全文获取类型
收费全文 | 134344篇 |
免费 | 11215篇 |
国内免费 | 5050篇 |
专业分类
150609篇 |
出版年
2023年 | 1004篇 |
2022年 | 2363篇 |
2021年 | 4165篇 |
2020年 | 2702篇 |
2019年 | 3299篇 |
2018年 | 3493篇 |
2017年 | 2663篇 |
2016年 | 3794篇 |
2015年 | 5804篇 |
2014年 | 6829篇 |
2013年 | 7793篇 |
2012年 | 9176篇 |
2011年 | 8740篇 |
2010年 | 5395篇 |
2009年 | 4857篇 |
2008年 | 6250篇 |
2007年 | 5809篇 |
2006年 | 5189篇 |
2005年 | 4556篇 |
2004年 | 4090篇 |
2003年 | 3751篇 |
2002年 | 3412篇 |
2001年 | 4258篇 |
2000年 | 3937篇 |
1999年 | 3258篇 |
1998年 | 1611篇 |
1997年 | 1433篇 |
1996年 | 1370篇 |
1995年 | 1233篇 |
1994年 | 1166篇 |
1993年 | 982篇 |
1992年 | 2150篇 |
1991年 | 1874篇 |
1990年 | 1738篇 |
1989年 | 1622篇 |
1988年 | 1400篇 |
1987年 | 1339篇 |
1986年 | 1175篇 |
1985年 | 1169篇 |
1984年 | 879篇 |
1983年 | 787篇 |
1979年 | 830篇 |
1978年 | 588篇 |
1977年 | 568篇 |
1975年 | 634篇 |
1974年 | 716篇 |
1973年 | 653篇 |
1972年 | 651篇 |
1971年 | 593篇 |
1970年 | 576篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
122.
123.
R. A. Tobey H. A. Crissman M. E. Wilder F. Traganos Z. Darzynkiewicz 《Cell proliferation》1987,20(3):363-366
Abstract. A population of line CHO Chinese hamster cells was synchronized by mitotic selection and allowed to enter early G1 , after which the largest and smallest cells in the population were sorted, irradiated, and their viability determined. Despite sizeable differences in volume, metabolic capability and cell cycle progression rates, an equivalent level of survival was obtained for the two populations, indicating that the factors responsible for the volume, metabolic and progression heterogeneity do not contribute greatly to radiation sensitivity. 相似文献
124.
Food sharing: a model of manipulation by harassment 总被引:3,自引:1,他引:2
Most analyses of food-sharing behavior invoke complex explanationssuch as indirect and delayed benefits for sharing via kin selectionand reciprocal altruism. However, food sharing can be a moregeneral phenomenon accounted for by more parsimonious, mutualisticexplanations. We propose a game theoretical model of a generalsharing situation in which food owners share because it is
in their own self-interestthey avoid high costs associatedwith beggar harassment. When beggars harass, owners may benefitfrom sharing part of the food if their consumption rate islow relative to the rate of cost accrual. Our model predictsthat harassment can be a profitable strategy for beggars if
they reap some direct benefits from harassing other than sharedfood (such as picking up scraps). Therefore, beggars may manipulatethe owner's fitness payoffs in such a way as to make sharingmutualistic. 相似文献
125.
126.
Kinetics and mechanism of acetohydroxy acid synthase isozyme III from Escherichia coli 总被引:5,自引:0,他引:5
Acetohydroxy acid synthase (AHAS, EC 4.1.3.18) isozyme III from Escherichia coli has been studied in steady-state kinetic experiments in which the rates of formation of acetolactate (AL) and acetohydroxybutyrate (AHB) have been determined simultaneously. The ratio between the rates of production of the two alternative products and the concentrations of the substrates pyruvate and 2-ketobutyrate (2KB) leading to them, R, VAHB/VAL = R[( 2KB]/[pyruvate]), was found to be 40 +/- 3 under a wide variety of conditions. Because pyruvate is a common substrate in the reactions leading to both products and competes with 2-ketobutyrate to determine whether AL or AHB is formed, steady-state kinetic studies are unusually informative for this enzyme. At a given pyruvate concentration, the sum of the rates of formation of AL and AHB was nearly independent of the 2-ketobutyrate concentration. On the basis of these results, a mechanism is proposed for the enzyme that involves irreversible and rate-determining reaction of pyruvate, at a site which accepts 2-ketobutyrate poorly, if at all, to form an intermediate common to all the reactions. In the second phase of the reaction, various 2-keto acids can compete for this intermediate to form the respective acetohydroxy acids. 2-Keto acids other than the natural substrates pyruvate and 2-ketobutyrate may also compete, to a greater or lesser extent, in the second phase of the reaction to yield alternative products, e.g., 2-ketovalerate is preferred by about 2.5-fold over pyruvate. However, the presence of an additional keto acid does not affect the relative specificity of the enzyme for pyruvate and 2-ketobutyrate; this further supports the proposed mechanism. The substrate specificity in the second phase is an intrinsic property of the enzyme, unaffected by pH or feedback inhibitors.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
127.
M T Hsieh H C Chen P H Hsu T Shibuya 《Proceedings of the National Science Council, Republic of China. Part B, Life sciences》1986,10(1):43-48
Previously, it was found that the ancient Chinese remedy of Suanzaorentang could be a promising anxiolytic drug (Chen and Hsieh, 1985a, Chen and Hsieh, 1985b). To understand the mechanism of the action of Suanzaorentang, the effects of Suanzaorentang on behavior changes and central monoamines and their metabolites were studied in rats. It was found that Suanzaorentang significantly (1) prolonged the period from the onset of clonic to tonic convulsions induced by pentylenetetrazol or picrotoxin, (2) prolonged the sleep duration induced by hexobarbital, (3) reduced locomotor activity, (4) enhanced the hypomotility induced by alpha-MT, (5) reduced the locomotor stimulation produced by levodopa plus benserazide, and (6) reduced central HVA, VMA, and 5-HIAA, but had no significant effects on central DA, NA, and 5-HT. These facts implied that Suanzaorentang decreased the turnover rate of central monoamines and central catecholaminergic activity. 相似文献
128.
129.
130.
Nucleotide sequence of the mouse ornithine decarboxylase gene. 总被引:6,自引:3,他引:3