全文获取类型
收费全文 | 1985篇 |
免费 | 260篇 |
出版年
2021年 | 20篇 |
2020年 | 16篇 |
2019年 | 19篇 |
2018年 | 20篇 |
2017年 | 24篇 |
2016年 | 34篇 |
2015年 | 58篇 |
2014年 | 44篇 |
2013年 | 82篇 |
2012年 | 92篇 |
2011年 | 77篇 |
2010年 | 72篇 |
2009年 | 57篇 |
2008年 | 68篇 |
2007年 | 76篇 |
2006年 | 72篇 |
2005年 | 67篇 |
2004年 | 73篇 |
2003年 | 65篇 |
2002年 | 78篇 |
2001年 | 58篇 |
2000年 | 67篇 |
1999年 | 58篇 |
1998年 | 34篇 |
1997年 | 33篇 |
1996年 | 20篇 |
1995年 | 28篇 |
1994年 | 19篇 |
1993年 | 27篇 |
1992年 | 47篇 |
1991年 | 47篇 |
1990年 | 44篇 |
1989年 | 41篇 |
1988年 | 39篇 |
1987年 | 41篇 |
1986年 | 45篇 |
1985年 | 28篇 |
1984年 | 20篇 |
1983年 | 26篇 |
1982年 | 23篇 |
1981年 | 23篇 |
1980年 | 24篇 |
1979年 | 35篇 |
1977年 | 19篇 |
1976年 | 15篇 |
1975年 | 27篇 |
1974年 | 25篇 |
1973年 | 27篇 |
1971年 | 18篇 |
1970年 | 20篇 |
排序方式: 共有2245条查询结果,搜索用时 15 毫秒
101.
102.
The effects of diet, lipolysis and limb ischaemia on the distribution of plasma tryptophan in the rat. 下载免费PDF全文
A non-linear relationship between the plasma non-esterified fatty acid concentration and the percentage of free plasma tryptophan was found in rats in different nutritional states, although non-esterified fatty acids are not the only factors determining the percentage of free tryptophan. This relationship was not seen in rats injured by limb ischaemia. The effect of drugs causing rapid increases in the plasma non-esterified fatty acid concentration was also studied. Isoprenaline decreased the total plasma tryptophan concentration. Dichloroisoprenaline caused a sustained increase in the plasma non-esterified fatty acid concentration which was accompanied by an increase in the concentration of free plasma tryptophan and followed by a fall in the concentration of total tryptophan. The loss of tryptophan from the plasma was attributed to an altered distribution of tryptophan in the extracellular space rather than to increased metabolism. This interpretation was supported by determinations of the irreversible disposal rate of plasma tryptophan which in uninjured rats was unaffected by the concentration of free plasma tryptophan. In the injured rats this rate was unaltered during limb ischaemia but was decreased after removal of the tourniquets; increased competition for tissue entry by other neutral amino acids and the fall in body temperature could be factors in this fall. 相似文献
103.
104.
105.
Antigenic characterization of infectious bronchitis virus 总被引:5,自引:0,他引:5
106.
Tonna O. Mojekwu Michael J. Cunningham Roger I. Bills Petrus C. Pretorius Thierry B. Hoareau 《Journal of fish biology》2021,98(2):498-506
The importance of Oreochromis in worldwide aquaculture and regional fisheries motivates the study of their genetic diversity in their native range. In this article, all mitochondrial cytochrome c oxidase subunit I gene (COI) sequences of Oreochromis species are retrieved from Barcode of Life Data system to quantify the available DNA barcoding information from wild individuals collected within the native ranges of the respective species. It is found that 70% of the known species in the genus still lack a COI barcode, and only 15% of the available sequences are from within the respective native ranges. Many of the available sequences have been produced from specimens acquired from aquaculture and introduced, naturalized populations, making the assessment of variation within the original native range challenging. Analyses of the wild-collected fraction of available sequences indicated the presence of cryptic lineages within Nile tilapia Oreochromis niloticus and O. schwebischi, the occurrence of potential introgressive hybridization between O. niloticus and blue tilapia O. aureus, and potential ancestral polymorphism between Karonga tilapia O. karongae and black tilapia O. placidus. This article also reports a case of misidentification of O. mweruensis as longfin tilapia O. macrochir. These results stress the importance of improving the knowledge of genetic variation within the native ranges of Oreochromis species for better-informed conservation of these natural resources. 相似文献
107.
