全文获取类型
收费全文 | 4583篇 |
免费 | 242篇 |
国内免费 | 304篇 |
学科分类
生物科学 | 5129篇 |
出版年
2023年 | 63篇 |
2022年 | 68篇 |
2021年 | 79篇 |
2020年 | 106篇 |
2019年 | 122篇 |
2018年 | 122篇 |
2017年 | 107篇 |
2016年 | 97篇 |
2015年 | 128篇 |
2014年 | 186篇 |
2013年 | 442篇 |
2012年 | 120篇 |
2011年 | 254篇 |
2010年 | 133篇 |
2009年 | 204篇 |
2008年 | 188篇 |
2007年 | 195篇 |
2006年 | 206篇 |
2005年 | 185篇 |
2004年 | 157篇 |
2003年 | 152篇 |
2002年 | 156篇 |
2001年 | 127篇 |
2000年 | 117篇 |
1999年 | 85篇 |
1998年 | 112篇 |
1997年 | 99篇 |
1996年 | 72篇 |
1995年 | 103篇 |
1994年 | 74篇 |
1993年 | 95篇 |
1992年 | 77篇 |
1991年 | 68篇 |
1990年 | 58篇 |
1989年 | 60篇 |
1988年 | 54篇 |
1987年 | 39篇 |
1986年 | 54篇 |
1985年 | 65篇 |
1984年 | 51篇 |
1983年 | 35篇 |
1982年 | 37篇 |
1981年 | 26篇 |
1980年 | 30篇 |
1979年 | 30篇 |
1978年 | 25篇 |
1977年 | 17篇 |
1976年 | 15篇 |
1974年 | 8篇 |
1973年 | 13篇 |
排序方式: 共有5129条查询结果,搜索用时 46 毫秒
1.
Bacillus sp. YX-1 glucose dehydrogenase (BsGDH) with good solvent resistance catalyzes the oxidation of β-d-glucose to d-glucono-1,5-lactone. Xylose is a recyclable resource from hemicellulase hydrolysis. In this work, to improve the preference of BsGDH for xylose, we designed seven mutants inside or adjacent to the substrate binding pocket using site-directed mutagenesis. Among all mutants, Ala258Phe mutant displayed the highest activity of 7.59 U mg−1 and nearly 8-folds higher kcat/Km value towards xylose than wild-type BsGDH. The kinetic constants indicated that the A258F mutation effectively altered the transition state. By analysis of modeled protein structure, Ala258Phe created a space to facilitate the reactivity towards xylose. A258F mutant retained good solvent resistance in glycol, ethyl caprylate, octane, decane, cyclohexane, nonane, etc. as with BsGDH. This work provides a protein engineering approach to modify the substrate stereo-preference of alcohol dehydrogenase and a promising enzyme for cofactor regeneration in chiral catalysis. 相似文献
2.
The NADPH-dependent reduction of some photosynthetic electron carriers in the dark, and the reduction of NADP+ associated
with the glycolytic sequence and the oxidative pentose phosphate pathway in chloroplasts are reviewed. The postulated pathways
of electron transports sensitive and insensitive to antimycin A are also evaluated. It is proposed that the electron flow,
predominantly through cytochrome bf complex, may be also involved in the pathway of NADPH-dependent and antimycin A-insensitive back electron transport. An information
on the chlororespiration in higher plants is also included.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
3.
Production of vitamin B12 in an upflow anaerobic filter continuous reactor using Acetobacterium sp. 总被引:2,自引:0,他引:2
Alberto Emilio Bainotti Belén Estebanez Hisashi Nagadomi Naomichi Nishio 《Biotechnology letters》2000,22(6):503-508
The accumulation of biofilm by Acetobacterium sp. during continuous culture in an upflow anaerobic filter (UAF) growing on methanol-formate was the result of space velocity and inlet concentrations of substrate and Co+2. To achieve good development of biofilm, a space velocity of 0.38 h–1, inlet substrate concentrations of 125 mM of both methanol and formate, and Co+2 at 0.16 mM were required. Cell productivities in the effluent of the UAF-reactor were about 6-fold higher than in chemostat cultures (0.20 g l–1 h–1 for UAF and 0.035 g l–1 h–1 for chemostat) (previous studies), and the maximum vitamin B12 specific concentration was 5.1 mg g cell–1. 相似文献
4.
A hybrid anaerobic solid-liquid (HASL) bioreactor is an enhanced two-phase anaerobic system, that consists of a solid waste reactor as the acidification reactor and a wastewater reactor, i.e. an upflow anaerobic sludge blanket (UASB) reactor as the methanogenic reactor. Food waste digestion in HASL bioreactors with pre-acidification and HASL operation stages was investigated in two separate runs. After 8 days of pre-acidification in Run A and 4 days in Run B, total volatile fatty acid (TVFA) and chemical oxygen demand (COD) concentrations in the leachates of both acidification reactors were similar. During HASL operation stage, TVFA and COD removal in the methanogenic phase were 77–100% and 75–95%, respectively. Some 99% of the total methane generated was from the methanogenic phase with a content of 68–70% methane. At the end of operation, about 59–60% of the added volatile solids (VS) were removed with a methane yield of 0.25 l g–1 VS. 相似文献
5.
