全文获取类型
收费全文 | 14569篇 |
免费 | 1052篇 |
国内免费 | 2篇 |
专业分类
15623篇 |
出版年
2024年 | 15篇 |
2023年 | 137篇 |
2022年 | 260篇 |
2021年 | 493篇 |
2020年 | 385篇 |
2019年 | 428篇 |
2018年 | 573篇 |
2017年 | 514篇 |
2016年 | 701篇 |
2015年 | 915篇 |
2014年 | 918篇 |
2013年 | 1040篇 |
2012年 | 1192篇 |
2011年 | 1118篇 |
2010年 | 683篇 |
2009年 | 616篇 |
2008年 | 749篇 |
2007年 | 715篇 |
2006年 | 669篇 |
2005年 | 453篇 |
2004年 | 438篇 |
2003年 | 403篇 |
2002年 | 348篇 |
2001年 | 292篇 |
2000年 | 256篇 |
1999年 | 203篇 |
1998年 | 98篇 |
1997年 | 78篇 |
1996年 | 71篇 |
1995年 | 51篇 |
1994年 | 54篇 |
1993年 | 51篇 |
1992年 | 86篇 |
1991年 | 57篇 |
1990年 | 45篇 |
1989年 | 62篇 |
1988年 | 48篇 |
1987年 | 40篇 |
1986年 | 31篇 |
1985年 | 34篇 |
1984年 | 38篇 |
1983年 | 31篇 |
1982年 | 24篇 |
1981年 | 16篇 |
1980年 | 14篇 |
1979年 | 21篇 |
1978年 | 21篇 |
1975年 | 14篇 |
1974年 | 22篇 |
1971年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Antônio A. Lins Mesquita Wanderley Gonçlves De Oliveira Rosa M. Taveira Neiva Otto R. Gottlieb 《Phytochemistry》1975,14(3):803-806
The wood of Tovomita pyrifolium (Guttiferae) contains the novel tovopyrifolins A [1,6-dihydroxy-7-methoxy-5-prenyl-6′,6′-dimethylpyrano (2′,3′:3,2)xanthone], B (1,5-dihydroxy-3,4-dimethoxyxanthone) and C (1,3,5-trihydroxy-2-methoxyxanthone) and also the known tovophyllins A and B [structure revised to 1,6-dihydroxy-5-prenyl-6′, 6′-dimethylpyrano(2′,3′:3,2)-6″,6″-dimethylpyrano(2″,3″:7,8)xanthone]. 相似文献
102.
P G Santos F Grande 《Proceedings of the Society for Experimental Biology and Medicine. Society for Experimental Biology and Medicine (New York, N.Y.)》1975,149(3):652-655
The lipolytic effect of glucagon was measured in vitro with adipose tissue of "young" (4-8 wk) and "old" (over 1 yr) geese. The response of the young geese tissue was about twice that observed with tissue of old geese, for glucagon concentrations of 0.05, 0.5, and 5.0 mug/ml. Our estimates indicate that the number of adipose cells per g of adipose tissue of young geese was three times that of the old geese tissue. This suggests that the greater lipolytic response to glucagon, observed in young geese adipose tissue, may possibly be due to its greater cellularity, rather than to a greater lipolytic response of the individual adipocyte. The lipolytic effect of glucagon in vivo, for each of the doses between 1.0 and 20.0 mug/kg, was significantly greater in the old than in the young geese. The slope of the linear equation relating log10 of glucagon dose and elevation of plasma FFA 5 min after injection, was significantly greater for the old than for the young geese. In the goose, therefore, the influence of age on the adipokinetic effect of glucagon appears to be mediated by factors operating in the whole animal, more than by changes in the adipose cell itself. A slower removal rate of circulating FFA by the old geese, could be one of these factors. 相似文献
103.
104.
Marina Rovani Drummond Luciene Silva dos Santos Renata Soalheiro Fvaro Raquel Silveira Bello Stucchi Ilka de Ftima Santana Ferreira Boin Paulo Eduardo Neves Ferreira Velho 《PLoS neglected tropical diseases》2022,16(7)
BackgroundThis study aimed to assess the prevalence of Bartonella sp.-DNA detection in blood and skin samples from patients with non-viral end-stage liver disease awaiting liver transplantation.Methodology/Principal findingsBlood samples and healthy skin fragments from 50 patients were tested using microbiological and molecular methods. Fifteen patients had cryptogenic hepatitis (CH) and 35 had alcoholic, drug-induced or autoimmune liver disease. DNA was extracted from whole blood and liquid culture samples, isolates, and skin fragments. Thirteen of the 50 patients (26%) had Bartonella henselae DNA detection in their blood (9/50) and/or skin (5/50) samples. Colonies were isolated in 3/50 (6%) and infection was detected in 7/50 (14%) of the 50 patients. B. henselae-DNA detection was more prevalent in patients with CH than in other patients (p = 0.040). Of 39 patients followed-up for at least two years, a higher mortality rate was observed among patients with CH infected with B. henselae (p = 0.039).Conclusions/SignificanceFurther studies assessing the role of B. henselae infection in the pathogenesis of hepatitis patients must be urgently conducted. 相似文献
105.
106.
