全文获取类型
收费全文 | 18339篇 |
免费 | 1555篇 |
国内免费 | 4篇 |
专业分类
19898篇 |
出版年
2023年 | 65篇 |
2022年 | 151篇 |
2021年 | 317篇 |
2020年 | 174篇 |
2019年 | 230篇 |
2018年 | 319篇 |
2017年 | 280篇 |
2016年 | 402篇 |
2015年 | 790篇 |
2014年 | 844篇 |
2013年 | 1063篇 |
2012年 | 1482篇 |
2011年 | 1452篇 |
2010年 | 876篇 |
2009年 | 865篇 |
2008年 | 1195篇 |
2007年 | 1211篇 |
2006年 | 1102篇 |
2005年 | 1118篇 |
2004年 | 985篇 |
2003年 | 925篇 |
2002年 | 900篇 |
2001年 | 189篇 |
2000年 | 138篇 |
1999年 | 187篇 |
1998年 | 271篇 |
1997年 | 159篇 |
1996年 | 144篇 |
1995年 | 142篇 |
1994年 | 124篇 |
1993年 | 115篇 |
1992年 | 96篇 |
1991年 | 82篇 |
1990年 | 91篇 |
1989年 | 87篇 |
1988年 | 77篇 |
1987年 | 55篇 |
1986年 | 74篇 |
1985年 | 76篇 |
1984年 | 97篇 |
1983年 | 75篇 |
1982年 | 86篇 |
1981年 | 104篇 |
1980年 | 88篇 |
1979年 | 56篇 |
1978年 | 66篇 |
1977年 | 53篇 |
1976年 | 38篇 |
1974年 | 56篇 |
1973年 | 35篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Ecdysteroid levels were determined during the period of the adult reproductive cycle in the ovovivparous fleshfly Sarcophaga bullata. Low levels were found in males during the 10 days following eclosion while the entire adult female showed a significant peak of ecdysteroid activity at 190 h post eclosion (i.e. during embryogenesis). When the female reproductive tract was analyzed it was observed that the ovaries became vitellogenic a short time after a protein meal was offered at 96 h post eclosion: at 140 h, ecdysteroid activity was recorded in the developing oöcytes. The major peak of ecdysteroids during the reproductive cycle was found in developing embryos at 190 h. The significance of these releases of ecdysteroids is discussed in relation to major embryonic developmental events. 相似文献
82.
83.
84.
J. Brian Parent Tet-Kin Yeol Kiang-Teck Yeol Kenneth Olden 《Molecular and cellular biochemistry》1986,72(1-2):21-33
To investigate our earlier hypothesis that carbohydrates play a regulatory role in the intracellular transport of secretory glycoproteins, we used 1-deoxynojirimycin (DNJ), and inhibitor of glucosidase I and II of the rough endoplasmic reticulum (RER), to modify the structure of N-linked glycan moieties of secretory glycoproteins of human hepatoma (Hep G2) cells in culture. Using a pulse-chase protocol, we found that treatment of Hep G2 cultures with 1.25 mM DNJ markedly reduced the rate of secretion of 1-protease inhibitor, ceruloplasmin, and 2-macroglobulin, but had no effect on the export of fibronectin, -fetoprotein and transferrin, nor on albumin which lacks carbohydrate. For example, 50% of newly synthesized 1-protease inhibitor, the glycoprotein most dramatically affected, was secreted by 27 min in control cultures versus 110 min in DNJ-treated cultures. Percoll gradient cell fractionation analyses revealed that DNJ inhibited transport of the affected secretory glycoproteins in the RER segment of the ER/Golgi pathway. For example, 50% of newly synthesized 1-protease inhibitor was lost from the RER fraction by 10 min in untreated cells, but 70 min was required for the transport of a similar amount of protein in DNJ-treated cells. DNJ treatment also inhibited the rate at which the N-linked glycan moieties of the affected glycoproteins became resistant to endo H in the Golgi. Since the glycan moiety of secreted forms of the affected glycoproteins were fully processed to the complex structure, suggesting escape from DNJ inhibition, we concluded that removal of terminal glucose residues from the glycan chain of secretory glycoproteins is required for their transport from the RER to the Golgi. We suggest that the oligosaccharide moieties on 1-protease inhibitor, ceruloplasmin and 2-macroglobulin form part of the binding site for a receptor which regulates transport of these glycoproteins. 相似文献
85.
