全文获取类型
收费全文 | 6893篇 |
免费 | 386篇 |
国内免费 | 20篇 |
专业分类
7299篇 |
出版年
2024年 | 11篇 |
2023年 | 96篇 |
2022年 | 253篇 |
2021年 | 371篇 |
2020年 | 336篇 |
2019年 | 501篇 |
2018年 | 378篇 |
2017年 | 268篇 |
2016年 | 353篇 |
2015年 | 378篇 |
2014年 | 464篇 |
2013年 | 600篇 |
2012年 | 560篇 |
2011年 | 549篇 |
2010年 | 293篇 |
2009年 | 252篇 |
2008年 | 271篇 |
2007年 | 279篇 |
2006年 | 245篇 |
2005年 | 203篇 |
2004年 | 151篇 |
2003年 | 107篇 |
2002年 | 100篇 |
2001年 | 17篇 |
2000年 | 17篇 |
1999年 | 22篇 |
1998年 | 21篇 |
1997年 | 16篇 |
1996年 | 15篇 |
1995年 | 6篇 |
1994年 | 7篇 |
1993年 | 13篇 |
1992年 | 6篇 |
1991年 | 14篇 |
1990年 | 5篇 |
1989年 | 10篇 |
1988年 | 4篇 |
1987年 | 12篇 |
1986年 | 12篇 |
1985年 | 13篇 |
1984年 | 12篇 |
1983年 | 8篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 6篇 |
1976年 | 5篇 |
1974年 | 7篇 |
1973年 | 3篇 |
1963年 | 2篇 |
1962年 | 2篇 |
排序方式: 共有7299条查询结果,搜索用时 0 毫秒
51.
Tree diversity and species identity effects on soil fungi,protists and animals are context dependent
Leho Tedersoo Mohammad Bahram Tomá? Cajthaml Sergei P?lme Indrek Hiiesalu Sten Anslan Helery Harend Franz Buegger Karin Pritsch Julia Koricheva Kessy Abarenkov 《The ISME journal》2016,10(2):346-362
Plant species richness and the presence of certain influential species (sampling effect) drive the stability and functionality of ecosystems as well as primary production and biomass of consumers. However, little is known about these floristic effects on richness and community composition of soil biota in forest habitats owing to methodological constraints. We developed a DNA metabarcoding approach to identify the major eukaryote groups directly from soil with roughly species-level resolution. Using this method, we examined the effects of tree diversity and individual tree species on soil microbial biomass and taxonomic richness of soil biota in two experimental study systems in Finland and Estonia and accounted for edaphic variables and spatial autocorrelation. Our analyses revealed that the effects of tree diversity and individual species on soil biota are largely context dependent. Multiple regression and structural equation modelling suggested that biomass, soil pH, nutrients and tree species directly affect richness of different taxonomic groups. The community composition of most soil organisms was strongly correlated due to similar response to environmental predictors rather than causal relationships. On a local scale, soil resources and tree species have stronger effect on diversity of soil biota than tree species richness per se. 相似文献
52.
Sabra Khorsand Ahmadi Maryam Mahmoodian Moghadam Parisa Mokaberi Mohammad Reza Saberi 《Journal of biomolecular structure & dynamics》2013,31(9):1880-1898
The interaction between bovin β-Lactoglobulin (β-LG) and retinol at two different pH values was investigated by multispectroscopic, zeta potential, molecular modeling, and conductometry measurements. The steady state and polarization fluorescence spectroscopy revealed that complex formation at two different pH values could occur through a remarkable static quenching. According to fluorescence quenching, one set of binding site at pH 2 and two sets of binding sites at pH 7 were introduced for binding of retinol to β-LG that show the enhancement of saturation score of β-LG to retinol in dimmer condition. The polarization fluorescence analysis represented that there is more affinity between β-LG and retinol at pH 7 rather than at pH 2. The effect of retinol on β-LG was studied by UV-visible, circular dichroism (CD), and synchronous fluorescence, which indicated that retinol induced more structural changes on β-LG at pH 7. β-LG–retinol complex formation at two different pH values was recorded via applying resonance light scattering (RLS) and zeta potential. Conductometry and RLS showed two different behaviors of interaction between β-LG and retinol at two different pH values; therefore, dimmer formation played important roles in different behaviors of interaction between β-LG and retinol. The zeta potential was the implied combination of electrostatic and hydrophobic forces which are involved in β-LG–retinol complex at two different pH values, and the hydrophobic interactions play a dominant role in complex formation. Molecular modeling was approved by all experimental results. The acquired results suggested that monomer and dimmer states of β-LG can be induced by retinol with different behaviors. 相似文献
53.
