全文获取类型
收费全文 | 1369篇 |
免费 | 113篇 |
国内免费 | 25篇 |
专业分类
1507篇 |
出版年
2023年 | 17篇 |
2022年 | 32篇 |
2021年 | 42篇 |
2020年 | 57篇 |
2019年 | 72篇 |
2018年 | 50篇 |
2017年 | 58篇 |
2016年 | 68篇 |
2015年 | 57篇 |
2014年 | 53篇 |
2013年 | 125篇 |
2012年 | 56篇 |
2011年 | 48篇 |
2010年 | 36篇 |
2009年 | 53篇 |
2008年 | 46篇 |
2007年 | 67篇 |
2006年 | 53篇 |
2005年 | 47篇 |
2004年 | 44篇 |
2003年 | 38篇 |
2002年 | 35篇 |
2001年 | 26篇 |
2000年 | 18篇 |
1999年 | 24篇 |
1998年 | 30篇 |
1997年 | 31篇 |
1996年 | 17篇 |
1995年 | 20篇 |
1994年 | 19篇 |
1993年 | 21篇 |
1992年 | 18篇 |
1991年 | 11篇 |
1990年 | 10篇 |
1989年 | 6篇 |
1988年 | 7篇 |
1987年 | 6篇 |
1986年 | 13篇 |
1985年 | 14篇 |
1984年 | 6篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1981年 | 4篇 |
1980年 | 10篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1976年 | 3篇 |
1972年 | 3篇 |
1971年 | 2篇 |
1970年 | 2篇 |
排序方式: 共有1507条查询结果,搜索用时 15 毫秒
71.
Arianne M. Neigh Matthew J. Zwiernik Alan L. Blankenship Patrick W. Bradley Denise P. Kay Monica A. MacCarroll 《人类与生态风险评估》2006,12(5):924-946
Dietary exposures of passerine birds at the Kalamazoo River, Michigan, were examined due to the presence of polychlorinated biphenyls (PCBs) in the terrestrial and aquatic food webs. Average potential daily doses in diets were 6- to 29-fold and 16- to 35-fold greater at a contaminated location than at a reference location for PCB exposures quantified as total PCBs and 2,3,7,8–tetrachlorodibenzo-p-dioxin equivalents (TEQs), respectively. Birds with diets comprised of primarily aquatic insects had greater dietary exposure than birds with diets of primarily terrestrial insects. Risk associated with dietary exposure varied with the selection of the threshold for effects including hazard quotients, which exceeded 1 in instances where the most conservative toxicity reference values were utilized. Risk based on concentrations of PCBs in the tissues indicated little risk to avian species, and co-located studies evaluating reproductive health did not suggest that observed incidences of diminished reproductive success were related to PCB exposure. Measures of risk based on comparison to toxicity reference values (TRVs) were consistent with direct measures of ecologically relevant endpoints of reproductive fitness, but uncertainty exists in the selection of threshold values for effects in these species especially based on TEQs. This is largely due to the absence of species-specific, dose-response relationships. Therefore, the best estimate of risk is through the application of multiple lines of evidence. 相似文献
72.
An ethnobotanical survey on the medicinal plant species marketed in Quanzhou, southern Fujian, was conducted in order to document traditional medicinal knowledge and application of medicinal plants in dietary. In addition to literature review, data was obtained using ethnobotanical tools and methods, including personal interviews during field trips, collecting herbarium specimens, and species identification by cross referencing. The results showed that: 1) There are 148 wild medicinal species, belonging to 56 plant families and 117 genera present in Quanzhou, southern Fujian. Among them, 46 species are edible wild herbs, 37 species are medicinal plants, 38 species are used as tea substituting plants or herbal tea,22 species are wild fruits, 4 species are used as spices and 1 species is used to make bean jelly; 2) The plant families which comprise a major part of the pool are Compositae, Lamiaceae, Rosaceae, Leguminosae, Rubiaceae, Liliaceae, Orchidaceae and Moraceae; 3) This study explored the utilization of the medicinal plants in Quanzhou area, and proposed methods to inherit the culture of using wild plants in dietary, as well as methods to conserve the edible medicinal wild plant resources. 相似文献
73.
