全文获取类型
收费全文 | 133693篇 |
免费 | 9520篇 |
国内免费 | 27篇 |
专业分类
143240篇 |
出版年
2023年 | 702篇 |
2022年 | 808篇 |
2021年 | 1670篇 |
2020年 | 1397篇 |
2019年 | 1582篇 |
2018年 | 3221篇 |
2017年 | 2921篇 |
2016年 | 4012篇 |
2015年 | 5830篇 |
2014年 | 6094篇 |
2013年 | 8077篇 |
2012年 | 9737篇 |
2011年 | 9144篇 |
2010年 | 5986篇 |
2009年 | 4729篇 |
2008年 | 7511篇 |
2007年 | 7392篇 |
2006年 | 6794篇 |
2005年 | 6233篇 |
2004年 | 5954篇 |
2003年 | 5407篇 |
2002年 | 5009篇 |
2001年 | 2953篇 |
2000年 | 2794篇 |
1999年 | 2443篇 |
1998年 | 1157篇 |
1997年 | 900篇 |
1996年 | 804篇 |
1995年 | 811篇 |
1994年 | 773篇 |
1993年 | 630篇 |
1992年 | 1548篇 |
1991年 | 1454篇 |
1990年 | 1288篇 |
1989年 | 1212篇 |
1988年 | 1116篇 |
1987年 | 989篇 |
1986年 | 897篇 |
1985年 | 960篇 |
1984年 | 805篇 |
1983年 | 698篇 |
1982年 | 561篇 |
1981年 | 507篇 |
1979年 | 706篇 |
1978年 | 524篇 |
1977年 | 471篇 |
1976年 | 464篇 |
1975年 | 524篇 |
1974年 | 568篇 |
1973年 | 559篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
931.
Martina Kyselková Jiří Janata Markéta Ságová-Marečková Jan Kopecký 《Archives of microbiology》2010,192(3):195-200
In acetohydroxy acid synthase from Streptomyces cinnamonensis mutants affected in valine regulation, the impact of mutations on interactions between the catalytic and the regulatory subunits
was examined using yeast two-hybrid system. Mutations in the catalytic and the regulatory subunits were projected into homology
models of the respective proteins. Two changes in the catalytic subunit, E139A (α domain) and ΔQ217 (β domain), both located
on the surface of the catalytic subunit dimer, lowered the interaction with the regulatory subunit. Three consecutive changes
in the N-terminal part of the regulatory subunit were examined. Changes G16D and V17D in a loop and adjacent α-helix of ACT
domain affected the interaction considerably, indicating that this region might be in contact with the catalytic subunit during
allosteric regulation. In contrast, the adjacent mutation L18F did not influence the interaction at all. Thus, L18 might participate
in valine binding or conformational change transfer within the regulatory subunits. Shortening of the regulatory subunit to
107 residues reduced the interaction essentially, suggesting that the C-terminal part of the regulatory subunit is also important
for the catalytic subunit binding. 相似文献
932.
Celia G. Jerez Carolina B. García Agustín Rearte Félix L. Figueroa 《Journal of applied phycology》2016,28(3):1635-1648
In order to estimate microalgal carbon assimilation or production of Chlorella fusca cultures based on electron transport rate (ETR) as in vivo chlorophyll a fluorescence, it is necessary to determine the photosynthetic yield and the absorbed quanta by measuring the incident irradiance and the fraction of absorbed light, i.e., absorptance or absorption coefficient in the photosynthetic active radiation (PAR) region of the spectra. Due to difficulties associated with the determination of light absorption, ETR is commonly expressed as relative units (rETR) although this is not a good estimator of the photosynthetic production since photobiological responses depend on the absorbed light. The quantitative filter technique (QFT) is commonly used to measure the absorbed quanta of cells retained on a filter (AbQf) as estimator of the absorbed quanta of cell suspensions (AbQs) determined by using integrating spheres. In this study, light attenuation of thin-layer cell suspensions is determined by using a measuring system designed to reduce the scattering. The light attenuation is related to the absorptance as the fraction of absorbed light by both indoor and outdoor C. fusca cultures of different cell densities. A linear relation between AbQf and AbQs (R 2?=?0.9902, p?<?0.01) was observed, AbQf?=?1.98?×?AbQs, being 1.98 an amplification factor to convert AbQs values into AbQf ones. On the other hand, depending on the culture system, the convenience of the use of the absorptance, light absorption or specific light absorption coefficient expressed per area (thin-layer cascade or flat panel cultivators), volume (cylindrical and tubular photobioreactors), or chlorophyll units (any type of cultivation system) is discussed. The procedure for the measurement of light absorption presented in this study for C. fusca could be applied in other phytoplankton groups. The absorbed quanta as determined in this study can be used to express absolute ETR instead of relative ETR, since the first one provides much more relevant photobiological information of microalgae culture systems. 相似文献
933.
934.
935.
