全文获取类型
收费全文 | 72篇 |
免费 | 7篇 |
出版年
2023年 | 1篇 |
2022年 | 1篇 |
2021年 | 1篇 |
2019年 | 2篇 |
2018年 | 5篇 |
2017年 | 2篇 |
2016年 | 6篇 |
2015年 | 1篇 |
2014年 | 2篇 |
2013年 | 2篇 |
2012年 | 1篇 |
2011年 | 1篇 |
2010年 | 1篇 |
2009年 | 1篇 |
2008年 | 1篇 |
2007年 | 3篇 |
2006年 | 3篇 |
2005年 | 3篇 |
2004年 | 1篇 |
2003年 | 2篇 |
2002年 | 1篇 |
2001年 | 1篇 |
2000年 | 2篇 |
1998年 | 1篇 |
1997年 | 1篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1989年 | 5篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1980年 | 1篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1973年 | 3篇 |
1972年 | 1篇 |
1969年 | 1篇 |
1968年 | 2篇 |
1967年 | 2篇 |
1966年 | 1篇 |
排序方式: 共有79条查询结果,搜索用时 15 毫秒
51.
Steric repulsion between phosphatidylcholine bilayers 总被引:12,自引:0,他引:12
The change in pressure needed to bring egg phosphatidylcholine bilayers into contact from their equilibrium separation in excess water has been determined as a function of both distance between the bilayers and water content. A distinct upward break in the pressure-distance relation appears at an interbilayer separation of about 5 A, whereas no such deviation is present in the pressure-water content relation. Thus, this break is not a property of the dehydration process per se, but instead is attributed to steric repulsion between the mobile lipid head groups that extend 2-3 A into the fluid space between bilayers. That is, electron density profiles of these bilayers indicate that the observed break in the pressure-spacing relation occurs at a bilayer separation where extended head groups from apposing bilayers come into steric hindrance. The pressure-spacing data are used to separate steric pressure from the repulsive hydration pressure, as well as to quantitate the range and magnitude of the steric interaction. An appreciable fraction of the measured steric energy can be ascribed to a decrease in configurational entropy due to restricted head-group motion as adjacent bilayers come together. 相似文献
52.
Lili Li Waleed Abu Al-Soud Lasse Bergmark Leise Riber Lars H. Hansen Jakob Magid Søren J. Sørensen 《Current microbiology》2013,67(4):423-430
Less than 1 % of bacterial populations present in environmental samples are culturable, meaning that cultivation will lead to an underestimation of total cell counts and total diversity. However, it is less clear whether this is also true for specific well-defined groups of bacteria for which selective culture media is available. In this study, we use culture dependent and independent techniques to describe whether isolation of Pseudomonas spp. on selective nutrient-poor NAA 1:100 agar-medium can reflect the full diversity, found by pyrosequencing, of the total soil Pseudomonas community in an urban waste field trial experiment. Approximately 3,600 bacterial colonies were isolated using nutrient-poor NAA 1:100 medium from soils treated with different fertilizers; (i) high N-level sewage sludge (SA), (ii) high N-level cattle manure (CMA), and (iii) unfertilized control soil (U). Based on Pseudomonas specific quantitative-PCR and Pseudomonas CFU counts, less than 4 % of Pseudomonas spp. were culturable using NAA 1:100 medium. The Pseudomonas selectivity and specificity of the culture medium were evaluated by 454 pyrosequencing of 16S rRNA gene amplicons generated using Bacteria- and Pseudomonas-specific primers. Pyrosequencing results showed that most isolates were Pseudomonas and that the culturable fraction of Pseudomonas spp. reflects most clusters of the total Pseudomonas diversity in soil. This indicates that NAA 1:100 medium is highly selective for Pseudomonas species, and reveals the ability of NAA 1:100 medium to culture mostly the dominant Pseudomonas species in soil. 相似文献
53.
Thomas Larsen Antonie Gorissen Paul Henning Krogh Marc Ventura Jakob Magid 《Plant and Soil》2007,295(1-2):253-264
It has been demonstrated that plant roots can take up small amounts of low-molecular weight (LMW) compounds from the surrounding
soil. Root uptake of LMW compounds have been investigated by applying isotopically labelled sugars or amino acids but not
labelled organic matter. We tested whether wheat roots took up LMW compounds released from dual-labelled (13C and 15N) green manure by analysing for excess 13C in roots. To estimate the fraction of green manure C that potentially was available for root uptake, excess 13C and 15N in the primary decomposers was estimated by analysing soil dwelling Collembola that primarily feed on fungi or microfauna.
The experimental setup consisted of soil microcosm with wheat and dual-labelled green manure additions. Plant growth, plant
N and recoveries of 13C and 15N in soil, roots, shoots and Collembola were measured at 27, 56 and 84 days. We found a small (<1%) but significant uptake
of green manure derived 13C in roots at the first but not the two last samplings. About 50% of green manure C was not recovered from the soil-plant
system at 27 days and additional 8% was not recovered at 84 days. Up to 23% of C in collembolans derived from the green manure
at 56 days (the 27 days sampling was lost). Using a linear mixing model we estimated that roots or root effluxes provided
the main C source for collembolans (54−79%). We conclude that there is no solid support for claiming that roots assimilated
green manure derived C due to very small or no recoveries of excess 13C in wheat roots. During the incubation the pool of green manure derived C available for root uptake decreased due to decomposition.