Oliveira MD Andrade CA Santos-Magalhães NS Coelho LC Teixeira JA Carneiro-da-Cunha MG Correia MT 《Letters in applied microbiology》2008,46(3):371-376
Aims: The aim of this work was to analyse the antimicrobial properties of a purified lectin from Eugenia uniflora L. seeds. Methods and Results: The E. uniflora lectin (EuniSL) was isolated from the seed extract and purified by ion‐exchange chromatography in DEAE‐Sephadex with a purification factor of 11·68. The purified lectin showed a single band on denaturing electrophoresis, with a molecular mass of 67 kDa. EuniSL agglutinated rabbit and human erythrocytes with a higher specificity for rabbit erythrocytes. The haemagglutination was not inhibited by the tested carbohydrates but glycoproteins exerted a strong inhibitory action. The lectin proved to be thermo resistant with the highest stability at pH 6·5 and divalent ions did not affect its activity. EuniSL demonstrated a remarkable nonselective antibacterial activity. EuniSL strongly inhibited the growth of Staphylococcus aureus, Pseudomonas aeruginosa and Klebsiella sp. with a minimum inhibitory concentration (MIC) of 1·5 μg ml?1, and moderately inhibited the growth of Bacillus subtilis, Streptococcus sp. and Escherichia coli with a MIC of 16·5 μg ml?1. Conclusions: EuniSL was found to be effective against bacteria. Significance and Impact of the Study: The strong antibacterial activity of the studied lectin indicates a high potential for clinical microbiology and therapeutic applications. 相似文献
107.
Tadeu Siqueira Luis Mauricio Bini Fabio Oliveira Roque Sheyla Regina Marques Couceiro Susana Trivinho‐Strixino Karl Cottenie 《Ecography》2012,35(2):183-192
Ecologists have long investigated why communities are composed of a few common species and many rare species. Most studies relate rarity to either niche differentiation among species or spatial processes. There is a parallel between these processes and the processes proposed to explain the structure of metacommunities. Based on a metacommunity perspective and on data on stream macroinvertebrates from different regions of Brazil, we answer two questions. 1) Are sets of common and rare species affected by similar niche and spatial processes? 2) How does the community composition of common and of rare species differ? The main hypothesis we test is that common species are mainly affected by environmental factors, whereas rare species are mostly influenced by dispersal limitation. We used variation partitioning to determine the proportion of variation explained by the environment and space in common and rare species matrices. Contrary to our expectations, evidence supported the idea that both common and rare species are affected mainly by environmental factors, even after controlling for the differing information content between common and rare species matrices. Moreover, the abundance of some common species is also a good predictor of variation in rare species matrices. Niche differences are unlikely to be the sole cause of patterns of rarity in these metacommunities. We suggest that sets of common and rare species react to similar major environmental gradients and that rare species also respond to processes that operate at a more fine‐grained spatial scale, particularly biotic interactions. We extend the view that species sorting is the dominant process structuring metacommunities and argue that future studies focusing on rarity would benefit from a metacommunity perspective. 相似文献
108.
Isabel C.M. Fensterseifer Mário R. Felício Eliane S.F. Alves Marlon H. Cardoso Marcelo D.T. Torres Carolina O. Matos Osmar N. Silva Timothy K. Lu Maurício V. Freire Natan C. Neves Sónia Gonçalves Luciano M. Lião Nuno C. Santos William F. Porto Cesar de la Fuente-Nunez Octavio L. Franco 《生物化学与生物物理学报:生物膜》2019,1861(7):1375-1387
Infections caused by Gram-negative bacteria, Escherichia coli and Pseudomonas aeruginosa foremost among them, constitute a major worldwide health problem. Bioinformatics methodologies are being used to rationally design new antimicrobial peptides, a potential alternative for treating these infections. One of the algorithms used to develop antimicrobial peptides is the Joker, which was used to design the peptide PaDBS1R6. This study evaluates the antibacterial activities of PaDBS1R6 in vitro and in vivo, characterizes the peptide interaction to target membranes, and investigates the PaDBS1R6 structure in contact with mimetic vesicles. Moreover, we demonstrate that PaDBS1R6 exhibits selective antimicrobial activity against Gram-negative bacteria. In the presence of negatively charged and zwitterionic lipids the structural arrangement of PaDBS1R6 transits from random coil to α-helix, as characterized by circular dichroism. The tertiary structure of PaDBS1R6 was determined by NMR in zwitterionic dodecylphosphocholine (DPC) micelles. In conclusion, PaDBS1R6 is a candidate for the treatment of nosocomial infections caused by Gram-negative bacteria, as template for producing other antimicrobial agents. 相似文献
109.
Hélène Cheap Sophie Bernad Valérie Derrien László Gerencsér Julia Tandori Pedro de Oliveira Deborah K. Hanson Péter Maróti Pierre Sebban 《BBA》2009,1787(12):1505-1515
Bacterial reaction centers use light energy to couple the uptake of protons to the successive semi-reduction of two quinones, namely QA and QB. These molecules are situated symmetrically in regard to a non-heme iron atom. Four histidines and one glutamic acid, M234Glu, constitute the five ligands of this atom. By flash-induced absorption spectroscopy and delayed fluorescence we have studied in the M234EH and M234EL variants the role played by this acidic residue on the energetic balance between the two quinones as well as in proton uptake. Delayed fluorescence from the P+QA? state (P is the primary electron donor) and temperature dependence of the rate of P+QA? charge recombination that are in good agreement show that in the two RC variants, both QA? and QB? are destabilized by about the same free energy amount: respectively ~ 100 ± 5 meV and 90 ± 5 meV for the M234EH and M234EL variants, as compared to the WT. Importantly, in the M234EH and M234EL variants we observe a collapse of the high pH band (present in the wild-type reaction center) of the proton uptake amplitudes associated with formation of QA? and QB?. This band has recently been shown to be a signature of a collective behaviour of an extended, multi-entry, proton uptake network. M234Glu seems to play a central role in the proton sponge-like system formed by the RC protein. 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