全文获取类型
收费全文 | 3059篇 |
免费 | 185篇 |
国内免费 | 252篇 |
专业分类
3496篇 |
出版年
2024年 | 8篇 |
2023年 | 46篇 |
2022年 | 55篇 |
2021年 | 71篇 |
2020年 | 78篇 |
2019年 | 131篇 |
2018年 | 84篇 |
2017年 | 140篇 |
2016年 | 169篇 |
2015年 | 149篇 |
2014年 | 143篇 |
2013年 | 216篇 |
2012年 | 127篇 |
2011年 | 121篇 |
2010年 | 96篇 |
2009年 | 196篇 |
2008年 | 190篇 |
2007年 | 189篇 |
2006年 | 150篇 |
2005年 | 124篇 |
2004年 | 95篇 |
2003年 | 102篇 |
2002年 | 77篇 |
2001年 | 76篇 |
2000年 | 62篇 |
1999年 | 49篇 |
1998年 | 57篇 |
1997年 | 39篇 |
1996年 | 34篇 |
1995年 | 37篇 |
1994年 | 38篇 |
1993年 | 36篇 |
1992年 | 34篇 |
1991年 | 26篇 |
1990年 | 32篇 |
1989年 | 27篇 |
1988年 | 20篇 |
1987年 | 25篇 |
1986年 | 8篇 |
1985年 | 21篇 |
1984年 | 16篇 |
1983年 | 9篇 |
1982年 | 18篇 |
1981年 | 14篇 |
1980年 | 12篇 |
1979年 | 7篇 |
1978年 | 8篇 |
1977年 | 9篇 |
1976年 | 11篇 |
1972年 | 3篇 |
排序方式: 共有3496条查询结果,搜索用时 0 毫秒
141.
Angel López Piñeiro Enrique Saiz 《International journal of biological macromolecules》1983,5(1):37-41
The Rotational Isomeric States model is applied to calculate dipole moments of polypeptides of the twenty natural α-amino acids in the random coil state. Dipole moments of each repeat unit (μi), are evaluated using a quantum mechanics procedure. Dipole moment ratios () of homopolypeptides are calculated and extrapolated to x →?. With a few exceptions, D? = 0.36 ± 0.1. Ten actual proteins and three enzymes are also studied; their dipole ratios (Dx′ =〈μ〉/x) range from 7.34 to 10.57 in 10?59 C2 m2 (6.6–9.5 D2). Diffferences in the values of Dx′ are due mainly to the different contributions, μi, of the amino acid residues contained in each polymer, whereas the sequence of amino acids has a very minor effect. 相似文献
142.
Correlations between environmental factors and the distribution of amphibian and reptile species richness were investigated
in a climate transition area, Peneda-Gerês National Park (PNPG), in North-Western Portugal. Using presence-data at a local-scale
(1 × 1 km), Ecological-Niche Factor Analysis (ENFA) identified a mixture of climatic (precipitation and number of days with
fog), topographical (altitude and relief) and habitat factors (number of watercourses and water surfaces, the type of the
largest water surface and tree diversity cover), as accurate predictors of species occurrence. Three factors were common for
both taxonomic groups, and consistently presented a positive relation with species occurrence: precipitation, number of water
surfaces, and tree diversity cover; suggesting a strong coincidence in the environmental correlates that influence amphibian
and reptile species richness. Distribution patterns of observed and predicted species richness were compared using a Geographical
Information System. Overall, three high species richness areas were predicted in common for both taxonomic groups and two
additional areas for amphibians only. These areas matched with the observed species richness but suggested larger areas of
high species richness. The location of the PNPG in a biogeographic crossroad, between Euro-Siberian and Mediterranean provinces,
emphasised species richness of amphibians and reptiles and suggests a high priority conservation status for this protected
area. Most of Central-Northern Portugal is located in a climatic transition area; therefore, increased species richness should
be expected for other areas. Local scale studies for other protected areas should be planned as a framework for the development
of multi-scale conservation planning by Portuguese authorities. 相似文献
143.
144.
