全文获取类型
收费全文 | 808篇 |
免费 | 53篇 |
出版年
2023年 | 2篇 |
2022年 | 7篇 |
2021年 | 21篇 |
2020年 | 18篇 |
2019年 | 25篇 |
2018年 | 14篇 |
2017年 | 14篇 |
2016年 | 29篇 |
2015年 | 28篇 |
2014年 | 31篇 |
2013年 | 42篇 |
2012年 | 53篇 |
2011年 | 61篇 |
2010年 | 41篇 |
2009年 | 33篇 |
2008年 | 43篇 |
2007年 | 37篇 |
2006年 | 31篇 |
2005年 | 27篇 |
2004年 | 30篇 |
2003年 | 28篇 |
2002年 | 27篇 |
2001年 | 31篇 |
2000年 | 26篇 |
1999年 | 18篇 |
1998年 | 12篇 |
1997年 | 9篇 |
1996年 | 7篇 |
1995年 | 4篇 |
1994年 | 8篇 |
1993年 | 8篇 |
1992年 | 14篇 |
1991年 | 12篇 |
1990年 | 6篇 |
1989年 | 7篇 |
1988年 | 2篇 |
1987年 | 6篇 |
1986年 | 10篇 |
1985年 | 6篇 |
1984年 | 8篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1978年 | 4篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1968年 | 1篇 |
1960年 | 1篇 |
排序方式: 共有861条查询结果,搜索用时 15 毫秒
71.
A transesterification process is analyzed in its multiple kinetic components that include the determination of the kinetic constants for both substrates, butyl acetate (BAc) and hexanol (H), involved in the alcoholysis reaction and for the products formed (hexyl acetate (HAc) and butanol (B)), participating into the reverse reaction. The order of magnitude of these constants is discussed in relation with the AOT/isooctane reverse micellar system under study. The values of the equilibrium conversion (X(e)) and constant (K(eq)) were also determined. Diffusional limitations were detected for H concentrations lower than 450 mM and the correspondent effectiveness factors were calculated. Above 450 mM H the reaction is kinetically controlled. The operation of a batch stirred tank reactor (BSTR) was modeled considering the integrated rate equation for reversible kinetics. 相似文献
72.
73.
Maria Do Carmo Bittencourt‐Oliveira Ariadne Do Nascimento Moura Mariana Cabral De Oliveira Nelson Sidnei Massola Jr. 《Journal of phycology》2009,45(3):716-725
Geitlerinema amphibium (C. Agardh ex Gomont) Anagn. and G. unigranulatum (Rama N. Singh) Komárek et M. T. P. Azevedo are morphologically close species with characteristics frequently overlapping. Ten strains of Geitlerinema (six of G. amphibium and four of G. unigranulatum) were analyzed by DNA sequencing and transmission electronic and optical microscopy. Among the investigated strains, the two species were not separated with respect to cellular dimensions, and cellular width was the most varying characteristic. The number and localization of granules, as well as other ultrastructural characteristics, did not provide a means to discriminate between the two species. The two species were not separated either by geography or environment. These results were further corroborated by the analysis of the cpcB‐cpcA intergenic spacer (PC‐IGS) sequences. Given the fact that morphology is very uniform, plus the coexistence of these populations in the same habitat, it would be nearly impossible to distinguish between them in nature. On the other hand, two of the analyzed strains were distinct from all others based on the PC‐IGS sequences, in spite of their morphological similarity. PC‐IGS sequences indicate that these two strains could be a different species of Geitlerinema. Using morphology, cell ultrastructure, and PC‐IGS sequences, it is not possible to distinguish G. amphibium and G. unigranulatum. Therefore, they should be treated as one species, G. unigranulatum as a synonym of G. amphibium. 相似文献
74.
