全文获取类型
收费全文 | 8594篇 |
免费 | 78篇 |
国内免费 | 54篇 |
专业分类
系统科学 | 53篇 |
丛书文集 | 90篇 |
教育与普及 | 14篇 |
理论与方法论 | 35篇 |
现状及发展 | 3685篇 |
研究方法 | 545篇 |
综合类 | 4215篇 |
自然研究 | 89篇 |
出版年
2016年 | 57篇 |
2014年 | 54篇 |
2012年 | 154篇 |
2011年 | 270篇 |
2010年 | 78篇 |
2009年 | 59篇 |
2008年 | 184篇 |
2007年 | 223篇 |
2006年 | 183篇 |
2005年 | 186篇 |
2004年 | 267篇 |
2003年 | 152篇 |
2002年 | 170篇 |
2001年 | 322篇 |
2000年 | 328篇 |
1999年 | 240篇 |
1992年 | 143篇 |
1991年 | 131篇 |
1990年 | 113篇 |
1989年 | 107篇 |
1988年 | 115篇 |
1987年 | 134篇 |
1986年 | 127篇 |
1985年 | 175篇 |
1984年 | 134篇 |
1983年 | 107篇 |
1982年 | 91篇 |
1981年 | 122篇 |
1980年 | 119篇 |
1979年 | 269篇 |
1978年 | 209篇 |
1977年 | 194篇 |
1976年 | 148篇 |
1975年 | 166篇 |
1974年 | 219篇 |
1973年 | 185篇 |
1972年 | 207篇 |
1971年 | 227篇 |
1970年 | 287篇 |
1969年 | 220篇 |
1968年 | 195篇 |
1967年 | 221篇 |
1966年 | 189篇 |
1965年 | 147篇 |
1959年 | 54篇 |
1958年 | 106篇 |
1957年 | 79篇 |
1956年 | 79篇 |
1955年 | 53篇 |
1954年 | 75篇 |
排序方式: 共有8726条查询结果,搜索用时 15 毫秒
11.
Vitamin B12 (methylcobalamin) was administered orally (3 mg/day) to 9 healthy subjects for 4 weeks. Nocturnal melatonin levels after exposure to bright light (ca. 2500 lx) were determined, as well as the levels of plasma melatonin over 24 h. The timing of sleep was also recorded. Vitamin B12 was given blind to the subjects and crossed over with placebo. We found that the 24-h melatonin rhythm was significantly phase-advanced (1.1 h) in the vitamin B12 trial as compared with that in the placebo trial. In addition, the 24-h mean of plasma melatonin level was much lower in the vitamin B12 trial than with the placebo. Furthermore, the nocturnal melatonin levels during bright light exposure were significantly lower in the vitamin B12 trial than with the placebo. On the other hand, vitamin B12 did not affect the timing of sleep. These findings raise the possibility that vitamin B12 phase-advances the human circadian rhythm by increasing the light sensitivity of the circadian clock. 相似文献
12.
13.
The timing of sleep and sleep EEG parameters in 10 healthy male subjects were investigated in four seasons under controlled conditions. The phase of nocturnal sleep was delayed about one and a half hours in winter as compared to that in summer. The duration of stage 4 sleep decreased and REM sleep increased significantly in winter compared with summer. The seasonality in the timing of sleep can be explained by photoperiodic time cues, but the changes in sleep EEG parameters are difficult to explain in terms of photoperiod. 相似文献
14.
DNA fingerprinting transforms the art of cell authentication. 总被引:4,自引:0,他引:4
The increasing diversity of new cell cultures is seriously stretching the capabilities of traditional methods of identification. DNA fingerprinting is set to play an important role in increasing confidence in the authenticity of cultures in research and industry. 相似文献
15.
The failing heart. 总被引:19,自引:0,他引:19
Cardiomyopathies are disorders affecting heart muscle that usually result in inadequate pumping of the heart. They are the most common cause of heart failure and each year kill more than 10,000 people in the United States. In recent years, there have been breakthroughs in understanding the molecular mechanisms involved in this group of conditions, with knowledge of the genetic basis for cardiomyopathies perhaps seeing the largest advance, enabling clinicians to devise improved diagnostic strategies and preparing the stage for new therapies. 相似文献
16.
N. Ait-Langomazino R. Sellier G. Jouquet M. Trescinski 《Cellular and molecular life sciences : CMLS》1991,47(6):533-539
Summary A blown bitumen Mexphalte R 90/40 with a high content of saturated hydrocarbons was degraded by several microorganisms to the same extent. In batch cultures ofSaccharomycopsis lipolytica, maximal biodegradation was estimated to be about 9% w/w, 3.2·10–3 g/cm2 and 3.1·10–3 cm of degraded bitumen. The Mexphalte R 90/40 degradation rate was closely coupled to biofilm formation. The microbial activity concerned predominantly the oxidation of saturated hydrocarbons. A direct distillation bitumen 80/100 with a low content of saturated hydrocarbons and a high content of aromatic hydrocarbons and resins was more resistant to biodegradation. 相似文献
17.
就近几年图书馆界关于新世纪馆员意识转换及社会角色转换等方面的研究进行了综述,指出新时期的馆员要实现自我社会角色的转换,首先是要明确转换的必要性,然后采取有效的策略逐步地、系统地推进转换,才能取得较好的效果。 相似文献
18.
19.
Structure of the detoxification catalyst mercuric ion reductase from Bacillus sp. strain RC607 总被引:10,自引:0,他引:10
Several hundred million tons of toxic mercurials are dispersed in the biosphere. Microbes can detoxify organo-mercurials and mercury salts through sequential action of two enzymes, organomercury lyase and mercuric ion reductase (MerA). The latter, a homodimer with homology to the FAD-dependent disulphide oxidoreductases, catalyses the reaction NADPH + Hg(II)----NADP+ + H+ + Hg(0), one of the very rare enzymic reactions with metal substrates. Human glutathione reductase serves as a reference molecule for FAD-dependent disulphide reductases and between its primary structure and that of MerA from Tn501 (Pseudomonas), Tn21 (Shigella), p1258 (Staphylococcus) and Bacillus, 25-30% of the residues have been conserved. All MerAs have a C-terminal extension about 15 residues long but have very varied N termini. Although the enzyme from Streptomyces lividans has no addition, from Pseudomonas aeruginosa Tn501 and Bacillus sp. strain RC607 it has one and two copies respectively of a domain of 80-85 residues, highly homologous to MerP, the periplasmic component of proteins encoded by the mer operon. These domains can be proteolytically cleaved off without changing the catalytic efficiency. We report here the crystal structure of MerA from the Gram-positive bacterium Bacillus sp. strain RC607. Analysis of its complexes with nicotinamide dinucleotide substrates and the inhibitor Cd(II) reveals how limited structural changes enable an enzyme to accept as substrate what used to be a dangerous inhibitor. Knowledge of the mode of mercury ligation is a prerequisite for understanding this unique detoxification mechanism. 相似文献
20.