This paper reports the response of isopods exposed to fallout of a municipal solid waste landfill located in central Italy. Soil samples and specimens of Armadillidium vulgare were collected at different distances from the landfill and analyzed to determine the concentrations of heavy elements such as As, Cd, Co, Cr, Cu, Ni, Pb, Sb, V and Zn. The isopod analysis was performed on unpurged and purged specimens. Analytical data indicate that the soil contents of heavy elements were quite uniform and within the respective local geochemical background. Slight enrichments of Cu and Pb were found in some soils collected within the solid waste. Purged isopods showed an accumulation of As, Co, Cr, Ni, Sb and V whose body levels decreased as the distance from the landfill increased. Cd, Cu, Pb and Zn concentrations in purged specimens were rather uniform and no significant variation trend occurred. This result probably was due to the fact that the isopods are provided with physiological mechanisms of regulation for these heavy elements. Analytical data also indicate the ability of A. vulgare to adsorb differently the heavy elements according to the following order: As > Co > Ni > Pb > V. The contents of heavy elements in unpurged specimens were higher than in purged ones. This finding suggested that the defecation has marked effects on the tissue levels of heavy elements in isopods. This study indicates that the isopods provide useful information about environmental quality in areas characterized by low and discontinuous emission of heavy elements and their low accumulation in soil. 相似文献
145.
The present study was designed to assess the preventive effect of licorice compounds glycyrrhizin and 18beta-glycyrrhetinic acid against mitochondrial damage and cell death in lung epithelial cells exposed to 3-morpholinosydnonime, a donor of nitric oxide and superoxide. Treatment of lung epithelial cells with 3-morpholinosydnonime resulted in the nuclear damage, decrease in the mitochondrial transmembrane potential, cytosolic accumulation of cytochrome c, activation of caspase-3, increase in the formation of reactive oxygen species and depletion of GSH. Treatment of glycyrrhizin and 18beta-glycyrrhetinic acid attenuated the 3-morpholinosydnonime-induced mitochondrial damage, formation of reactive oxygen species and GSH depletion and revealed a maximal inhibitory effect at 10 and 1 muM, respectively; beyond these concentrations the inhibitory effect declined. Melatonin, carboxy-PTIO, rutin and uric acid reduced the 3-morpholinosydnonime-induced cell death. The results show that glycyrrhizin and 18beta-glycyrrhetinic acid seem to prevent the toxic effect of 3-morpholinosydnonime against lung epithelial cells by suppressing the mitochondrial permeability transition that leads to the release of cytochrome c and activation of caspase-3. The preventive effect may be ascribed to the inhibitory action on the formation of reactive oxygen species and depletion of GSH. The findings suggest that licorice compounds seem to prevent the nitrogen species-mediated lung cell damage. 相似文献
146.
Occurrences of harmful algal blooms (HABs) associated with ocean environments in the South China Sea 总被引:2,自引:0,他引:2
SuFen Wang DanLing Tang FangLiang He Yasuwo Fukuyo Rhodora V. Azanza 《Hydrobiologia》2008,596(1):79-93
Harmful algal blooms (HABs) occur frequently in the South China Sea (SCS), causing enormous economic losses in aquaculture.
We analyzed historical HAB records during the period from 1980 to 2003 in SCS. We found that HABs-affected areas have expanded
and the frequency of HABs varied during this period. The seasonal and annual variations, as well as causative algal species
of HABs are different among the four regions. Areas with frequent HABs include the Pearl River Estuary (China), the Manila
Bay (the Philippines), the Masinloc Bay (the Philippines), and the western coast of Sabah (Malaysia). HABs occurred frequently
during March–May in the northern region of SCS, May–July in the eastern region, July in the western region, and year-round
in the southern region. Among the species that cause HABs, Noctiluca scintillans dominated in the northern region, and Pyrodinium bahamense in the southern and eastern regions. Causative species also varied in different years for the entire SCS. Both P. bahamense and N. scintillans were the dominant species during 1980–2003. Some species not previously recorded formed blooms during 1991–2003, including
Phaeocystis globosa, Scrippsiella trochoidea, Heterosigma akashiwo, and Mesodinium rubrum. Variations in HABs are related to various regional conditions, such as a reversed monsoon wind in the entire SCS, river
discharges in the northern area, upwelling in Vietnam coastal waters during southwest winds and near Malaysia coastal waters
during northeast winds, and eutrophication from coastal aquaculture in the Pearl River estuary, Manila Bay, and Masinloc Bay.
