全文获取类型
收费全文 | 19659篇 |
免费 | 1654篇 |
国内免费 | 5篇 |
出版年
2023年 | 78篇 |
2022年 | 52篇 |
2021年 | 351篇 |
2020年 | 233篇 |
2019年 | 306篇 |
2018年 | 423篇 |
2017年 | 353篇 |
2016年 | 636篇 |
2015年 | 1030篇 |
2014年 | 1169篇 |
2013年 | 1409篇 |
2012年 | 1747篇 |
2011年 | 1649篇 |
2010年 | 1062篇 |
2009年 | 936篇 |
2008年 | 1264篇 |
2007年 | 1291篇 |
2006年 | 1105篇 |
2005年 | 1060篇 |
2004年 | 1034篇 |
2003年 | 936篇 |
2002年 | 889篇 |
2001年 | 201篇 |
2000年 | 139篇 |
1999年 | 146篇 |
1998年 | 216篇 |
1997年 | 139篇 |
1996年 | 129篇 |
1995年 | 124篇 |
1994年 | 112篇 |
1993年 | 101篇 |
1992年 | 80篇 |
1991年 | 62篇 |
1990年 | 67篇 |
1989年 | 57篇 |
1988年 | 49篇 |
1987年 | 53篇 |
1986年 | 41篇 |
1985年 | 46篇 |
1984年 | 43篇 |
1983年 | 61篇 |
1982年 | 37篇 |
1981年 | 32篇 |
1980年 | 36篇 |
1979年 | 28篇 |
1978年 | 42篇 |
1977年 | 35篇 |
1976年 | 32篇 |
1975年 | 25篇 |
1974年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Karen L. Lindsay Christian Hellmuth Olaf Uhl Claudia Buss Pathik D. Wadhwa Berthold Koletzko Sonja Entringer 《PloS one》2015,10(12)
Pregnancy is characterized by a complexity of metabolic processes that may impact fetal development and ultimately, infant health outcomes. However, our understanding of whole body maternal and fetal metabolism during this critical life stage remains incomplete. The objective of this study is to utilize metabolomics to profile longitudinal patterns of fasting maternal metabolites among a cohort of non-diabetic, healthy pregnant women in order to advance our understanding of changes in protein and lipid concentrations across gestation, the biochemical pathways by which they are metabolized and to describe variation in maternal metabolites between ethnic groups. Among 160 pregnant women, amino acids, tricarboxylic acid (TCA) cycle intermediates, keto-bodies and non-esterified fatty acids were detected by liquid chromatography coupled with mass spectrometry, while polar lipids were detected through flow-injected mass spectrometry. The maternal plasma concentration of several essential and non-essential amino acids, long-chain polyunsaturated fatty acids, free carnitine, acetylcarnitine, phosphatidylcholines and sphingomyelins significantly decreased across pregnancy. Concentrations of several TCA intermediates increase as pregnancy progresses, as well as the keto-body β-hydroxybutyrate. Ratios of specific acylcarnitines used as indicators of metabolic pathways suggest a decreased beta-oxidation rate and increased carnitine palmitoyltransferase-1 enzyme activity with advancing gestation. Decreasing amino acid concentrations likely reflects placental uptake and tissue biosynthesis. The absence of any increase in plasma non-esterified fatty acids is unexpected in the catabolic phase of later pregnancy and may reflect enhanced placental fatty acid uptake and utilization for fetal tissue growth. While it appears that energy production through the TCA cycle increases as pregnancy progresses, decreasing patterns of free carnitine and acetylcarnitine as well as increased carnitine palmitoyltransferase-1 rate and β-hydroxybutyrate levels suggest a concomitant upregulation of ketogenesis to ensure sufficient energy supply in the fasting state. Several differences in metabolomic profiles between Hispanic and non-Hispanic women demonstrate phenotypic variations in prenatal metabolism which should be considered in future studies. 相似文献
82.
