全文获取类型
收费全文 | 4179篇 |
免费 | 209篇 |
国内免费 | 278篇 |
专业分类
4666篇 |
出版年
2024年 | 10篇 |
2023年 | 57篇 |
2022年 | 103篇 |
2021年 | 95篇 |
2020年 | 105篇 |
2019年 | 124篇 |
2018年 | 110篇 |
2017年 | 95篇 |
2016年 | 89篇 |
2015年 | 127篇 |
2014年 | 134篇 |
2013年 | 297篇 |
2012年 | 122篇 |
2011年 | 120篇 |
2010年 | 117篇 |
2009年 | 169篇 |
2008年 | 198篇 |
2007年 | 196篇 |
2006年 | 211篇 |
2005年 | 200篇 |
2004年 | 193篇 |
2003年 | 160篇 |
2002年 | 170篇 |
2001年 | 135篇 |
2000年 | 96篇 |
1999年 | 101篇 |
1998年 | 107篇 |
1997年 | 94篇 |
1996年 | 90篇 |
1995年 | 95篇 |
1994年 | 89篇 |
1993年 | 102篇 |
1992年 | 66篇 |
1991年 | 88篇 |
1990年 | 55篇 |
1989年 | 54篇 |
1988年 | 26篇 |
1987年 | 51篇 |
1986年 | 23篇 |
1985年 | 29篇 |
1984年 | 38篇 |
1983年 | 11篇 |
1982年 | 25篇 |
1981年 | 15篇 |
1980年 | 11篇 |
1979年 | 23篇 |
1978年 | 9篇 |
1977年 | 7篇 |
1976年 | 8篇 |
1973年 | 4篇 |
排序方式: 共有4666条查询结果,搜索用时 15 毫秒
21.
22.
Roles of iron in neoplasia 总被引:5,自引:0,他引:5
Eugene D. Weinberg 《Biological trace element research》1992,34(2):123-140
Research and clinical observations during the past six decades have shown that: 1. Iron promotes cancer cell growth; 2. Hosts attempt to withhold or withdraw iron from cancer cells; and 3. Iron is a factor in prevention and in therapy of neoplastic disease. Although normal and neoplastic cells have similar qualitative requirements for iron, the neoplastic cells have more flexibility in acquisition of the metal. Excessive iron levels in animals and humans are associated with enhanced neoplastic cell growth. In invaded hosts, cytokine-activated macrophages increase intracellular ferritin retention of the metal, scavenge iron in areas of tumor growth, and secrete reactive nitrogen intermediates to effect efflux of nonheme iron from tumor cells. Procedures associated with lowering host intake of excess iron can assist in prevention and in management of neoplastic disease. Chemical methods for prevention of iron assimilation by neoplastic cells are being developed in experimental and clinical protocols. The antineoplastic activity of a considerable variety of chemicals, as well as of radiation, is modulated by iron. The present article focuses on recent findings and suggests directions for further cancer-iron research. 相似文献
23.
An ecological study of the rotifer species Trochosphaera solstitialis (Thorpe, 1893) was conducted for a period of eight months in a temporary pond in eastern Texas, U.S.A. The pond was found to be environmentally stressed and contained large amounts of decomposing vegetation. Physico-chemical factors contributing to the stressed conditions were low dissolved oxygen concentrations, low pH values, high ammonia concentrations, and high color values caused by large concentrations of iron, tannin and lignin. Large concentrations of iron, tannin, and lignin seem to be highly correlated with T. solstitialis populations. Physicochemical conditions probably eliminated predators of the rotifer, such as fish. Males of T. solstitialis were observed which never left the body cavity of the female. Males probably do not feed with an apparent rudimentary digestive system. 相似文献
24.
Sperm whale apomyoglobin was reconstituted with selectively deuterated D6-2,4-diacetyldeuterohemin in which the 2H label was confined to the methyl groups of the acetyl moieties. A single resonance was observed in 2H NMR of the cyanoferrimyoglobin derivative, with a chemical shift 0.80 ppm downfield of external D12-TMS at pH 6.7. The corresponding chemical shift of D6-2,4-diacetyldeuterohemin-OMe as the cyanide complex in pyridine-water was 0.96 ppm downfield of external D12-TMS. The prominent HOD peak was well separated at 4.4 ppm downfield. The line width of the porphyrin 2H resonances in both the protein and free solvent environments yields evidence of considerable rotational freedom of the -CD3 groups about their axes. 相似文献
25.
