全文获取类型
收费全文 | 929篇 |
免费 | 33篇 |
国内免费 | 38篇 |
出版年
2024年 | 1篇 |
2023年 | 13篇 |
2022年 | 9篇 |
2021年 | 15篇 |
2020年 | 26篇 |
2019年 | 28篇 |
2018年 | 27篇 |
2017年 | 28篇 |
2016年 | 24篇 |
2015年 | 22篇 |
2014年 | 26篇 |
2013年 | 93篇 |
2012年 | 16篇 |
2011年 | 13篇 |
2010年 | 16篇 |
2009年 | 18篇 |
2008年 | 24篇 |
2007年 | 21篇 |
2006年 | 39篇 |
2005年 | 36篇 |
2004年 | 48篇 |
2003年 | 39篇 |
2002年 | 33篇 |
2001年 | 40篇 |
2000年 | 29篇 |
1999年 | 37篇 |
1998年 | 22篇 |
1997年 | 26篇 |
1996年 | 25篇 |
1995年 | 12篇 |
1994年 | 20篇 |
1993年 | 18篇 |
1992年 | 22篇 |
1991年 | 8篇 |
1990年 | 20篇 |
1989年 | 7篇 |
1988年 | 21篇 |
1987年 | 21篇 |
1986年 | 14篇 |
1985年 | 7篇 |
1984年 | 3篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 8篇 |
1979年 | 7篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有1000条查询结果,搜索用时 766 毫秒
91.
The radioactive multitracer technique was applied to the simultaneous determination of the uptake of 17 trace elements (Be,
Na, Sc, V, Cr, Mn, Fe, Co, Zn, As, Se, Rb, Sr, Y, Zr, Nb, and Ru) in the liver, kidney, and blood of hypercholesterolemic
model mice. The uptakes of Be, Sc, V, Cr, Fe, As, Rb, Y, Zr, Nb, and Ru in liver increased with an increasing feeding period
of a cholesterol-rich diet, whereas the uptakes of Zn and Se decreased. Feeding of the diet resulted in a marked increase
in serum total cholesterol, triglycerides, and low-density lipoprotein cholesterol. The metabolism of trace elements between
cholesterolemic and normal mice was compared with respect to their serum cholesterol levels. A significant positive correlation
was found between the concentration of serum triglycerides and liver uptakes of Cr, Fe, and As and a negative correlation
for the uptake of Zn. A significant positive correlation was found between the concentrations of serum high- and low-density
lipoprotein cholesterols and kidney uptakes of Cr and Rb. A negative correlation was found between the uptake of Be in the
blood and the concentration of serum triglycerides. These results suggest that cholesterolemia have some specific effects
on the metabolism of some elements. 相似文献
92.
Copper plays an important role in the structure and function of metalloproteins and in the absorption of iron. The present
study deals with the effects of excessive copper intake on hematological and hemorheological parameters.
Drinking water containing 250 μg/mL copper for a period of 9 wk, Wistar albino rats showed increased erythrocyte count, blood
viscosity, and hematocrit values (p<0.05) and lower hemoglobin (p<0.05) than controls fed a normal diet. The two groups also had differences in the erythrocyte deformability index.
The results suggest that excessive copper intake results in hematological and hemorheological changes affecting both the protein
content of the erythrocyte membrane and heme synthesis. 相似文献
93.
Transport characteristics of certain inorganic elements such as copper, magnesium, selenium and iron have been studied in maternal-fetal direction in normal pregnancies, using in vitro perfusion of isolated placental lobules. Copper, selenium, magnesium and iron salts corresponding to twice physiological concentrations were injected as a 100 l bolus, into the maternal arterial perfusate. Serial perfusate samples were collected from venous outflows for a study period of 5 min. Concentrations of various inorganic elements and their transport kinetics were determined. Transport fractions of copper, selenium, magnesium and iron averaged 0.14, 0.19, 0.06 and 0.23% of maternal load respectively. The pharmacokinetic parameters such as area under the curve, clearance, elimination constant, and time for maximum response showed some significant differences between the various elements. We speculate that copper and selenium share the same transport pathway along a concentration gradient in maternal-fetal direction, while for iron and magnesium, active transport plays a predominant role for element transfer across the human placental membrane. 相似文献
94.
