全文获取类型
收费全文 | 2340篇 |
免费 | 227篇 |
国内免费 | 1篇 |
专业分类
2568篇 |
出版年
2021年 | 32篇 |
2020年 | 24篇 |
2019年 | 29篇 |
2018年 | 32篇 |
2017年 | 16篇 |
2016年 | 43篇 |
2015年 | 87篇 |
2014年 | 106篇 |
2013年 | 136篇 |
2012年 | 116篇 |
2011年 | 129篇 |
2010年 | 86篇 |
2009年 | 66篇 |
2008年 | 110篇 |
2007年 | 118篇 |
2006年 | 101篇 |
2005年 | 118篇 |
2004年 | 102篇 |
2003年 | 85篇 |
2002年 | 84篇 |
2001年 | 42篇 |
2000年 | 38篇 |
1999年 | 43篇 |
1998年 | 32篇 |
1997年 | 18篇 |
1996年 | 24篇 |
1995年 | 20篇 |
1994年 | 17篇 |
1993年 | 27篇 |
1992年 | 37篇 |
1991年 | 22篇 |
1990年 | 19篇 |
1989年 | 26篇 |
1988年 | 23篇 |
1987年 | 15篇 |
1985年 | 21篇 |
1984年 | 19篇 |
1983年 | 23篇 |
1982年 | 28篇 |
1981年 | 24篇 |
1980年 | 28篇 |
1979年 | 22篇 |
1978年 | 22篇 |
1977年 | 21篇 |
1976年 | 16篇 |
1975年 | 15篇 |
1974年 | 18篇 |
1973年 | 19篇 |
1971年 | 25篇 |
1970年 | 15篇 |
排序方式: 共有2568条查询结果,搜索用时 15 毫秒
101.
Ecological restoration of rich fens in Europe and North America: from trial and error to an evidence‐based approach
下载免费PDF全文
![点击此处可从《Biological reviews of the Cambridge Philosophical Society》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Leon P. M. Lamers Melanie A. Vile Ab P. Grootjans Mike C. Acreman Rudy van Diggelen Martin G. Evans Curtis J. Richardson Line Rochefort Annemieke M. Kooijman Jan G. M. Roelofs Alfons J. P. Smolders 《Biological reviews of the Cambridge Philosophical Society》2015,90(1):182-203
Fens represent a large array of ecosystem services, including the highest biodiversity found among wetlands, hydrological services, water purification and carbon sequestration. Land‐use change and drainage has severely damaged or annihilated these services in many parts of North America and Europe; restoration plans are urgently needed at the landscape level. We review the major constraints on the restoration of rich fens and fen water bodies in agricultural areas in Europe and disturbed landscapes in North America: (i) habitat quality problems: drought, eutrophication, acidification, and toxicity, and (ii) recolonization problems: species pools, ecosystem fragmentation and connectivity, genetic variability, and invasive species; and here provide possible solutions. We discuss both positive and negative consequences of restoration measures, and their causes. The restoration of wetland ecosystem functioning and services has, for a long time, been based on a trial‐and‐error approach. By presenting research and practice on the restoration of rich fen ecosystems within agricultural areas, we demonstrate the importance of biogeochemical and ecological knowledge at different spatial scales for the management and restoration of biodiversity, water quality, carbon sequestration and other ecosystem services, especially in a changing climate. We define target processes that enable scientists, nature managers, water managers and policy makers to choose between different measures and to predict restoration prospects for different types of deteriorated fens and their starting conditions. 相似文献
102.