Phospholipase D (PLD) is emerging as a major player in many novel signaling pathways. Based on recent studies correlating membrane composition with enzyme function, we speculated that feeding of dietary lipids to the newborns has a major impact on brain PLD activity. To test this hypothesis, the rat dams were fed fat-free powder containing either safflower oil or fish oil, and a control powdered chow. The pups were weaned onto the diet and sacrificed at 30 days of age. PLD activity was measured by transphosphatidylation assays using rat brain membranes. This study shows that microsome GTPS-dependent PLD activity in rats fed safflower oil or fish oil was significantly reduced by 38% and 30% respectively compared to controls. Oleate-dependent PLD activity in the safflower oil group, however, was significantly increased by 38%. In contrast, synaptosome membrane (P2) GTPS-dependent PLD activity in rats consuming safflower oil was significantly increased by 29%, but there was no difference in oleate-dependent PLD activity. Likewise, no difference was observed in microsome oleate-dependent PLD and P2 GTPS-dependent PLD activity between the fish oil and the control groups. These results indicate that dietary lipid intake appears to modulate phospholipid metabolism and differential expression of PLD isozymes in the brain. 相似文献
74.
The effect of dietary fibres on constituents of complex carbohydrates in various tissues of streptozotocin induced diabetic rats is presented by analysing different constituents of complex carbohydrates in presence and absence of dietary fibre. Wheat bran was effective in preventing the decrease (14%) in total sugars in spleen and an increase in total sugars in stomach (33%) during diabetes. Decrease in uronic acid content during diabetes in spleen was prevented to the extent of 25% by the presence of wheat bran in the diet. The other parameters which were affected by the presence of wheat bran in the diet during diabetes are amino sugar (brain and stomach), sulphates (liver) and protein (lungs and stomach). Guar gum was effective in preventing the decrease in total sugar content in spleen by 28% and sulphate content in liver by 14% during diabetes. Variation in protein content in lungs was observed in diabetes. The results indicated beneficial role of dietary fibres like wheat bran and guar gum on complex carbohydrates to varying extents in different tissues. 相似文献
75.
Nitrogen availability influences the biochemical composition and photosynthesis of tank-cultivated Ulva rigida (Chlorophyta) 总被引:5,自引:0,他引:5
Juan Luis Gómez Pinchetti Elena del Campo Fernández Paula Moreno Díez Guillermo García Reina 《Journal of applied phycology》1998,10(4):383-389
Physiological and biochemical changes in relation to inorganic nitrogen availability were studied for tank-cultivated Ulva
rigida grown under nitrogen- enriched and nitrogen-depleted seawater. U. rigida was initially cultivated in nitrogen-enriched
seawater (daily concentrations of NH4+ and NO3- + NO2- ranged between 0.5–1.7 and 0.06–0.15 mg L-1, respectively), then transferred
to nitrogen-depleted seawater where photosynthetic capacity decreased to zero after 23 d. At the time (14 d) when photosynthetic
rates were lower than 2.0 μmol O2 g-1 FW min-1 and strong bleaching had occurred, some algae were returned to the initial
nitrogen-enriched seawater to study recovery from N-limited growth. Data on biochemical composition (chlorophylls, ash, caloric
content, fatty acids and dietary fibres) and colouration varied significantly depending on the nitrogen conditions. C:N ratios
correlated significantly with biochemical parameters. Fatty acid (FA) synthesis continued during the N-starvation period;
saturated and mono-unsaturated FA increased to a maximun of 72.2%, while poly-unsaturated fatty acids (PUFA) decreased to
27.7%. During the N-enriched recovery period, the reverse was found. C:N ratios above 10 correlated with carbohydrate synthesis
as shown by the dietary fibre level. Under nitrogen enriched conditions, C:N ratios decreased along with a decrease in fibre
level. Under controlled conditions, nitrogen represents a major influence on the development of intensive tank cultivation
of Ulva rigida, not only by affecting parameters closely related to nitrogen metabolism but also some clearly influenced by
carbon uptake.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
76.
The justification for providing dietary guidance for the nutritional intake of boron 总被引:1,自引:0,他引:1
Nielsen FH 《Biological trace element research》1998,66(1-3):319-330
Because a biochemical function has not been defined for boron (B), its nutritional essentiality has not been firmly established.
Nonetheless, dietary guidance should be formulated for B, because it has demonstrated beneficial, if not essential, effects
in both animals and humans. Intakes of B commonly found with diets abundant in fruits, vegetables, legumes, pulses, and nuts
have effects construed to be beneficial in macromineral, energy, nitrogen, and reactive oxygen metabolism, in addition to
enhancing the response to estrogen therapy and improving psychomotor skills and cognitive processes of attention and memory.