It has long been assumed that the extant bilaterian phyla generally have their origin in the Cambrian explosion, when they appear in an essentially modern form. Both these assumptions are questionable. A strict application of stem- and crown-group concepts to phyla shows that although the branching points of many clades may have occurred in the Early Cambrian or before, the appearance of the modern body plans was in most cases later: very few bilaterian phyla sensu stricto have demonstrable representatives in the earliest Cambrian. Given that the early branching points of major clades is an inevitable result of the geometry of clade diversification, the alleged phenomenon of phyla appearing early and remaining morphologically static is seen not to require particular explanation. Confusion in the definition of a phylum has thus led to attempts to explain (especially from a developmental perspective) a feature that is partly inevitable, partly illusory. We critically discuss models for Proterozoic diversification based on small body size, limited developmental capacity and poor preservation and cryptic habits, and show that the prospect of lineage diversification occurring early in the Proterozoic can be seen to be unlikely on grounds of both parsimony and functional morphology. Indeed, the combination of the body and trace fossil record demonstrates a progressive diversification through the end of the Proterozoic well into the Cambrian and beyond, a picture consistent with body plans being assembled during this time. Body-plan characters are likely to have been acquired monophyletically in the history of the bilaterians, and a model explaining the diversity in just one of them, the coelom, is presented. This analysis points to the requirement for a careful application of systematic methodology before explanations are sought for alleged patterns of constraint and flexibility. 相似文献
95.
Yildirim G Baştuğ M Akçil IE Fiçicilar H Koç E Zaloğlu N 《Biological trace element research》2000,78(1-3):7-12
The platelet-activating factor (PAF) is an inflammatory mediator and it may exert some of its effects by reactive oxygen species
(ROS). We investigated the effects of PAF and hyperbaric oxygenation (HBO) on copper (Cu) and zinc (Zn) levels in plasma and
the intracellular antioxidant enzyme activities of rats. PAF administration caused a decrease in erythrocyte catalase (CAT)
and glutathione peroxidase (GPx) activities and in the plasma zinc level. Following PAF administration, exposure to HBO also
caused a decrease in erythrocyte GPx activity. These results support the hypothesis that PAF may produce free oxygen radicals
and HBO enhances this effect. The enzyme activities of the antioxidant defense system were found to be affected by these oxidative
processes. This is likely to be the result of excessive production of ROS or overutilization and/or inhibition of the antioxidant
enzymes. 相似文献
96.