van der Werf MJ Overkamp KM Muilwijk B Coulier L Hankemeier T 《Analytical biochemistry》2007,370(1):17-25
Achieving metabolome data with satisfactory coverage is a formidable challenge in metabolomics because metabolites are a chemically highly diverse group of compounds. Here we present a strategy for the development of an advanced analytical platform that allows the comprehensive analysis of microbial metabolomes. Our approach started with in silico metabolome information from three microorganisms-Escherichia coli, Bacillus subtilis, and Saccharomyces cerevisiae-and resulted in a list of 905 different metabolites. Subsequently, these metabolites were classified based on their physicochemical properties, followed by the development of complementary gas chromatography-mass spectrometry and liquid chromatography-mass spectrometry methods, each of which analyzes different metabolite classes. This metabolomics platform, consisting of six different analytical methods, was applied for the analysis of the metabolites for which commercial standards could be purchased (399 compounds). Of these 399 metabolites, 380 could be analyzed with the platform. To demonstrate the potential of this metabolomics platform, we report on its application to the analysis of the metabolome composition of mid-logarithmic E. coli cells grown on a mineral salts medium using glucose as the carbon source. Of the 431 peaks detected, 235 (=176 unique metabolites) could be identified. These include 61 metabolites that were not previously identified or annotated in existing E. coli databases. 相似文献
103.
Hydrogen peroxide induced the accumulation of free benzoic acid (BA) and salicylic acid (SA) in tobacco (Nicotiana tabacum L. cv Xanthi-nc) leaves. Six hours after infiltration with 300 mM H2O2, the levels of BA and SA in leaves increased 5-fold over the levels detected in control leaves. The accumulation of BA and SA was preceded by the rapid activation of benzoic acid 2-hydroxylase (BA2H) in the H2O2-infiltrated tissues. This enzyme catalyzes the formation of SA from BA. Enzyme activation could be reproduced in vitro by addition of H2O2 or cumene hydroperoxide to the assay mixture. H2O2 was most effective in vitro when applied at 6 mM. In vitro activation of BA2H by peroxides was inhibited by the catalase inhibitor 3-amino-1,2,4-triazole. We suggest that H2O2 activates SA biosynthesis via two mechanisms. First, H2O2 stimulates BA2H activity directly or via the formation of its substrate, molecular oxygen, in a catalase-mediated reaction. Second, higher BA levels induce the accumulation of BA2H protein in the cells and provide more substrate for this enzyme. 相似文献
104.
Critical evaluation of organic acid mediated iron dissolution in the rhizosphere and its potential role in root iron uptake 总被引:7,自引:0,他引:7
Both experimental extractions and theoretical calculations were undertaken to assess whether organic acid-mediated Fe dissolution could play a significant role in elevating the concentration of Fe-complexes in the rhizosphere, and further, whether this could satisfy the Fe demands of a plant utilizing ferric reduction to acquire Fe. Using a mathematical computer model, it was predicted that organic acids released from and diffusing away from the root would result in a solution organic acid concentration at the root surface of between 1 to 50 M. Over 99% of the organic acids lost by the root were predicted to remain within 1 mm of the root surface. The experimental results indicated that citrate-mediated Fe dissolution of amorphous Fe(OH)3, was rapid in comparison with citrate dissolution of the Fe-oxides, Fe2O3 and Fe3O4. The rate of citrate and malate mediated Fe-dissolution was dependent on many factors such as pH, metal cations and phosphate saturation of the Fe(OH)3 surface. At pH values 6.8, citrate formed stable complexes with Fe and dissolution proceeded rapidly. Under optimal growth conditions for a plant utilizing a reductive-bound mechanism of Fe acquisition (dicots and non-grass monocots), it can be expected that citrate and malate may be able to satisfy a significant proportion of the plant's Fe demand through the formation of plant-available organic-Fe3+ complexes in the rhizosphere. In high pH soils (pH7.0), the plant must rely on other sources of Fe, as citrate-mediated Fe dissolution is slow and Fe-citrate complexes are unstable. Alternatively, the root acidification of the rhizosphere could allow the formation of stable Fe-organic complexes. ei]H Marschner 相似文献
105.
