全文获取类型
收费全文 | 1004篇 |
免费 | 41篇 |
专业分类
1045篇 |
出版年
2022年 | 6篇 |
2021年 | 13篇 |
2020年 | 5篇 |
2019年 | 13篇 |
2018年 | 20篇 |
2017年 | 15篇 |
2016年 | 33篇 |
2015年 | 33篇 |
2014年 | 51篇 |
2013年 | 61篇 |
2012年 | 72篇 |
2011年 | 74篇 |
2010年 | 43篇 |
2009年 | 34篇 |
2008年 | 60篇 |
2007年 | 58篇 |
2006年 | 60篇 |
2005年 | 49篇 |
2004年 | 50篇 |
2003年 | 54篇 |
2002年 | 64篇 |
2001年 | 8篇 |
2000年 | 5篇 |
1999年 | 7篇 |
1998年 | 14篇 |
1997年 | 6篇 |
1996年 | 7篇 |
1995年 | 7篇 |
1994年 | 5篇 |
1993年 | 7篇 |
1992年 | 8篇 |
1991年 | 4篇 |
1990年 | 3篇 |
1988年 | 5篇 |
1986年 | 10篇 |
1985年 | 9篇 |
1984年 | 13篇 |
1982年 | 5篇 |
1981年 | 7篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1977年 | 3篇 |
1975年 | 4篇 |
1973年 | 5篇 |
1972年 | 3篇 |
1970年 | 2篇 |
1964年 | 2篇 |
1960年 | 2篇 |
1956年 | 2篇 |
1953年 | 2篇 |
排序方式: 共有1045条查询结果,搜索用时 15 毫秒
101.
102.
Bernhard Egger Robert Gschwentner Michael W Hess Katharina T Nimeth Zbigniew Adamski Maxime Willems Reinhard Rieger Willi Salvenmoser 《BMC developmental biology》2009,9(1):41-15
Background
Macrostomum lignano is a small free-living flatworm capable of regenerating all body parts posterior of the pharynx and anterior to the brain. We quantified the cellular composition of the caudal-most body region, the tail plate, and investigated regeneration of the tail plate in vivo and in semithin sections labeled with bromodeoxyuridine, a marker for stem cells (neoblasts) in S-phase. 相似文献103.
Aggregation of Abeta peptides is a seminal event in Alzheimer's disease. Detailed understanding of Abeta assembly would facilitate the targeting and design of fibrillogenesis inhibitors. Here comparative conformational and aggregation studies using CD spectroscopy and thioflavine T fluorescence assay are presented. As a model peptide, the 11-28 fragment of Abeta was used. This model peptide is known to contain the core region responsible for Abeta aggregation. The structural and aggregational behaviour of the peptide was compared with the properties of its variants corresponding to natural, clinically relevant mutants at positions 21-23 (A21G, E22K, E22G, E22Q and D23N). In HFIP (hexafluoro-2-propanol), a strong alpha-helix inducer, the CD spectra revealed an unexpectedly high amount of beta-sheet conformation. The aggregation process of Abeta(11-28) variants provoked by water addition to HFIP was found to be consistent with a model of an alpha-helix-containing intermediate. The aggregation propensity of all Abeta(11-28) variants was also compared and discussed. 相似文献
104.
Shinsuke Hara H. Dorota Halicka Silvia Bruno Jianping Gong Frank Traganos Zbigniew Darzynkiewicz 《Experimental cell research》1996,223(2):372
Proteolysis is an early event of apoptosis which appears to be associated with activation of the endonuclease which is responsible for internucleosomal DNA cleavage. The present study was designed to reveal the possible role of proteolysis in other early events, such as chromatin condensation, nuclear breakdown, and destabilization ofin situDNA double-stranded structure. Apoptosis of human leukemic HL-60 cells and rat thymocytes was induced by different agents, including DNA topoisomerase inhibitors, an RNA antimetabolite, and the glucocorticosteroid, prednisolone. DNA degradation was evaluated by pulsed field and conventional gel electrophoresis and by the presence ofin situDNA strand breaks. DNA stability was estimated by the measure of its sensitivityin situto denaturation. Chromatin condensation, nuclear breakdown, and other morphological changes were monitored by interference contrast and UV microscopy following cell staining with the DNA-specific fluorochrome 4′,6-diamidino-2-phenylindole. Several irreversible or reversible serine protease inhibitors prevented internucleosomal DNA degradation, nuclear breakdown, and destabilization of DNA double-stranded structure. The effective inhibitors, however, did not prevent the onset of chromatin condensation, nor the loss of the fine structural framework, nor the initial step of DNA cleavage generating DNA fragments of ≥50 kb in size. The data indicate that in both cell systems the activity of proteases sensitive to the inhibitors tested is needed for internucleosomal DNA cleavage to occur. The data also suggest that these proteases may be involved in dissolution of the nuclear envelope. Because nuclear matrix proteins and histones stabilize DNAin situ,and the decrease in DNA stability which occurs during apoptosis is precluded by the inhibitors, it is likely that serine proteases may degrade DNA stabilizing proteins. The activity of these proteases, however, appears needed neither for DNA cleavage to ≥50-kb fragments nor for the onset of chromatin condensation which is associated with dissolution of the structural framework of the nucleus. 相似文献
105.
