全文获取类型
收费全文 | 7034篇 |
免费 | 501篇 |
出版年
2024年 | 4篇 |
2023年 | 36篇 |
2022年 | 103篇 |
2021年 | 192篇 |
2020年 | 105篇 |
2019年 | 133篇 |
2018年 | 196篇 |
2017年 | 179篇 |
2016年 | 267篇 |
2015年 | 385篇 |
2014年 | 438篇 |
2013年 | 543篇 |
2012年 | 716篇 |
2011年 | 649篇 |
2010年 | 389篇 |
2009年 | 348篇 |
2008年 | 455篇 |
2007年 | 423篇 |
2006年 | 375篇 |
2005年 | 355篇 |
2004年 | 293篇 |
2003年 | 264篇 |
2002年 | 231篇 |
2001年 | 34篇 |
2000年 | 34篇 |
1999年 | 48篇 |
1998年 | 49篇 |
1997年 | 29篇 |
1996年 | 30篇 |
1995年 | 29篇 |
1994年 | 22篇 |
1993年 | 19篇 |
1992年 | 21篇 |
1991年 | 12篇 |
1990年 | 13篇 |
1989年 | 15篇 |
1988年 | 6篇 |
1987年 | 9篇 |
1986年 | 12篇 |
1985年 | 7篇 |
1984年 | 14篇 |
1983年 | 9篇 |
1982年 | 7篇 |
1981年 | 8篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1978年 | 4篇 |
1977年 | 4篇 |
1973年 | 3篇 |
1972年 | 1篇 |
排序方式: 共有7535条查询结果,搜索用时 15 毫秒
951.
Patrizia Morena Baldini Paolo De Vito Daniela Vismara Claudia Bagni Francesca Zalfa Marilena Minieri Paolo Di Nardo 《Cellular physiology and biochemistry》2005,15(1-4):77-88
AIMS: The present study was performed to evaluate Atrial Natriuretic Peptide (ANP) effects on intracellular pH, phospholipase D and ROS production and the possible relationship among them in HepG2 cells. Cancer extracellular microenvironment is more acidic than normal tissues and the activation of NHE-1, the only system able to regulate pHi homeostasis in this condition, can represent an important event in cell proliferation and malignant transformation. METHODS: The ANP effects on pHi were evaluated by fluorescence spectrometry. The effects on p38 MAPK and ROS production were evaluated by immunoblots and analysis of DCF-DA fluorescence, respectively. RT-PCR analysis and Western blotting were used to determine the ANP effect on mRNA NHE-1 expression and protein levels. PLD-catalyzed conversion of phosphatidylcholine to phosphatydilethanol (PetOH), in the presence of ethanol, was monitored by thin layer chromatography. RESULTS: A significant pHi decrease was observed in ANP-treated HepG2 cells and this effect was paralleled by the enhancement of PLD activity and ROS production. The ANP effect on pHi was coupled to an increased p38 MAPK phosphorylation and a down-regulation of mRNA NHE-1 expression and protein levels. Moreover, the relationship between PLD and ROS production was demonstrated by calphostin-c, a potent inhibitor of PLD. At the same time, all assessed ANP-effects were mediated by NPR-C receptors. CONCLUSION: Our results indicate that ANP recruits a signal pathway associated with p38 MAPK, NHE-1 and PLD responsible for ROS production, suggesting a possible role for ANP as novel modulator of ROS generation in HepG2 cells. 相似文献
952.
Leaf phyllotaxis: Does it really affect light capture? 总被引:1,自引:0,他引:1
The intriguing mathematical properties of leaf phyllotaxis still attract scientific attention after centuries of research.
Phyllotaxis, and in particular the divergence angle between successive leaves, have been frequently interpreted in terms of
maximization of light capture, although certain model simulations of light capture by vertical shoots revealed minor effects
of phyllotaxis in comparison with the effect of other morphological features of the plant. However, these simulations assumed
a number of simplifications, did not take into account diffuse light, and were not based on real plants with their natural
range of morphological variation. This study was aimed at filling these gaps by examining the influence on light harvesting
of shoot architecture and divergence angle in four species with spiral phyllotaxis (Quercus ilex, Arbutus unedo, Heteromeles arbutifolia and Daphne gnidium) with a realistic 3-D model (Y-plant). A wide range of divergence angles (from 100° to 154°) was observed within each species,
with 144° being the most frequent one. These different divergence angles rendered very different vertical projections of the
shoot due to contrasting patterns of leaf overlap as seen from above, but they rendered indistinguishable light interception
efficiencies (Ea). Setting the leaves with an opposite-decussate phyllotaxis led, however, to a 40–50% decrease of Ea. The
interplay of internode length, leaf size and shape, and leaf elevation angle led to significant species differences in Ea.
