全文获取类型
收费全文 | 3345篇 |
免费 | 466篇 |
国内免费 | 359篇 |
出版年
2024年 | 6篇 |
2023年 | 75篇 |
2022年 | 48篇 |
2021年 | 91篇 |
2020年 | 175篇 |
2019年 | 177篇 |
2018年 | 163篇 |
2017年 | 153篇 |
2016年 | 164篇 |
2015年 | 159篇 |
2014年 | 157篇 |
2013年 | 199篇 |
2012年 | 158篇 |
2011年 | 165篇 |
2010年 | 141篇 |
2009年 | 193篇 |
2008年 | 187篇 |
2007年 | 189篇 |
2006年 | 173篇 |
2005年 | 155篇 |
2004年 | 125篇 |
2003年 | 107篇 |
2002年 | 101篇 |
2001年 | 78篇 |
2000年 | 76篇 |
1999年 | 61篇 |
1998年 | 58篇 |
1997年 | 46篇 |
1996年 | 40篇 |
1995年 | 44篇 |
1994年 | 29篇 |
1993年 | 32篇 |
1992年 | 38篇 |
1991年 | 24篇 |
1990年 | 32篇 |
1989年 | 27篇 |
1988年 | 15篇 |
1987年 | 27篇 |
1986年 | 28篇 |
1985年 | 23篇 |
1984年 | 46篇 |
1983年 | 40篇 |
1982年 | 40篇 |
1981年 | 23篇 |
1980年 | 28篇 |
1979年 | 19篇 |
1978年 | 7篇 |
1977年 | 8篇 |
1976年 | 5篇 |
1973年 | 7篇 |
排序方式: 共有4170条查询结果,搜索用时 500 毫秒
61.
【背景】高温引起的微生物活性降低是限制园林绿化废弃物堆肥过程中木质素降解的主要因素。【目的】驯化一株木质素降解菌芽孢杆菌NO.2,提高其在高温下的微生物活性,探究其生长情况及酶学特性。【方法】采用温度梯度方法驯化菌株,以菌株生长曲线、酶活力、木质素降解率为评价指标,探究驯化前后菌株间差异,以及驯化后菌株所产木质素降解酶的酶促反应温度和pH范围。【结果】与原菌株相比,驯化后菌株在60℃培养时最大生物量间差异不显著;漆酶(laccase,Lac)、锰过氧化物酶(manganeseperoxidase,MnP)和木质素过氧化物酶(ligninperoxidase,LiP)酶活力得到进一步提高,分别提高了30.75%、35.98%和29.62%,木质素降解率提高60.52%。酶学性质研究表明,驯化后菌株所产Lac、MnP和LiP在20-60℃、pH 3.0-9.0范围内酶活力均较高,而且具有较好的稳定性,稳定性依次为Lac>LiP>MnP。【结论】温度梯度驯化方法可有效提高微生物对高温环境的适应性,扩大木质素降解酶的酶促反应温度和pH范围,在进一步自主研制专用降解园林废弃物微生物菌... 相似文献
62.
63.
Douglas P. Swain 《Evolution; international journal of organic evolution》1992,46(4):987-997
Previous studies have demonstrated selective predation for vertebral traits of larvae in the stickleback Gasterosteus aculeatus. I tested the hypothesis that this selection results from a direct functional advantage to particular vertebral phenotypes by direct measurement of the burst swimming performance of larvae. Within a narrow window of lengths, burst speed did depend on vertebral phenotype. As in the previous predation experiments, performance was related more directly to the ratio of abdominal to caudal vertebrae (VR) than to the total number of vertebrae (VN), and the optimal VR decreased as larval length increased. Changes with length in the vertebral phenotype frequencies of wild larvae provided evidence of selection for VR and for VN in the wild. Larvae with particular VR increased in frequency in the wild at just those lengths when their relative performance was superior in the laboratory. The observed pattern of length-dependent selection for vertebral number provides an explanation for the widespread trends in vertebral number that occur among populations of related fishes. 相似文献
64.
65.
