全文获取类型
收费全文 | 822篇 |
免费 | 49篇 |
出版年
2023年 | 6篇 |
2022年 | 10篇 |
2021年 | 26篇 |
2020年 | 19篇 |
2019年 | 17篇 |
2018年 | 27篇 |
2017年 | 35篇 |
2016年 | 37篇 |
2015年 | 42篇 |
2014年 | 44篇 |
2013年 | 58篇 |
2012年 | 73篇 |
2011年 | 60篇 |
2010年 | 41篇 |
2009年 | 31篇 |
2008年 | 44篇 |
2007年 | 46篇 |
2006年 | 37篇 |
2005年 | 31篇 |
2004年 | 28篇 |
2003年 | 26篇 |
2002年 | 19篇 |
2001年 | 18篇 |
2000年 | 11篇 |
1999年 | 7篇 |
1998年 | 12篇 |
1997年 | 2篇 |
1996年 | 3篇 |
1995年 | 7篇 |
1994年 | 5篇 |
1993年 | 7篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1982年 | 1篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1978年 | 2篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1973年 | 2篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1970年 | 1篇 |
1969年 | 1篇 |
1967年 | 1篇 |
1938年 | 1篇 |
排序方式: 共有871条查询结果,搜索用时 406 毫秒
1.
2.
Three protocols for the determination of inorganic and organic sulfur fractions were tested for their suitability to estimate total indigenous organic sulfur (Sorg) and35Sorg formed from added35SO4
2– in sediments of chemically dilute lakes in the ELA. The protocols tested have all been reported in the literature. It was found that two protocols involving sequential analyses for S fractions following acid treatment gave estimates of both Sorg and35Sorg up to 87% lower than a non-sequential protocol. The low estimates were largely due to hydrolysis and solubilization of solid phase S which was then removed in a rinsing step. The non-sequential protocol, in which total reduced inorganic sulfur and total sulfur were determined on separate aliquots, is recommended as the most reliable of the three. Individual analyses in this protocol were verified for these lake sediments using a variety of S standards. 相似文献
3.
Application of rRNA-based probes for observing marine nanoplanktonic protists. 总被引:11,自引:5,他引:6 下载免费PDF全文
The use of small-subunit rRNA-based oligonucleotides as probes for detecting marine nanoplanktonic protists was examined with a ciliate (an Uronema sp.), a flagellate (a Cafeteria sp.), and mixed assemblages of protists from enrichment cultures and natural seawater samples. Flow cytometry and epifluorescence microscopy analyses demonstrated that hybridizations employing fluorescein-labeled, eukaryote-specific probes intensely stained logarithmically growing protists, whereas these same protist strains in late stationary growth were barely detectable. The fluorescence intensity due to probe binding was significantly enhanced by the use of probes end labeled with biotin, which were detected by fluorescein-labeled avidin. The degree of signal amplification ranged from two- to fivefold for cultured protists in both logarithmic and stationary growth phases. Mixed assemblages of heterotrophic protists from enrichment cultures were also intensely labeled by rRNA-targeted oligonucleotide probes by the biotin-avidin detection system. Protists in late stationary growth phase and natural assemblages of protists that were otherwise undetectable when hybridized with fluorescein-labeled probes were easily visualized by this approach. In the latter samples, hybridization with multiple, biotin-labeled probes was necessary for detection of naturally occurring marine protists by epifluorescence microscopy. The signal amplification obtained with the biotin-avidin system should increase the utility of rRNA-targeted probes for identifying protists and facilitate characterization of the population structure and distribution of protists in aquatic environments. 相似文献
4.
