全文获取类型
收费全文 | 5280篇 |
免费 | 526篇 |
出版年
2022年 | 41篇 |
2021年 | 85篇 |
2020年 | 49篇 |
2019年 | 57篇 |
2018年 | 57篇 |
2017年 | 59篇 |
2016年 | 123篇 |
2015年 | 234篇 |
2014年 | 249篇 |
2013年 | 290篇 |
2012年 | 406篇 |
2011年 | 390篇 |
2010年 | 229篇 |
2009年 | 219篇 |
2008年 | 331篇 |
2007年 | 277篇 |
2006年 | 284篇 |
2005年 | 291篇 |
2004年 | 290篇 |
2003年 | 255篇 |
2002年 | 284篇 |
2001年 | 64篇 |
2000年 | 39篇 |
1999年 | 52篇 |
1998年 | 74篇 |
1997年 | 44篇 |
1996年 | 47篇 |
1995年 | 53篇 |
1994年 | 51篇 |
1993年 | 44篇 |
1992年 | 33篇 |
1991年 | 36篇 |
1990年 | 49篇 |
1989年 | 30篇 |
1988年 | 32篇 |
1987年 | 29篇 |
1986年 | 29篇 |
1985年 | 41篇 |
1984年 | 46篇 |
1983年 | 45篇 |
1982年 | 35篇 |
1981年 | 48篇 |
1980年 | 40篇 |
1979年 | 35篇 |
1978年 | 27篇 |
1977年 | 37篇 |
1976年 | 29篇 |
1974年 | 30篇 |
1973年 | 28篇 |
1972年 | 27篇 |
排序方式: 共有5806条查询结果,搜索用时 15 毫秒
51.
During the last decade increasing attention has been given to the role of free radicals in biological oxidations. The subject has been of increasing interest to both the food scientist and the physiologist. Free radical scavengers in the form of both indigenous and added antioxidants are necessary for the successful preservation of food; free radicals are increasingly being implicated in the onset of, among others, ischaemic heart disease and for protection against these diseases it is suggested that the dietary intake of the antioxidant vitamins should be increased especially for diets high in polyunsaturated fats. 1,2 Convenience and snack foods which absorb substantial amounts of frying oils are being increasingly consumed. Since poly-unsaturated fatty acids are particularly susceptible to oxidation by free radicals during the storage, cooking and frying of foods, the potential risk of exposure to lipid degradation products' is likely to have increased. In foods the non-enzymic and lipoxygenase catalysed oxidation of polyunsaturated fatty acids, β-carotene and vitamin A can result in the loss of essential nutrients and the development of off-flavours. 相似文献
52.
Inhibition of lignin biosynthesis in Triticum aestivum L. rootsby Mn deficiency has been suggested as the mechanism of reducedresistance of Mn-deficient wheat roots to infection by the take-allfungus (Gaeumannomyces graminis var. tritici). This study evaluatedphenolics and lignin accumulation in roots of wheat genotypesdiffering in Mn efficiency (measured as growth and yield inMn-deficient soils) and take-all resistance. Seedlings of theMn-inefficient, take-all sensitive genotype Bayonet and theMn-efficient, more take-all resistant genotype C8MM were grownin nutrient solution without added Mn for 18 d and then transferredto a Mn-deficient sandy soil fertilized with Mn at 0 or 30 mgkg-1. Both genotypes had Mn-deficient roots and shoots at thetime of transfer to the soil. Roots of both genotypes were inoculatedwith the take-all fungus 0, 1, 3 and 7 d after transfer. Twenty-fourhours after inoculation, take-all fungus penetrated the rootstele of take-all sensitive Bayonet but not of more resistantC8MM wheat. Rates of phenolics and lignin accumulation in rootsdeclined steadily during growth in soil for up to 8 d, werehigher in mature, fully differentiated parts of the root systemcompared to distal, younger root tissue, and were higher inBayonet than in C8MM. Manganese fertilization did not significantlyinfluence rates of phenolics and lignin accumulation but reduceddepth of radial penetration by hyphae in both genotypes. Therate of phenolics accumulation was positively (r = 0·91to 0·96) correlated with the rate of lignin accumulation.Mn-efficient C8MM had a higher rate of lignin accumulation perunit of phenolics than Mn-inefficient Bayonet over a wide rangeof phenolics synthesis rates. From this we suggest that C8MMhas a more efficient mechanism for conversion of phenolics tolignin, the trait which appears related to higher take-all resistanceof this genotype.Copyright 1994, 1999 Academic Press Gaeumannomyces graminis var. tritici, lignin, manganese, phenolics, resistance, roots, Triticum aestivum 相似文献
53.