Makoto Kato 《Population Ecology》1996,38(1):27-40
Population dynamics of a leafminer,Chromatomyia suikazurae (Agromyzidae, Diptera) and its parasitoid community were studied for ten years at seven natural populations along an altitudinal gradient in Japan. This species which mines leaves of a forest shrub,Lonicera gracilipes (Caprifoliaceae), was attacked by 25 hymenopterous parasitoid species. Annually, the parasitoid community structure varied less within a population than among populations. The seven parasitoid communities were clustered into three groups corresponding to the altitudinal gradient: (a) lowland communities dominated by late-attacking, generalist pupal idiobiont eulophids and with highest species diversity, (b) hillside communities dominated by an early-attacking, specialist larval-pupal koinobiont braconid and (c) highland communities dominated by an early-attacking, generalist larval idiobiont eulophid. Annual changes of the host larval densities among the local populations were largely synchronous rather than cyclic. Among these populations, host density levels and mortality patterns greatly varied. By analyzing these inter-populational differences of host mortality patterns, the following conclusions were drawn: (1) The host mortality patterns were determined by the host utilization patterns of the locally dominant species. (2) The host pupal mortality but not larval mortality was related to species diversity but not to species richness itself of each parasitoid community. (3) Density dependence was detected only in pupal mortality at a lowland population dominated by late-attacking pupal parasitoids. These results suggest that interspecific interactions of parasitoids add additive effects to host population dynamics dissimilarly among local populations with different parasitoid communities. 相似文献
66.
Summary Triple-resonance experiments can be designed to provide useful information on spin-system topologies. In this paper we demonstrate optimized proton and carbon versions of PFG-CT-HACANH and PFG-CT-HACA(CO)NH straight-through triple-resonance experiments that allow rapid and almost complete assignments of backbone H, 13C, 15N and HN resonances in small proteins. This work provides a practical guide to using these experiments for determining resonance assignments in proteins, and for identifying both intraresidue and sequential connections involving glycine residues. Two types of delay tunings within these pulse sequences provide phase discrimination of backbone Gly C and H resonances: (i) C–H phase discrimination by tuning of the refocusing period a_f; (ii) C–C phase discrimination by tuning of the 13C constant-time evolution period 2Tc. For small proteins, C–C phase tuning provides better S/N ratios in PFG-CT-HACANH experiments while C–H phase tuning provides better S/N ratios in PFG-CT-HACA(CO)NH. These same principles can also be applied to triple-resonance experiments utilizing 13C-13C COSY and TOCSY transfer from peripheral side-chain atoms with detection of backbone amide protons for classification of side-chain spin-system topologies. Such data are valuable in algorithms for automated analysis of resonance assignments in proteins. 相似文献
67.
Abstract. We studied floristic and diversity patterns and their environmental controls in two landscapes of contrasting topography in the Patagonian steppe. The analyses were focused on the effects of water availability gradients and landscape configuration on plant species distribution and coexistence. Floristic variation was investigated using Correspondence Analysis. The relationship between floristic and environmental variation was analyzed using Canonical Correspondence Analysis and correlation tests. We explored diversity patterns by relating spatial distance to floristic dissimilarities. The floristic gradient was determined by shrub and grass species and was related to precipitation in the flat area, and to precipitation, elevation and potential radiation in the mountain area. Site species richness increased with water availability in both areas. Mean site species richness and species turnover in space was higher in the mountain than in the flat area. Landscape species richness and floristic gradients were more concentrated in the mountain than in the flat area. In contrast to shrubs and grasses, forb species distributions were uncoordinated and probably independent of any environmental gradient. Our results suggest (1) that landscape configuration affects species composition and diversity through its direct effect on abiotic environmental heterogeneity, and (2) that the environmental controls of the community composition vary depending on the plant functional type considered. 相似文献
68.
