全文获取类型
收费全文 | 2596篇 |
免费 | 174篇 |
国内免费 | 1篇 |
专业分类
2771篇 |
出版年
2023年 | 26篇 |
2022年 | 30篇 |
2021年 | 67篇 |
2020年 | 53篇 |
2019年 | 61篇 |
2018年 | 70篇 |
2017年 | 62篇 |
2016年 | 79篇 |
2015年 | 118篇 |
2014年 | 148篇 |
2013年 | 193篇 |
2012年 | 224篇 |
2011年 | 207篇 |
2010年 | 126篇 |
2009年 | 116篇 |
2008年 | 154篇 |
2007年 | 160篇 |
2006年 | 118篇 |
2005年 | 103篇 |
2004年 | 130篇 |
2003年 | 108篇 |
2002年 | 110篇 |
2001年 | 36篇 |
2000年 | 13篇 |
1999年 | 19篇 |
1998年 | 18篇 |
1997年 | 23篇 |
1996年 | 19篇 |
1995年 | 15篇 |
1994年 | 11篇 |
1993年 | 17篇 |
1992年 | 16篇 |
1991年 | 10篇 |
1990年 | 7篇 |
1989年 | 4篇 |
1988年 | 8篇 |
1987年 | 7篇 |
1986年 | 6篇 |
1985年 | 14篇 |
1984年 | 9篇 |
1983年 | 10篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1976年 | 5篇 |
1973年 | 3篇 |
1968年 | 4篇 |
1965年 | 2篇 |
排序方式: 共有2771条查询结果,搜索用时 15 毫秒
51.
Antioxidant activity and neuroprotective effects of zolpidem and several synthesis intermediates 总被引:2,自引:0,他引:2
García-Santos G Herrera F Martín V Rodriguez-Blanco J Antolín I Fernández-Marí F Rodriguez C 《Free radical research》2004,38(12):1289-1299
Structural relationship between the antioxidant melatonin and the non-benzodiazepine hypnotic zolpidem (ZPD) suggests possible direct antioxidant and neuroprotective properties of this compound. In the present work, these effects were analyzed for zolpidem and four of its synthesis intermediates. In vitro assays include lipid peroxidation and protein oxidation studies in liver and brain homogenates. Intracellular antioxidant effects were analyzed by evaluation of free radical formation prevention in HT-22 hippocampal cells treated with glutamate 10mM and measured by flow cytometer DCF fluorescence. The neuroprotective effect of these compounds was evaluated as neuronal death prevention of HT-22 cells treated with the same concentration of glutamate. Zolpidem was found to prevent induced lipid peroxidation in rat liver and brain homogenates showing figures similar to melatonin, although it failed to prevent protein oxidation. ZPD-I was the most effective out of the several zolpidem intermediates studied as it prevented lipid peroxidation with an efficiency higher than melatonin or zolpidem and with an effectiveness similar to estradiol and trolox. ZPD-I prevents protein oxidation, which trolox is known to be unable to prevent. When cellular experiments were undertaken, ZPD-I prevented totally the increase of intracellular free radicals induced by glutamate 10mM in culture medium for 12h, while zolpidem and ZPD-III partially prevented this increase. Also the three compounds protected hippocampal neurons from glutamate-induced death in the same conditions, being their comparative efficacy, ZPD-III > ZPD-I = ZPD. 相似文献
52.
Dosio GA Rey H Lecoeur J Izquierdo NG Aguirrezábal LA Tardieu F Turc O 《Journal of experimental botany》2003,54(392):2541-2552
Common features in the time-course of expansion of leaves which considerably differed in final area, due to phytomer position, growing conditions and genotype, were identified. Leaf development consisted of two phases of exponential growth, followed by a third phase of continuous decrease of the relative expansion rate. The rate and the duration of the first exponential phase were common to all phytomers, growing conditions and genotypes. Leaves differed in the rate and the duration of the second exponential phase. The decrease of the relative expansion rate during the third phase depended on neither genotype nor growing conditions. It was phytomer-dependent and was deduced from the rate of the second phase via a parameter common to all cases studied. Differences in final leaf area among growing conditions were linked to different expansion rates during the second exponential phase. The duration of the phases at any given phytomer position was the same for the two hybrids in different growing conditions. The dates of developmental events (initiation, end of the two exponential phases, full expansion), and the rate of the second exponential phase, were related to phytomer position, defining a strict pattern of leaf development at the whole plant level. Using this framework simplified the analysis of the response of leaf expansion to genotype and environment. 相似文献
53.
54.
