全文获取类型
收费全文 | 4543篇 |
免费 | 327篇 |
国内免费 | 1篇 |
出版年
2023年 | 23篇 |
2022年 | 49篇 |
2021年 | 88篇 |
2020年 | 58篇 |
2019年 | 64篇 |
2018年 | 94篇 |
2017年 | 88篇 |
2016年 | 155篇 |
2015年 | 201篇 |
2014年 | 222篇 |
2013年 | 311篇 |
2012年 | 369篇 |
2011年 | 353篇 |
2010年 | 225篇 |
2009年 | 193篇 |
2008年 | 287篇 |
2007年 | 301篇 |
2006年 | 262篇 |
2005年 | 248篇 |
2004年 | 211篇 |
2003年 | 190篇 |
2002年 | 196篇 |
2001年 | 42篇 |
2000年 | 21篇 |
1999年 | 37篇 |
1998年 | 61篇 |
1997年 | 25篇 |
1996年 | 37篇 |
1995年 | 45篇 |
1994年 | 23篇 |
1993年 | 39篇 |
1992年 | 26篇 |
1991年 | 22篇 |
1990年 | 15篇 |
1989年 | 8篇 |
1988年 | 12篇 |
1987年 | 21篇 |
1986年 | 15篇 |
1985年 | 16篇 |
1984年 | 15篇 |
1983年 | 15篇 |
1982年 | 14篇 |
1981年 | 17篇 |
1980年 | 17篇 |
1977年 | 10篇 |
1976年 | 12篇 |
1975年 | 16篇 |
1974年 | 10篇 |
1973年 | 10篇 |
1972年 | 11篇 |
排序方式: 共有4871条查询结果,搜索用时 15 毫秒
991.
Frederick L. Crane Iris L. Sun Rita Barr Hans Löw 《Journal of bioenergetics and biomembranes》1991,23(5):773-803
Transplasma membrane electron transport in both plant and animal cells activates proton release. The nature and components of the electron transport system and the mechanism by which proton release is activated remains to be discovered. Reduced pyridine nucleotides are substrates for the plasma membrane dehydrogenases. Both plant and animal membranes have unusual cyanide-insensitive oxidases so oxygen can be the natural electron acceptor. Natural ferric chelates or ferric transferrin can also act as electron acceptors. Artificial, impermeable oxidants such as ferricyanide are used to probe the activity. Since plasma membranes containb cytochromes, flavin, iron, and quinones, components for electron transport are present but their participation, except for quinone, has not been demonstrated. Stimulation of electron transport with impermeable oxidants and hormones activates proton release from cells. In plants the electron transport and proton release is stimulated by red or blue light. Inhibitors of electron transport, such as certain antitumor drugs, inhibit proton release. With animal cells the high ratio of protons released to electrons transferred, stimulation of proton release by sodium ions, and inhibition by amilorides indicates that electron transport activates the Na+/H+ antiport. In plants part of the proton release can be achieved by activation of the H+ ATPase. A contribution to proton transfer by protonated electron carriers in the membrane has not been eliminated. In some cells transmembrane electron transport has been shown to cause cytoplasmic pH changes or to stimulate protein kinases which may be the basis for activation of proton channels in the membrane. The redox-induced proton release causes internal and external pH changes which can be related to stimulation of animal and plant cell growth by external, impermeable oxidants or by oxygen. 相似文献
992.
Rita M. Ulloa Gerardo C. Glikin María T. Téllez-Ĩnón Héctor N. Torres Norberto D. Judewicz 《Molecular and cellular biochemistry》1987,77(1):11-17
Summary An endogenous thermostable activator of Protein kinase III (PKIII) was purified from 100000 × g supernatants of Neurospora crassa mycelial extracts. This 38 000 dalton polypeptide, clearly separable from calmodulin on P-60 gel filtration, specifically stimulated N. crassa PKIII activity on casein or phosvitin in vitro phosphorylation.The factor was only present in the initial growth phase of the fungus. The mechanism of PKIII activation and its possible regulatory role are discussed.Abbreviations PK
protein kinase
- MES
2-N-Morpholino ethane-sulfonic acid
- PMSF
phenylmethylsulfonyl fluoride
- S100
100000 × g Supernatant 相似文献
993.