However, the isotopic composition in Collembola indicated that there was a considerable fraction of green manure derived C
in the decomposer system at 56 days thus supporting the premise that LMW compounds containing C from the green manure was
released throughout the incubation.
Responsible Editor: A. C. Borstlap. 相似文献
54.
Seasonal variation in organic and inorganic phosphorus fractions of temperate-climate sandy soils 总被引:3,自引:0,他引:3
Soils from an arable plot, a grassland plot and pasture plot were sampled over an 18-month period. Inorganic (Pi) and organic (Po) soil phosphorus fractions were extracted sequentially with resin, NaHCO3, and NaOH. Soil solution was sampled on the arable plot and pasture plot during 12 months with teflon suction cups, and the contents of Pi and Po were determined.The patterns of the variation for all soil fractions were similar for the three plots. All soil Pi fractions were at minimum in the cool moist winter period. The soil Po fractions varied less systematically than Pi fractions. The sum of Po fractions had a winter maximum and a spring minimum. For all soil P fractions temporal variation was highly significant (p<0.0001). The magnitude of change in Pi and Po soil fractions was 4–40 times greater than what would be expected from the magnitude of new N mineralization.The content of P in the inorganic soil P fractions was negatively correlated with soil moisture. The variation in organic soil P could not be explained by any single factor, but it is suggested that the variation is caused by changes in solubility rather than by biological transformations. Thus, physicochemical processes masked the impact of biological transformations on the temporal variation of soil phosphorus fractions.Both soil solution Pi and Po varied significantly with time on field scale. In contrast to soil Pi fractions, solution Pi was initially low in the early autumn, increased by a factor 4 during the following 6 weeks, and thereafter decreased to a low level by the end of the sampling period. Soil solution Po had several fluctuations during the sampling period. 相似文献
55.
The inheritance of red blood cell levels of carbonic anhydrase isozymes (CA I and CA II) has been studied in different carbonic anhydrase I genotypes of the pig-tailed macaque, Macaca nemestrina. Quantitation of CA I isozymes in a series of animals indicates that the total CA I concentration is the sum of the average effects of each CA I structural allele and that the average effects are independent of the various allelic combinations. The relative average effects were 0.32:0.95:1.0 for the CA I
a, CA Ib, and CA I
c structural genes, respectively. It is also demonstrated that the level of CA II is related to the CA I genotypes. Multiple regression analysis demonstrated that each dose of CA I-deficiency gene present decreased the CA II concentration by approximately 30%, with this decrease in CA II level being solely related to the dose of CA I-deficiency gene and not to the level of CA I. The CA I-deficient animals produce CA I products that are similar to the common CA Ia, CA Ib, CA Ic electrophoretic types. Limited mating data indicate that the CA I components in CA I-deficient animals are inherited codominantly.Supported by U.S. Public Health Service Research Grant GM-15419.This report is a portion of a dissertation submitted to the University of Michigan in partial fulfillment of the requirements for the Doctor of Philosophy degree.U.S. Public Health Service Predoctoral Trainee (GM-71-14). 相似文献
56.
- The respiratory behaviour and patterns of oxygen consumption of three Nile species have been investigated.
- Tilapia nilotica showed a typical pattern of oxygen consumption with an ambient region, adaptive plateau and lethal region (Fig. 2).
- Specimens of Polypterus senegalus and Clarias lazera (body weights 20–30 and 30–45 g respectively) showed patterns of consumption comparable to that of Tilapia (Fig. 3a and 4a). In larger specimens of the two species the adaptive plateau was either insignificant or completely absent.
- Specimens of Polypterus and Clarias (20–30 g and 30–45 g respectively) could survive in waters saturated with oxygen (7.4 mg/l) but their tolerance to lower oxygen concentrations was limited. Larger specimens of Polypterus and Clarias failed to survive in oxygen saturated waters.
- The tolerance of Tilapia nilotica to extremely low oxygen concentration is an adaptation of a tropical and completely aquatic species. Polypterus and Clarias resort to their compensatory mechanisms only when the aquatic respiratory surface fails to satisfy their oxygen requirements.
57.