Nuno Santos Catarina Santos Teresa Valente Christian Gortázar Virgílio Almeida Margarida Correia-Neves 《PloS one》2015,10(11)
Environmental contamination with Mycobacterium tuberculosis complex (MTC) has been considered crucial for bovine tuberculosis persistence in multi-host-pathogen systems. However, MTC contamination has been difficult to detect due to methodological issues. In an attempt to overcome this limitation we developed an improved protocol for the detection of MTC DNA. MTC DNA concentration was estimated by the Most Probable Number (MPN) method. Making use of this protocol we showed that MTC contamination is widespread in different types of environmental samples from the Iberian Peninsula, which supports indirect transmission as a contributing mechanism for the maintenance of bovine tuberculosis in this multi-host-pathogen system. The proportion of MTC DNA positive samples was higher in the bovine tuberculosis-infected than in presumed negative area (0.32 and 0.18, respectively). Detection varied with the type of environmental sample and was more frequent in sediment from dams and less frequent in water also from dams (0.22 and 0.05, respectively). The proportion of MTC-positive samples was significantly higher in spring (p<0.001), but MTC DNA concentration per sample was higher in autumn and lower in summer. The average MTC DNA concentration in positive samples was 0.82 MPN/g (CI95 0.70–0.98 MPN/g). We were further able to amplify a DNA sequence specific of Mycobacterium bovis/caprae in 4 environmental samples from the bTB-infected area. 相似文献
83.
Stephan Francke Karine Clement Christian Dina Hiroshi Inoue Philip Behn Vincent Vatin Arnaud Basdevant Bernard Guy-Grand M. Alan Permutt Philippe Froguel J. Hager 《Human genetics》1997,100(5-6):491-496
Family studies have shown that in some populations up to 75% of the variation of body mass index can be explained by genetic
factors. However, in humans, no major obesity gene has been identified to date. In contrast, there are a number of genetically
well defined animal models for obesity. In two of those models (ob/ob and db/db), defects in the same pathway are responsible
for obesity. Recently, some evidence has been found for the OB gene also being involved in human obesity. In this study we
investigated the potential role of the OB receptor (OBR) in the etiology of massive obesity in humans using familial linkage
analyses and case-control association studies. The typing of two microsatellite markers (D1S198 and D1S209), flanking the
OBR gene, in 256 sib pairs showed no evidence for linkage with obesity. In order to be able to detect small gene effects,
association studies with a 3′-UTR insertion/deletion polymorphism were carried out. The results of these analyses remained
non-significant (χ2 = 3.442, P = 0.18). However, subjects heterozygous for the insertion/deletion polymorphism showed a slight trend towards lower insulin
values 30 min after an oral glucose load compared to homozygous individuals (P = 0.02). In summary, our results do not support a major role of the human OBR gene in the development of morbid obesity in
our population.
Received: 4 December 1996 / Accepted: 25 June 1997 相似文献
84.
85.
Rafael O. Wüest Niklaus E. Zimmermann Damaris Zurell Jake M. Alexander Susanne A. Fritz Christian Hof Holger Kreft Signe Normand Juliano Sarmento Cabral Eniko Szekely Wilfried Thuiller Martin Wikelski Dirk Nikolaus Karger 《Journal of Biogeography》2020,47(1):1-12
Recent years have seen an exponential increase in the amount of data available in all sciences and application domains. Macroecology is part of this “Big Data” trend, with a strong rise in the volume of data that we are using for our research. Here, we summarize the most recent developments in macroecology in the age of Big Data that were presented at the 2018 annual meeting of the Specialist Group Macroecology of the Ecological Society of Germany, Austria and Switzerland (GfÖ). Supported by computational advances, macroecology has been a rapidly developing field over recent years. Our meeting highlighted important avenues for further progress in terms of standardized data collection, data integration, method development and process integration. In particular, we focus on (a) important data gaps and new initiatives to close them, for example through space- and airborne sensors, (b) how various data sources and types can be integrated, (c) how uncertainty can be assessed in data-driven analyses and (d) how Big Data and machine learning approaches have opened new ways of investigating processes rather than simply describing patterns. We discuss how Big Data opens up new opportunities, but also poses new challenges to macroecological research. In the future, it will be essential to carefully assess data quality, the reproducibility of data compilation and analytical methods, and the communication of uncertainties. Major progress in the field will depend on the definition of data standards and workflows for macroecology, such that scientific quality and integrity are guaranteed, and collaboration in research projects is made easier. 相似文献
86.