J. Carlos Roseiro Francisco M. Gírio Alida Kará M.T. Amaral Collaço 《Journal of applied microbiology》1993,75(4):381-386
By reducing the concentration of nitrogen (from 5.0 to 2.5 mmol 1-1 ), batch cultures of Xanthomonas campestris induced the enzyme UDP-glucose dehydrogenase and stimulated the Entner-Doudoroff pathway enzyme glucose-6-P dehydrogenase. The surplus energy generation was directed to xanthan biosynthesis resulting in a 10% polysaccharide increase. The nitrogen restriction led to a higher consumption of nitrogen (93%) whereas glucose consumption did not surpass 75% utilization. Low concentrations of both magnesium and sulphur exerted a negative effect on xanthan formation. Both restrictions reduced the phosphomannose isomerase enzyme activity by 10-fold turning the mannose transference presumably into the rate-limiting step for xanthan biosynthesis. Conversely, the rate of synthesis of glucuronic acid residues did not affect the rate of xanthan biosynthesis. Polysaccharide synthesis in magnesium and sulphur cultures was negatively affected in comparison with cell formation as the cell volumetric production rate increased from 0.037 to 0.091 g 1-1 h-1 and the xanthan volumetric production rate dropped from 0.133 g 1-1 h-1 to the minimum obtained at 0.083 g 1-1 h-1 . The efficiency of the carbon substrate conversion was also greatly changed. 相似文献
5.
Increased ability to predict protein structures is moving research focus towards understanding protein dynamics. A promising approach is to represent protein dynamics through networks and take advantage of well-developed methods from network science. Most studies build protein dynamics networks from correlation measures, an approach that only works under very specific conditions, instead of the more robust inverse approach. Thus, we apply the inverse approach to the dynamics of protein dihedral angles, a system of internal coordinates, to avoid structural alignment. Using the well-characterized adhesion protein, FimH, we show that our method identifies networks that are physically interpretable, robust, and relevant to the allosteric pathway sites. We further use our approach to detect dynamical differences, despite structural similarity, for Siglec-8 in the immune system, and the SARS-CoV-2 spike protein. Our study demonstrates that using the inverse approach to extract a network from protein dynamics yields important biophysical insights. 相似文献
6.
Variation in heat shock proteins within tropical and desert species of poeciliid fishes 总被引:8,自引:0,他引:8
Norris CE; diIorio PJ; Schultz RJ; Hightower LE 《Molecular biology and evolution》1995,12(6):1048-1062
The 70-kilodalton heat shock protein (hsp70) family of molecular
chaperones, which contains both stress-inducible and normally abundant
constitutive members, is highly conserved across distantly related taxa.
Analysis of this protein family in individuals from an outbred population
of tropical topminnows, Poeciliopsis gracilis, showed that while
constitutive hsp70 family members showed no variation in protein isoforms,
inducibly synthesized hsp70 was polymorphic. Several species of
Poeciliopsis adapted to desert environments exhibited lower levels of
inducible hsp70 polymorphism than the tropical species, but constitutive
forms were identical to those in P. gracilis, as they were in the
confamilial species Gambusia affinis. These differences suggest that
inducible and constitutive members of this family are under different
evolutionary constraints and may indicate differences in their function
within the cell. Also, northern desert species of Poeciliopsis synthesize a
subset of the inducible hsp70 isoforms seen in tropical species. This
distribution supports the theory that ancestral tropical fish migrated
northward and colonized desert streams; the subsequent decrease in
variation of inducible hsp70 may have been due to genetic drift or a
consequence of adaptation to the desert environment. Higher levels of
variability were found when the 30- kilodalton heat shock protein (hsp30)
family was analyzed within different strains of two desert species of
Poeciliopsis and also in wild-caught individuals of Gambusia affinis. In
both cases the distribution of hsp30 isoform diversity was similar to that
seen previously with allozyme polymorphisms.
相似文献
7.
L E Hollister D J Jenden J R Amaral J D Barchas K L Davis P A Berger 《Life sciences》1978,23(1):17-22
Plasma choline levels were measured in patients being treated with choline chloride for movement disorders. Following single doses of 5 g given orally in aqueous solution, plasma concentrations rose to a peak within four hours and then rapidly declined. The degree of increase was variable both between and within patients. During chronic treatment, plasma choline concentrations tended to rise as the dose increased, although the relationship was not strong. The highest concentrations attained by patients were always at a dose of 16 or 20 g daily. Following chronic treatment, the disappearance of choline from plasma was rapid, with most patients reaching baseline by four days. Choline chloride is generally given in four divided doses, which seems reasonable in the early stages of treatment. Most therapeutic effect is seen when patients are treated with daily doses in the 12 to 20 g range, doses likely to produce substantial increases in plasma choline concentration. However, the relationship of plasma choline concentration to clinical efficacy may be tenuous. Following discontinuation of treatment, clinical improvement tends to persist long after plasma choline has returned to baseline concentrations. 相似文献
8.
