全文获取类型
收费全文 | 1640篇 |
免费 | 75篇 |
国内免费 | 132篇 |
专业分类
1847篇 |
出版年
2023年 | 25篇 |
2022年 | 14篇 |
2021年 | 35篇 |
2020年 | 29篇 |
2019年 | 47篇 |
2018年 | 48篇 |
2017年 | 50篇 |
2016年 | 42篇 |
2015年 | 46篇 |
2014年 | 73篇 |
2013年 | 94篇 |
2012年 | 51篇 |
2011年 | 62篇 |
2010年 | 45篇 |
2009年 | 68篇 |
2008年 | 72篇 |
2007年 | 112篇 |
2006年 | 70篇 |
2005年 | 70篇 |
2004年 | 101篇 |
2003年 | 68篇 |
2002年 | 58篇 |
2001年 | 46篇 |
2000年 | 36篇 |
1999年 | 32篇 |
1998年 | 37篇 |
1997年 | 43篇 |
1996年 | 20篇 |
1995年 | 30篇 |
1994年 | 23篇 |
1993年 | 21篇 |
1992年 | 22篇 |
1991年 | 22篇 |
1990年 | 14篇 |
1989年 | 5篇 |
1988年 | 8篇 |
1987年 | 7篇 |
1986年 | 10篇 |
1985年 | 21篇 |
1984年 | 27篇 |
1983年 | 23篇 |
1982年 | 20篇 |
1981年 | 20篇 |
1980年 | 13篇 |
1979年 | 17篇 |
1978年 | 11篇 |
1977年 | 11篇 |
1976年 | 5篇 |
1975年 | 7篇 |
1973年 | 11篇 |
排序方式: 共有1847条查询结果,搜索用时 15 毫秒
51.
Parabiotic ants—ants that share their nest with another ant species—need to tolerate not only conspecific nestmates, but also
nestmates of a foreign species. The parabiotic ants Camponotus rufifemur and Crematogaster modiglianii display high interspecific tolerance, which exceeds their respective partner colony and extends to alien colonies of the
partner species. The tolerance appears to be related to unusual cuticular substances in both species. Both species possess
hydrocarbons of unusually high chain lengths. In addition, Cr. modiglianii carries high quantities of hereto unknown compounds on its cuticle. These unusual features of the cuticular profiles may
affect nestmate recognition within both respective species as well. In the present study, we therefore examined inter-colony discrimination within the two parabiotic
species in relation to chemical differentiation. Cr. modiglianii was highly aggressive against workers from alien conspecific colonies in experimental confrontations. In spite of high inter-colony
variation in the unknown compounds, however, Cr. modiglianii failed to differentiate between intracolonial and allocolonial unknown compounds. Instead, the cuticular hydrocarbons functioned
as recognition cues despite low variation across colonies. Moreover, inter-colony aggression within Cr. modiglianii was significantly influenced by the presence of two methylbranched alkenes acquired from its Ca. rufifemur partner. Ca. rufifemur occurs in two varieties (‘red’ and ‘black’) with almost no overlap in their cuticular hydrocarbons. Workers of this species
showed low aggression against conspecifics from foreign colonies of the same variety, but attacked workers from the respective
other variety. The low inter-colony discrimination within a variety may be related to low chemical differentiation between
the colonies. Ca. rufifemur majors elicited significantly more inter-colony aggression than medium-sized workers. This may be explained by the density
of recognition cues: majors carried significantly higher quantities of cuticular hydrocarbons per body surface. 相似文献
52.
The component hydrocarbons, sterols, alcohols, monobasic, α,ω-dibasic and ω-hydroxy acids of the fresh hand decayed leaves and the pneumatophores of the mangrove Avicennia marina are reported in detail. From the quantitative comparisons which can be drawn, relative changes in the lipid classes occurring during leaf decay can be highlighted. These base-line data are important to our understanding of inputs to marine intertidal sediments. During leaf decay the only significant changes were a reduction in the total absolute concentrations of monobasic acids due largely to a decrease in concentration of the C18 polyunsaturated fatty acids, and an enhancement of the concentrations of the long-chain monobasic acids, ω-hydroxy acids and α,ω-dibasic acids. This resistance to degradation shown by the cutin derived acids (α,ω-dibasic, ω-hydroxy and long-chain monobasic acids) relative to the cellular and wax derived lipids may allow these cutin components to be used as quantitative markers of A. marina in mangrove associated sediments. 相似文献
53.
M. Ardhaoui A. Falcimaigne J. M. Engasser P. Moussou G. Pauly M. Ghoul 《Biocatalysis and Biotransformation》2004,22(4):253-259
Enzymatic acylation of rutin and esculin with aromatic, aliphatic and aryl-aliphatic acids using Candida antarctica lipase in tert amyl alcohol as solvent was investigated under low water content. Whatever the acyl donor used, the conversion yields and initial rates for esculin were higher than for rutin. For a given flavonoid, the performance of these reactions depended on the acyl donor structures. For aliphatic acids, conversion yields and initial rates of both flavonoids were respectively in the ranges of 68-90% and of 9.5×10-2-72×10-2 mmol l-1 h-1. For aromatic acids, the reaction occurred only with the aryl subgroup (cinnamic, hydrocinnamic, 3,4-dihydroxyhydrocinnamic and 4-hydroxyphenyl acetic acids) and was drastically influenced by the presence of side chain and substitution patterns of the aromatic ring. Except for hydrocinnamic acid (75%, 23.4×10-2 mmol l-1 h-1), with these acids the conversion yields and initial rates were lower and in the range of 10-45% and of 0.7×10-2 to 12.1×10-2 mmol l-1 h-1. Unsaturation of the side chain of the hydrocinnamic acid decreased the esculin conversion rate from 75 to 13% and initial rate from 23.4 to 1.76×10-2 mmol l-1 h-1. The presence of hydroxyl or nitro-groups on the aromatic ring of the aryl aliphatic acid also reduced conversion yields and initial rates. Even without a spacer, the non-phenolic ring acid (quinic acid) was reactive and lead to conversion yields of about 20 and 23% respectively for rutin and esculin. 相似文献
54.
