全文获取类型
收费全文 | 4756篇 |
免费 | 349篇 |
国内免费 | 1篇 |
出版年
2023年 | 9篇 |
2022年 | 19篇 |
2021年 | 58篇 |
2020年 | 44篇 |
2019年 | 58篇 |
2018年 | 75篇 |
2017年 | 78篇 |
2016年 | 139篇 |
2015年 | 214篇 |
2014年 | 202篇 |
2013年 | 325篇 |
2012年 | 362篇 |
2011年 | 321篇 |
2010年 | 247篇 |
2009年 | 199篇 |
2008年 | 308篇 |
2007年 | 297篇 |
2006年 | 279篇 |
2005年 | 238篇 |
2004年 | 231篇 |
2003年 | 243篇 |
2002年 | 228篇 |
2001年 | 52篇 |
2000年 | 50篇 |
1999年 | 45篇 |
1998年 | 67篇 |
1997年 | 40篇 |
1996年 | 29篇 |
1995年 | 45篇 |
1994年 | 38篇 |
1993年 | 35篇 |
1992年 | 32篇 |
1991年 | 32篇 |
1990年 | 43篇 |
1989年 | 31篇 |
1988年 | 22篇 |
1987年 | 19篇 |
1986年 | 22篇 |
1985年 | 32篇 |
1984年 | 35篇 |
1983年 | 28篇 |
1982年 | 26篇 |
1981年 | 34篇 |
1980年 | 17篇 |
1979年 | 15篇 |
1978年 | 18篇 |
1977年 | 19篇 |
1976年 | 15篇 |
1975年 | 23篇 |
1974年 | 11篇 |
排序方式: 共有5106条查询结果,搜索用时 468 毫秒
991.
Abstract. In this paper we present an application of aerial remote sensing to the analysis of spatial information in a mountainous area of central Italy by applying texture measures and landscape indices. Land cover data acquired in different time periods are used to calculate measures of landscape pattern and structure at pixel level (tone and texture variables) and patch‐level (landscape indices). Images of the patches from the 1950s, 1980s and 1990s have been derived from the tone‐texture classification of scanned black‐and‐white photographs. Multitemporal analysis of landscape indices has been performed to detect changes of landscape elements and related effects on vegetation dynamics due to the reduction of human impact. 相似文献
992.
Direct Association and Nuclear Import of the Hepatitis B Virus X Protein with the NF-κB Inhibitor IκBα 总被引:8,自引:0,他引:8 下载免费PDF全文
993.
Anna Palumbo Anna Di Cosmo Annarita Poli Carlo Di Cristo & Marco d'Ischia† 《Journal of neurochemistry》1999,73(3):1254-1263
Chemical, biochemical, and immunohistochemical evidence is reported demonstrating the presence in the brain of the cuttlefish Sepia officinalis of a Ca2+-dependent nitric oxide synthase, NMDAR2/3 receptor subunits, and glutamate, occurring in neurons and fibers functionally related to the inking system. Nitric oxide synthase activity was concentrated for the most part in the cytosolic fraction and was masked by other citrulline-forming enzyme(s). The labile nitric oxide synthase could be partially purified by ammonium sulfate precipitation of tissue extracts, followed by affinity chromatography on 2',5'-ADP-agarose and calmodulin-agarose. The resulting activity, immunolabeled at 150 kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis by antibodies to rat neuronal nitric oxide synthase, depended on NADPH and tetrahydro-L-biopterin, and was inhibited by N(G)-nitro-L-arginine. NMDAR2/3 subunit-immunoreactive proteins migrating at 170 kDa could also be detected in brain extracts, along with glutamate (whole brain: 0.32 +/- 0.03 micromol of glutamate/mg of protein; optic lobes: 0.22 +/- 0.04; vertical complex: 0.65 +/- 0.06; basal lobes: 0.58 +/- 0.04; brachial lobe: 0.77 +/- 0.06; pedal lobe: 1.04 +/- 0.08; palliovisceral lobe: 0.86 +/- 0.05). Incubation of intact brains with 1.5 mM glutamate or NMDA or the nitric oxide donor 2-(N,N-diethylamino)diazenolate-2-oxide caused a fivefold rise in the levels of cyclic GMP, indicating operation of the glutamate-nitric oxide-cyclic GMP signaling pathway. Immunohistochemical mapping of Sepia CNS showed specific localization of nitric oxide synthase-like and NMDAR2/3-like immunoreactivities in the lateroventral palliovisceral lobe, the visceral lobe, and the pallial and visceral nerves, as well as in the sphincters and wall of the ink sac. 相似文献
994.