The karyotypes of 10 species and one variety of South American Lathyrus were determined and compared with those obtained of five entities from the Northern Hemisphere. Although all the species have a chromosome number of 2n = 14, they could be differentiated by their karyotype formula and quantitative parameters of the karyotypes. Phenetic distance and principal component analysis showed that in spite of the differences observed among entities, they can be grouped in clusters that coincide with the taxonomic sections established by F. K. Kupicha and with the life cycle of the species. South American species form a homogeneous group and can be distinguished by the presence of a subtelocentric pair, which has a macrosatellite in the long arm, and the lack of a short metacentric pair characteristic of most species of the Northern Hemisphere. From an evolutionary point of view, variation in total chromosome length without major changes in the karyotype formula suggests that changes in the amounts of genomic DNA are proportional to the relative length of each chromosome arm and that species of Notolathyrus evolved in a concerted fashion. Variation in genome size, however, is congruent with morphological variation of some reproductive organs as well as with the life cycle and minimum generation time, as predicted by the nucleotype hypothesis. 相似文献
55.
García-Barrios L Mayer-Foulkes D Franco M Urquijo-Vásquez G Franco-Pérez J 《Bulletin of mathematical biology》2001,63(3):507-526
Spatial disposition of plants in intercrops, and differences in sowing time between species, can strongly affect their ecological
interactions and, in consequence, the system’s viability and performance. Empirical exploration of a wide range of spatial
and temporal plant arrangements is costly and time-consuming. Modelling the growth of mixed crops is a tool which, combined
with empirical tests, can greatly reduce the time and investment required for this task. Spatially explicit, individual-based
dynamic models seem well suited for this purpose; their exploration and experimental validation for the case of simple, two-species,
artificial plant communities, can also provide further insight as to how the spatial and temporal scales of a plant’s multispecific
neighbourhood affect its growth and performance. The aim of this investigation was to further develop a published spatially
explicit individual-based mixed crop growth model [Vandermeer, J. H. (1989). The Ecology of Intercropping, Cambridge, U.K.: Cambridge University Press, p. 237], and to validate it experimentally. With this purpose in mind: (1)
computer programs to simulate individual plant growth and to perform statistical analysis of both deterministic and stochastic
versions of the model were developed; (2) the model was parametrized using a complex experimental diculture with several cohorts
and spatial arrangements; (3) the predictive capacity of the model was tested using independent spatio-temporal experimental
arrangements; (4) a modified version of the model was written, which abandons the assumption of linearity of the neighbourhood
index at the cost of increasing the number of parameters; (5) The performance of stochastic versions of both Vandermeer’s
and our modified model were compared, employing a non-parametric measure of goodness of fit. We conclude that this approach
to modelling plant growth subject to intra and interspecific competition is a remarkably efficient, general, conceptually
elegant, heuristic tool whose predictive power can be further improved when nonlinear terms are introduced into the neighbourhood
competition index, as done in our modified version of Vandermeer’s model. 相似文献
56.
Cabrera PV Blanco G Ernst G Alvarez E Cooper EL Hajos S 《Journal of invertebrate pathology》2002,79(2):111-119
Cell migration is a key event in the invertebrate immuno-defense system. Microbial products like lipopolysacharide (LPS) and formyl-methyl-leucyl-phenylalanine (fMLP) promote cell recruitment to sites of infection. In mammals, complement activation by factors such as zymosan induces C5a production, which influences leukocyte migration. The endogenous factor hyaluronic acid (HA), an extracellular matrix component, also promotes cell migration through its receptor CD44. We evaluated whether coelomocytes from the sipunculan worm T. petricola migrated towards LPS, fMLP, or zymosan treated plasma (ZTP) and if HA was involved in coelomocyte migration and adhesion. We also evaluated if antibodies specific for mouse HA receptor CD44 inhibited any of the effects induced by HA. Using microchemotaxis chambers we found that coelomocytes migrated towards exogenously and endogenously derived chemoattractants. We also observed that HA was a potent chemotactic signal and that coelomocytes adhered strongly to plates coated with LMW-HA but not with HMW-HA. In addition we found that these HA mediated effects were blocked by the monoclonal antibody IM7 directed to mouse CD44, suggesting that a CD44-like cross-reactive antigen might play a role in HA mediated coelomocyte locomotion. 相似文献
57.
58.
