全文获取类型
收费全文 | 223979篇 |
免费 | 14977篇 |
国内免费 | 45篇 |
专业分类
239001篇 |
出版年
2021年 | 1773篇 |
2018年 | 3125篇 |
2017年 | 3023篇 |
2016年 | 3666篇 |
2015年 | 3201篇 |
2014年 | 4437篇 |
2013年 | 6321篇 |
2012年 | 7542篇 |
2011年 | 8060篇 |
2010年 | 5722篇 |
2009年 | 4876篇 |
2008年 | 7198篇 |
2007年 | 7479篇 |
2006年 | 7179篇 |
2005年 | 6705篇 |
2004年 | 6850篇 |
2003年 | 6568篇 |
2002年 | 6526篇 |
2001年 | 9275篇 |
2000年 | 8956篇 |
1999年 | 6988篇 |
1998年 | 2122篇 |
1997年 | 1954篇 |
1996年 | 1820篇 |
1992年 | 5467篇 |
1991年 | 5677篇 |
1990年 | 5650篇 |
1989年 | 5620篇 |
1988年 | 5212篇 |
1987年 | 4862篇 |
1986年 | 4442篇 |
1985年 | 4677篇 |
1984年 | 3696篇 |
1983年 | 3025篇 |
1982年 | 2013篇 |
1981年 | 1722篇 |
1980年 | 1725篇 |
1979年 | 3338篇 |
1978年 | 2576篇 |
1977年 | 2375篇 |
1976年 | 2330篇 |
1975年 | 2826篇 |
1974年 | 3117篇 |
1973年 | 3084篇 |
1972年 | 2746篇 |
1971年 | 2604篇 |
1970年 | 2212篇 |
1969年 | 2158篇 |
1968年 | 2031篇 |
1967年 | 1797篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
971.
V Bocci K von Bremen F Corradeschi E Luzzi L Paulesu 《Journal of biological regulators and homeostatic agents》1991,5(4):121-124
No one has ever doubted that maternal milk, in comparison to formula milk, has a far superior nutritional value. Colostrum has a well acknowledge crucial value for the survival of the animal species that cannot receive immunoglobulins through the placenta. Until recently the presence of cytokines in colostrum was unsuspected but it has been now clarified that normally there are at least four cytokines, namely interleukin 1 and 6, tumor necrosis factor and interferon gamma, that may exert an important immunostimulatory role particularly on the oropharyngeal-associated lymphoid tissue. As a corollary, physiological concentration of cytokines administered per os may exert a useful adjuvant activity in aged or immunodeficient people. 相似文献
972.
S S Bo?ko V P Zherdev A A Dvorianinov I Janku E Buchar 《Biulleten' eksperimental'no? biologii i meditsiny》1990,110(11):501-503
Pharmacokinetics of novel immunostimulating drug kamantane was studied by using gas-liquid chromatography in experiments on rats. It was found that kemantane biotransformed rapidly after oral administration with the forming of active metabolite. Kemantane and its metabolites are distributed rapidly from the blood to organs. The drug is eliminated from the organism of rats as metabolite. 相似文献
973.
George V. Sharonov Eduard V. Bocharov Peter M. Kolosov Maria V. Astapova Alexander S. Arseniev Alexey V. Feofanov 《The Journal of biological chemistry》2014,289(21):14955-14964
The EphA2 receptor tyrosine kinase plays a central role in the regulation of cell adhesion and guidance in many human tissues. The activation of EphA2 occurs after proper dimerization/oligomerization in the plasma membrane, which occurs with the participation of extracellular and cytoplasmic domains. Our study revealed that the isolated transmembrane domain (TMD) of EphA2 embedded into the lipid bicelle dimerized via the heptad repeat motif L535X3G539X2A542X3V546X2L549 rather than through the alternative glycine zipper motif A536X3G540X3G544 (typical for TMD dimerization in many proteins). To evaluate the significance of TMD interactions for full-length EphA2, we substituted key residues in the heptad repeat motif (HR variant: G539I, A542I, G553I) or in the glycine zipper motif (GZ variant: G540I, G544I) and expressed YFP-tagged EphA2 (WT, HR, and GZ variants) in HEK293T cells. Confocal microscopy revealed a similar distribution of all EphA2-YFP variants in cells. The expression of EphA2-YFP variants and their kinase activity (phosphorylation of Tyr588 and/or Tyr594) and ephrin-A3 binding were analyzed with flow cytometry on a single cell basis. Activation of any EphA2 variant is found to occur even without ephrin stimulation when the EphA2 content in cells is sufficiently high. Ephrin-A3 binding is not affected in mutant variants. Mutations in the TMD have a significant effect on EphA2 activity. Both ligand-dependent and ligand-independent activities are enhanced for the HR variant and reduced for the GZ variant compared with the WT. These findings allow us to suggest TMD dimerization switching between the heptad repeat and glycine zipper motifs, corresponding to inactive and active receptor states, respectively, as a mechanism underlying EphA2 signal transduction. 相似文献
974.