R. M. Joeckel J. M. Cunningham R. G. Corner G. W. Brown P. L. Phillips G. A. Ludvigson 《Ichnos》2013,20(3-4):275-284
At least 22 tridactyl dinosaur tracks, poorly preserved in various degrees of expression, have recently been found at an exposure in the Dakota Formation (Lower Cretaceous, Albian) in Jefferson County, Nebraska. These tracks generally have broad, blunt digits and a broad posterior margin. The largest of the tracks measures 57 cm in length and 58 cm in width. All of the tracks lie within a stratigraphic horizon of 40 cm or less, but they do not form a single trackway. We interpret the trackmakers to have been ornithopods. The Jefferson County tracks are in a well-cemented sandstone with oscillation ripples, at a stratigraphic level between two well-established sequence boundaries. Channel forms and lateral accretion units are common in the stratigraphic interval enclosing the tracks, and the site is interpreted as a bar or sand flat in a tidally influenced river. The Jefferson County tracks are only the second known occurrence of large Mesozoic tetrapod tracks east of the Rocky Mountain Front-High Plains Margin, including the Black Hills of South Dakota, west of the Atlantic Coastal Plain, and north of the Gulf Coastal Plain. Further, this paper is the first documentation of in situdinosaur fossils from the Nebraska-Iowa area. 相似文献
108.
Philip S. Barton Matthew J. Colloff Kimberi R. Pullen Saul A. Cunningham 《Journal of Insect Conservation》2013,17(6):1209-1219
Patch size is known to affect biodiversity in fragmented landscapes, but is usually examined in systems where the surrounding matrix habitat is unfavourable. We examined beetle diversity in a floodplain ecosystem that is characterised by naturally occurring grassland patches within a dominant matrix of contrasting yet habitable forest. We asked whether differences in the beetle assemblage between grassland and forest vegetation depended on the area of the grassland patch, which is a function of its flooding frequency and duration: smaller grasslands tend to be higher on the floodplain and are flooded less often and for shorter periods than larger grasslands. We found a negative relationship between grassland area and beetle abundance and species richness, and a positive relationship between grassland area and compositional dissimilarity from the surrounding forest. As expected, we found an overall difference in composition between forest and grassland assemblages, with five beetle species more common in the grasslands. Our study indicates that floodplain grasslands not only support beetle assemblages that are distinct from the surrounding forest, but that assemblages from the larger grasslands are compositionally more distinct than those from smaller grasslands. A likely cause of this pattern is the reduced edge effects and greater environmental contrast between forest and large grasslands that may be exposed to greater variation in local climate. Ongoing changes to flood regimes and potential encroachment of forest plants may decrease grassland area in the future, which may reduce spatial heterogeneity in the insect community in this unique floodplain ecosystem. 相似文献
109.
Margaret R. Cunningham Shaista P. Nisar Alexandra E. Cooke Elizabeth D. Emery Stuart J. Mundell 《Traffic (Copenhagen, Denmark)》2013,14(5):585-598
P2Y12 receptor internalization and recycling play an essential role in ADP‐induced platelet activation. Recently, we identified a patient with a mild bleeding disorder carrying a heterozygous mutation of P2Y12 (P341A) whose P2Y12 receptor recycling was significantly compromised. Using human cell line models, we identified key proteins regulating wild‐type (WT) P2Y12 recycling and investigated P2Y12‐P341A receptor traffic. Treatment with ADP resulted in delayed Rab5‐dependent internalization of P341A when compared with WT P2Y12. While WT P2Y12 rapidly recycled back to the membrane via Rab4 and Rab11 recycling pathways, limited P341A recycling was observed, which relied upon Rab11 activity. Although minimal receptor degradation was evident, P341A was localized in Rab7‐positive endosomes with considerable agonist‐dependent accumulation in the trans‐Golgi network (TGN). Rab7 activity is known to facilitate recruitment of retromer complex proteins to endosomes to transport cargo to the TGN. Here, we identified that P341A colocalized with Vps26; depletion of which blocked limited recycling and promoted receptor degradation. This study has identified key points of divergence in the endocytic traffic of P341A versus WT‐P2Y12. Given that these pathways are retained in human platelets, this research helps define the molecular mechanisms regulating P2Y12 receptor traffic and explain the compromised receptor function in the platelets of the P2Y12‐P341A‐expressing patient. 相似文献
110.