A Colotto P Mariani M G Ponzi Bossi F Rustichelli G Albertini L Q Amaral 《Biochimica et biophysica acta》1992,1107(1):165-174
The ternary system constituted by distearoylphosphatidylcholine, pindolol (a vasodilator drug) and water has been investigated by using X-ray diffraction and calorimetric techniques. The structural modifications induced by the drug have been determined and a possible interaction model has been derived. In particular, the pindolol content-temperature dependent phase diagram shows the occurrence of two new phases: the first is an interdigitated gel, and the second is a lamellar structure presenting an unusual mixed disordered-ordered conformation of the hydrocarbon chains (L alpha beta). The comparative analysis of electron density profiles relative to the L alpha beta phase, reveals significant modifications in the paraffinic region of the lipid layer. In agreement with thermodynamic results, the structural data suggest that the drug induces a stiffening and a tightening of the hydrocarbon chains. Moreover, the hydrophilic properties of the membrane (particularly in P beta, and L alpha beta phases) present an evident dependence with the drug concentration. 相似文献
9.
Control of cell volume in the J774 macrophage by microtubule disassembly and cyclic AMP 总被引:8,自引:5,他引:3 下载免费PDF全文
We have explored the possibilities that cell volume is regulated by the status of microtubule assembly and cyclic AMP metabolism and may be coordinated with shape change. Treatment of J774.2 mouse macrophages with colchicine caused rapid microtubule disassembly and was associated with a striking increase (from 15-20 to more than 90 percent) in the proportion of cells with a large protuberance at one pole. This provided a simple experimental system in which shape changes occurred in virtually an entire cell population in suspension. Parallel changes in cell volume could then be quantified by isotope dilution techniques. We found that the shape change caused by colchicine was accompanied by a decrease in cell volume of approximately 20 percent. Nocodozole, but not lumicolchicine, caused identical changes in both cell shape and cell volume. The volume loss was not due to cell lysis nor to inhibition of pinocytosis. The mechanism of volume loss was also examined. Colchicine induced a small but reproducible increase in activity of the ouabain-sensitive Na(+), K(+)-dependent ATPase. However, inhibition of this enzyme/transport system by ouabain did not change cell volume nor did it block the colchicines-induced decrease in volume. One the other hand, SITS (4’acetamido, 4-isothiocyano 2,2’ disulfonic acid stilbene), an inhibitor of anion transport, inhibited the effects of colchicines, thus suggesting a role for an anion transport system in cell volume regulation. Because colchicine is known to activate adenylate cyclase in several systems and because cell shape changes are often induced by hormones that elevate cyclic AMP, we also examined the effects of cyclic AMP on cell volume. Agents that act to increase syclic AMP (cholera toxin, which activates adenylate cyclase; IBMX, and inhibitor of phosphodiesterase; and dibutyryl cyclic AMP) all caused a volume decrease comparable to that of colchicine. To define the effective metabolic pathway, we studied two mutants of J774.2, one deficient in adenylate cyclase and the other exhibiting markedly reduced activity of cyclic AMP-dependent protein kinase. Cholera toxin did not produce a volume change in either mutant. Cyclic AMP produced a decrease in the cyclase-deficient line comparable to that in wild type, but did not cause a volume change in the kinase- deficient line. This analysis established separate roles for cyclic AMP and colchicine. The volume decrease induced by cyclic AMP requires the action of a cyclic AMP-dependent protein kinase. Colchicine, on the other hand, induced a comparable volume change in both mutants and wild type, and thus does not require the kinase. 相似文献