Leandro C. de Oliveira Viviam M. da Silva Francieli Colussi Aline D. Cabral Mario de Oliveira Neto Fabio M. Squina Wanius Garcia 《PloS one》2015,10(2)
Endo-β-1, 4-mannanase from Thermotoga petrophila (TpMan) is a modular hyperthermostable enzyme involved in the degradation of mannan-containing polysaccharides. The degradation of these polysaccharides represents a key step for several industrial applications. Here, as part of a continuing investigation of TpMan, the region corresponding to the GH5 domain (TpManGH5) was characterized as a function of pH and temperature. The results indicated that the enzymatic activity of the TpManGH5 is pH-dependent, with its optimum activity occurring at pH 6. At pH 8, the studies demonstrated that TpManGH5 is a molecule with a nearly spherical tightly packed core displaying negligible flexibility in solution, and with size and shape very similar to crystal structure. However, TpManGH5 experiences an increase in radius of gyration in acidic conditions suggesting expansion of the molecule. Furthermore, at acidic pH values, TpManGH5 showed a less globular shape, probably due to a loop region slightly more expanded and flexible in solution (residues Y88 to A105). In addition, molecular dynamics simulations indicated that conformational changes caused by pH variation did not change the core of the TpManGH5, which means that only the above mentioned loop region presents high degree of fluctuations. The results also suggested that conformational changes of the loop region may facilitate polysaccharide and enzyme interaction. Finally, at pH 6 the results indicated that TpManGH5 is slightly more flexible at 65°C when compared to the same enzyme at 20°C. The biophysical characterization presented here is well correlated with the enzymatic activity and provide new insight into the structural basis for the temperature and pH-dependent activity of the TpManGH5. Also, the data suggest a loop region that provides a starting point for a rational design of biotechnological desired features. 相似文献
75.
Katia M. S. Cabral Diana P. Raymundo Viviane S. Silva Laura A. G. Sampaio Laizes Johanson Luis Fernando Hill Fabio C. L. Almeida Yraima Cordeiro Marcius S. Almeida 《PloS one》2015,10(9)
BEX3 (Brain Expressed X–linked protein 3) is a member of a mammal-specific placental protein family. Several studies have found the BEX proteins to be associated with neurodegeneration, the cell cycle and cancer. BEX3 has been predicted to be intrinsically disordered and also to represent an intracellular hub for cell signaling. The pro-apoptotic activity of BEX3 in association with a number of additional proteins has been widely supported; however, to the best of our knowledge, very limited data are available on the conformation of any of the members of the BEX family. In this study, we structurally characterized BEX3 using biophysical experimental data. Small angle X-ray scattering and atomic force microscopy revealed that BEX3 forms a specific higher-order oligomer that is consistent with a globular molecule. Solution nuclear magnetic resonance, partial proteinase K digestion, circular dichroism spectroscopy, and fluorescence techniques that were performed on the recombinant protein indicated that the structure of BEX3 is composed of approximately 31% α-helix and 20% β-strand, contains partially folded regions near the N- and C-termini, and a core which is proteolysis-resistant around residues 55–120. The self-oligomerization of BEX3 has been previously reported in cell culture and is consistent with our in vitro data. 相似文献
76.
77.
The virtual ecologist approach: simulating data and observers 总被引:3,自引:0,他引:3
Damaris Zurell Uta Berger Juliano S. Cabral Florian Jeltsch Christine N. Meynard Tamara Münkemüller Nana Nehrbass Jörn Pagel Björn Reineking Boris Schröder Volker Grimm 《Oikos》2010,119(4):622-635
Ecologists carry a well‐stocked toolbox with a great variety of sampling methods, statistical analyses and modelling tools, and new methods are constantly appearing. Evaluation and optimisation of these methods is crucial to guide methodological choices. Simulating error‐free data or taking high‐quality data to qualify methods is common practice. Here, we emphasise the methodology of the ‘virtual ecologist’ (VE) approach where simulated data and observer models are used to mimic real species and how they are ‘virtually’ observed. This virtual data is then subjected to statistical analyses and modelling, and the results are evaluated against the ‘true’ simulated data. The VE approach is an intuitive and powerful evaluation framework that allows a quality assessment of sampling protocols, analyses and modelling tools. It works under controlled conditions as well as under consideration of confounding factors such as animal movement and biased observer behaviour. In this review, we promote the approach as a rigorous research tool, and demonstrate its capabilities and practical relevance. We explore past uses of VE in different ecological research fields, where it mainly has been used to test and improve sampling regimes as well as for testing and comparing models, for example species distribution models. We discuss its benefits as well as potential limitations, and provide some practical considerations for designing VE studies. Finally, research fields are identified for which the approach could be useful in the future. We conclude that VE could foster the integration of theoretical and empirical work and stimulate work that goes far beyond sampling methods, leading to new questions, theories, and better mechanistic understanding of ecological systems. 相似文献
78.