A functional differential equation which is nonlinear and involves forward and backward deviating arguments is solved numerically. The equation models conduction in a myelinated nerve axon in which the myelin completely insulates the membrane, so that the potential change jumps from node to node. The equation is of first order with boundary values given at t=±. The problem is approximated via a difference scheme which solves the problem on a finite interval by utilizing an asymptotic representation at the endpoints, cubic interpolation and iterative techniques to approximate the delays, and a continuation method to start the procedure. The procedure is tested on a class of problems which are solvable analytically to access the scheme's accuracy and stability, then applied to the problem that models propagation in a myelinated axon. The solution's dependence on various model parameters of physical interest is studied. This is the first numerical study of myelinated nerve conduction in which the advance and delay terms are treated explicitly.Supported in part by NSF Grant MCS8301724 and by a Biomedical Research Support Grant 2SO7RR0706618 from NIH 相似文献
86.
Site-related differences in the localization of the monoclonal antibody OX7 in SL2 and SL1 lymphomas
Howard Sands Peter L. Jones William P. Neacy Sudhir A. Shah Brian M. Gallagher 《Cancer immunology, immunotherapy : CII》1986,22(3):169-175
Summary The uptake of a monoclonal antibody (OX7) by murine lymphomas (SL1, SL2) growing in two sites in the mouse were compared. SL2 tumors grown in the subrenal site showed greater specific antibody uptake than did the same tumor grown in the subcutaneous site. Major differences in membrane bound antibody, in vitro antibody binding patterns, and gamma scintillation camera imaging were also observed between the two sites. These differences may be due to the greater blood flow measured in tumors growing in the subrenal capsule than those growing at the subcutaneous site. The differences observed in antibody uptake of the same tumor growing in two different sites raises questions concerning the choice of animal model systems that can be used to predict clinical utility. 相似文献
87.
Synopsis We tested the hypothesis that anadromous salmonids are guided on their homeward migration by population-specific pheromones.
Our findings do not support the hypothesis. Wild migrant Arctic charr,Salvelinus alpinus, from Ikarut River, Labrador were transferred and held in a tributary previously uninhabited by anadromous fish. None of
the charr migrating up Ikarut River entered the tributary after fish were transferred. Similarly, migrant charr, which were
caught in Ikarut River and released in the tributary below the captive fish, did not remain in the tributary. We re-evaluated
the data which have been used to uphold the concept of pheromone attraction in salmonid migration and concluded that support
for the hypothesis is unsubstantiated. 相似文献
88.
89.
J. Allan Downie Gerd Hombrecher Qing-Sheng Ma Celia D. Knight Brian Wells Andrew W. B. Johnston 《Molecular & general genetics : MGG》1983,190(3):359-365
Summary Three nodulation-deficient (nod) mutants of Rhizobium leguminosarum were isolated following insertion of the transposon Tn5 into pRL1JI, the R. leguminosarum plasmid known to carry the nodulation genes. DNA adjacent to the nod: Tn5 alleles was subcloned and used to probe a cosmid clone bank containing DNA from a Rhizobium strain carrying pRL1JI. Two cosmid clones which showed homology with the probe contained about 10 kb of DNA in common. The R. leguminosarum host-range determinants were found to be present within this 10 kb common region since either of the cosmid clones could enable a cured R. phaseoli strain to nodulate peas instead of Phaseolus beans, its normal host. Electron microscopy of nodules induced by Rhizobium strains cured of their normal symbiotic plasmid but containing either of the two cosmid clones showed bacteroid-forms surrounded by a peri-bacteroid membrane, indicating that normal infection had occurred. Thus it is clear that this 10 kb region of nodDNA carries the genes that determine host range and that relatively few bacterial genes may be involved in nodule and bacteroid development. 相似文献
90.
Mesophyll cells isolated from Phaseolus vulgaris and Lycopersicon esculentum show decreasing photosynthetic rates when suspended in media containing increasing concentrations of osmoticum. The photosynthetic activity was sensitive to small changes in osmotic potential over a range of sorbitol concentrations from 0.44 M (−1.08 MPa) to 0.77 M (−1.88 MPa). Photorespiration assayed by 14 CO2 release in CO2 -free air and by 14 CO2 release from the oxidation of [1–14 C] glycolate also decreased as the osmotic potential of the incubation medium was reduced. The CO2 compensation points of the cells increased with increasing concentration of osmoticum from approximately 60 μ I−1 1 at −1.08 MPa to 130 μl 1−1 for cells stressed at −1.88 MPa. Changes in photosynthetic and photorespiratory activities occurred at moderate osmotic potentials in these cells suggesting that in whole leaves during a reduction in water potential, non- stomatal inhibition of CO2 assimilation and glycolate pathway metabolism occurs simultaneously with stomatal closure. 相似文献