Neda Milinkovi Milica Zekovi Margarita Dodevska Briita orevi Branimir Radosavljevi Svetlana Ignjatovi Nevena Ivanovi 《Journal of Medical Biochemistry》2022,41(3):316
BackgroundLiterature data indicate the benefit of magnesium (Mg) supplementation. The aim of this study was to examine the effect of short-term Mg supplementation on iron status in healthy female participants.MethodsOne hundred healthy female students of the University of Belgrade - Faculty of Pharmacy participated the study during eleven intervention days. Students ingested Mg preparations with the same dose of the active substance. The analysis included the measurement of serum iron, unsaturated iron binding capacity (UIBC), total iron binding capacity (TIBC), total Mg (tMg), ionized Mg (iMg), complete blood count, met-, carboxyand oxy-haemoglobin (metHgb, COHgb, O2Hgb). Transferrin concentrations and percentage of transferrin saturation (SAT) were calculated manually. The association among the analyzed biochemical parameters was examined using polynomial regression. A principal component analysis (PCA) was used for the evaluation of interdependence between the analyzed parameters.ResultsA statistically significant trend for change in O2Hgb (%) by tertiles of iMg concentrations was found (P = 0.029). Serum tMg reached significant positive correlation with the SAT at concentration levels greater than 0.9 mmol/L, after 11 days of intervention (R2=0.116). Ionized Mg in a concentration higher than 0.6 mmol/L is positively correlated with SAT and serum Fe (R2=0.214; 0.199, respectively). PCA revealed variability of 64.7% for two axes after 11 days.ConclusionsMg supplementation leads to an improvement in the certain iron status parameters even in individuals with optimal levels of these indices. However, caution should be exercised when supplementing Mg, and laboratory monitoring of the interaction is required. 相似文献
26.
27.
Metal oxyhydroxide precipitates that form from acid mine drainage (AMD) may indirectly limit periphyton by sorbing nutrients, particularly P. We examined effects of nutrient addition on periphytic algal biomass (chl a), community structure, and carbon and nitrogen content along an AMD gradient. Nutrient diffusing substrata with treatments of +P, +NP and control were placed at seven stream sites. Conductivity and SO4 concentration ranged over an order of magnitude among sites and were used to define the AMD gradient, as they best indicate mine discharge sources of metals that create oxyhydroxide precipitates. Aqueous total phosphorous (TP) ranged from 2 to 23 μg · L?1 and significantly decreased with increasing SO4. Mean chl a concentrations at sites ranged from 0.2 to 8.1 μg · cm?2. Across all sites, algal biomass was significantly higher on +NP than control treatments (Co), and significantly increased with +NP. The degree of nutrient limitation was determined by the increase in chl a concentration on +NP relative to Co (response ratio), which ranged from 0.6 to 9.7. Response to nutrient addition significantly declined with increasing aqueous TP, and significantly increased with increasing SO4. Thus, nutrient limitation of algal biomass increased with AMD impact, indicating metal oxyhydroxides associated with AMD likely decreased P availability. Algal species composition was significantly affected by site but not nutrient treatment. Percent carbon content of periphyton on the Co significantly increased with AMD impact and corresponded to an increase in the relative abundance of Chlorophytes. Changes in periphyton biomass and cellular nutrient content associated with nutrient limitation in AMD streams may affect higher trophic levels. 相似文献
28.
Leon P.M. Lamers G. Els Ten Dolle Serge T.G. Van Den Berg Sebastiaan P.J. Van Delft Jan G.M. Roelofs 《Biogeochemistry》2001,55(1):87-101
Sulphate (SO4
2-)reduction rates are generally low in freshwaterwetlands and are regulated by the scarceavailability of the ion. Increasedconcentrations of this electron acceptor due tosulphur (S) pollution of groundwater andsurface water may, however, lead to highSO4
2- reduction rates now regulatedby the availability of appropriate electrondonors. Due to variations in this availability,the response to S pollution (e.g. from surfacewater or groundwater) is expected to differbetween soils. This hypothesis was tested inlaboratory mesocosm experiments by comparingtwo wetland soil types with distinctlydifferent humus profiles: a Hydromoder and aRhizomull type. In the first type, expected tohave a higher availability of degradable soilorganic matter (SOM), SO4
2-availability appeared to be rate limiting forSO4
2- reduction. In the Rhizomullsoils, in contrast, the electron acceptor didnot limit SO4
2- reduction rates athigher concentrations. These differences inresponse could not, however, be attributed todifferences in the various SOM fractions or inSOM densities. Eutrophication and free sulphideaccumulation, two major biogeochemical problemscaused by SO4
2- pollution, occurredin both types. The absolute extent ofphosphorus mobilisation was determined by theconcentration of this element in the soil (C/Pratio), while the level of sulphideaccumulation was governed by the concentrationof dissolved iron in the pore water. It wastherefore concluded that neither the humusprofile nor the concentrations of different SOMfractions in the soils are reliable indicatorsfor the sensitivity of wetland types to Spollution. 相似文献
29.
鸡减蛋综合征病毒(EDSV—76)末端前体蛋白的基因结构分析 总被引:1,自引:0,他引:1
从中国发病鸡群中分离的鸡减蛋综合征病毒弱毒株AA-2,经常规方法提取其病毒核酸后,组建了完整的限制性内切酶PstI及HingⅢ水解片段的基因文库,并对其中HindⅢ,-SacⅠ进行了序列测定。同源比较分析证明:其L链含编码病毒末端前体蛋白,容量为580个氨基酸残基的开放读码框架。 相似文献
30.
The ectomycorrhizal fungus Cenococcum geophilum was grown in low-iron medium and the excreted siderophores were extracted, purified and analyzed by HPLC. The principal hydroxamate siderophore produced, was identified as ferricrocin as confirmed by analytical HPLC, FAB-mass spectrometry and 1H- and 13C-NMR spectra. Although the occurrence of ferricrocin has been shown earlier to occur in the ericoid mycorrhizal fungi, this is the first report of ferricrocin in a true ectomycorrhizal fungus which is taxonomically related to the ascomycetes. 相似文献