Thus, only particular phyllotaxis (e.g., decussate) might be functionally inefficient under certain combinations of the various
morphological variables that influence light capture of a shoot.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
953.
Giampietro Viola Laura Facciolo Daniela Vedaldi Silvia Disarò Giuseppe Basso Francesco Dall'Acqua 《Photochemical & photobiological sciences》2004,3(3):237-239
Treatment of cells with UVA radiation in combination with linear psoralens induces a cell-cycle block and subsequent apoptosis, whereas angular derivatives produce apoptosis without affecting the cellular cycle. 相似文献
954.
Improved Production of Heterologous Proteins by a Glucose Repression-Defective Mutant of Kluyveromyces lactis 总被引:1,自引:0,他引:1
下载免费PDF全文
![点击此处可从《Applied microbiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Claudia Donnini Francesca Farina Barbara Neglia Maria Concetta Compagno Daniela Uccelletti Paola Goffrini Claudio Palleschi 《Applied microbiology》2004,70(5):2632-2638
The secreted production of heterologous proteins in Kluyveromyces lactis was studied. A glucoamylase (GAA) from the yeast Arxula adeninivorans was used as a reporter protein for the study of the secretion efficiencies of several wild-type and mutant strains of K. lactis. The expression of the reporter protein was placed under the control of the strong promoter of the glyceraldehyde-3-phosphate dehydrogenase of Saccharomyces cerevisiae. Among the laboratory strains tested, strain JA6 was the best producer of GAA. Since this strain is known to be highly sensitive to glucose repression and since this is an undesired trait for biomass-oriented applications, we examined heterologous protein production by using glucose repression-defective mutants isolated from this strain. One of them, a mutant carrying a dgr151-1 mutation, showed a significantly improved capability of producing heterologous proteins such as GAA, human serum albumin, and human interleukin-1β compared to the parent strain. dgr151-1 is an allele of RAG5, the gene encoding the only hexokinase present in K. lactis (a homologue of S. cerevisiae HXK2). The mutation in this strain was mapped to nucleotide position +527, resulting in a change from glycine to aspartic acid within the highly conserved kinase domain. Cells carrying the dgr151-1 allele also showed a reduction in N- and O-glycosylation. Therefore, the dgr151 strain may be a promising host for the production of heterologous proteins, especially when the hyperglycosylation of recombinant proteins must be avoided. 相似文献
955.
Daniela M. Simeonovska-Nikolova 《Acta theriologica》2004,49(2):157-165
Vocal sounds of 15 individuals ofCrocidura leucodon (Hermann, 1780) emitted during agonistic and amicable interactions in male-male, female-female and male-female dyadic encounters,
and intraspecific cage groups, were studied. An analysis of spectral properties, along with an examination of context and
function, were performed. The sounds registered during agonistic interactions showed an increase, then a decrease in frequency,
often followed a complex undulating curve. The dominant frequency (DF) was 10.7±0.05 kHz, and duration (DU) was 104.2±4.4
ms. The calls emitted by shrews at investigation and grooming often ended with chirping notes (DF=4.8±0.2 kHz, DU=23.5±1.1
ms). While clustering and huddling up to the partner, the animals produced calls consisting of short notes with a very low
intensity (DF=1.0±0.07 kHz, DU=35.2±0.8 ms). Based on these finding, it can be assumed that threatening sounds, emitted in
agonistic encounters, allow shrews to avoid conflicts, while those, emitted when clustering and huddling up to the partner,
contribute to maintenance of group cohesiveness. The variability of frequency ranges and intensity of sounds probably reflects
the territorial and gregarious behaviour ofC. leucodon and adaptation to communication in variable acoustic environments. 相似文献
956.
957.