Influence of hydrological connectivity of riverine wetlands on nitrogen removal via denitrification 总被引:1,自引:0,他引:1
E. Racchetti Marco Bartoli E. Soana D. Longhi R. R. Christian M. Pinardi P. Viaroli 《Biogeochemistry》2011,103(1-3):335-354
Wetland ecosystems in agricultural areas often become progressively more isolated from main water bodies. Stagnation favors the accumulation of organic matter as the supply of electron acceptors with water renewal is limited. In this context it is expected that nitrogen recycling prevails over nitrogen dissipation. To test this hypothesis, denitrification rates, fluxes of dissolved oxygen (SOD), inorganic carbon (DIC) and nitrogen and sediment features were measured in winter and summer 2007 on 22 shallow riverine wetlands in the Po River Plain (Northern Italy). Fluxes were determined from incubations of intact cores by measurement of concentration changes or isotope pairing in the case of denitrification. Sampled sites were eutrophic to hypertrophic; 10 were connected and 12 were isolated from the adjacent rivers, resulting in large differences in nitrate concentrations in the water column (from <5 to 1,133 μM). Benthic metabolism and denitrification rates were investigated by two overarching factors: season and hydrological connectivity. SOD and DIC fluxes resulted in respiratory quotients greater than one at most sampling sites. Sediment respiration was coupled to both ammonium efflux, which increased from winter to summer, and nitrate consumption, with higher rates in river-connected wetlands. Denitrification rates measured in river-connected wetlands (35–1,888 μmol N m?2 h?1) were up to two orders of magnitude higher than rates measured in isolated wetlands (2–231 μmol N m?2 h?1), suggesting a strong regulation of the process by nitrate availability. These rates were also significantly higher in summer (9–1,888 μmol N m?2 h?1) than in winter (2–365 μmol N m?2 h?1). Denitrification supported by water column nitrate (DW) accounted for 60–100% of total denitrification (Dtot); denitrification coupled to nitrification (DN) was probably controlled by limited oxygen availability within sediments. Denitrification efficiency, calculated as the ratio between N removal via denitrification and N regeneration, and the relative role of denitrification for organic matter oxidation, were high in connected wetlands but not in isolated sites. This study confirms the importance of restoring hydraulic connectivity of riverine wetlands for the maintenance of important biogeochemical functions such as nitrogen removal via denitrification. 相似文献
87.
88.
We describe the design and characterization of a set of fusion proteins of the Escherichia coli lactose (lac) permease in which a set of five different soluble “carrier” proteins (cytochromeb562, flavodoxin, T4 lysozyme, β-lactamase and 70 kDa heat shock ATPase domain) were systematically inserted into selected loop positions of the transporter. The design goal was to increase the exposed hydrophilic surface area of the permease, while minimizing the internal flexibility of the resulting fusion proteins in order to improve the crystallization properties of the membrane protein. Fusion proteins with insertions into the central hydrophilic loop of the lac permease were active in transport lactose, although only the fusion proteins with E. coli cytochromeb562, E. coli flavodoxin or T4 lysozyme were expressed at near wild-type lac permease levels. Eight other loop positions were tested with these three carriers, leading to the identification of additional fusion proteins that were active and well-expressed. By combining the results from the single carrier insertions, we have expressed functional “double fusion” proteins containing cytochromeb562 domains inserted in two different loop positions. 相似文献
89.