Pedro Silva-Pinheiro Carlos Pardo-Hernndez Aurelio Reyes Lisa Tilokani Anup Mishra Raffaele Cerutti Shuaifeng Li Dieu-Hien Rozsivalova Sebastian Valenzuela Sukru A Dogan Bradley Peter Patricio Fernndez-Silva Aleksandra Trifunovic Julien Prudent Michal Minczuk Laurence Bindoff Bertil Macao Massimo Zeviani Maria Falkenberg Carlo Viscomi 《Nucleic acids research》2021,49(18):10803
995.
Eric W. Bridgeford Shangsi Wang Zeyi Wang Ting Xu Cameron Craddock Jayanta Dey Gregory Kiar William Gray-Roncal Carlo Colantuoni Christopher Douville Stephanie Noble Carey E. Priebe Brian Caffo Michael Milham Xi-Nian Zuo Consortium for Reliability Reproducibility Joshua T. Vogelstein 《PLoS computational biology》2021,17(9)
Replicability, the ability to replicate scientific findings, is a prerequisite for scientific discovery and clinical utility. Troublingly, we are in the midst of a replicability crisis. A key to replicability is that multiple measurements of the same item (e.g., experimental sample or clinical participant) under fixed experimental constraints are relatively similar to one another. Thus, statistics that quantify the relative contributions of accidental deviations—such as measurement error—as compared to systematic deviations—such as individual differences—are critical. We demonstrate that existing replicability statistics, such as intra-class correlation coefficient and fingerprinting, fail to adequately differentiate between accidental and systematic deviations in very simple settings. We therefore propose a novel statistic, discriminability, which quantifies the degree to which an individual’s samples are relatively similar to one another, without restricting the data to be univariate, Gaussian, or even Euclidean. Using this statistic, we introduce the possibility of optimizing experimental design via increasing discriminability and prove that optimizing discriminability improves performance bounds in subsequent inference tasks. In extensive simulated and real datasets (focusing on brain imaging and demonstrating on genomics), only optimizing data discriminability improves performance on all subsequent inference tasks for each dataset. We therefore suggest that designing experiments and analyses to optimize discriminability may be a crucial step in solving the replicability crisis, and more generally, mitigating accidental measurement error. 相似文献
996.
Desiccation and osmotic stress increase the abundance of mRNA of the tonoplast aquaporin BobTIP26-1 in cauliflower cells 总被引:8,自引:0,他引:8
François Barrieu Danièle Marty-Mazars Dominique Thomas François Chaumont Maryse Charbonnier Francis Marty 《Planta》1999,209(1):77-86
Changes in vacuolar structure and the expression at the RNA level of a tonoplast aquaporin (BobTIP26-1) were examined in
cauliflower (Brassicaoleracea L. var. botrytis) under water-stress conditions. Gradual drying out of slices of cauliflower floret tissue caused its collapse, with a shrinkage
in tissue and cell volumes and an apparent vesiculation of the central vacuole, whereas osmotic stress resulted in plasmolysis
with a collapse of the cytoplasm and the central vacuole within. Osmotic stress caused a rapid and substantial increase in
BobTIP26 mRNA in slices of floret tissue. Exposure of tissue slices to a regime of desiccation showed a slower but equally large rise
in BobTIP26 mRNA followed by a rapid decline upon rehydration. In situ hybridization showed that BobTIP26-2 mRNA is expressed most highly in meristematic and expanding cells of the cauliflower florets and that desiccation strongly
increased the expression in those cells and in differentiated cells near the xylem vessels. These data indicate that under
water-deficit conditions, expression of the tonoplast aquaporin gene in cauliflower is subject to a precise regulation that
can be correlated with important cytological changes in the cells.
Received: 21 October 1998 / Accepted: 10 February 1999 相似文献
997.