Cholesterol modifies the short-range repulsive interactions between phosphatidylcholine membranes 总被引:11,自引:0,他引:11
Pressure versus distance relationships have been obtained for egg phosphatidylcholine bilayers containing a range of cholesterol concentrations. Water was removed from between adjacent bilayers by the application of osmotic pressures in the range of 0.4-2600 atm (4 x 10(5)-2.6 x 10(9) dyn/cm2), and the distance between adjacent bilayers was obtained by Fourier analysis of X-ray diffraction data. For applied pressures up to about 50 atm and bilayer surface separations of 15-5 A, the incorporation of up to equimolar cholesterol has little influence on plots of pressure versus bilayer separation. However, for the higher applied pressures, cholesterol reduces the interbilayer separation distance by an amount that depends on the cholesterol concentration in the bilayer. For example, the incorporation of equimolar cholesterol reduces the distance between bilayers by as much as 6 A at an applied pressure of 2600 atm. At this applied pressure, electron density profiles show that the high-density head-group peaks from apposing bilayers have merged. This indicates that equimolar concentrations of cholesterol spread the lipid molecules apart in the plane of the bilayer enough to allow the phosphatidylcholine head groups from apposing bilayers to interpenetrate as the bilayers are squeezed together. All of these X-ray and pressure-distance data indicate that, by reducing the volume fraction of phospholipid head groups, cholesterol markedly reduces the steric repulsion between apposing bilayers but has a much smaller effect on the sum of the longer ranged repulsive hydration and fluctuation pressures. Increasing concentrations of cholesterol monotonically increase the dipole potential of egg phosphatidylcholine monolayers, from 415 mV with no cholesterol to 493 mV with equimolar cholesterol.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
58.
The effect of oxidative stress on endothelium-dependent and nitric oxide donor-induced relaxation: implications for nitrate tolerance. 总被引:2,自引:0,他引:2
Increased inactivation of nitric oxide (NO) by superoxide has been implicated in nitrate tolerance. Here, we set out to compare the inhibitory effect of superoxide on endothelium-dependent, acetylcholine (ACh)-mediated vascular relaxation with that on the endothelium-independent effects of glyceryl trinitrate (GTN) and another NO donor drug, S-nitrosoglutathione (GSNO). Rings of thoracic aorta from adult male Wistar rats (350-450 g) were precontracted with phenylephrine (approximately EC(90)) prior to cumulative additions (10 nM/L-10 microM/L) of GTN, GSNO, or ACh. Rings were then treated with the superoxide generator pyrogallol (300 micromol/L) alone or following pretreatment with the Cu/Zn superoxide dismutase inhibitor diethyldithiocarbamate (DETCA; 100 micromol/L), and cumulative additions of the vasodilators were repeated. All experiments were conducted in the presence of catalase (3000 U/ml) to prevent accumulation of hydrogen peroxide. Relaxation to ACh was abolished by pyrogallol-derived superoxide. Relaxation to GSNO was significantly inhibited by superoxide (P < 0.05, n = 8) and was more pronounced at lower GSNO concentrations. However, GTN was relatively resistant to inhibition by superoxide with modest inhibition only occurring in rings pretreated with DETCA prior to pyrogallol (P < 0.05; n = 8). In contrast to GSNO, the inhibitory effect was more pronounced with high concentrations of GTN, suggesting that the mechanism underlying superoxide-mediated inhibition is different for the two NO donor drugs. Further experiments showed that vascular responses to ACh were not inhibited (P > 0.05, n = 6) in aortic rings made tolerant to GTN (10 micromol/L, 2-h incubation) and that treatment of vessels with the antioxidant vitamin C (1 mmol/L) successfully prevented the development of tolerance. Taken together, these results suggest that superoxide is not a major factor in tolerance in vitro and imply that the protective actions of vitamin C are unrelated to its antioxidant activity in this setting. 相似文献
59.
Aisha A.K. Al-Ashmawy Fatma A. Ragab Khaled M. Elokely Manal M. Anwar Oscar Perez-Leal Mario C. Rico John Gordon Eugeney Bichenkov George Mateo Emad M.M. Kassem Gehan H. Hegazy Magid Abou-Gharbia Wayne Childers 《Bioorganic & medicinal chemistry letters》2017,27(14):3117-3122
PI3Kα/mTOR ATP-competitive inhibitors are considered as one of the promising molecularly targeted cancer therapeutics. Based on lead compound A from the literature, two similar series of 2-substituted-4-morpholino-pyrido[3,2-d]pyrimidine and pyrido[2,3-d]pyrimidine analogs were designed and synthesized as PI3Kα/mTOR dual inhibitors. Interestingly, most of the series gave excellent inhibition for both enzymes with IC50 values ranging from single to double digit nM. Unlike many PI3Kα/mTOR dual inhibitors, our compounds displayed selectivity for PI3Kα. Based on its potent enzyme inhibitory activity, selectivity for PI3Kα and good therapeutic index in 2D cell culture viability assays, compound 4h was chosen to be evaluated in 3D culture for its IC50 against MCF7 breast cancer cells as well as for docking studies with both enzymes. 相似文献
60.
Chemical investigation of the methanol extract of the leaves of Caledonian Guioa crenulata led to the isolation and characterisation of four farnesyl diglycosides, crenulatosides A, B, C and D, along with three known flavonol glycosides and one known trimeric proanthocyanidin possessing a doubly linked structure. The structures of these compounds were determined on the basis of spectroscopic studies and chemical evidence. The ethanol and ethyl acetate extracts of the leaves exhibited no cytotoxic activity and no inhibition of acetylcholinesterase. 相似文献