全文获取类型
收费全文 | 18316篇 |
免费 | 1656篇 |
国内免费 | 850篇 |
专业分类
20822篇 |
出版年
2024年 | 33篇 |
2023年 | 206篇 |
2022年 | 379篇 |
2021年 | 641篇 |
2020年 | 440篇 |
2019年 | 560篇 |
2018年 | 581篇 |
2017年 | 565篇 |
2016年 | 746篇 |
2015年 | 1138篇 |
2014年 | 1230篇 |
2013年 | 1426篇 |
2012年 | 1711篇 |
2011年 | 1606篇 |
2010年 | 1007篇 |
2009年 | 947篇 |
2008年 | 1096篇 |
2007年 | 988篇 |
2006年 | 955篇 |
2005年 | 793篇 |
2004年 | 744篇 |
2003年 | 638篇 |
2002年 | 598篇 |
2001年 | 184篇 |
2000年 | 108篇 |
1999年 | 142篇 |
1998年 | 171篇 |
1997年 | 139篇 |
1996年 | 99篇 |
1995年 | 91篇 |
1994年 | 78篇 |
1993年 | 92篇 |
1992年 | 66篇 |
1991年 | 64篇 |
1990年 | 56篇 |
1989年 | 41篇 |
1988年 | 36篇 |
1987年 | 34篇 |
1986年 | 27篇 |
1985年 | 54篇 |
1984年 | 45篇 |
1983年 | 26篇 |
1982年 | 28篇 |
1981年 | 33篇 |
1980年 | 28篇 |
1979年 | 19篇 |
1978年 | 17篇 |
1977年 | 20篇 |
1976年 | 18篇 |
1973年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Ferreira-Gomes MS González-Lebrero RM de la Fuente MC Strehler EE Rossi RC Rossi JP 《The Journal of biological chemistry》2011,286(37):32018-32025
In this work, we set out to identify and characterize the calcium occluded intermediate(s) of the plasma membrane Ca(2+)-ATPase (PMCA) to study the mechanism of calcium transport. To this end, we developed a procedure for measuring the occlusion of Ca(2+) in microsomes containing PMCA. This involves a system for overexpression of the PMCA and the use of a rapid mixing device combined with a filtration chamber, allowing the isolation of the enzyme and quantification of retained calcium. Measurements of retained calcium as a function of the Ca(2+) concentration in steady state showed a hyperbolic dependence with an apparent dissociation constant of 12 ± 2.2 μM, which agrees with the value found through measurements of PMCA activity in the absence of calmodulin. When enzyme phosphorylation and the retained calcium were studied as a function of time in the presence of La(III) (inducing accumulation of phosphoenzyme in the E(1)P state), we obtained apparent rate constants not significantly different from each other. Quantification of EP and retained calcium in steady state yield a stoichiometry of one mole of occluded calcium per mole of phosphoenzyme. These results demonstrate for the first time that one calcium ion becomes occluded in the E(1)P-phosphorylated intermediate of the PMCA. 相似文献
72.
目的本试验旨在研究不同蛋白水平对实验藏酋猴生长性能和血液生化指标的影响并确定幼年实验藏酋猴的蛋白营养需要量。方法选用15只幼年实验藏酋猴随机分为5个处理组,每个处理3个重复,分别饲喂蛋白水平为11.1%、16.1%、20.5%、24.5%和29.8%的日粮,试验期90d。结果随着日粮中蛋白水平的提高,实验藏酋猴的增重、血清白蛋白、球蛋白、天门冬氨酸氨基转移酶、丙氨酸氨基转移酶和尿素氮含量显著增加(P〈0.05),而血清总蛋白、甘油三酯、γ-谷氨酰胺转移酶和碱性磷酸酶无显著差异。应用折线法确定幼年实验藏酋猴的蛋白营养需要量为23.69%。结论日粮蛋白水平对实验藏酋猴生长性能有显著影响,从生物学和经济学角度考虑,初步认为在本试验条件下,未成年实验藏酋猴日粮蛋白水平以23.69%最合适。 相似文献
73.
74.
75.
76.
Expression of baculovirus anti-apoptotic genes p35 and op-iap in cotton (Gossypium hirsutum L.) enhances tolerance to verticillium wilt 总被引:1,自引:0,他引:1
Background
Programmed cell death plays an important role in mediating plant adaptive responses to the environment such as the invasion of pathogens. Verticillium wilt, caused by the necrotrophic pathogen Verticillium dahliae, is a serious vascular disease responsible for great economic losses to cotton, but the molecular mechanisms of verticillium disease and effective, safe methods of resistance to verticillium wilt remain unexplored.Methodology/Principal Findings
In this study, we introduced baculovirus apoptosis inhibitor genes p35 and op-iap into the genome of cotton via Agrobacterium-mediated transformation and analyzed the response of transgenic plants to verticillium wilt. Results showed that p35 and op-iap constructs were stably integrated into the cotton genome, expressed in the transgenic lines, and inherited through the T3 generation. The transgenic lines had significantly increased tolerance to verticillium wilt throughout the developmental stages. The disease index of T1–T3 generation was lower than 19, significantly (P<0.05) better than the negative control line z99668. After treatment with 250 mg/L VD-toxins for 36 hours, DNA from negative control leaves was fragmented, whereas fragmentation in the transgenic leaf DNA did not occur. The percentage of cell death in transgenic lines increased by 7.11% after 60 mg/L VD-toxin treatment, which was less than that of the negative control lines''s 21.27%. This indicates that p35 and op-iap gene expression partially protects cells from VD-toxin induced programmed cell death (PCD).Conclusion/Significance
Verticillium dahliae can trigger plant cells to die through induction of a PCD mechanism involved in pathogenesis. This paper provides a potential strategy for engineering broad-spectrum necrotrophic disease resistance in plants. 相似文献77.