Use of Genetic and Physical Mapping to Locate the Spinal Muscular Atrophy Locus between Two New Highly Polymorphic DNA Markers 总被引:9,自引:2,他引:7 下载免费PDF全文
Olivier Clermont Philippe Burlet Lydie Burglen Suzie Lefebvre Fabrice Pascal John McPherson John J. Wasmuth Daniel Cohen Denis Le Paslier Jean Weissenbach Mark Lathrop Arnold Munnich Judith Melki 《American journal of human genetics》1994,54(4):687-694
The gene for autosomal recessive forms of spinal muscular atrophy (SMA) has recently been mapped to chromosome 5ql3, within a 4-cM region between the blocks D5S465/D5S125 and MAP-1B/D5S112. We identified two new highly polymorphic microsatellite DNA markers—namely, AFM265wf5 (D5S629) and AFM281yh9 (D5S637)—which are the closest markers to the SMA locus. Multilocus analysis by the location-score method was used to establish the best estimate of the SMA gene location. Our data suggest that the most likely location for SMA is between locus D5S629 and the block D5S637/D5S351/MAP-1B/D5S112/D5S357. Genetic analysis of inbred SMA families, based on homozygosity by descent and physical mapping using mega-YACs, gave additional information for the loci order as follows: cen–D5S6–D5S125/D5S465–D5S435–D5S629–SMA–D5S637–D5S351–MAP–1B/D5S112–D5S357–D5S39–tel. These data give the direction for bidirectional walking in order to clone this interval and isolate the SMA gene. 相似文献
54.
55.
Zoophthora radicans Brefeld was tested for control of Plutella xylostella L. in a caged‐field trial. No infection was seen in control cages, but between 36% and 68% of larvae in live samples, and 38% and 55% of larvae at the final harvest, were infected in inoculated cages, suggesting the biological control potential of this fungus. 相似文献
56.
Astrocytes are important in regulating the microencironment of neurons both by catabolic and synthetic pathways. The glutamine synthetase (GS) activity observed in astrocytes affects neurons by removing toxic substances, NH3 and glutamate; and by providing an important neuronal substrate, glutamine. This glutamate cycle might play a critical role during periods of hypoxia and ischemia, when an increase in extracellular excitatory amino acids is observed. It was previously shown in our laboratory that fructose-1,6-bisphosphate (FBP) protected cortical astrocyte cultures from hypoxic insult and reduced ATP loss following a prolonged (18–30 hrs) hypoxia. In the present study we established the effects of FBP on the level of glutamate uptake and GS activity under normoxic and hypoxic conditions. Under normoxic conditions, [U-14C]glutamate uptake and glutamine production were independent of FBP treatment; whereas under hypoxic conditions, the initial increase in glutamate uptake and an overall increase in glutamine production in astrocytes were FBP-dependent. Glutamine synthetase activity was dependent on FBP added during the 22 hours of either normoxic- or hypoxic-treatment, hence significant increases in activity were observed due to FBP regardless of the oxygen/ATP levels in situ. These studies suggest that activation of GS by FBP may provide astrocytic protection against hypoxic injury. 相似文献
57.