G M Ka?nova D Markovska D Staneva V E Kagan 《Biulleten' eksperimental'no? biologii i meditsiny》1990,109(1):35-37
Looking for an appropriate model of accelerated aging in vivo we investigated the content of endogenous products of lipid peroxidation (LP) in the rat brain after single or 4 day-lasting intramuscular injection of complex-bind iron (ferum Hausman, 50 mg/kg body weight) like promoter of LP. We found that the single administration of this iron complex fails to induce endogenous LP; after 4 day-application of iron we observed significant increase in content of primary (lipid peroxides) and final (fluorescent) products of LP. Iron-promoted activation of endogenous LP could be abolished by animal pretreatment with the natural antioxidant alpha-tocopherol. The calcium antagonist nifedipine didn't affect the content of endogenous LP products neither alone nor in combination with alpha-tocopherol. 相似文献
975.
N Schiering V S Stoll J S Blanchard E F Pai 《The Journal of biological chemistry》1989,264(35):21144-21145
NADH peroxidase from Streptococcus faecalis 10C1 has been crystallized from ammonium sulfate solutions using the hanging drop vapor diffusion method. Depending on pH, the crystals grew in the orthorhombic space group I222 or one of its subgroups P222 or P2(1)2(1)2 (or one of its two permutations). In both cases the unit cell axes are a = 76.6 A, b = 132.9 A, and c = 145.7 A. There are two monomers/asymmetric unit in the body-centered crystal form and four in the primitive one. The enzyme is catalytically active in the crystalline state. The crystals diffract to at least 2.5 A resolution; they are stable in the x-ray beam and hence suitable for detailed three-dimensional structure determination. 相似文献
976.
977.
Karthish Manthiram Brandon J. Beberwyck Dmitri V. Talapin A. Paul Alivisatos 《Journal of visualized experiments : JoVE》2013,(82)
We demonstrate a method for the synthesis of multicomponent nanostructures consisting of CdS and CdSe with rod and tetrapod morphologies. A seeded synthesis strategy is used in which spherical seeds of CdSe are prepared first using a hot-injection technique. By controlling the crystal structure of the seed to be either wurtzite or zinc-blende, the subsequent hot-injection growth of CdS off of the seed results in either a rod-shaped or tetrapod-shaped nanocrystal, respectively. The phase and morphology of the synthesized nanocrystals are confirmed using X-ray diffraction and transmission electron microscopy, demonstrating that the nanocrystals are phase-pure and have a consistent morphology. The extinction coefficient and quantum yield of the synthesized nanocrystals are calculated using UV-Vis absorption spectroscopy and photoluminescence spectroscopy. The rods and tetrapods exhibit extinction coefficients and quantum yields that are higher than that of the bare seeds. This synthesis demonstrates the precise arrangement of materials that can be achieved at the nanoscale by using a seeded synthetic approach. 相似文献
978.
V.V. Ivanov 《Journal of Russian & East European Psychology》2013,51(5-6):179-191
Ivanov's article presents background information and a methodological approach to Sergei Eisenstein's two-volume work on the method of art. Eisenstein himself considered film to be an art in which the basic problem of art is clearly visible. He argued that successfully combining regressive and progressive elements in art can solve the problem of form and content. The article demonstrates the personal cooperation that took place between Eisenstein, Marr, Luria, and Vygotsky at the beginning of 1930's. 相似文献
979.
Michelle de C. S. Azevedo Patricia S. Rosa Cleverson T. Soares Luciana R. V. Fachin Ida Maria F. D. Baptista William J. Woods Gustavo P. Garlet Ana Paula F. Trombone Andrea de F. F. Belone 《PloS one》2015,10(12)
Jorge Lobo’s disease (JLD) is a chronic infection that affects the skin and subcutaneous tissues. Its etiologic agent is the fungus Lacazia loboi. Lesions are classified as localized, multifocal, or disseminated, depending on their location. Early diagnosis and the surgical removal of lesions are the best therapeutic options currently available for JLD. The few studies that evaluate the immunological response of JLD patients show a predominance of Th2 response, as well as a high frequency of TGF-β and IL-10 positive cells in the lesions; however, the overall immunological status of the lesions in terms of their T cell phenotype has yet to be determined. Therefore, the objective of this study was to evaluate the pattern of Th1, Th2, Th17 and regulatory T cell (Treg) markers mRNA in JLD patients by means of real-time PCR. Biopsies of JLD lesions (N = 102) were classified according to their clinical and histopathological features and then analyzed using real-time PCR in order to determine the expression levels of TGF-β1, FoxP3, CTLA4, IKZF2, IL-10, T-bet, IFN-γ, GATA3, IL-4, IL-5, IL-13, IL-33, RORC, IL-17A, IL-17F, and IL-22 and to compare these levels to those of healthy control skin (N = 12). The results showed an increased expression of FoxP3, CTLA4, TGF-β1, IL-10, T-bet, IL-17F, and IL-17A in lesions, while GATA3 and IL-4 levels were found to be lower in diseased skin than in the control group. When the clinical forms were compared, TGF-β1 was found to be highly expressed in patients with a single localized lesion while IL-5 and IL-17A levels were higher in patients with multiple/disseminated lesions. These results demonstrate the occurrence of mixed T helper responses and suggest the dominance of regulatory T cell activity, which could inhibit Th-dependent protective responses to intracellular fungi such as L. loboi. Therefore, Tregs may play a key role in JLD pathogenesis. 相似文献
980.
Comparative sequence analysis of Escherichia coli ATP-dependent La protease led to the suggestion that Ser679 is the catalytically active enzyme residue. Site-directed mutagenesis Ser679----Ala, investigation of the cells containing the mutant plasmid, and study of the partially purified mutant protein produced results in favour of this suggestion. 相似文献