全文获取类型
收费全文 | 2659篇 |
免费 | 162篇 |
出版年
2023年 | 21篇 |
2022年 | 50篇 |
2021年 | 93篇 |
2020年 | 62篇 |
2019年 | 67篇 |
2018年 | 99篇 |
2017年 | 71篇 |
2016年 | 121篇 |
2015年 | 147篇 |
2014年 | 159篇 |
2013年 | 193篇 |
2012年 | 209篇 |
2011年 | 200篇 |
2010年 | 156篇 |
2009年 | 113篇 |
2008年 | 136篇 |
2007年 | 136篇 |
2006年 | 121篇 |
2005年 | 99篇 |
2004年 | 90篇 |
2003年 | 59篇 |
2002年 | 74篇 |
2001年 | 46篇 |
2000年 | 28篇 |
1999年 | 47篇 |
1998年 | 22篇 |
1997年 | 14篇 |
1996年 | 9篇 |
1995年 | 13篇 |
1994年 | 11篇 |
1993年 | 7篇 |
1992年 | 14篇 |
1991年 | 15篇 |
1990年 | 8篇 |
1989年 | 10篇 |
1988年 | 7篇 |
1987年 | 6篇 |
1986年 | 6篇 |
1985年 | 5篇 |
1984年 | 6篇 |
1983年 | 7篇 |
1982年 | 7篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1974年 | 3篇 |
1971年 | 5篇 |
1970年 | 3篇 |
1969年 | 4篇 |
排序方式: 共有2821条查询结果,搜索用时 31 毫秒
991.
We have examined the effect of the uncharged species of lidocaine (LDC) and etidocaine (EDC) on the acyl chain moiety of egg phosphatidylcholine liposomes. Changes in membrane organization caused by both anesthetics were detected through the use of EPR spin labels (5, 7 and 12 doxyl stearic acid methyl ester) or fluorescence probes (4, 6, 10, 16 pyrene-fatty acids). The disturbance caused by the LA was greater when the probes were inserted in more external positions of the acyl chain and decreased towards the hydrophobic core of the membrane. The results indicate a preferential insertion of LDC at the polar interface of the bilayer and in the first half of the acyl chain, for EDC. Additionally, (2)H NMR spectra of multilamellar liposomes composed by acyl chain-perdeutero DMPC and EPC (1:4 mol%) allowed the determination of the segmental order (S(mol)) and dynamics (T(1)) of the acyl chain region. In accordance to the fluorescence and EPR results, changes in molecular orientation and dynamics are more prominent if the LA preferential location is more superficial, as for LDC while EDC seems to organize the acyl chain region between carbons 2-8, which is indicative of its positioning. We propose that the preferential location of LDC and EDC inside the bilayers creates a "transient site", which is related to the anesthetic potency since it could modulate the access of these molecules to their binding site(s) in the voltage-gated sodium channel. 相似文献
992.
Avilez IM Hori TS de Almeida LC Hackbarth A da Cunha Bastos Neto J da Cunha Bastos VL Moraes G 《Comparative biochemistry and physiology. Toxicology & pharmacology : CBP》2008,148(2):136-142
Parameters of the antioxidant defense systems of Brycon amazonicus (matrinx?--a neotropical fish) exposed to phenol for 96 h plus the recovery over 1 and 2 weeks were studied in erythrocytes and liver. Hematocrit increase was observed during phenol exposure and recovery for 1 week. Total superoxide dismutases (SOD), glutathione peroxidase (GPx) and reduced glutathione (GSH) did not change during phenol exposure. Erythrocyte glucose-6-phosphate dehydrogenase (G6PDH) increased during that period while catalase (CAT) activity decreased during phenol exposure and recovery for 2 weeks. In the liver, SOD and CAT did not change, whereas GPx increased in the first week of recovery and decreased after 2 weeks. A late response was observed for G6PDH activity which increased only at the second week. Ascorbate concentration in the brain decreased during phenol exposure and increased over recovery. From our results it appears that the oxidative stress was limited in matrinx? exposed to phenol, but seemed to occur during the recovery period. 相似文献
993.
Carlos A. Fernandes Catarina Ginja Iris Pereira Rogério Tenreiro Michael W. Bruford Margarida Santos-Reis 《Conservation Genetics》2008,9(3):681-690
Genetic species identification of non-invasively collected samples has become an important tool in ecological research, management
and conservation and wildlife forensics. This is especially true for carnivores, due to their elusive nature, and is crucial
when several ecologically and phylogenetically close species, with similar faeces, hairs, bones and/or pelts, occur in sympatry.