Bermudo E Busquets R Barceló-Barrachina E Puignou L Santos FJ Galceran MT 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》2004,802(1):61-68
The present paper describes the preparation of a suitable laboratory reference material (LRM) to validate analytical methods for the determination of heterocyclic amines (HAs) in foods. Three different lots of reference material were prepared using a beef extract which was contaminated with a well-known quantity of amines at different levels ranging from 10 to 75 ng/g. These materials were then lyophilised under determined conditions and, after grinding and sieving, homogenised and, finally, bottled and labelled. Homogeneity and stability studies were performed and no statistical differences were observed in the analysis of variances for within- and between-bottle results, thus demonstrating the homogeneity of the material. Stability at different storage temperatures (-18, +4, +25 and +40 degrees C) and times (1, 2, 3 and 6 months) was also tested. Therefore, the material can be considered homogeneous and stable and can be proposed for use in inter-comparison exercises for the determination of HAs. 相似文献
78.
Infantes S Lorente E Cragnolini JJ Ramos M García R Jiménez M Iborra S Del Val M López D 《Immunology and cell biology》2011,89(4):558-565
Short viral antigens bound to human major histocompatibility complex (HLA) class I molecules are presented on infected cells. Vaccine development frequently relies on synthetic peptides to identify optimal HLA class I ligands. However, when natural peptides are analyzed, more complex mixtures are found. By immunoproteomics analysis, we identify in this study a physiologically processed HLA ligand derived from the human respiratory syncytial virus matrix protein that is very different from what was expected from studies with synthetic peptides. This natural HLA-Cw4 class I ligand uses alternative interactions to the anchor motifs previously described for its presenting HLA-Cw4 class I molecule. Finally, this octameric peptide shares its C-terminal core with the H-2D(b) nonamer ligand previously identified in the mouse model. These data have implications for the identification of antiviral cytotoxic T lymphocyte responses and for vaccine development. 相似文献
79.
Abnormal fragmentation during cyanogen bromide polypeptide cleavage rarely occurs, although parallel side reactions are known to typically accompany normal cleavage. We have observed that cyanogen bromide cleavage of highly hydrophobic fusion proteins utilized for production of recombinant peptides results in almost complete abolishment of the expected reaction products when the reaction is carried out in 70% trifluoroacetic acid. On the basis of mass spectrometric analysis of the reaction products, we have identified a number of fragments whose origin can be attributed to incomplete fragmentation of the fusion protein, and to unspecific degradation affecting the carrier protein. Substituting the solvent in the reaction media with 70% formic acid or with a matrix composed of 6M guanidinium hydrochloride in 0.1M HCl, however, was found to alleviate polypeptide cleavage. We have attributed the poor yields of the CNBr cleavage carried out in 70% TFA to the increased hydrophobicity of our particular fusion proteins, and to the poor solubilizing ability of this reaction medium. We propose the utilization of chaotropic agents in the presence of diluted acids as the preferred cyanogen bromide cleavage medium of fusion proteins in order to maximize cleavage efficiency of hydrophobic sequences and to prevent deleterious degradation and structural modifications of the target peptides. 相似文献
80.
Jorge S. Gutiérrez José M. Abad-Gómez Auxiliadora Villegas Juan M. Sánchez-Guzmán José A. Masero 《Oecologia》2013,171(1):61-69
Salt stress can suppress the immune function of fish and other aquatic animals, but such an effect has not yet been examined in air-breathing vertebrates that frequently cope with waters (and prey) of contrasting salinities. We investigated the effects of seawater salinity on the strength and cost of mounting an immune response in the dunlin Calidris alpina, a long-distance migratory shorebird that shifts seasonally from freshwater environments during the breeding season to marine environments during migration and the winter period. Phytohaemagglutinin (PHA)-induced skin swelling, basal metabolic rate (BMR), body mass, fat stores, and plasma ions were measured in dunlins acclimated to either freshwater or seawater (salinity: 0.3 and 35.0 ‰, respectively). Seawater-acclimated dunlins mounted a PHA-induced swelling response that was up to 56 % weaker than those held under freshwater conditions, despite ad libitum access to food. Freshwater-acclimated dunlins significantly increased their relative BMR 48 h after PHA injection, whereas seawater-acclimated dunlins did not. However, this differential immune and metabolic response between freshwater- and seawater-acclimated dunlins was not associated with significant changes in body mass, fat stores or plasma ions. Our results indicate that the strength of the immune response of this small-sized migratory shorebird was negatively influenced by the salinity of marine habitats. Further, these findings suggest that the reduced immune response observed under saline conditions might not be caused by an energy or nutrient limitation, and raise questions about the role of osmoregulatory hormones in the modulation of the immune system. 相似文献