An essential role for Prox1 in the induction of the lymphatic endothelial cell phenotype 总被引:55,自引:0,他引:55
Wigle JT Harvey N Detmar M Lagutina I Grosveld G Gunn MD Jackson DG Oliver G 《The EMBO journal》2002,21(7):1505-1513
The process of angiogenesis has been well documented, but little is known about the biology of lymphatic endothelial cells and the molecular mechanisms controlling lymphangiogenesis. The homeobox gene Prox1 is expressed in a subpopulation of endothelial cells that, after budding from veins, gives rise to the mammalian lymphatic system. In Prox1(-)(/-) embryos, this budding becomes arrested at around embryonic day (E)11.5, resulting in embryos without lymphatic vasculature. Unlike the endothelial cells that bud off in E11.5 wild-type embryos, those of Prox1-null embryos did not co-express any lymphatic markers such as VEGFR-3, LYVE-1 or SLC. Instead, the mutant cells appeared to have a blood vascular phenotype, as determined by their expression of laminin and CD34. These results suggest that Prox1 activity is required for both maintenance of the budding of the venous endothelial cells and differentiation toward the lymphatic phenotype. On the basis of our findings, we propose that a blood vascular phenotype is the default fate of budding embryonic venous endothelial cells; upon expression of Prox1, these budding cells adopt a lymphatic vasculature phenotype. 相似文献
59.
García-Fernández MO Bodega G Solano RM Ruíz-Villaespesa A Sánchez-Chapado M Carmena MJ Prieto JC 《Regulatory peptides》2002,103(1):9-15
The therapeutic potential of the intestinotrophic mediator glucagon-like peptide-2 (1-33) [GLP-2 (1-33)] has increased interest in the pharmacokinetics of the peptide. This study was undertaken to investigate whether the primary degradation product GLP-2 (3-33) interacts with the GLP-2 receptor. Functional (cAMP) and binding in vitro studies were carried out in cells expressing the transfected human GLP-2 receptor. Furthermore, a biologic response of GLP-2 (3-33) was tested in vivo. Mice were allocated to groups treated for 10 days (twice daily) with: (1) 5 microg GLP-2 (1-33), (2) 25 microg GLP-2 (3-33), (3) 5 microg GLP-2 (1-33)+100 microg GLP-2 (3-33), or (4) 5 microg GLP-2 (1-33)+500 microg GLP-2 (3-33). The intestine was investigated for growth changes. GLP-2 (3-33) bound to the GLP-2 receptor with a binding affinity of 7.5% of that of GLP-2 (1-33). cAMP accumulation was stimulated with an efficacy of 15% and a potency more than two orders of magnitude lower than that of GLP-2 (1-33). Increasing doses of GLP-2 (3-33) (10(-7)-10(-5) M) caused a shift to the right in the dose-response curve of GLP-2 (1-33). Treatment of mice with either GLP-2 (1-33) or (3-33) induced significant growth responses in both the small and large intestines, but the response induced by GLP-2 (3-33) was much smaller. Co-administration of 500 microg of GLP-2 (3-33) and 5 microg GLP-2 (1-33) resulted in a growth response that was smaller than that of 5 microg GLP-2 (1-33) alone. Consistent with the observed in vivo activities, our functional studies and binding data indicate that GLP-2 (3-33) acts as a partial agonist with potential competitive antagonistic properties on the GLP-2 receptor. 相似文献
60.
Hernández-Alcántara G Garza-Ramos G Hernández GM Gómez-Puyou A Pérez-Montfort R 《Biochemistry》2002,41(13):4230-4238
In homodimeric triosephosphate isomerase from Trypanosoma brucei (TbTIM), cysteine 14 of each the two subunits forms part of the dimer interface. This residue is central for the catalysis and stability of TbTIM. Cys14 was changed to the other 19 amino acids to determine the characteristics that the residue must have to yield catalytically competent stable enzymes. C14A, C14S, C14P, C14T, and C14V TbTIMs were essentially wild type in activity and stability. Mutants with Asn, Arg, and Gly had low activities and stabilities. The other mutants had less than 1% of the activity of TbTIM. One of the latter enzymes (C14F) was purified to homogeneity. Size exclusion chromatography and equilibrium sedimentation studies showed that C14F TbTIM is a monomer, with a k(cat) approximately 1000 times lower and a K(m) approximately 6 times higher than those of TbTIM. In C14F TbTIM, the ratio of the elimination (methylglyoxal and phosphate formation) to isomerization reactions was higher than in TbTIM. Its secondary structure was very similar to that of TbTIM; however, the quantum yield of its aromatic residues was lower. The analysis of the data with the 19 mutants showed that to yield enzymes similar to the wild type, the residue must have low polarity and a van der Waals volume between 65 and 110 A(3). The results with C14F TbTIM illustrate that the secondary structure of TbTIM can be formed in the absence of intersubunit contacts, and that it has sufficient tertiary structure to support catalysis. 相似文献