全文获取类型
收费全文 | 9461篇 |
免费 | 770篇 |
国内免费 | 705篇 |
专业分类
10936篇 |
出版年
2024年 | 30篇 |
2023年 | 133篇 |
2022年 | 292篇 |
2021年 | 503篇 |
2020年 | 345篇 |
2019年 | 391篇 |
2018年 | 437篇 |
2017年 | 273篇 |
2016年 | 402篇 |
2015年 | 595篇 |
2014年 | 670篇 |
2013年 | 658篇 |
2012年 | 864篇 |
2011年 | 793篇 |
2010年 | 445篇 |
2009年 | 391篇 |
2008年 | 471篇 |
2007年 | 375篇 |
2006年 | 355篇 |
2005年 | 301篇 |
2004年 | 238篇 |
2003年 | 184篇 |
2002年 | 177篇 |
2001年 | 181篇 |
2000年 | 155篇 |
1999年 | 169篇 |
1998年 | 78篇 |
1997年 | 83篇 |
1996年 | 73篇 |
1995年 | 89篇 |
1994年 | 88篇 |
1993年 | 61篇 |
1992年 | 88篇 |
1991年 | 77篇 |
1990年 | 68篇 |
1989年 | 45篇 |
1988年 | 55篇 |
1987年 | 36篇 |
1986年 | 42篇 |
1985年 | 53篇 |
1984年 | 26篇 |
1983年 | 17篇 |
1982年 | 21篇 |
1981年 | 11篇 |
1979年 | 11篇 |
1978年 | 14篇 |
1977年 | 10篇 |
1974年 | 11篇 |
1969年 | 5篇 |
1968年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The gamma-aminobutyric acid type C (GABA(C)) receptor is a ligand-gated chloride channel with distinct physiological and pharmacological properties. Although the exact subunit composition of native GABA(C) receptors has yet to be firmly established, there is general agreement that GABA rho subunits participate in their formation. Recent studies on white perch suggest that certain GABA rho subunits can co-assemble with the GABA(A) receptor gamma2 subunit to form a heteromeric receptor with electrophysiological properties that correspond more closely to the native GABA(C) receptor on retinal neurons than any of the homomeric rho receptors. In the present study we examined the interactions among various perch GABA rho and gamma2 subunits. When co-expressed in Xenopus oocytes, the gamma2 subunit co-immunoprecipitated with Flag-tagged perch rho1A, rho1B, and rho2B subunits, but not with the Flag-tagged perch rho2A subunit. Immunocytochemical studies indicated that the membrane surface expression of the gamma2 subunit was detected only when it was co-expressed with perch rho1A, rho1B, or rho2B subunit, but not with the perch rho2A subunit or when expressed alone. In addition, co-immunoprecipitation of perch rho1B and gamma2 subunits was also detected in protein samples of the teleost retina. Taken together, these findings suggest that a heteromeric rho(gamma2) receptor could represent one form of GABA(C) receptor on retinal neurons. 相似文献
992.
The parasite Plasmodium falciparum, responsible for the most deadly form of human malaria, is one of the extremely AT-rich genomes sequenced so far and known
to possess many atypical characteristics. Using multivariate statistical approaches, the present study analyzes the amino
acid usage pattern in 5038 annotated protein-coding sequences in P. falciparum clone 3D7. The amino acid composition of individual proteins, though dominated by the directional mutational pressure, exhibits
wide variation across the proteome. The Asn content, expression level, mean molecular weight, hydropathy, and aromaticity
are found to be the major sources of variation in amino acid usage. At all stages of development, frequencies of residues
encoded by GC-rich codons such as Gly, Ala, Arg, and Pro increase significantly in the products of the highly expressed genes.
Investigation of nucleotide substitution patterns in P. falciparum and other Plasmodium species reveals that the nonsynonymous sites of highly expressed genes are more conserved than those of the lowly expressed
ones, though for synonymous sites, the reverse is true. The highly expressed genes are, therefore, expected to be closer to
their putative ancestral state in amino acid composition, and a plausible reason for their sequences being GC-rich at nonsynonymous
codon positions could be that their ancestral state was less AT-biased. Negative correlation of the expression level of proteins
with respective molecular weights supports the notion that P. falciparum, in spite of its intracellular parasitic lifestyle, follows the principle of cost minimization.
[Reviewing Editor : Dr. Richard Kliman] 相似文献
993.