This is the case of the Iberian Peninsula, a region with a carnivore community of 16 species—about two-thirds of the European
carnivore fauna. Here we present a simple, efficient and reliable PCR-based protocol, using a novel set of species-specific
primers, for the unambiguous identification to species of non-invasively collected samples or forensic materials from Iberian
carnivores. For each species, from the consensus of all cytochrome b haplotypes, found here and previously reported, we designed species-specific primer pairs for short fragments, the most likely
to persist in low-quantity and degraded DNA samples. The predicted specificity of each primer pair was assessed through PCR
of positive DNA extracts from the carnivore species, from an exhaustive array of potential prey and from humans. The robustness
of PCR amplification for non-invasively sampled DNA was tested with scat samples. The primers did not produce false positives
and correctly identified all carnivore samples to the species level. In comparison with sequencing and PCR-RFLP assays, our
method is, respectively, cost- and time-effective, and is especially suited for monitoring surveys targeting multiple populations/species.
It also introduces an approach that works for a whole community of carnivores living sympatrically over a large geographic
area. 相似文献
994.
Michelle Andrade Furtado Lenita Caetano Fernandes de Almeida Ricardo Andrade Furtado Wilson Roberto Cunha Denise Crispim Tavares 《Mutation Research - Genetic Toxicology and Environmental Mutagenesis》2008,657(2):150-154
Rosmarinic acid (RA) is a natural phenolic compound which presents different biological activities such as antitumor, antibacterial, anti-inflammatory, hepatoprotective and cardioprotective properties. In view of its important biological activities, the study of the effects of RA on genetic material becomes relevant. Thus, the objective of the present study was to evaluate the mutagenic and/or antimutagenic potential of RA on peripheral blood cells of Swiss mice using the micronucleus assay. Three doses of RA (50, 100 and 200 mg/kg body weight, b.w.) were used for the evaluation of its mutagenic potential. In the antimutagenicity assays, the different concentrations of RA were combined with the chemotherapeutic agent doxorubicin (DXR, 15 mg/kg b.w.). Peripheral blood samples were collected 24, 48 and 72 h after treatment for the evaluation of micronucleated polychromatic erythrocytes (MNPCEs). The results of the mutagenicity assays showed no increase in the frequency of micronuclei in animals treated with different concentrations of RA when compared to the negative controls. Treatment with different concentrations of RA combined with DXR revealed a significant reduction in the frequency of micronuclei compared to animals treated with DXR only. Although the mechanisms underlying the protective effect of RA are not completely understood, the putative antioxidant activity of RA might explain its effect on DXR mutagenicity. 相似文献
995.
Buckup L Dutra BK Ribarcki FP Fernandes FA Noro CK Oliveira GT Vinagre AS 《Comparative biochemistry and physiology. Part A, Molecular & integrative physiology》2008,149(1):59-67
The crayfish Parastacus defossus occurs in Uruguay and the state of Rio Grande do Sul in Brazil. It lives in swamps and lakes, where it digs sloped subterranean tunnels that are used as burrows. Because there is little information about the biology, physiology and ecology of this species, the aim of this study was to identify the seasonal variations of its carbohydrate and lipid metabolism. Crayfish were collected monthly (from November 2002 to February 2004) in the Lami region, Porto Alegre municipality (30 degrees 11'41'S-51 degrees 06'00'W). Haemolymph samples, used for determination of glucose, total proteins, triglycerides, total cholesterol and total lipids, were collected in the field using potassium oxalate as an anti-clotting agent. The animals and haemolymph samples were immediately frozen in the field. In the laboratory, the hepatopancreas, gills and abdominal muscles were removed for determination of glycogen, triglycerides, total cholesterol and total lipids. The findings suggest that in P. defossus, lipids are an important reserve of energy used during reproduction in both males and females; whereas glycogen may be used during periods of intense activity or environmental stress. 相似文献
996.