Summary Growth hormone-releasing factor (GRF)-like immunoreactivity has been demonstrated in the trigeminal and spinal ganglia of fetal, young and adult rats by use of peroxidase-antiperoxidase immunohistochemistry. GRF-like-immunoreactive cells first appear during the second half of embryonic life, as early as day 17. In untreated animals the GRF-immunoreactive elements form approximately 1% of all ganglion cells in the trigeminal and spinal ganglia; their numbers do not change significantly during development. The granular immunoreaction product is confined to perikarya, especially to the perinuclear region. Nerve fibers displaying GRF-like immunoreactivity were found neither in the ganglia, nor in the corresponding central and peripheral areas of termination. The possible role of GRF in sensory ganglia is discussed. 相似文献
994.
Jose L. Subiza Javier Coll Rita Alvarez Manuel Valdivieso Emilio G. de la Concha 《Cancer immunology, immunotherapy : CII》1987,25(2):87-92
Summary Antibody response and protection against Ehrlich ascites tumor (EAT) was studied in eight EAT-immunized strains of mice (AL/N, BALB/C, C57BL/6J, F1(C57BL/6×BALB/C), C57BL/10J, B10.BR, CBA/Ca, SW). The results showed a close association between IgM response and resistance to subsequent tumor challenge. Thus, protection was only achieved in those animals giving a measurable IgM response against EAT cell surface antigens, i.e., all inbred strains of mice tested, except CBA/Ca, and some outbred SW mice. The lack of IgM response to these antigens in CBA/Ca was not linked to the strain H-2 haplotype. Resistance could be passively transferred to nonimmunized mice by means of serum, or purified IgM, from protected immune animals. Moreover, complement depletion by cobra venom factor treatment did not modify the protection afforded to those mice. IgM reactivity to EAT cells was completely abolished by previous cell trypsinization. Trypsin removed but did not destroy the antigen(s) recognized by the IgM, since all its activity could be absorbed with the supernatant of the EAT cell trypsinization. Absorption assays with this supernatant treated with different agents, showed that lipids, simple peptides and nucleic acids were not important components of the antigenic determinants. On the contrary, its susceptibility to -galactosidase and particularly to a mild periodate oxidation, suggested that determinants recognized by the IgM against the EAT cell surface are carbohydrate in nature. 相似文献
995.
996.
William K. Fitt Michael P. Labare W. Claiborne Fuqua Marianne Walch Steven L. Coon Dale B. Bonar Rita R. Colwell Ronald M. Weiner 《Microbial ecology》1989,17(3):287-298
Dissolved chemical inducers of settlement behavior of veliger larvae of the oysterCrassostrea gigas are found in supernatants of both pigmented species of bacteria (Alteromonas colwelliana, Vibrio cholerae strain HTX) as well as nonpigmented bacteria (Excherichia coli, Vibrio cholerae strain 596-B). Usually less than 10% of veligers exhibited settlement behavior in response to supernatants from the early bacterial growth phases, whereas 30–90% of larvae responded when exposed to supernatant from late-log and stationary phase cultures. Percentages of larvae exhibiting settlement behavior were inversely correlated with oxygen levels in the culture. Furthermore, the behavioral response decreased with pigment formation, suggesting that quantities of noxious compounds such as quinones may build up in the supernatants of cultures of pigmented bacteria. Tyrosinase, an enzyme that converts L-tyrosine to L-DOPA in the first step of melanogenesis, was detected both in the bacterial pellet and the supernatant during growth of the pigmented species. The enzyme is not required for the production of settlement inducer as the nonpigmented speciesE. coli andV. cholerae (596-B) also released inducer into the supernatant and had no detectable tyrosinase. The data suggest either that there is more than one inducer of settlement behavior found in bacterial supernatants or that the inducer is not L-DOPA or an L-DOPA-mimetic associated with the melanin biochemical pathway. 相似文献
997.