In vitro assays have demonstrated that peptides derived from the recently-identified proSAAS precursor inhibit prohormone convertase 1 (PC1) suggesting that this novel peptide may function as an endogenous inhibitor of PC1. To further understand the role of proSAAS in vivo, we have investigated the expression of proSAAS mRNA and processing of proSAAS during pre- and early postnatal rodent development. In situ hybridization showed that, by embryonic day 12.5 (e12.5) in the rat, proSAAS mRNA was present in essentially all differentiating neurons in the mantle layer of the myelencephalon, metencephalon, diencephalon, spinal cord and several sympathetic ganglia. During later stages of prenatal development, widespread proSAAS expression continues in post-mitotic neurons of both the CNS and PNS and begins in endocrine cells of the anterior and intermediate pituitary. Although proSAAS expression overlaps with PC1 in several regions, its overall expression pattern is significantly more extensive, suggesting that proSAAS may be multifunctional during development. Processed forms of proSAAS are present by at least mid-gestation with marked accumulation of two C-terminal forms, comprising the PC1 inhibitory fragment of proSAAS. 相似文献
994.
995.
996.
Isolation, purification and structural investigation of a water-soluble polysaccharide from Solanum lyratum Thunb 总被引:1,自引:0,他引:1
Yalin W Yuanjiang P Cuirong S 《International journal of biological macromolecules》2005,36(4):241-245
Polysaccharides, extracted from the herbs of Chinese natural Solanum lyratum Thunb (SLT), which is a traditional Chinese medicine with hot water and 0.04 M sodium hydroxide successively, were fractionated and purified by ion-exchange and gel-filtration chromatography. According to methylation, periodate oxidation, NMR spectroscopy, partial and graded acid and enzymic hydrolysis analysis, the results indicated the D-glucan to be linear and to contain both (1-->3)- and (1-->4)-linkages. The anomeric NMR and IR spectra measurements confirmed that the sugar residues were beta-glycosidically linked. 相似文献
997.
The Ahmed glaucoma valve (AGV) is a popular glaucoma drainage device, allowing maintenance of normal intraocular pressure in patients with reduced trabecular outflow facility. The uniquely attractive feature of the AGV, in contrast to other available drainage devices, is its variable resistance in response to changes in flow rate. As a result of this variable resistance, the AGV maintains a pressure drop between 7 and 12 mm Hg for a wide range of aqueous humor flow rates. In this paper, we demonstrate that the nonlinear behavior of the AGV is a direct result of the flexibility of the valve material. Due to the thin geometry of the system, the leaflets of the AGV were modeled using the von Kármán plate theory coupled to a Reynolds lubrication theory model of the aqueous humor flow through the valve. The resulting two-dimensional coupled steady-state partial differential equation system was solved by the finite element method. The Poisson's ratio of the valve was set to 0.45, and the modulus was regressed to experimental data, giving a best-fit value 4.2 MPa. Simulation results compared favorably with previous experimental studies and our own pressure-drop/flow-rate data. For an in vitro flow of 1.6 microL/min, we calculated a pressure drop of 5.8 mm Hg and measured a pressure drop of 5.2 +/- 0.4 mm Hg. As flow rate was increased, pressure drop rose in a strongly sublinear fashion, with a flow rate of 20 microL/min giving a predicted pressure drop of only 10.9 mm Hg and a measured pressure drop of 10.5 +/- 1.1 mm Hg. The AGV model was then applied to simulate in vivo conditions. For an aqueous humor flow rate of 1.5-3.0 microL/min, the calculated pressure drops were 5.3 and 6.3 mm Hg. 相似文献
998.
The oxidative stress of heroin administered mice via intraperitoneal injection, and the therapeutic effects of exogenous antioxidants on the restrain of the oxidative damage of biomolecules and withdrawal syndrome were studied. After administered with heroin, mice showed decrease of total antioxidant capacity in blood, increase of reactive oxygen species production in white blood cells, and increase of oxidative damages of protein and lipid in brain and liver, but not in heart. On the other hand, exogenous antioxidants could restrain the oxidative stress, even alleviate withdrawal syndrome. 相似文献
999.
V.?P.?KorobovEmail author A.?V.?Titova L.?M.?Lemkina T.?V.?Polyudova N.?V.?Pan’kova 《Microbiology》2005,74(2):136-140
The bactericidal effect of the polycationic peptide warnerin, produced by Staphylococcus warneri IEGM KL-1, was found to depend on the energy state of susceptible Staphylococcus epidermidis cells. The pretreatment of these cells with compounds that diminish the proton-motive force of plasma membranes enhanced cell tolerance to warnerin. The components and pH of the membrane proton potential influenced the antibacterial activity of warnerin in different ways. In particular, the antibacterial activity of warnerin decreased when the electric component of the proton-motive force of target membranes declined.__________Translated from Mikrobiologiya, Vol. 74, No. 2, 2005, pp. 166–171.Original Russian Text Copyright © 2005 by Korobov, Titova, Lemkina, Polyudova, Pankova. 相似文献
1000.
INTRODUCTIONCytokinesis is a very complicated and carefully orches-trated process. During the last step of this process, anintercellular bridge is formed between the two daughtercells. A number of studies suggest that this intercellularbridge is not merel… 相似文献