Short-range molecular rearrangements in ion channels detected by tryptophan quenching of bimane fluorescence
下载免费PDF全文
![点击此处可从《The Journal of general physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Ion channels are allosteric membrane proteins that open and close an ion-permeable pore in response to various stimuli. This gating process provides the regulation that underlies electrical signaling events such as action potentials, postsynaptic potentials, and sensory receptor potentials. Recently, the molecular structures of a number of ion channels and channel domains have been solved by x-ray crystallography. These structures have highlighted a gap in our understanding of the relationship between a channel's function and its structure. Here we introduce a new technique to fill this gap by simultaneously measuring the channel function with the inside-out patch-clamp technique and the channel structure with fluorescence spectroscopy. The structure and dynamics of short-range interactions in the channel can be measured by the presence of quenching of a covalently attached bimane fluorophore by a nearby tryptophan residue in the channel. This approach was applied to study the gating rearrangements in the bovine rod cyclic nucleotide-gated ion channel CNGA1 where it was found that C481 moves towards A461 during the opening allosteric transition induced by cyclic nucleotide. The approach offers new hope for elucidating the gating rearrangements in channels of known structure. 相似文献
106.
Comparative structural analysis of glycoprotein gD of herpes simplex virus types 1 and 2. 总被引:7,自引:25,他引:7
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
We studied the synthesis and processing of the type-common glycoprotein gD in herpes simplex virus type 2 (HSV-2) and compared it structurally to glycoprotein gD of herpes simplex virus type 1 (HSV-1). We demonstrated that in HSV-2, gD undergoes posttranslational processing from a lower-molecular-weight precursor (pgD51) to a higher-molecular-weight product (gD56). Tryptic peptide analysis by cation-exchange chromatography indicated that this processing step altered neither the methionine nor the arginine tryptic peptide profile of gD of HSV-2. Comparative tryptic peptide analysis of gD of HSV-1 and HSV-2 showed that the methionine and arginine tryptic peptide profiles of these two proteins were very similar, but not identical. Some of the resolved peptides coeluted from the cation-exchange column, suggesting that some amino acid sequences of the two proteins might be very similar. However, each protein also appeared to possess several type-specific tryptic peptides. The structural similarity of these two glycoproteins correlates well with their antigenic cross-reactivity since monoprecipitin antibody to gD of HSV-1 also immunoprecipitates gD of HSV-2 and neutralizes the infectivity of both viruses to approximately the same extent. 相似文献
107.
Localization and synthesis of an antigenic determinant of herpes simplex virus glycoprotein D that stimulates the production of neutralizing antibody. 总被引:8,自引:46,他引:8
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
G H Cohen B Dietzschold M Ponce de Leon D Long E Golub A Varrichio L Pereira R J Eisenberg 《Journal of virology》1984,49(1):102-108
An antigenic determinant capable of inducing type-common herpes simplex virus (HSV)-neutralizing antibodies has been located on glycoprotein D (gD) of HSV type 1 (HSV-1). A peptide of 16 amino acids corresponding to residues 8 to 23 of the mature glycoprotein (residues 33 to 48 of the predicted gD-1 sequence) was synthesized. This peptide reacted with an anti-gD monoclonal antibody (group VII) previously shown to neutralize the infectivity of HSV-1 and HSV-2. The peptide was also recognized by polyclonal antibodies prepared against purified gD-1 but was less reactive with anti-gD-2 sera. Sera from animals immunized with the synthetic peptide reacted with native gD and neutralized both HSV-1 and HSV-2. 相似文献
108.
Melissa E. Carew Leon Metzeling Rosalind St Clair Ary A. Hoffmann 《Molecular ecology resources》2017,17(5):915-930
Invertebrate biodiversity measured at mostly family level is widely used in biological monitoring programmes to assess anthropogenic impacts on ecosystems. However, next‐generation sequencing (NGS) could allow development of new more sensitive biomonitoring tools by allowing rapid species identification. This could be accelerated if archived invertebrate collections and environmental information from past programmes are used to understand species distributions and their environmental responses. In this study, we take archived macroinvertebrate samples from two sites collected on multiple occasions and test whether NGS can successfully detect species. Samples had been stored in 70% ethanol at room temperature for up to 12 years. Three amplicons ranging from 197 to 274 bps within the DNA barcode region were amplified from samples and compared to DNA barcoding libraries to identify species. We were able to amplify partial DNA barcodes from most samples, and species were often detected with multiple amplicons. However, some singletons and taxa poorly covered by DNA barcoding were missed. This suggests additional DNA barcodes will be required to fill ‘gaps’ in current DNA barcode libraries for aquatic macroinvertebrates and/or that it may not be possible to detect all taxa in a sample. Furthermore, older samples often detected fewer taxa and were less reliable for amplification, suggesting NGS is best used on samples within 8 years of collection. Nevertheless, many common taxa with existing DNA barcodes were reliably identified with NGS and were often present at sites across multiple years, showing the potential of NGS for detecting common and abundant species in archived material. 相似文献
109.