Carbon Dioxide Variation in a Hardwood Forest Stream: An Integrative Measure of Whole Catchment Soil Respiration 总被引:8,自引:3,他引:5
The concentration of CO2 in stream water is a product of not only instream metabolism but also upland, riparian, and groundwater processes and as
such can provide an integrative measure of whole catchment soil respiration. Using a 5-year dataset of pH, alkalinity, Ca2+, and Mg2+ in surface water of the West Fork of Walker Branch in eastern Tennessee in conjunction with a hydrological flowpath chemistry
model, we investigated how CO2 concentrations and respiration rates in stream, bedrock, and soil environments vary seasonally and interannually. Dissolved
inorganic carbon concentration was highest in summer and autumn (P < 0.05) although the proportion as free CO2 (pCO2) did not vary seasonally (P > 0.05). Over the 5 years, pCO2 was always supersaturated with respect to the atmosphere ranging from 374 to 3626 ppmv (1.0- to 10.1-fold greater than atmospheric
equilibrium), and CO2 evasion from the stream to the atmosphere ranged from 146 to 353 mmol m−2 d−1. Whereas pCO2 in surface water exhibited little intra-annual or interannual variation, distinct seasonal patterns in soil and bedrock pCO2 were revealed by the catchment CO2 model. Seasonally, soil pCO2 increased from a winter low of 8167 ppmv to a summer high of 27,068 ppmv. Driven by the seasonal variation in gas levels,
evasion of CO2 from soils to the atmosphere ranged from 83 mmol m−2 d−1 in winter to 287 mmol m−2 d−1 in summer. The seasonal variation in soil CO2 tracked soil temperature (r
2= 0.46, P < 0.001) and model-derived estimates of CO2 evasion rate from soils agreed with previously reported fluxes measured using chambers (Pearson correlation coefficient =
0.62, P < 0.05) supporting the model assumptions. Although rates of CO2 evasion were similar between the stream and soils, the overall rate of evasion from the channel was only 0.4% of the 70,752
mol/d that evaded from soils due to the vastly different areas of the two subsystems. Our model provides a means to assess
whole catchment CO2 dynamics from easily collected and measured stream-water samples and an approach to study catchment scale variation in soil
ecosystem respiration.
Received 24 July 1997; accepted 14 November 1997. 相似文献
97.
98.
Effect of EDTA and Tannic Acid on the Removal of Cd,Ni, Pb and Cu from Artificially Contaminated Soil by Althaea rosea Cavan 总被引:1,自引:0,他引:1
Seydahmet Cay Ahmet Uyanik Mehmet Soner Engin Hamdi Guray Kutbay 《International journal of phytoremediation》2015,17(6):568-574
In this study an ornamental plant of Althaea rosea Cavan was investigated for its potential use in the removal of Cd, Ni, Pb and Cu from an artificially contaminated soil. Effect of two different chelating agents on the removal has also been studied by using EDTA (ethylenediaminetetracetic acid) and TA (tannic acid). Both EDTA and TA have led to higher heavy metal concentration in shoots and leaves compared to control plants. However EDTA is generally known as an effective agent in metal solubilisation of soil, in this study, TA was found more effective to induce metal accumulation in Althaea rosea Cavan under the studied conditions. In addition to this, EDTA is toxic to some species and restraining the growth of the plants. The higher BCF (Bio Concentration Factor) and TF (Translocation Factor) values obtained from stems and leaves by the effects of the chemical enhancers (EDTA and TA) show that Althaea rosea Cavan is a hyper accumulator for the studied metals and may be cultivated to clean the contaminated soils. 相似文献
99.
Cantarelli MA Pellerano RG Del Vitto LA Marchevsky EJ Camiña JM 《Phytochemical analysis : PCA》2010,21(6):550-555
Introduction – The chemometric characterisation of two plants frequently used as food and medicinal species, Achyrocline satureioides and Achyrocline venosa (Asteraceae: Gnaphalieae), was carried out based on their mineral composition. Both species, known by the common name of ‘marcelas’, are very similar in their morphological features but they have different medicinal and food properties. Objective – To develop multivariate models for the classification of A. satureiodes and A. venosa based on their mineral content. Methodology – The analytic determinations were made by means of inductively coupled plasma optical emission spectrometry from aerial parts of the plants. An internal standard was used to evaluate the accuracy in the sample treatment and the recovery of toxic elements was studied. The multivariate methods used include principal components analysis, cluster analysis and linear discriminant analysis. Results – Classification for both A. satureioides and A. venosa was successful in all cases using only four variables: aluminium, iron, magnesium and sulphur content. The concentrations of the following elements were determined: Al, As, Ba, Ca, Cd, Co, Cr, Cu, Fe, K, La, Mg, Mn, Mo, Na, Ni, P, Pb, S, Sr, Ti, V, Y and Zn. Conclusions – This method is useful to identify both species in raw material in order to detect eventual errors of selection. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
100.