全文获取类型
收费全文 | 100075篇 |
免费 | 766篇 |
国内免费 | 889篇 |
专业分类
101730篇 |
出版年
2023年 | 42篇 |
2022年 | 93篇 |
2021年 | 158篇 |
2020年 | 84篇 |
2019年 | 129篇 |
2018年 | 11948篇 |
2017年 | 10771篇 |
2016年 | 7658篇 |
2015年 | 927篇 |
2014年 | 665篇 |
2013年 | 787篇 |
2012年 | 4760篇 |
2011年 | 13327篇 |
2010年 | 12342篇 |
2009年 | 8500篇 |
2008年 | 10173篇 |
2007年 | 11721篇 |
2006年 | 700篇 |
2005年 | 880篇 |
2004年 | 1337篇 |
2003年 | 1352篇 |
2002年 | 1086篇 |
2001年 | 327篇 |
2000年 | 206篇 |
1999年 | 97篇 |
1998年 | 81篇 |
1997年 | 103篇 |
1996年 | 68篇 |
1995年 | 36篇 |
1994年 | 53篇 |
1993年 | 70篇 |
1992年 | 52篇 |
1991年 | 68篇 |
1990年 | 34篇 |
1989年 | 35篇 |
1988年 | 43篇 |
1987年 | 34篇 |
1986年 | 22篇 |
1985年 | 30篇 |
1984年 | 36篇 |
1983年 | 41篇 |
1982年 | 37篇 |
1981年 | 29篇 |
1980年 | 26篇 |
1975年 | 17篇 |
1973年 | 18篇 |
1972年 | 252篇 |
1971年 | 280篇 |
1965年 | 18篇 |
1962年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
M Fondi V Orlandini G Emiliani MC Papaleo I Maida E Perrin M Vaneechoutte L Dijkshoorn R Fani 《Journal of bacteriology》2012,194(17):4771-4772
We report the draft genome sequence of Acinetobacter venetianus strain RAG-1(T), which is able to degrade hydrocarbons and to synthesize a powerful biosurfactant (emulsan) that can be employed for oil removal and as an adjuvant for vaccine delivery. The genome sequence of A. venetianus RAG-1(T) might be useful for bioremediation and/or clinical purposes. 相似文献
972.
M. Ciaffi D. Lafiandra E. Porceddu S. Benedettelli 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1993,86(4):518-525
Genetic diversity in the seed storage-proteins encoded at theGlu-A1,Glu-B1 andGli-B1/Glu-B3 loci was studied electrophoretically in 315 individuals belonging to nine populations ofT. dicoccoides from Jordan and three from Turkey. The inter- and intra-population distribution of seed storage-protein alleles at the considered loci and its link with geographical factors were investigated. Population differentiation in seed storage-proteins was in some cases very high with very weak correlations with geographic distance. Greater gene differentiation was found within and between populations which were geographically very close in Jordan than between those from Jordan and Turkey. However the distribution of alleles appeared to be non random. Samples collected from populations at locations over 900 m above sea level were less polymorphic than those collected at lower altitudes (500–700 m), whereas the relative genetic differentiation between populations was greater between those collected at higher altitudes. Seed storage-protein differentiation was significantly correlated with the altitude of the collecting sites. Although it is difficult to point out the selective pressure of altitude per se, altitude can reflect an integration of several environmental parameters. The possible adaptive value of seed storage-proteins is discussed. 相似文献
973.
974.
We evaluated the association between GSTM1 and GSTT1 gene polymorphisms and susceptibility to chronic obstructive pulmonary
disease (COPD) in smokers. A meta-analysis of the published case–control studies was performed. Published literature was retrieved
from PubMed, EMBASE, and China National Knowledge Infrastructure (CNKI), with last update in February, 2011. Data were extracted
and a fixed- or random-effects model was used to calculate pooled odds ratios with 95% confidence intervals depending on statistical
heterogeneity. Fourteen eligible studies, comprising 1,665 COPD cases and 1,614 controls, were included in the meta-analysis.
The combined analyses showed that there was a significant difference in GSTM1 genotype distribution between COPD cases and
controls among Caucasians, but not among Asians. The combined GSTM1/GSTT1 null genotype conferred a 1.36-fold greater risk
for COPD in Asian smokers. The GSTT1 null genotype alone was not associated with enhanced risk for COPD. The GSTM1 null genotype
is significantly associated with an increasing susceptibility to COPD in Caucasian smokers, but not in Asian smokers. The
GSTM1/GSTT1 null genotype is a significant risk factor for developing COPD in Asian smokers. The GSTT1 null genotype, however,
was not associated with COPD. 相似文献
975.