Siváková D Lajdová A Basistová Z Cvícelová M Karabová P Blazícek P 《Anthropologischer Anzeiger; Bericht über die biologisch-anthropologische Literatur》2008,66(2):199-209
Epidemiological studies have demonstrated that several specific environmental factors and candidate genes influence the human variation in blood pressure. The aim of this study was to investigate variables associated with blood pressure; with a particular emphasis on the differences in insertion/deletion (I/D) polymorphism of the human angiotensin-converting enzyme (ACE), the body composition and the recognized risk factors for atherosclerosis among elderly males and females. A total of 374 participants (174 males and 200 females) aged from 60 to 90 years were recruited from different parts of Slovakia. The elderly were not bed-ridden, nor mentally impaired, they were able to manage their daily activities by themselves. The ACE I/D polymorphism was determined by PCR amplification of the ACE gene sequence. Body composition variables were obtained by bioelectrical impedance analysis, using the BIA 101 soft tissue-body impedance analyzer (Akern, S.r.l.). The subjects were determined to be hypertensive (blood pressure > or = 140/90 mm Hg) or normotensive (blood pressure < or = 140/90 mm Hg ). These two subgroups of males and females did not differ significantly in their mean ages. As expected, the hypertensive subjects of both sexes showed significantly higher mean values in systolic (SBP) and diastolic blood pressure (DBP), in body mass index (BMI), and in the mean values of their plasma glucose and extracellular water (ECW). The genotype distribution and allele frequencies in the whole sample (D = 0.5474, I = 0.4526) fell within the Hardy-Weinberg equilibrium. The frequency of the deleterious D allele in the normotensive (0.5532) and hypertensive (0.5516) subjects was not significantly different. The ACE I/D genotypes did not associate either with the systolic (p = 0.836) or diastolic BP (p = 0.629). From the other variables that may induce differences in blood pressure, a statistical effect was detected for glucose, Na/K, and Apo A1/ApoB ratios and physical activity on SBP, and for ApoA1, physical activity, BMI and total cholesterol on DBP. 相似文献
958.
Numerous studies show that miss ratios at forward proxies are typically at least 40–50%. This paper proposes and evaluates a new approach for improving the throughput of Web proxy systems by reducing the overhead of handling cache misses. Namely, we propose to front-end a Web proxy with a high performance node that filters the requests, processing the misses and forwarding the hits and the new cacheable content to the proxy. Requests are filtered based on hints of the proxy cache content. This system, called Proxy Accelerator, achieves significantly better communications performance than a traditional proxy system. For instance, an accelerator can be built as an embedded system optimized for communication and HTTP processing, or as a kernel-mode HTTP server. Scalability with the Web proxy cluster size is achieved by using several accelerators. We use analytical models, trace-based simulations, and a real implementation to study the benefits and the implementation tradeoffs of this new approach. Our results show that a single proxy accelerator node in front of a 4-node Web proxy can improve the cost-performance ratio by about 40%. Hint-based request filter implementation choices that do not affect the overall hit ratio are available. An implementation of the hint management module integrated in Web proxy software is presented. Experimental evaluation of the implementation demonstrates that the associated overheads are very small. 相似文献
959.
Nadja E. Schoemaker Enrico Frigerio Daniela Fraier Jan H. M. Schellens Hilde Rosing Sindy Jansen Jos H. Beijnen 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》2001,763(1-2)
A selective HPLC assay is described for the determination of free and total (free plus polymer-bound) camptothecin (CPT) in human plasma after administration of the anti-tumor drug MAG-CPT (polymer bound camptothecin). Total CPT levels were determined after hydrolysis and free CPT was extracted from acidified plasma using Oasis solid-phase extraction material. Extracts were analyzed on a Zorbax SB-C8 analytical column, using a mixture of acetonitrile–25 mM phosphate buffer (pH 4.0) as the eluent. Detection was performed fluorimetrically. Concentrations of polymer-bound CPT were calculated by subtraction of free from total CPT. The lower limits of quantitation of the methods were 100 ng/ml for total and 1.0 ng/ml for free CPT using 50 μl and 250 μl plasma, respectively. Special attention was paid to the stability of the analytes. The presented method was successfully applied in a clinical pharmacokinetic study in our institute. 相似文献
960.
Lorenzo Lusini Ranieri Rossi Daniela Giustarini Paolo Di Simplicio 《Chemico-biological interactions》2002,139(1):97-114
Menadione is selectively toxic to erythrocytes. Although GSH is considered a primary target of menadione, intraerythrocyte thiolic alterations consequent to menadione exposure are only partially known. In this study alterations of GSH and protein thiols (PSH) and their relationship with methemoglobin formation were investigated in human and rat red blood cells (RBC) exposed to menadione. In both erythrocyte types, menadione caused a marked increase in methemoglobin associated with GSH depletion and increased oxygen consumption. However, in human RBC, GSH formed a conjugate with menadione, whereas, in rat RBC it was converted to GSSG, concomitantly with a loss of protein thiols (corresponding to menadione arylation), and an increase in glutathione-protein mixed disulfides (GS-SP). Such differences were related to the presence of highly reactive cysteines, which characterize rat hemoglobin (cys beta125). In spite of the greater thiol oxidation in rat than in human RBC, methemoglobin formation and the rate of oxygen consumption elicited by menadione in both species were rather similar. Moreover, in repeated experiments under N2 or CO-blocked heme, it was found that menadione conjugation (arylation) in both species was not dependent on the presence of oxygen or the status of heme. Therefore, we assumed that GSH (human RBC) and protein (rat RBC) arylation was equally responsible for increased oxygen consumption and Hb oxidation. Moreover, thiol oxidation of rat RBC was strictly related to methemoglobin formation. 相似文献