A nuclear mutant of Arabidopsis thaliana selected for enhanced sensitivity to light-chill stress is altered in PSII electron transport activity 总被引:1,自引:0,他引:1
Delia Tarantino Alberto Vianelli Luisa Carraro Carlo Soave 《Physiologia plantarum》1999,107(3):361-371
Chilling in the light imposes a considerable level of stress on the photosynthetic apparatus, resulting in a decrease of photosystem II activity and the quenching of maximum and variable fluorescence. We selected in a fah - 1 mutagenized population of Arabidopsis thaliana , which permits a direct visible evaluation of the intensity of chlorophyll (Chl) fluorescence, a monogenic recessive nuclear mutant hypersensitive to photoinhibition induced by light and cold. The major phenotypic trait of the mutant is the appearance of chlorotic areas on developed leaves. Photochemical analyses indicate that the mutant is hypersensitive to photoinhibition in excess light in the cold but also at room temperature. The susceptibility to photoinhibition is a consequence of perturbations in photochemistry already present in unstressed plants. Such perturbations result in a greater fraction of the primary acceptor QA remaining in the reduced state even at low light fluxes. From estimates of the number of total and functional PSII units and measurements of PSII quantum yield and QA − reoxidation kinetics, the basic lesion of the mutant seems restricted to PSII photochemistry likely affecting the rate of electron transport from QA − to QB . 相似文献
998.
Masmoudi Raja Rival Alain Nato Aimé Lavergne Danièle Drira Nourredine Ducreux Georges 《Plant Cell, Tissue and Organ Culture》1999,57(2):139-143
While describing major trends of carbon metabolism during the initiation and expression of somatic embryogenesis in date palm
(Phoenix dactylifera L., cv. Deglet Nour), we have investigated the role of two carboxylases, namely PEPC (Phosphoenolpyruvate
carboxylase, EC 4.1.1.31) and RubisCO (Ribulose 1,5-bisphosphate carboxylase/oxygenase, EC 4.1.1.39), in embryogenic and non-embryogenic
cultures. The detection of PEPC activity on polyacrylamide native gels after electrophoresis revealed the presence of 3 active
isoforms in crude extracts from the embryogenic (E) callus strain, whereas only a single band was present in the non-embryogenic
(NE) one. The level of PEPC specific capacity was of the same order (3.9 ± 1.2 μmol CO2 h−1 mg−1 TSP) in both types of cultures. Further changes in carboxylase (PEPC and RubisCO) activities during the growth and development
of somatic embryo–derived plantlets were also analysed. The PEPC/RubisCO ratio was found to progressively decrease (from 17.7
to 0.2) throughout the in vitro development of plantlets, due to a substantial depletion of PEPC activity, which decreased
from 5.3 to 1.2 μmol CO2 h−1 mg−1 TSP. Concomitantly, RubisCO assumed greater importance (from 0.3 to 5.3 μmol CO2 h−1 mg−1
TSP
) and became the main route for inorganic carbon fixation. Western blot analysis using polyclonal antibodies raised against
PEPC and RubisCO purified from tobacco leaves confirmed this trend in terms of relative enzyme abundance.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
999.
Claudia Galoppini Riccardo Patacchini Stefania Meini Stefania Viganò Mariella Tancredi Laura Quartara Antonio Triolo Carlo A. Maggi Paolo Rovero 《Letters in Peptide Science》1999,6(2-3):123-127
It has recently been shown that the biological activity of the second generation kinin B1 receptor selective antagonist, desArg10 HOE 140, can be improved by specific amino acid substitutions. Starting from this observation, we undertook a systematic structure-activity relationship study of this antagonist, based on the alanine-scan technique, in order to obtain useful information for the rational design of more analogues. Our data indicate that the sequence of desArg10 HOE 140 does not tolerate the replacement by Ala of most of its residues, with the exception of Ser in position 7 and, to a lesser extent, D-Arg in position 1 and Hyp in position 4. The most critical residues appear to be Pro in position 3 and the C-terminal dipeptide DTic-Oic; Ala replacement at these positions resultes in a total loss of activity. Moreover, replacement by Ala of Gly in position 5 reverts the activity of desArg10 HOE 140 to that of an agonist. 相似文献
1000.
Greater than one-half of the world's population harbors Helicobacter pylori. The majority of infected individuals, however, remain asymptomatic, with only 10% to 20% developing diseases, including peptic ulcer disease, gastric cancer, and gastric mucosa–associated lymphoid tissue lymphoma. This article reviews host factors that may predispose an individual to both the acquisition of H. pylori infection and subsequent clinical outcome. Individuals with specific blood group antigens and human leukocyte antigen genotypes may be more susceptible to H. pylori infection. Additional factors, such as the age of acquisition, the host immune response, the site of infection, acid secretion, and interactions with nonhost factors (including bacterial virulence factors and environmental influences) may play a role in determining clinical outcome. Further investigation is required to clarify the mechanisms by which these interactions occur and, more critically, to determine their relative importance. This knowledge will enable the identification of individuals at risk of developing clinical disease with H. pylori infection. 相似文献