Thermoluminescence experiments have been carried out to study the effect of a transmembrane proton gradient on the recombination properties of the S2 and S3 states of the oxygen evolving complex with QA
- and QB
-, the reduced electron acceptors of Photosystem II. We first determined the properties of the S2QA
- (Q band), S2QB
- and S3QB
- (B bands) recombinations in the pH range 5.5 to 9.0, using uncoupled thylakoids. The, a proton gradient was created in the dark, using the ATP-hydrolase function of ATPases, in coupled unfrozen thylakoids. A shift towards low temperature of both Q and B bands was observed to increase with the magnitude of the proton gradient measured by the fluorescence quenching of 9-aminoacridine. This downshift was larger for S3QB
- than for S2QB
- and it was suppressed by nigericin, but not by valinomycin. Similar results were obtained when a proton gradient was formed by photosystem I photochemistry. When Photosystem II electron transfer was induced by a flash sequence, the reduction of the plastoquinone pool also contributed to the downshift in the absence of an electron acceptor. In leaves submitted to a flash sequence above 0°C, a downshift was also observed, which was supressed by nigericin infiltration. Thus, thermoluminescence provides direct evidence on the enhancing effect of lumen acidification on the S3S2 and S2S1 reverse-transitions. Both reduction of the plastoquinone pool and lumen acidification induce a shift of the Q and B bands to lower temperature, with a predominance of lumen acidification in non-freezing, moderate light conditions.Abbreviations 9-AA
9-aminoacridine
- EA
activation energy
- F0
constant fluorescence level
- FM
maximum fluorescence, when all PS-II centers are closed
- FV
variable fluorescence (FM–F0)
- PS I, PS II
Photosystem I, photosystem II
- PQ
plastoquinone
- TL
thermoluminescence 相似文献
69.
E. García-Martín F. J. Martín-Romero C. Gutiérrez-Merino 《Journal of neurochemistry》1995,65(6):2757-2764
Abstract: Synaptosomes can be loaded with mag-fura-2 without significant perturbation of their ATP content by incubation for 10 min at 37°C with 10 µM mag-fura-2 acetoxymethyl ester in Hanks'-HEPES buffer (pH 7.45). The intrasynaptosomal free Mg2+ concentration ([Mg2+]i) was found to be dependent on external Mg2+ concentration, increasing from 0.8 to 1.25 mM when the concentration of Mg2+ in the incubation medium increased from 1 to 8 mM. Dissipation of the Na+ gradient across the plasma membrane of synaptosomes by treatment with the Na+ ionophore monensin (0.2 mM) or with veratridine (0.2 mM) and ouabain (0.6 mM) produced a moderate increase of [Mg2+]i, from 1.0 to 1.2–1.3 mM in an incubation medium containing 5 mM Mg2+. Plasma membrane depolarization by incubation of synaptosomes in a medium containing 68 mM KCl and 68 mM NaCl had no effect on [Mg2+]i. Reversal of the Na+ gradient by incubation of synaptosomes in a medium in which external Na+ was replaced by choline increased [Mg2+]i up to 1.6 and 2.2 mM for extrasynaptosomal Mg2+ concentrations of 1 and 8 mM, respectively. We conclude that a Na+/Mg2+ exchange operates in the plasma membrane of synaptosomes. In the presence of Mg2+ in the incubation medium, extrasynaptosomal ATP, but not ADP or adenosine, increased [Mg2+]i from 1.1 ± 0.1 up to 1.6 ± 0.1 mM. The nonhydrolyzable ATP analogue adenosine 5′-(βγ-imido)triphosphate antagonized the effect of ATP, but had no effect by itself on [Mg2+]i. It is concluded that Mg2+ transport across the plasma membrane of synaptosomes is modulated by the activity of an ecto-ATPase or an ecto-protein kinase. 相似文献
70.
Generating developmental gradients by temperature gradients established within a developing organism is an easy, non-invasive technique to study physiological interdependencies between locally separated subsystems. A linear temperature gradient of about 10° C/mm was maintained up to 5 h in either direction along the long axis of a long-germ-type hymenopteran egg, which was simultaneously filmed by the 16 mm timelapse technique. The result was a dramatic desynchronization of development, which between the egg poles could reach up to 9.3 h relative to normal development. Within the same egg, up to seven mitotic waves (i.e. eight different nuclear generations) were observed at the same time, and the subsequent cellularization process was extremely asynchronous. The initial regions of the mitotic waves, the fountain flow of the ooplasm, and the gastrulation process were shifted towards the egg pole kept at higher temperatures. Developmental processes occurring successively in normal development now took place simultaneously, with either direction of the temperature gradient. For instance, while gastrulation had started in the warm region, midblastula transition and cellularization were in progress in the middle of the egg, and intravitelline nuclear multiplication occurred at the cold pole, by rapid and still biphasic cell cycles. In some respects, development resembled that of a short-germ-type insect egg. Nevertheless, the developmental processes were resynchronized after the temperature gradient was switched off. Surprisingly, the extreme desynchronization during early development did not affect the segment pattern of the resulting embryos. The technique of inducing well-defined developmental asynchronies might be applied in Drosophila to analyse the subtle interplay between maternal and zygotic gene activities described in this species. 相似文献