Hakimelahi GH Shia KS Pasdar M Hakimelahi S Khalafi-Nezhad A Soltani MN Mei NW Mei HC Saboury AA Rezaei-Tavirani M Moosavi-Movahedi AA 《Bioorganic & medicinal chemistry》2002,10(9):2927-2932
A novel cephalosporin derivative of monohydroguaiaretic acid (cephem-M(3)N, 7) was synthesized and found to possess anticancer activity against human leukemia (K562), breast carcinoma (MCF7), human lung cancer (A549), human colon cancer (Colo205) and pancreatic cancer cells (Capan2 and MiaPaCa2). A tumor targeting fusion protein (dsFv3-beta-lactamase) was also used in conjunction with cephem-based M(3)N 7 and its potency toward K562, MCF7, A549, Colo205, Capan2, and MiaPaCa2 was found to approach that of the free M(3)N (4). In the presence of dsFv3-beta-lactamase, tumor cells were found to be much more susceptible to conjugate 7 than normal human embryonic lung (HEL) cells and normal fibroblasts (Hef522). These notions provide a new approach to the use of nordihydroguaiaretic acid (NDGA) and its derivatives for antitumor therapy. 相似文献
54.
Hadi Mozafari Zohreh Rahimi Azadeh Heidarpour Mahsa Fallahi Adraiana Muniz 《Molecular biology reports》2009,36(8):2361-2364
It has been suggested that the allele frequency of thrombophilic mutations is affected by glucose-6-phosphate dehydrogenase
(G6PD) deficiency. The prevalence of thrombophilic mutations were studied in sixty G6PD deficient individuals including 57
males and three females with the mean age of 15 ± 3.08 and 110 age and sex matched healthy individuals consisted of 95 males and 15 females with
the mean age of 16.19 ± 2.17 from the Kermanshah Province of Iran. Using a combination of PCR-RFLP technique, single strand
conformation polymorphism (SSCP) analysis and DNA sequencing polymorphic G6PD mutations were identified. The factor V Leiden,
prothrombin G20210A and methylenetetrahydrofolate reductase (MTHFR) C677T were detected by PCR-RFLP method using MnlI, HindIII
and HinfI restriction enzymes, respectively. Three mutations, G6PD Mediterranean, G6PD Chatham and G6PD Cosenza were identified
in 60 G6PD deficient individuals with highest prevalence of G6PD Mediterranean (91.6%). In G6PD deficient individuals the
prevalence of factor V Leiden tended to be higher (5%) compared to healthy individuals (2.7%). The prevalence of prothrombin
G20210A mutation in G6PD deficient individuals was 1.7%. However, in normal subjects the prevalence of this mutation was 2.7%.
The frequency of T allele in G6PD deficient individuals were insignificantly higher (29.16%) than those in healthy individuals
(26.8%). Our finding indicates that the prevalence of factor V Leiden, prothrombin G20210A and MTHFR C677T in G6PD deficient
individuals is not statistically different compared to normal subjects and G6PD deficiency is not associated with these thrombophilic
mutations in Western Iran. 相似文献
55.
Hameed Gul Mengya Qian Mohammad G. Arabzai Tianhui Huang Qiannan Ma Fangyu Xing Wan Cao Tingting Liu Hong Duan Qianlin Xiao Zhizhai Liu 《Phyton》2022,91(7):1429-1443
Kernel size-related traits, including kernel length, kernel width, and kernel thickness, are critical components in determining yield and kernel quality in maize (Zea mays L.). Dissecting the phenotypic characteristics of these traits, and discovering the candidate chromosomal regions for these traits, are of potential importance for maize yield and quality improvement. In this study, a total of 139 F2:3 family lines derived from EHel and B73, a distinct line with extremely low ear height (EHel), was used for phenotyping and QTL mapping of three kernel size-related traits, including 10-kernel length (KL), 10-kernel width (KWid), and 10-kernel thickness (KT). The results showed that only one QTL for KWid, i.e., qKWid9 on Chr9, with a phenotypic variation explained (PVE) of 13.4% was detected between SNPs of AX-86298371 and AX-86298372, while no QTLs were detected for KL and KT across all 10 chromosomes. Four bulked groups of family lines, i.e., Groups I to IV, were constructed with F2:3 family lines according to the phenotypic comparisons of KWid between EHel and B73. Among these four groups, Group I possessed a significantly lower KWid than EHel (P = 0.0455), Group II was similar to EHel (P = 0.34), while both Group III and Group IV were statistically higher than EHel (P < 0.05). Besides, except Group IV exhibited a similar KWid to B73 (P = 0.11), KWid of Groups I to III were statistically lower than B73 (P < 0.00). By comparing the bulked genotypes of the four groups to EHel and B73, a stable chromosomal region on Chr9 between SNPs of AX-86298372 to AX-86263154, entirely covered by qKWid9, was identified to link KWid with the positive allele of increasing phenotypic effect to KWid from B73, similar to that of qKWid9. A large amount of enzyme activity and macromolecule binding-related genes were annotated within this chromosomal region, suggesting qKWid9 as a potential QTL for KWid in maize. 相似文献
56.