The Na(+)-K(+)-ATPase alpha2 gene and trainability of cardiorespiratory endurance: the HERITAGE family study. 总被引:5,自引:0,他引:5
T Rankinen L Pérusse I Borecki Y C Chagnon J Gagnon A S Leon J S Skinner J H Wilmore D C Rao C Bouchard 《Journal of applied physiology》2000,88(1):346-351
The Na(+)-K(+)-ATPase plays an important role in the maintenance of electrolyte balance in the working muscle and thus may contribute to endurance performance. This study aimed to investigate the associations between genetic variants at the Na(+)-K(+)-ATPase alpha2 locus and the response (Delta) of maximal oxygen consumption (VO(2 max)) and maximal power output (W(max)) to 20 wk of endurance training in 472 sedentary Caucasian subjects from 99 families. VO(2 max) and W(max) were measured during two maximal cycle ergometer exercise tests before and again after the training program, and restriction fragment length polymorphisms at the Na(+)-K(+)-ATPase alpha2 (exons 1 and 21-22 with Bgl II) gene were typed. Sibling-pair linkage analysis revealed marginal evidence for linkage between the alpha2 haplotype and DeltaVO(2 max) (P = 0.054) and stronger linkages between the alpha2 exon 21-22 marker (P = 0.005) and alpha2 haplotype (P = 0.003) and DeltaW(max). In the whole cohort, DeltaVO(2 max) in the 3.3-kb homozygotes of the exon 1 marker (n = 5) was 41% lower than in the 8.0/3.3-kb heterozygotes (n = 87) and 48% lower than in the 8.0-kb homozygotes (n = 380; P = 0.018, adjusted for age, gender, baseline VO(2 max), and body weight). Among offspring, 10.5/10.5-kb homozygotes (n = 14) of the exon 21-22 marker showed a 571 +/- 56 (SE) ml O(2)/min increase in VO(2 max), whereas the increases in the 10.5/4.3-kb (n = 93) and 4.3/4.3-kb (n = 187) genotypes were 442 +/- 22 and 410 +/- 15 ml O(2)/min, respectively (P = 0.017). These data suggest that genetic variation at the Na(+)-K(+)-ATPase alpha2 locus influences the trainability of VO(2 max) in sedentary Caucasian subjects. 相似文献
110.
de Leon PM Campos VF Corcini CD Santos EC Rambo G Lucia T Deschamps JC Collares T 《Theriogenology》2012,77(1):21-27
The objective was to evaluate the effect of three cryopreservation methods on the in vitro maturation (IVM) and membrane integrity (MIn) of immature equine oocytes. An open pulled straw (OPS) method, a novel solid surface vitrification (SSV) process, and the addition of a synthetic ice blocker were evaluated. Compared with the control group (N = 269), the OPS (N = 159) and the SSV (N = 202) cryopreservation methods decreased both IVM (50.9 vs. 13.3 and 9.4%, respectively; P < 0.001) and MIn (76.6 vs. 31.1 and 33.7%; P < 0.001) of immature equine oocytes. However, inclusion of 0.1% ice blocker in the OPS vitrification process increased the rates of both IVM (30.5%; P < 0.01) and MIn (45.8%; P < 0.05) of the oocytes (N = 59). Including 0.1% ice blocker in the SSV process improved the IVM rate (20.9%; P < 0.05), whereas MIn remained compromised in this group (N = 67). However, increasing the concentration of the ice blocker (to 1.0%) in the cryopreservation methods did not significantly improve rates of IVM. In conclusion, the addition of a synthetic ice blocker (0.1%) to both cryopreservation processes significantly increased rates of both IVM and MIn of immature equine oocytes cryopreserved by OPS. 相似文献