David Crews Judith M. Bergeron James J. Bull Deborah Flores Alan Tousignant James K. Skipper Thane Wibbels 《Genesis (New York, N.Y. : 2000)》1994,15(3):297-312
In many egg-laying reptiles, the incubation temperature of the egg determines the sex of the offspring, a process known as temperature-dependent sex determination (TSD). In TSD sex determination is an “all or none” process and intersexes are rarely formed. How is the external signal of temperature transduced into a genetic signal that determines gonadal sex and channels sexual development? Studies with the red-eared slider turtle have focused on the physiological, biochemical, and molecular cascades initiated by the temperature signal. Both male and female development are active processes—rather than the crganized/default system characteristic of vertebrates with genotypic sex determination—that require simultaneous activation and suppression of testis- and ovary-determining cascades for normal sex determination. It appears that temperature accomplishes this end by acting on genes encoaing for steroidogenic enzymes and steroid hormone receptors and modifying the endocrine microenvironment in the embryo. The temperature experienced in development also has long-term functional outcomes in addition to sex determination. Research with the leopard gecko indicates that incubation temperature as well as steroid hormones serve as organizers in shaping the adult phenotype, with temperature modulating sex hormone action in sexual differentiation. Finally, practical applications of this research have emerged for the conservation and restoration of endangered egg-laying reptiles as well as the embryonic development of reptiles as biomarkers to monitor the estrogenic effects of common environmental contaminants. © 1994 Wiley-Liss, Inc. 相似文献
58.
59.
Carlos A. Pérez-Muñoz Judith A. Jernstedt Barbara D. Webster 《American journal of botany》1993,80(10):1183-1192
Light and electron microscope observations characterized the layers that comprise Vigna vexillata L. pollen walls, and identified the timing of their development. Exine sculpturings form an unusually coarse ektexinous reticulum. The structure of the ektexine is granular; this differs from the columellate/tectate type of structure typical of most angiosperm pollen. The ektexine overlies a homogeneous-to-lamellar, electron-dense endexine, which in turn surrounds a thick, microfibrillar intine. Pollen grains are triporate and operculate, with Zwischenkörper and thickened intine underlying the apertures. The ektexine forms during the tetrad period of microspore development, the endexine and Zwischenkörper during the free microspore stage, and the intine during the bicelled (pollen) stage. Coarsely reticulate exine sculpturings and the granular structure of the patterned exine wall of the pollen grains are features that make this species suitable for detailed studies of pollen wall pattern formation. 相似文献
60.
Unfolding of an α-helix in peptide crystals by solvation: Conformational fragility in a heptapeptide
The structure of the peptide Boc-Val-Ala-Leu-Aib-Val-Ala-Leu-OMe has been determined in crystals obtained from a dimethylsulfoxide–isopropanol mixture. Crystal parameters are as follows: C38H69N7O10 · H2O · 2C3H7OH, space group P21, a = 10.350 (2) Å, b = 26.084 (4) Å, c = 10.395(2) Å, β = 96.87(12), Z = 2, R = 8.7% for 2686 reflections observed > 3.0 σ (F). A single 5 → 1 hydrogen bond is observed at the N-terminus, while two 4 → 1 hydrogen bonds characteristic of a 310-helix are seen in the central segment. The C-terminus residues, Ala(6) and Leu(7) are expended, while Val(5) is considerably distorted from a helical conformation. Two isopropanol molecules make hydrogen bonds to the C-terminal segment, while a water molecule interacts with the N-terminus. The structure is in contrast to that obtained for the same peptide in crystals from methanol-water [ I. L. Karle, J. L. Flippen-Anderson, K. Uma, and P. Balaram (1990) Proteins: Structure, Function and Genetics, Vol. 7, pp. 62–73] in which two independent molecules reveal an almost perfect α-helix and a helix penetrated by a water molecule. A comparison of the three structures provides a snapshot of the progressive effects of solvation leading to helix unwinding. The fragility of the heptapeptide helix in solution is demonstrated by nmr studies in CDC13 and (CD3)2SO. A helical conformation is supported in the apolar solvent CDCl3, whereas almost complete unfolding is observed in the strongly solvating medium (CD3)2SO. © 1993 John Wiley & Sons, Inc. 相似文献