High-level production of 1,3-propanediol from crude glycerol by <Emphasis Type="Italic">Clostridium butyricum</Emphasis> AKR102a 总被引:1,自引:0,他引:1
Wilkens E Ringel AK Hortig D Willke T Vorlop KD 《Applied microbiology and biotechnology》2012,93(3):1057-1063
The aim of this study was to optimize a biotechnological process for the production of 1,3-propanediol (1,3-PD) based on low-quality
crude glycerol derived from biodiesel production. Clostridium butyricum AKR102a was used in fed-batch fermentations in 1-L and 200-L scale. The newly discovered strain is characterized by rapid
growth, high product tolerance, and the ability to use crude glycerol at the lowest purity directly gained from a biodiesel
plant side stream. Using pure glycerol, the strain AKR102 reached 93.7 g/L 1,3-PD with an overall productivity of 3.3 g/(L*h).
With crude glycerol under the same conditions, 76.2 g/L 1,3-PD was produced with a productivity of 2.3 g/(L*h). These are
among the best results published so far for natural producers. The scale up to 200 L was possible. Due to the simpler process
design, only 61.5 g/L 1,3-PD could be reached with a productivity of 2.1 g/(L*h). 相似文献
976.
Ozaki TJ Sato N Kitajo K Someya Y Anami K Mizuhara H Ogawa S Yamaguchi Y 《Cognitive neurodynamics》2012,6(2):185-198
An ambiguous figure such as the Necker cube causes spontaneous perceptual switching (SPS). The mechanism of SPS in multistable
perception has not yet been determined. Although early psychological studies suggested that SPS may be caused by fatigue or
satiation of orientation, the neural mechanism of SPS is still unknown. Functional magnetic resonance imaging (fMRI) has shown
that the dorsal attention network (DAN), which mainly controls voluntary attention, is involved in bistable perception of
the Necker cube. To determine whether neural dynamics along the DAN cause SPS, we performed simultaneous electroencephalography
(EEG) and fMRI during an SPS task with the Necker cube, with every SPS reported by pressing a button. This EEG–fMRI integrated
analysis showed that (a) 3–4 Hz spectral EEG power modulation at fronto-central, parietal, and centro-parietal electrode sites
sequentially appeared from 750 to 350 ms prior to the button press; and (b) activations correlating with the EEG modulation
traveled along the DAN from the frontal to the parietal regions. These findings suggest that slow oscillation initiates SPS
through global dynamics along the attentional system such as the DAN. 相似文献
977.
Jered M. Studinski Kyle J. Hartman Jonathan M. Niles Patrick Keyser 《Hydrobiologia》2012,686(1):107-117
Forested headwater streams rely on their riparian areas for temperature regulation, woody debris inputs, and sediment retention. These products and services may be altered by disturbances such as timber harvest, windthrow, or development. This study investigated the effects of riparian forest disturbance by removing trees using 50 and 90% basal area harvests and by directly felling some trees into eight streams in eastern West Virginia. On summer afternoons, water temperature increased in the 50 and 90% BAH treatments at average rates of 0.18 and 0.79°C/100 m, respectively. The 90% BAH treatments had the potential to disrupt fish and invertebrate communities via increased water temperature. New roads and log landings associated with the riparian logging had no detectable effect on sedimentation or turbidity. Large woody debris (LWD) additions increased habitat complexity but no net increase in pool area was observed. Greater morphological instability was observed within the LWD addition sections as pools were both created and destroyed at significantly higher rates. Experimentally manipulating small riparian patches may be an analog for small-scale natural and anthropogenic disturbances. These common events are assumed to alter streams, but there are few experimental studies quantifying their effects. 相似文献
978.