Perhaps the best-documented beneficial effect of B is on calcium (Ca) metabolism or utilization, and thus, bone calcification
and maintenance. The paradigm emerging for the provision of dietary guidance that includes consideration of the total health
effects of a nutrient, not just the prevention of a deficiency disease, has resulted in dietary guidance for chromium (Cr)
and fluoride; both of these elements have beneficial effects in humans, but neither has a defined biochemical function. Knowledge
of B nutritional effects in humans equals or is superior to that of Cr and fluoride; thus, establishing a dietary reference
intake for B is justified. An analysis of both human and animal data suggests that an acceptable safe range of population
mean intakes of B for adults could well be 1–13 mg/d. Recent findings indicate that a significant number of people do not
consistently consume more than 1 mg B/d; this suggests that B could be a practical nutritional or clinical concern. 相似文献
77.
Knowledge of daily boron (B) intakes will assist researchers in establishing B requirements and elucidating the metabolic
role of B in humans. B concentrations in commonly consumed foods were utilized to approximate the B intake of selected US
populations. Triplicate food samples were digested at low temperatures with 16M HNO3 and 30% H2O2, and analyzed by inductively coupled plasma spectroscopy (ICP). Milk and dairy products, juices, and beverages were the largest
contributors to dietary B. Total B content of average daily diets ranged from 548 μg for toddlers (2 yr old) to 883 μg for
mature males (60–65 yr old), excluding tap water contributions. Toddlers consumed 3.7 times more B than mature males when
adjusted for body weight (body wt) and consumed the diet with the highest boron density (1.8 μg/kJ or 0.43 μg/kcal). Adolescent
females consumed a diet with the lowest B density (1.12 μg/kJ or 0.26 μg/kcal). Food B concentrations applied to diet records
(1020 μg/d) and ICP analysis of the corresponding food composites (1170 μg/d) were comparable (p < 0.05). Current estimations of B in US diets are consistent with reports in the literature stating that normal adult daily
B intakes are approx 1 mg.
Presented at the symposium “Second International Symposium on the Health Effects of Boron and Its Compounds” held in Irvine,
CA, on October 23, 1997. This symposium was presented by the University of California, Irvine. The publication of the symposium
was supported by U.S. Borax Inc. Guest editors for this symposium were B. Dwight Culver (University of California, Irvine,
CA) and James R. Coughlin (Coughlin and Associates, Newport Coast, CA).
US Department of Agriculture, Agricultural Research Service, Northern Plains Area is an equal opportunity/affirmative action
employer, and all agency services are available without discrimination. 相似文献
78.
79.
80.
Matthew J. Ravosa Rachel A. Menegaz Jeremiah E. Scott David J. Daegling Kevin R. McAbee 《Biological reviews of the Cambridge Philosophical Society》2016,91(4):883-898
Experimental analyses directly inform how an anatomical feature or complex functions during an organism's lifetime, which serves to increase the efficacy of comparative studies of living and fossil taxa. In the mammalian skull, food material properties and feeding behaviour have a pronounced influence on the development of the masticatory apparatus. Diet‐related variation in loading magnitude and frequency induce a cascade of changes at the gross, tissue, cellular, protein and genetic levels, with such modelling and remodelling maintaining the integrity of oral structures vis‐à‐vis routine masticatory stresses. Ongoing integrative research using rabbit and rat models of long‐term masticatory plasticity offers unique insight into the limitations of functional interpretations of fossilised remains. Given the general restriction of the palaeontological record to bony elements, we argue that failure to account for the disparity in the hierarchical network of responses of hard versus soft tissues may overestimate the magnitude of the adaptive divergence that is inferred from phenotypic differences. Second, we note that the developmental onset and duration of a loading stimulus associated with a given feeding behaviour can impart large effects on patterns of intraspecific variation that can mirror differences observed among taxa. Indeed, plasticity data are relevant to understanding evolutionary transformations because rabbits raised on different diets exhibit levels of morphological disparity comparable to those found between closely related primate species that vary in diet. Lastly, pronounced variation in joint form, and even joint function, can also characterise adult conspecifics that differ solely in age. In sum, our analyses emphasise the importance of a multi‐site and hierarchical approach to understanding determinants of morphological variation, one which incorporates critical data on performance. 相似文献