55.
多环芳烃微生物降解基因的研究进展 总被引:10,自引:2,他引:10
多环芳烃(PAHs)是环境中普遍存在的一类有机污染物,微生物的降解是PAHs去除的主要途径。近年来,有关PAHs微生物降解途径和代谢产物的研究已有很多报道。小分子PAHs一般可以直接被微生物降解,而大分子PAHs则需要微生物以共代谢的方式降解。在过去20年中,微生物降解PAHs的基因相继被发现,各种基因在调控PAHs降解过程中的功能也越来越清晰。本文概述了PAHs微生物降解基因方面的研究进展,详细介绍了微生物对萘、菲的降解基因,最后对PAHs微生物降解基因的应用前景进行了展望。 相似文献
56.
Hydrocarbon composition of newly isolated strains of the green microalga Botryococcus braunii 总被引:1,自引:0,他引:1
Four new strains of Botryococcus braunii were isolated from Japanese waters and cultured under defined conditions. Their hydrocarbon content and composition were analyzed and compared with those of the Darwin and Berkeley strains. The Yamanaka strain produced only alkadienes characteristic of the A race, whereas the others, the Yayoi, Kawaguchi-1 and -2 strains as well as the Darwin and Berkeley strains, produced botryococcenes peculiar to the B race. The hydrocarbon content of the Yamanaka strain was 16.1 % dry weight and that of the B race strains ranged from 9.7 to 37.9%. Botryococcene composition of the Japanese strains differed from each other as well as from the Darwin and Berkeley strains. More than 50% of the hydrocarbons in the Yayoi, Darwin, and Berkeley strains were composed of C34H58, but the main components were different from one another as isomers. The Kawaguchi-1 and -2 strains did not have a high level of C34 botryococcenes, C32 ones being the main components. In these strains significant amounts of squalene-related compounds were detected. 相似文献
57.
58.
Surfactant aided biodegradation of pyrene using immobilized cells of Mycobacterium frederiksbergense 总被引:2,自引:0,他引:2
Saurabh Jyoti SarmaKannan Pakshirajan 《International biodeterioration & biodegradation》2011,65(1):73-77
Low aqueous phase solubility is the major limiting factor in successful biodegradation of pyrene and other polycyclic aromatic hydrocarbons (PAH), which can, however, be overcome by using a suitable surfactant. Biodegradation of pyrene by immobilized cells of Mycobacterium frederiksbergense in presence of non-ionic surfactant Tween 80 was evaluated. For cell immobilization, beads were prepared using calcium alginate as the immobilizing material based on immobilized cell viability and mechanical stability of the beads. Complete degradation of pyrene was achieved employing the immobilized cells in batch shake flask experiments for all four different initial concentrations of the PAH at 100 mg l−1, 200 mg l−1, 400 mg l−1 and 1000 mg l−1. The experimental results of biodegradation of pyrene at very high initial concentration of 1000 mg l−1 using the cell immobilized beads was further investigated in a 3 l fermentor operated at controlled conditions of 150 rpm, 28 °C, pH 7 and 1.5 l min−1 aeration. The results confirmed complete degradation of the PAH with a very higher degradation rate of 250 mg l−1 d−1, which is so far the highest value reported for pyrene biodegradation. 相似文献
59.
From the non-alkaloidal fractions of Delphinium venulosum, four known aromatic compounds cis and trans p-coumaric acids, p-hydroxybenzoic acid, protocatechuic acid methyl ester and a new aromatic compound 2,5,6-trihydroxypiperonylic acid methyl ester were isolated together with kaempferol, sitosterol and sitosteryl 3-glucoside. The structures of the compounds were established by spectral data. 相似文献
60.
Degradation of aromatic hydrocarbons by white-rot fungi in a historically contaminated soil 总被引:10,自引:0,他引:10
D'Annibale A Ricci M Leonardi V Quaratino D Mincione E Petruccioli M 《Biotechnology and bioengineering》2005,90(6):723-731
Phanerochaete chrysosporium NRRL 6361 and Pleurotus pulmonarius CBS 664.97 were tested for their ability to grow under nonsterile conditions and to degrade various aromatic hydrocarbons in an aged contaminated soil that also contained high concentrations of heavy metals. After 24 days fungal incubation, carbon-CO2 liberated, an indicator of microbial activity, reached a plateau. At the end of the incubation time (30 days), fungal colonization was clearly visible and was confirmed by ergosterol and cell organic carbon determinations. In spite of unfavorable pH (around 7.4) and the presence of heavy metals, both fungi produced Mn-peroxidase activity. In contrast, laccase and aryl-alcohol oxidase were detected only in the soil treated with P. pulmonarius CBS 664.97 and lignin-peroxidase in that with P. chrysosporium NRRL 6361. No lignin-modifying enzyme activities were present in non-inoculated soil incubated for 30 days (control microcosm). Regardless of the fungus employed, a total removal of naphtalene, tetrachlorobenzene, and dichloroaniline isomers, diphenylether and N-phenyl-1-naphtalenamine, was observed. Significant release of chloride ions was also observed in fungal-treated soil, in comparison with that recorded in the control microcosm. Both fungi led to a significant decrease in soil toxicity, as assessed using two different soil contact assays, including the Lepidium sativum L. germination test and the Collembola mortality test. 相似文献