The objectives of this study were to assess the effects of induced testicular degeneration in Bos taurus indicus (Nellore) bulls on changes in seminal characteristics and fertilizing ability of sperm. Four Nellore bulls (30–36-month-old, 500–550 kg) with good seminal quality (>80% motile and morphologically normal sperm) had scrotal insulation applied for 5 d. Semen was collected by electroejaculation and cryopreserved at the pre-insulation moment, and 7, 14, and 21 d after insulation was removed. Gross motility, vigor of sperm movement (1–5), acrosome integrity, sperm morphology (phase-contrast microscopy), nuclear vacuoles and abnormal chromatin (Feulgen-stain) were determined after sperm preparations for in vitro fertilization (IVF). Prior to IVF, sperm were separated using a Percoll gradient (45% and 90%). Normal sperm decreased (P < 0.05) 14 and 21 d after insulation was removed. On 14 and 21 d, the incidence of head defects (9.7 ± 0.6 and 17.0 ± 0.8, respectively; mean ± S.E.M.) was higher (P < 0.05) in agreement with the incidence of nuclear vacuoles (14.0 ± 5.0 and 12.3 ± 2.3) and abnormal chromatin (24.4 ± 7.2 and 30.8 ± 2.8). Although the frequency of cleaved oocytes decreased only on 21 d (P < 0.05), blastocyst rates were lower (P < 0.05) than pre-insulation on 14 and 21 d. In regression analyses, only nuclear vacuoles, head defects and intact acrosome accounted for differences in cleavage (R2 = 0.38, 0.48, and 0.30, respectively) and blastocyst rates (R2 = 0.35, 0.37, and 0.44). Abnormal chromatin was associated only with blastocyst rates (R2 = 0.35). In conclusion, blastocyst rate was more sensitive than cleavage rate and the assessment of nuclear integrity is recommended to predict the fertilizing ability of bull sperm. 相似文献
997.
Bacterial RNA chaperones confer abiotic stress tolerance in plants and improved grain yield in maize under water-limited conditions 总被引:6,自引:0,他引:6
998.
A total of 98 Manihot species have been recognized in the genus. All of them are native to the tropics of the New World, particularly Brazil and Mexico. The cultigen, Manihot esculenta Crantz (cassava), grows throughout the lowland tropics. Wild species vary in growth habit from acaulescent or short shrubs to tree-like. Because of their adaptations to different conditions, they are gene reservoirs for tackling many abiotic and biotic stresses such as improving root quality and resistance to diseases. They have been used successfully by the first author for improving protein content, seed-fertility, apomixis, resistance to mealy bug, and tolerance to drought. A table of the most important species from an economic viewpoint is presented. 相似文献
999.
Ferreira JM Drumond BP Guedes MI Pascoal-Xavier MA Almeida-Leite CM Arantes RM Mota BE Abrahão JS Alves PA Oliveira FM Ferreira PC Bonjardim CA Lobato ZI Kroon EG 《PloS one》2008,3(8):e3043
Brazilian Vaccinia virus had been isolated from sentinel mice, rodents and recently from humans, cows and calves during outbreaks on dairy farms in several rural areas in Brazil, leading to high economic and social impact. Some phylogenetic studies have demonstrated the existence of two different populations of Brazilian Vaccinia virus strains circulating in nature, but little is known about their biological characteristics. Therefore, our goal was to study the virulence pattern of seven Brazilian Vaccinia virus strains. Infected BALB/c mice were monitored for morbidity, mortality and viral replication in organs as trachea, lungs, heart, kidneys, liver, brain and spleen. Based on the virulence potential, the Brazilian Vaccinia virus strains were grouped into two groups. One group contained GP1V, VBH, SAV and BAV which caused disease and death in infected mice and the second one included ARAV, GP2V and PSTV which did not cause any clinical signals or death in infected BALB/c mice. The subdivision of Brazilian Vaccinia virus strains into two groups is in agreement with previous genetic studies. Those data reinforce the existence of different populations circulating in Brazil regarding the genetic and virulence characteristics. 相似文献
1000.
A. Cruz P. Fernandes J. M. S. Cabral H. M. Pinheiro 《Journal of Molecular Catalysis .B, Enzymatic》2001,11(4-6):579-585
The selective cleavage of the β-sitosterol side-chain by free Mycobacterium sp. NRRL B-3805 cells was used as a model system for the study of solvent effects in a whole-cell bioconversion in two phase aqueous–organic media. This multi-step degradation pathway leads to the production of 4-androstene-4,17-dione (AD) and 1,4-androstadiene-3,17-dione (ADD) as a minor product. In an attempt to correlate the substrate and cell partition effects and solvent hydrophobicity (log P) with biocatalytic activity, 15 carboxylic acid esters with log P values between 3 and 10 were screened. The results indicated that the toxicity of the tested solvents in this system could not be correlated to their log P, but seemed to depend on their ability to accumulate in the cells, as these showed a strong affinity towards the organic phase. Different solvent/aqueous ratios and hydrodynamic conditions were further tested in the solvent systems (phthalates) showing significant biodegradation activity. The bioconversion rate was generally not much affected by the stirring speed in the employed range (150–300 rpm) but was strongly influenced by the aqueous/organic phase ratio. Results suggest that the bioconversion takes place at the interphase, its rate being possibly limited by mass transport inside the organic phase. 相似文献