Kataoka R Siddiqui ZA Kikuchi J Ando M Sriwati R Nozaki A Futai K 《Journal of microbiology (Seoul, Korea)》2012,50(2):199-206
The fungus Tricholoma matsutake forms an ectomycorrhizal relationship with pine trees. Its sporocarps often develop in a circle, which is commonly known
as a fairy ring. The fungus produces a solid, compact, white aggregate of mycelia and mycorrhizae beneath the fairy ring,
which in Japanese is called a ’shiro’. In the present study, we used soil dilution plating and molecular techniques to analyze
the bacterial communities within, beneath, and outside the T. matsutake fairy ring. Soil dilution plating confirmed previous reports that bacteria and actinomycetes are seldom present in the soil
of the active mycorrhizal zone of the T. matsutake shiro. In addition, the results showed that the absence of bacteria was strongly correlated with the presence of T. matsutake mycorrhizae. The results demonstrate that bacteria, especially aerobic and heterotrophic forms, and actinomycetes, are strongly
inhibited by T. matsutake. Indeed, neither bacteria nor actinomycetes were detected in 11.3% of 213 soil samples from the entire shiro area by culture-dependent
methods. However, molecular techniques demonstrated that some bacteria, such as individual genera of Sphingomonas and Acidobacterium, were present in the active mycorrhizal zone, even though they were not detected in soil assays using the dilution plating
technique. 相似文献
979.
Grasses and browsers reinforce landscape heterogeneity by excluding trees from ecosystem hotspots 总被引:1,自引:0,他引:1
Spatial heterogeneity in woody cover affects biodiversity and ecosystem function, and may be particularly influential in savanna
ecosystems. Browsing and interactions with herbaceous plants can create and maintain heterogeneity in woody cover, but the
relative importance of these drivers remains unclear, especially when considered across multiple edaphic contexts. In African
savannas, abandoned temporary livestock corrals (bomas) develop into long-term, nutrient-rich ecosystem hotspots with unique
vegetation. In central Kenya, abandoned corral sites persist for decades as treeless ‘glades’ in a wooded matrix. Though glades
are treeless, areas between adjacent glades have higher tree densities than the background savanna or areas near isolated
glades. The mechanisms maintaining these distinctive woody cover patterns remain unclear. We asked whether browsing or interactions
with herbaceous plants help to maintain landscape heterogeneity by differentially impacting young trees in different locations.
We planted the mono-dominant tree species (Acacia drepanolobium) in four locations: inside glades, far from glades, at edges of isolated glades and at edges between adjacent glades. Within
each location, we assessed the separate and combined effects of herbivore exclusion (caging) and herbaceous plant removal
(clearing) on tree survival and growth. Both caging and clearing improved tree survival and growth inside glades. When herbaceous
plants were removed, trees inside glades grew more than trees in other locations, suggesting that glade soils were favorable
for tree growth. Different types of glade edges (isolated vs. non-isolated) did not have significantly different impacts on
tree performance. This represents one of the first field-based experiments testing the separate and interactive effects of
browsing, grass competition and edaphic context on savanna tree performance. Our findings suggest that, by excluding trees
from otherwise favorable sites, both herbaceous plants and herbivores help to maintain functionally important landscape heterogeneity
in African savannas. 相似文献
980.
G protein-coupled receptors (GPCRs) are transmembrane proteins, which transduce signals from extracellular ligands to intracellular
G protein. Automatic classification of GPCRs can provide important information for the development of novel drugs in pharmaceutical
industry. In this paper, we propose an evolutionary approach, GPCR-MPredictor, which combines individual classifiers for predicting
GPCRs. GPCR-MPredictor is a web predictor that can efficiently predict GPCRs at five levels. The first level determines whether
a protein sequence is a GPCR or a non-GPCR. If the predicted sequence is a GPCR, then it is further classified into family,
subfamily, sub-subfamily, and subtype levels. In this work, our aim is to analyze the discriminative power of different feature
extraction and classification strategies in case of GPCRs prediction and then to use an evolutionary ensemble approach for
enhanced prediction performance. Features are extracted using amino acid composition, pseudo amino acid composition, and dipeptide
composition of protein sequences. Different classification approaches, such as k-nearest neighbor (KNN), support vector machine (SVM), probabilistic neural networks (PNN), J48, Adaboost, and Naives Bayes, have been used to classify GPCRs. The proposed hierarchical GA-based ensemble classifier exploits
the prediction results of SVM, KNN, PNN, and J48 at each level. The GA-based ensemble yields an accuracy of 99.75, 92.45, 87.80, 83.57, and 96.17% at the five levels, on
the first dataset. We further perform predictions on a dataset consisting of 8,000 GPCRs at the family, subfamily, and sub-subfamily
level, and on two other datasets of 365 and 167 GPCRs at the second and fourth levels, respectively. In comparison with the
existing methods, the results demonstrate the effectiveness of our proposed GPCR-MPredictor in classifying GPCRs families.
It is accessible at . 相似文献