Laura Fiorucci Gabriella De Renzis Rita Businaro Lorenzo Fumagalli Evandro Fioretti Bruno Giardina Franca Ascoli 《The Histochemical journal》1989,21(12):721-730
Summary Three isoinhibitors of bovine pancreatic trypsin inhibitor (BPTI) have been identified and isolated from bovine pituitary gland. The results of the purification process by affinity chromatography on immobilized trypsin, the electrophoretic mobility in non-denaturing conditions, the antiproteolytic activity and the immunochemical reactions indicate that these inhibitors correspond to those previously isolated from bovine spleen and lung. In addition, immunohistochemical experiments show that the isoinhibitors and BPTI are exclusively localized in the mast cells, and not in the endocrine cells, of the pars intermedia and posterior lobe (neurohypophysis) of the pituitary gland. The physiological implications of these findings are discussed. 相似文献
998.
Valentina Camomilla Marco Donati Rita Stagni Aurelio Cappozzo 《Journal of biomechanics》2009,42(7):931-937
In the movement analysts community, the assessment of the displacement of skin photogrammetric markers relative to the underlying bone (soft tissue displacement, STD) is considered to be a priority. The aim of this study is to present a non-invasive method that allows for the characterization of STD for any marker location, subject, and motor task. In particular, this method provides an estimate of the STD vector in a bone-embedded frame. The body segment under analysis is endowed with the largest possible number of skin markers located over all areas of interest. Any given STD vector is observed from all the marker cluster frames that can be built by suitably combining all the available markers. A subset of the latter frames is identified that is made of frames endowed with uncorrelated local movements. The estimate of a given STD vector is determined through the coherent average of the vectors reconstructed using the above-mentioned independent frames. This estimate is affected by a 180° phase indeterminacy.The proposed method and the underlying hypotheses were validated using markers located on the thighs of two female subjects treated for a total knee replacement. The relevant STD estimates, STDm, were compared with those directly observed using photogrammetry combined with 2D fluoroscopic projections and the prosthesis CAD model (STDf). Recordings were made while the volunteers performed step up/down motor tasks.The root mean square value of STDm was found in the range 2.5–23.0 mm and was consistent with the RMS values of STDf and with other results reported in the literature and obtained in similarly unconstrained conditions. Moreover, STDm and STDf showed a pattern similarity measured by a correlation coefficient equal to 0.83 (±0.13) and by a normalised root mean square distance equal to 27% (±16%).The described estimate of the STD pattern and magnitude, even with the above-mentioned indeterminacies, constitutes valuable information when aiming at optimal marker placement and is an indispensable prerequisite for bone pose estimator design and assessment. 相似文献
999.
Aldo Andreani Silvia Burnelli Massimiliano Granaiola Alberto Leoni Alessandra Locatelli Rita Morigi Mirella Rambaldi Lucilla Varoli Laura Landi Cecilia Prata Francesco Vieceli Dalla Sega Cristiana Caliceti Robert H. Shoemaker 《Bioorganic & medicinal chemistry》2010,18(9):3004-3011
This paper reports the synthesis of new derivatives (formed by two indole systems separated by a central moiety) analogous of potent antitumor agents previously described. The activity of the bis-indoles bearing a pyridine core confirms the good result described in the previous paper and compound 4c was chosen for the first in vivo experiment (Hollow Fiber Assay). COMPARE analysis and structure–activity relationships were also considered. Contrary to data reported by other Authors, no correlations were found between antitumor activity and NQO1 induction. 相似文献
1000.
Emmanouil Fokas Gerhard Kraft Hanxiang An Rita Engenhart-Cabillic 《生物化学与生物物理学报:癌评论》2009,1796(2):216-229
High-energy protons and carbon ions exhibit an inverse dose profile allowing for increased energy deposition with penetration depth. Additionally, heavier ions like carbon beams have the advantage of a markedly increased biological effectiveness characterized by enhanced ionization density in the individual tracks of the heavy particles, where DNA damage becomes clustered and therefore more difficult to repair, but is restricted to the end of their range. These superior biophysical and biological profiles of particle beams over conventional radiotherapy permit more precise dose localization and make them highly attractive for treating anatomically complex and radioresistant malignant tumors but without increasing the severe side effects in the normal tissue. More than half a century since Wilson proposed their use in cancer therapy, the effects of particle beams have been extensively investigated and the biological complexity of particle beam irradiation begins to unfold itself. The goal of this review is to provide an as comprehensive and up-to-date summary as possible of the different radiobiological aspects of particle beams for effective application in cancer treatment. 相似文献