Effects of extremely low frequency magnetic field on the development of tolerance to the analgesic effect of morphine in rats 下载免费PDF全文
57.
Didymocarpus pedicellata R. Br. (Gesneriaceae) is widely used in traditional Indian medicines against renal afflictions. In the present study, we have revealed ethanolic extract of aerial parts of D. pedicellata to possess significant antioxidant activity and protect against ferric nitrilotriacetate (Fe-NTA) mediated renal oxidative stress, nephrotoxicity and tumor promotion response. D. pedicellata extract was found to possess a high content of total polyphenolics, exhibit potent reducing power and significantly scavenge free radicals including several reactive oxygen species (ROS) and reactive nitrogen species (RNS). The extract also significantly and dose-dependently protected against Fe-NTA plus H(2)O(2)-mediated damage to lipids and DNA. Protective efficacy of the extract was also tested in vivo against Fe-NTA mediated nephrotoxicity and tumor promotion response. Administration of Fe-NTA (9 mg/kg body weight, i.p.) to Swiss albino mice depleted renal glutathione content and activities of antioxidant and phase II metabolizing enzymes with concomitant induction of oxidative damage. Fe-NTA also incited hyperproliferation response elevating ornithine decarboxylase activity and [(3)H]-thymidine incorporation into DNA. Elevation in serum creatinine (SCr) and blood urea nitrogen (BUN), and histopathological changes were also evident and suggested Fe-NTA to afflict damage to kidney. Pretreatment of mice with D. pedicellata extract (100-200 mg/kg body weight) for 7 days not only restored antioxidant armory near normal values but also significantly protected against renal oxidative stress and damage restoring normal renal architecture and levels of renal damage markers, viz., BUN and SCr. The results of the present study indicate D. pedicellata to possess potent antioxidant and free radical scavenging activities and preclude oxidative damage and hyperproliferation in renal tissues. 相似文献
58.
A number of new imine derivatives of 5-amino-1, 3, 4-thiadiazole-2-thiol have been synthesized, and their anti-depressant activity was tested using imipramine as reference drug. Two compounds namely 5-{[1-(4-chlorophenyl)-3-(4-methoxy-phenyl)prop-2-en-1-ylidene]-amino}-5-benzylthio-1, 3,4 -thiadiazole 4i(b) and 5-{[1-(4-chlorophenyl)-3-(4-dimethyl-aminophenyl)-prop-2-en-1-ylidene]amino}-5-benzylthio-1,3,4-thiadiazole 4i(c) have shown significant anti-depressant activity, which decreased immobility time by 77.99% and 76.26% compared to the standard imipramine (82%). All the compounds in the series have passed neurotoxicity tests. 相似文献
59.
Mojtaba Amiri Leila Farzin Mohammad Esmail Moassesi Fattaneh Sajadi 《Biological trace element research》2010,135(1-3):38-44
Febrile convulsion is the most common disorder in childhood with good prognosis. There are different hypotheses about neurotransmitters and trace element changes in biological fluids which can have a role in pathogenesis of febrile convulsion. In this study, serum selenium, zinc, and copper were measured by atomic absorption spectrometry in the children with febrile convulsion (n?=?30) and in the control group (n?=?30). The age and sex of the subjects were registered. Selenium and zinc were found to be significantly lower in febrile convulsion cases than in the control group (p?<?0.0001 and p?<?0.0001, respectively). There was no significant difference in the value of copper between the two groups (p?=?0.16). While selenium and zinc levels were 44.92?±?10.93 μg/l and 66.13?±?18.97 μg/dl in febrile convulsion, they were found to be 62.98?±?9.80 μg/l and 107.87?±?28.79 μg/dl in healthy children. Meanwhile, copper levels were 146.40?±?23.51 μg/dl in the patients and 137.63?±?24.19 μg/dl in the control group, respectively. This study shows that selenium and zinc play an important role in the pathogenesis of febrile convulsion. 相似文献