全文获取类型
收费全文 | 51758篇 |
免费 | 3920篇 |
国内免费 | 3395篇 |
专业分类
59073篇 |
出版年
2024年 | 157篇 |
2023年 | 1033篇 |
2022年 | 1480篇 |
2021年 | 1963篇 |
2020年 | 1855篇 |
2019年 | 2435篇 |
2018年 | 2047篇 |
2017年 | 1582篇 |
2016年 | 1627篇 |
2015年 | 1862篇 |
2014年 | 3364篇 |
2013年 | 4204篇 |
2012年 | 2496篇 |
2011年 | 2986篇 |
2010年 | 2176篇 |
2009年 | 2455篇 |
2008年 | 2476篇 |
2007年 | 2457篇 |
2006年 | 2200篇 |
2005年 | 2007篇 |
2004年 | 1765篇 |
2003年 | 1554篇 |
2002年 | 1297篇 |
2001年 | 1101篇 |
2000年 | 891篇 |
1999年 | 869篇 |
1998年 | 853篇 |
1997年 | 745篇 |
1996年 | 646篇 |
1995年 | 691篇 |
1994年 | 702篇 |
1993年 | 588篇 |
1992年 | 573篇 |
1991年 | 391篇 |
1990年 | 379篇 |
1989年 | 270篇 |
1988年 | 253篇 |
1987年 | 210篇 |
1986年 | 198篇 |
1985年 | 322篇 |
1984年 | 317篇 |
1983年 | 236篇 |
1982年 | 280篇 |
1981年 | 208篇 |
1980年 | 197篇 |
1979年 | 161篇 |
1978年 | 139篇 |
1977年 | 94篇 |
1976年 | 78篇 |
1974年 | 72篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
The three-dimensional structure of the photosynthetic reaction center fromRhodobacter sphaeroides is described. The reaction center is a transmembrane protein that converts light into chemical energy. The protein has three subunits: L, M, and H. The mostly helical L and M subunits provide the scaffolding and the finely tuned environment in which the chromophores carry out electron transfer. The details of the protein-chromophore interactions are from studies of a trigonal crystal form that diffracted to 2.65-Å resolution. Functional studies of the multi-subunit complex by site-specific replacement of key amino acid residues are summarized in the context of the molecular structure.This work was supported in part by the U.S. Department of Energy, Office of Health and Environmental Research, under Contract No. W-31-109-ENG-38 and by Public Health Service Grant GM36598. 相似文献
82.
Abstract: Alzheimer's disease (AD) is identified by the accumulation of amyloid plaques, neurofibrillary degeneration, and the accompanying neuronal loss. AD amyloid assembles into compact fibrous deposits from the amyloid β(Aβ) protein, which is a proteo-lytic fragment of the membrane-associated amyloid precursor protein. To examine the effects of amyloid on neuron growth, a hybrid mouse motoneuron cell line (NSC34) exhibiting spontaneous process formation was exposed to artificial "plaques" created from aggregated synthetic Aβ peptides. These correspond to full-length Aβ residues 1–40 (Aβ1–40), an internal β-sheet region comprising residues 11–28 (Aβ11–28), and a proposed toxic fragment comprising residues 25–35 (Aβ25–35). Fibers were immobilized onto culture dishes, and addition of cells to these in vitro plaques revealed that Aβ was not a permissive substrate for cell adhesion. Neurites in close contact with these deposits displayed abnormal swelling and a tendency to avoid contact with the Aβ fibers. In contrast, Aβ did not affect the adhesion or growth of rat astrocytes, implicating a specific Aβ-neuron relationship. The inhibitory effects were also unique to Aβ as no response was observed to deposits of pancreatic islet amyloid poly-peptide fibers. Considering the importance of cell adhesion in neurite elongation and axonal guidance, the antiadhesive properties of Aβ amyloid plaques found in vivo may contribute to the neuronal loss responsible for the clinical manifestations of AD. 相似文献
83.