Handling editor: D. Hamilton 相似文献
147.
Dai Xiao-bing 《植被学杂志》1993,4(1):109-114
Vegetation and biomass in six plots, each with five quadrats, in Vitex shrubland in northern China were related to type and degree of human disturbance. Canonical correspondence analysis of the vegetation and environmental data showed two lines of variation, the first one highly correlated with the degree of disturbance, but the second one appearing within the disturbed plots cannot yet be fully explained. Changes in life-form spectrum were small, but the geographical spectrum changed under heavy disturbance towards a higher proportion of tropical, subtropical and cosmopolitan species. The number of shrub species also decreased noticeably. The above-ground biomass of both Vitex and the accompanying species and the total above-ground biomass of the shrubland was negatively correlated with the intensity of disturbance. The root/stem biomass ratio in Vitex reaches values of over 5 in disturbed plots, but only between 1 and 2 in the protected plots. 相似文献
148.
As environmental problems become more and more serious all over the world, environmental efficiency evaluation has drawn increasing interests of many scholars and governments’ decision makers. Data envelopment analysis (DEA) as a non-parametric approach for evaluating the relative efficiency of a group of decision making units (DMUs) has been widely extended and applied in many areas. Among various DEA models, enhanced Russell measure (ERM) model can measure the inefficiencies from input orientation and output orientation simultaneously. In this paper, we proposed a new non-orientation DEA approach based on enhanced Russell measure for measuring the environmental efficiency of a DMU and meanwhile, provided the closest target for the evaluated DMU to efficient with less effort. At last, our approach was applied to a practical example about thermal power enterprises. The results show that our model can provide a much easier way for the inefficient enterprises to improve their efficiencies than the enhanced Russell measure method. Moreover, the provided benchmarks for enterprises through our model can be used to further rank the efficient ones. 相似文献
149.
Sylvie A. Quideau Anne C.S. McIntosh Charlotte E. Norris Emily Lloret Mathew J.B. Swallow Kirsten Hannam 《Journal of visualized experiments : JoVE》2016,(114)
Phospholipid fatty acids (PLFAs) are key components of microbial cell membranes. The analysis of PLFAs extracted from soils can provide information about the overall structure of terrestrial microbial communities. PLFA profiling has been extensively used in a range of ecosystems as a biological index of overall soil quality, and as a quantitative indicator of soil response to land management and other environmental stressors.The standard method presented here outlines four key steps: 1. lipid extraction from soil samples with a single-phase chloroform mixture, 2. fractionation using solid phase extraction columns to isolate phospholipids from other extracted lipids, 3. methanolysis of phospholipids to produce fatty acid methyl esters (FAMEs), and 4. FAME analysis by capillary gas chromatography using a flame ionization detector (GC-FID). Two standards are used, including 1,2-dinonadecanoyl-sn-glycero-3-phosphocholine (PC(19:0/19:0)) to assess the overall recovery of the extraction method, and methyl decanoate (MeC10:0) as an internal standard (ISTD) for the GC analysis. 相似文献
150.
Erica Fleishman Daniel P. Costa John Harwood Scott Kraus David Moretti Leslie F. New Robert S. Schick Lisa K. Schwarz Samantha E. Simmons Len Thomas Randall S. Wells 《Marine Mammal Science》2016,32(3):1004-1021
We provide guidance for monitoring whether human activities affect the physiology or behavior of marine mammals and, if so, whether those effects may lead to changes in survival and reproduction at the population level. We suggest that four elements be included in designing and implementing such a monitoring program. The first is development of a theory of change: a set of mechanistic hypotheses that outline why a given activity might be expected to have one or more measurable effects on individuals and populations, and ideally the magnitude, timing, and duration of the effects. The second element, definition of biologically meaningful effect sizes, ultimately facilitates the development of a monitoring program that can detect those magnitudes of effect with the desired levels of precision. The third element, selection of response variables for monitoring, allows inference to whether observed changes in the status of individuals or populations are attributable to a given activity. Visual observations, passive acoustic and tagging instruments, and direct physical measurements all can provide data that facilitate quantitative hypothesis testing. The fourth element is specification of the temporal sequence of monitoring. These elements also can be used to inform monitoring of the responses of other taxonomic groups to human activities. 相似文献