Haiwei Luo Bradley B Tolar Brandon K Swan Chuanlun L Zhang Ramunas Stepanauskas Mary Ann Moran James T Hollibaugh 《The ISME journal》2014,8(3):732-736
Previous studies based on analysis of amoA, 16S ribosomal RNA or accA gene sequences have established that marine Thaumarchaeota fall into two phylogenetically distinct groups corresponding to shallow- and deep-water clades, but it is not clear how water depth interacts with other environmental factors, including light, temperature and location, to affect this pattern of diversification. Earlier studies focused on single-gene distributions were not able to link phylogenetic structure to other aspects of functional adaptation. Here, we analyzed the genome content of 46 uncultivated single Thaumarchaeota cells sampled from epi- and mesopelagic waters of subtropical, temperate and polar oceans. Phylogenomic analysis showed that populations diverged by depth, as expected, and that mesopelagic populations from different locations were well mixed. Functional analysis showed that some traits, including putative DNA photolyase and catalase genes that may be related to adaptive mechanisms to reduce light-induced damage, were found exclusively in members of the epipelagic clade. Our analysis of partial genomes has thus confirmed the depth differentiation of Thaumarchaeota populations observed previously, consistent with the distribution of putative mechanisms to reduce light-induced damage in shallow- and deep-water populations. 相似文献
6.
An attempt has been made to determine the location of the site at which the metabolism of ethanol interacts with that of choline to produce an increase in the oxidation of choline. The first enzyme in the oxidation pathway for choline, choline dehydrogenase, was assayed using a newly developed spectro-photometric assay and freshly isolated intact rat liver mitochondria. No changes were observed in either the ‘apparent’ V or the ‘apparent’ Km values of choline dehydrogenase for choline after ethanol ingestion. However, when the choline oxidase system was assayed, a 28% decrease in ‘apparent’ Km for choline and a 53% increase in ‘apparent’ V was observed. The effects of ATP on choline oxidase were studied further, and a 29.4% decrease was observed in mitochondrial ATP levels from freshly isolated mitochondria from the ethanoltreated rats. In vitro aging of mitochondria further decreased the level of ATP, and the rate of decrease was considerably faster during the first hour in the mitochondria from the ethanol-treated animals. The decreases in ATP from both control and experimental mitochondria were accompanied by increases in choline oxidase activity. The initial decrease in ATP was correlated with an increase in mitochondrial ATPase activity which may be related to an increase in mitochondrial Mg2+. Because chronic ethanol ingestion has resulted in decreased oxidation rates of succinate and β-hydroxybutyrate while at the same time increasing the oxidation rates of choline, the studies reported here suggest that the effect of chronic ethanol ingestion is primarily on a step that is unique to choline and which probably exists prior to the electron transport chain. 相似文献
7.
Biodiversity is claimed to be essential for ecosystem functioning, but is threatened by anthropogenic disturbances. Prokaryotes have been assumed to be functionally redundant and virtually inextinguishable. However, recent work indicates that microbes may well be sensitive to environmental disturbance. Focusing on methane-oxidizing bacteria as model organisms, we simulated disturbance-induced mortality by mixing native with sterilized paddy soil in two ratios, 1:4 and 1:40, representing moderate and severe die-offs. Disturbed microcosms were compared with an untreated control. Recovery of activity and populations was followed over 4 months by methane uptake measurements, pmoA-qPCR, pmoA-based terminal restriction fragment length polymorphism and a pmoA-based diagnostic microarray. Diversity and evenness of methanotrophs decreased in disturbed microcosms, but functioning was not compromised. We consistently observed distinctive temporal shifts between type I and type II methanotrophs, and a rapid population growth leading to even higher cell numbers comparing disturbed microcosms with the control. Overcompensating mortality suggested that population size in the control was limited by competition with other bacteria. Overall, methanotrophs showed a remarkable ability to compensate for die-offs. 相似文献
8.
9.
Jerry W. McClure 《Phytochemistry》1974,13(7):1065-1069
Five-day-old etiolated barley shoots respond to brief illumination with red light by increasing their level of PAL ca 50% within 5 hr. When assayed s 相似文献
10.
Thermoanaerobacter brockii 《Anaerobe》1997,3(6)
Thermoanaerobacter brockiifermented serine to acetate and ethanol. It oxidized leucine to isovalerate, isoleucine to 2-methylbutyrate, and valine to isobutyrate only in the presence of thiosulfate, or when co-cultured withMethanobacteriumsp. This oxidative deamination was rendered thermodynamically possible by the ability ofT. brockiito reduce thiosulfate to sulfide or the transfer of reducing equivalents to the hydrogenotrophic methanogen. The results suggest thatT. brockiimay be of ecological significance in thermal environments in the turnover of amino acids, especially with thiosulfate or H2-utilizing methanogens are present. 相似文献