Plant functional traits are increasingly used in restoration ecology because they have the potential to guide restoration practices at a broad scale. This article presents a trait‐based multi‐criteria framework to evaluate and predict the performance of 17 plant seedlings to improve ecological restoration of marly eroded areas in the French Southern Alps. The suitability of these species to limit soil erosion was assessed by studying both their response to erosive forces and their effect on erosion dynamics. We assumed that species efficiency could be explained and predicted from plant traits and we looked for trait‐performance relationships. Our results showed that root slenderness ratio, the percentage of fine roots and root system topology, were the three root morphology traits best describing anchorage strength. Root system characterized by a long and thin tap root and many fine lateral ramifications would be the best to resist concentrated runoff. Species response to burial mainly depended on growth form and morphological flexibility. The abilities of species in reinforcing the soil and reducing erosion rates were negatively correlated to root diameter and positively to the percentage of fine roots. Moreover, root system density and root tensile strength also influenced root reinforcement. Finally, the ability to trap sediment was positively correlated to leaf area and canopy density. Species were then scored and classified in four clusters according to their global performance. This method allows identifying species that possess both response and effect traits related to the goal of preventing erosion during ecological restoration. 相似文献
936.
M. José Feito Mercedes Jiménez Concha Fernández-Cabrera Germán Rivas Guillermo Giménez-Gallego Rosa M. Lozano 《Analytical biochemistry》2011,411(1):1
Here we describe, for the first time, the design and characterization of a bona fide fluorescently labeled mutant of the human acidic fibroblast growth factor (aFGF). The aFGF–Cys2 mutant was recombinantly synthesized by substituting the second amino acid with a reactive cysteine whose sulfhydryl group’s side chain reactivity facilitated the covalent binding of a fluorescent probe as a thiolyte monobromobimane. Using a combination of biophysical and functional assays, we found that the fluorescently labeled mutant aFGF is characterized by essentially the same global folding, mitogenic activity, and association behavior with heparin, its physiological activator, as the unlabeled wild-type protein. We used this new tracer protein mutant to determine the association behavior of aFGF with heparin in the presence of high concentrations of albumin that mimicked more closely the plasma medium in which aFGF is naturally located and in which it has evolved to function. By exposing the aFGF–Cys2–heparin complex to increasing concentrations of albumin up to physiological plasma levels, we were able to demonstrate that macromolecular crowding does not affect the stoichiometry of the interaction. In summary, the dimeric aFGF–Cys2–heparin complex might represent a biologically relevant complex in physiological media. 相似文献
937.
Tony Robinet Eric Feunteun Philippe Keith Gérard Marquet Jean-Michel Olivier Elodie Réveillac Pierre Valade 《Environmental Biology of Fishes》2007,78(2):93-105
Anguillid eels were sampled from permanent rivers in the Réunion and Mauritius islands, western Indian Ocean, with a standardized
electrofishing method. A. marmorata was very dominant, corresponding to 91.7 and 90.7% of all the eels collected in Réunion and Mauritius, respectively. Three
other species (A. mossambica, A. bicolor bicolor and A. nebulosa labiata) were also present in both islands. A. marmorata showed a strong altitudinal gradient of densities from the lower to upper zones, especially in the younger stages (TL <250 mm),
while A. mossambica was only found in the upper zones and A. bicolor bicolor occurred only in the lower zones (A. nebulosa labiata was rare). The eel species composition in freshwaters of both islands is very similar because these two adjoining islands
are located in the same trail of drifting marine larvae. Mean estimated eel biomasses were noticeably low (11.1 and 22.2 kg ha−1 in Réunion and Mauritius islands, respectively), especially when compared to those of other tropical insular systems without
any eel fishery (Comoros or Polynesia, more than 100 kg ha−1). Nevertheless, the fluvial recruitment of A. marmorata seemed to be regular during the surveyed period, staggering from October to April. The obvious lack of large eels in Mauritius
but more significantly in Réunion suggests a high pressure from traditional fishery, and the local reproductive turnover is
uncertain. Because sexual maturation seems to occur at a large body size for A. marmorata, as for temperate species, the Réunion and Mauritius rivers may only have a weak contribution to the regional production
of spawners. However, the giant mottled eel population in the western Indian Ocean is believed to be panmictic at the regional
scale, and may not rely exclusively on these islands’ contribution. A comparison is made with those of freshwater systems
in other tropical islands. 相似文献
938.
939.
940.
María Crespo López Carlos Hornillos JerezMónica Bernaldo De Quirós Aragón M. Mar Gómez Gutiérrez 《Revista espa?ola de geriatría y gerontología》2011,46(6):319
Quality of life (QoL) is one of the most important outcome variables in the study of the efficacy of interventions with people with dementia. However, its assessment is difficult 1) because it is a complex construct for which there is no unified theoretical or conceptual approach, and 2) because of the inherent difficulties in the cognitive impairments of the people under study. In this work different methods and instruments to this end are reviewed, and related findings are discussed. It is important to take into account the subjective view of the assessed person, as assessments done by proxies tend to underestimate QoL. In spite of the need for further development in this field, it is concluded that the instrument of choice is the QOL-AD, as it is change-sensitive, it correlates with health measurements, it is translated into several languages and it can be administered to people with low MMSE scores. 相似文献