86Rb+ efflux from preloaded islets, did not modify glucose utilization and markedly increased insulin release. Its effects on release were inhibited by cobalt, but not by D600. These results show that insulin release can occur without electrical activity in B cells and suggest that cyclic AMP not only mobilizes intracellular Ca, but also facilitates Ca2+ influx in insulin secreting cells. 相似文献
24.
Felix Bronner Danielle Pansu Arlette Bosshard Jeffrey H. Lipton 《Journal of cellular physiology》1983,116(3):322-328
Intestinal cells were isolated by a combination of mechanical and enzymatic means, and their calcium uptake was assayed by a rapid filtration procedure. Calcium uptake was a time- and concentration-dependent process that was markedly elevated at 25 and 37°C, as compared to 0°C. Cells isolated from rat duodenum exhibited higher uptakes than cells from jejunum, which in turn took up more calcium than cells from the ileurn. Duodenal cells from vitamin D-deficient animals took up less calcium than cells from vitamin D-replete cells. In vivo vitamin D repletion with 1,25-dihydroxyvitamin D3 raised calcium uptake by duodenal cells from treated animals toward that of cells from replete rats. Furthermore, calcium uptake by duodenal cells from vitamin D-deficient animals approximated that of ileal cells from replete rats. These findings with isolated cells parallel prior findings of tissue calcium transport and suggest that cellular calcium uptake may be related to the saturable component of intestinal calcium absorption. Isolated intestinal cells may therefore constitute one experimental model for the study of transcellular calcium transport. 相似文献
25.
26.
27.
28.
We have examined the nature of the circular dichroism band at 330 mμ for a series of copolymers of β-p-nitrobenzyl L -aspartate with β-benzyl L -aspartate. The circular dichroism band arises from an electronic transition in the nitroaromatic group. In order to interpret the effect quantitatively, we employed a simplified statistical treatment and curve fitting for six copolymers. Both approaches gave consistent results, which indicates that the dichroism comes from pairwise interactions between two nitrobenzyl groups. We constructed a molecular model that meets the constraints and requirements of the analyses developed in this paper. In this model, it is proposed that the main chain forms a right-handed α-helix and that nitrobenzyl groups separated by four residues interact with each other. 相似文献
29.
The control by bone metabolism of the blood calcium level in young rats may be described in terms of a regulator-type system. The model presented here comprises a feedback loop involving only a proportional control in thyroparathyroidectomized, and a combination of proportional and integral controls in normal animals. It accounts for the variations observed when the system was subjected to a variety of experimental constraints. The implications, limitations, and possible extensions of the model are discussed. 相似文献
30.
Heungsoon Felix Lee Roger Vivian Johnson 《Flexible Services and Manufacturing Journal》1991,3(2):91-120
We present a rough-cut analysis tool that quickly determines a few potential cost-effective designs at the initial design stage of flexible assembly systems (FASs) prior to a detailed analysis such as simulation. It uses quantitative methods for selecting and configuring the components of an FAS suitable for medium to high volumes of several similar products. The system is organized as a series of assembly stations linked with an automated material-handling system moving parts in a unidirectional flow. Each station consists of a single machine or of identical parallel machines. The methods exploit the ability of flexible hardware to switch almost instantaneously from product to product. Our approach is particularly suitable where the product mix is expected to be stable, since we combine the hardware-configuration phase with the task-allocation phase. For the required volume of products, we use integer programming to select the number of stations and the number of machines at each station and to allocate tasks to stations. We use queueing network analysis, which takes into account the mean and variance of processing times among different products to determine the necessary capacity of the material-handling system. We iterate between the two analyses to find the combined solution with the lowest costs. Work-in-process costs are also included in the analysis. Computational results are presented. 相似文献