84.
85.
Zhao Ying Hiromasa Tojo Takanori Komatsubara Manabu Nakagawa Masami Inada Sumio Kawata Yuji Matsuzawa Mitsuhiro Okamoto 《生物化学与生物物理学报:疾病的分子基础》1994,1226(2):201-205
Enzyme activity, protein contents, and mRNA contents of group II phospholipase A2 (PLA2) in hepatocellular carcinoma (HCC) surgically obtained from 8 patients were compared with those in either its neighboring liver tissues or control liver tissues. The PLA2 specific activity towards the mixed micelles of 1-palmitoyl-2-oleoyl-phosphatidylglycerol and cholate was significantly greater in the tumor tissues (6.62 ± 1.46 nmol/min/mg) than those in the surrounding liver tissues (1.33 ± 0.22 nmol/min/mg) and controls (0.43 ± 0.04 nmol/min/mg). The results of immunoblot analysis using a specific anti-human group II PLA2 antibody and of Northern blot analysis using a human group II PLA2 cDNA as a probe demonstrated that group II PLA2 was responsible for the increased enzyme activity. The contents of immunoreactive group II PLA2 in the tumor tissues (8.81 ± 1.24 ng/mg) were significantly higher than those in the surrounding liver tissues (1.77 ± 0.27 ng/mg); those in the control tissues were below the analytical range of the method used. The group II PLA2 mRNA was also significantly increased in the tumor tissues, compared with that in the surrounding liver tissues, whereas it was not detectable in th controls. This indicates that group II PLA2 in HCC is induced at the pretranslational level. 相似文献
86.
87.
We use the electron-conformational interaction approach to develop a physical model which self-consistently describes the photomobilized electron transfer kinetics and structure conformational transitions in reaction centers (RCs) of purple bacteria. We consider the kinetics of electron transition from pigment onto primary acceptor and the subsequent charge recombination accounting for the change of distance between the above-mentioned cofactors. It is shown that, given natural values of RC parameters, the kinetic constant's dependence on the acting light intensity is monotone. As opposed to the previous case, similar dependencies for the chain of electron transfer between primary and secondary quinone acceptors revealed anS-like relationship. This can lead to bistability of the RC optical transmission coefficient and a fundamental dependence of charge recombination kinetics upon the prehistory of the RC's interaction with exciting radiation. 相似文献
88.
Sieghart Sopper Susanne Hemm Jürgen Meixensberger Cheik Coulibaly Christiane Stahl-Hennig Gerhard Hunsmann Bernhard Fleckenstein Volker ter Meulen Rüdiger Drries 《Journal of medical primatology》1993,22(2-3):138-146
Paired sera and CSF samples were collected from SIVmac-infected macaques. Animals infected with SIVmac251 maintained low gag and high env-specific antibody levels in plasma. Increasing env-specific antibody titers in CSF were associated in one animal with strong intrathecal synthesis. SIVmac239-infected monkeys revealed high antibody titers of gag and env-specificity, in one animal accompanied by weak intrathecal synthesis of virus-specific antibodies. In all animals, the CD4/CD8 ratio in CSF decreased faster compared to blood. 相似文献
89.
T. C. Pan T. H. Lin C. L. Tseng M. H. Yang C. W. Huang 《Biological trace element research》1993,39(2-3):117-128
Blackfoot disease is a peripheral vascular disease resulting in gangrene of the lower extremities. Though extensive epidemiological
study has implicated that high arsenic content in artesian well water of the endemic area, bears some important connection
with the disease, the etiology of the disease is still unknown. In this study, attention is paid to multielement determination
in order to find out whether the trace elements in hair of Blackfoot disease patients are different from those of the controls.
Experimental results indicate that the concentrations of As and Se in hair of patients are significantly higher than those
of the controls, but Ca and Zn are significantly lower than those of the controls. The possible connection of these elements
with the etiology of the disease is discussed. 相似文献
90.