Spatial and temporal distribution patterns of the macrozoobenthos assemblage in the salt marshes of Tejo estuary (Portugal) 总被引:1,自引:0,他引:1
The present study focuses on the spatial and temporal distribution of the macroinvertebrate community of the salt marsh areas
of the Tejo estuary, based on surveys conducted from autumn 1998 to summer 2000. Samples were collected quarterly in five
different intertidal areas along an elevation gradient in: mudflats, creek mouths, creeks, pioneer salt marsh areas and middle
marsh areas. A total of 36 benthic invertebrate taxa were identified. Insect larvae were the most well represented group,
with 10 taxa identified. Oligochaetes and ostracods were the most numerically abundant taxa, whereas bivalves dominated in
biomass. Benthic macroinvertebrate assemblages were dominated, both in number and biomass, by deposit feeders. Three distinct
macroinvertebrate assemblages were distinguished along the elevation gradient, based on species presence, density and biomass:
the unvegetated muddy areas with a macrobenthic assemblage composed mostly by infauna; the salt marsh pioneer areas of Spartina maritima in which several epibenthic taxa were found, as well as endobenthic taxa characteristic of muddy sediment; and the creek
margins, with epifauna taxa such as insect larvae and crustaceans and a low abundance of benthic infauna. Total biomass in
the unvegetated and Spartina areas was higher during spring and summer mainly due to the increase in biomass of Scrobicularia plana and Hydrobia ulvae. No decreases in the salt marsh macroinvertebrate biomass values were observed during the highest densities of their potential
nektonic predators (summer). This fact might indicate that macroinvertebrates are not a limiting resource for the nektonic
species, and that the natural biomass increment of these invertebrate species could be masking the predation/disturbance caused
by the nektonic species. 相似文献
79.
The trophic structure of benthic communities in the Tagus estuary and adjacent coastal shelf was characterized according to
a functional guild approach, based on sampling surveys conducted between 1987 and 2000. Macrobenthic organisms were assigned
to seven distinct trophic groups (herbivorous, filter feeders, surface deposit feeders, subsurface deposit feeders, carnivores,
filter feeders/detritivores, carnivores/detritivores) and the dominance of these groups was related to environmental variables
using multivariate ordination techniques. Surface-deposit feeders were numerically dominant in the Tagus estuary, making up
52% of the benthic communities, while in the adjacent coastal shelf the assemblage was dominated by both surface-deposit feeders
and filter feeders (37% and 33%, respectively). When biomass was considered, filter feeders and filter feeders/detritivores
were the dominant groups in the estuary, while for the adjacent coastal shelf filter feeders represented 83% of the total
biomass. Salinity, depth and sediment composition were the main factors structuring spatial distribution. Surface-deposit
feeders were the most abundant macrobenthos of the upper estuary. Surface deposit feeders also dominated the middle and the
lower estuary but the proportion of filter feeders as well as other trophic groups increased with salinity. Generally, a more
even distribution of trophic structure was found at stations with high salinity. In the adjacent coastal shelf, the trophic
diversity decreased with depth. The trophic structure revealed that filter feeders dominated in abundance and biomass in shallow
sandy sediments (<25 m), while in deeper sandy mud and muddy habitats (>50 m to 260 m), deposit feeders and carnivores were
the most important groups in abundance and biomass, respectively. 相似文献
80.
Phytosterols: applications and recovery methods 总被引:5,自引:0,他引:5
Phytosterols, or plant sterols, are compounds that occur naturally and bear close structural resemblance to cholesterol, but have different side-chain configurations. Phytosterols are relevant in pharmaceuticals (production of therapeutic steroids), nutrition (anti-cholesterol additives in functional foods, anti-cancer properties), and cosmetics (creams, lipstick). Phytosterols can be obtained from vegetable oils or from industrial wastes, which gives an added value to the latter. Considerable efforts have been recently dedicated to the development of efficient processes for phytosterol isolation from natural sources. The present work aims to summarize information on the applications of phytosterols and to review recent approaches, mainly from the industry, for the large-scale recovery of phytosterols. 相似文献