Marieke De Craemer Mina Lateva Violeta Iotova Ellen De Decker Ma?té Verloigne Ilse De Bourdeaudhuij Odysseas Androutsos Piotr Socha Zbigniew Kulaga Luis Moreno Berthold Koletzko Yannis Manios Greet Cardon the ToyBox-study group 《PloS one》2015,10(3)
Background
The aim of the current study was to compare levels of energy balance-related behaviours (physical activity, sedentary behaviour, and dietary behaviours (more specifically water consumption, sugar-sweetened beverage consumption and unhealthy snacking)) in four- to six-year-old preschoolers from six European countries (Belgium, Bulgaria, Germany, Greece, Poland, and Spain) within the ToyBox cross-sectional study.Methods
A sample of 4,045 preschoolers (4.77 ± 0.43 years; 52.2% boys) had valid physical activity data (steps per day), parents of 8,117 preschoolers (4.78 ± 0.46 years; 53.0% boys) completed a parental questionnaire with questions on sedentary behaviours (television viewing, computer use, and quiet play), and parents of 7,244 preschoolers (4.77 ± 0.44 years; 52.0% boys) completed a food frequency questionnaire with questions on water consumption, sugar-sweetened beverage consumption and unhealthy snacking.Results
The highest levels of physical activity were found in Spain (12,669 steps/day on weekdays), while the lowest levels were found in Bulgaria and Greece (9,777 and 9,656 steps/day on weekdays, respectively). German preschoolers spent the least amount of time in television viewing (43.3 min/day on weekdays), while Greek preschoolers spent the most time in television viewing (88.5 min/day on weekdays). A considerable amount of time was spent in quiet play in all countries, with the highest levels in Poland (104.9 min/day on weekdays), and the lowest levels in Spain (60.4 min/day on weekdays). Belgian, German, and Polish preschoolers had the lowest intakes of water and the highest intakes of sugar-sweetened beverages. The intake of snacks was the highest in Belgian preschoolers (73.1 g/day) and the lowest in Greek preschoolers (53.3 g/day).Conclusions
Across six European countries, differences in preschoolers’ energy balance-related behaviours were found. Future interventions should target European preschoolers’ energy balance-related behaviours simultaneously, but should apply country-specific adaptations. 相似文献106.
Non-photochemical fluorescence quenching and the diadinoxanthin cycle in a marine diatom 总被引:7,自引:0,他引:7
Miguel Olaizola Julie La Roche Zbigniew Kolber Paul G. Falkowski 《Photosynthesis research》1994,41(2):357-370
The diadinoxanthin cycle (DD-cycle) in chromophyte algae involves the interconversion of two carotenoids, diadinoxanthin (DD) and diatoxanthin (DT). We investigated the kinetics of light-induced DD-cycling in the marine diatom Phaeodactylum tricornutum and its role in dissipating excess excitation energy in PS II. Within 15 min following an increase in irradiance, DT increased and was accompanied by a stoichiometric decrease in DD. This reaction was completely blocked by dithiothreitol (DTT). A second, time-dependent, increase in DT was detected 20 min after the light shift without a concomitant decrease in DD. DT accumulation from both processes was correlated with increases in non-photochemical quenching of chlorophyll fluorescence. Stern-Volmer analyses suggests that changes in non-photochemical quenching resulted from changes in thermal dissipation in the PS II antenna and in the reaction center. The increase in non-photochemical quenching was correlated with a small decrease in the effective absorption cross section of PS II. Model calculations suggest however that the changes in cross section are not sufficiently large to significantly reduce multiple excitation of the reaction center within the turnover time of steady-state photosynthetic electron transport at light saturation. In DTT poisoned cells, the change in non-photochemical quenching appears to result from energy dissipation in the reaction center and was associated with decreased photochemical efficiency. D1 protein degradation was slightly higher in samples poisoned with DTT than in control samples. These results suggest that while DD-cycling may dynamically alter the photosynthesis-irradiance response curve, it offers limited protection against photodamage of PS II reaction centers at irradiance levels sufficient to saturate steady-state photosynthesis.Abbreviations CAP
chloramphenicol
- D1
PS II reaction center protein
- DD
diadinoxanthin
- DD
cycle-diadinoxanthin cycle
- DT
diatoxanthin
- DTT
dithiothreitol
- FCP
fucoxanthin chlorophyll a-c protein
- Fm
maximum fluorescence yield in the dark-adapted state
- Fo
minimum fluorescence yield in the dark-adapted state
- Fm and Fo
maximum and minimum fluorescence yields respectively in some light adapted state
- Fv
maximum variable fluorescence yield in the dark-adapted state
- Ik
Irradiance at the intercept of the initial slope of the photosynthesis-irradiance curve and the maximum photosynthetic rate
- kD
first order rate constant for nonradiative de-excitation of excitions in the PS II antenna
- kd
first order rate constant for non-radiative de-excitation of excitons in the PS II reaction center
- kF
first order rate constant for fluorescence
- kT
first order rate constant for exciton transfer to the reaction center
- kt
first order rate constant for exciton transfer from the reaction center to the antenna
- Rubisco
ribulose bisphosphate carboxylase
- SVm
Stern-Volmer quenching coefficient of the maximum fluorescence yield
- SVo
Stern-Volmer quenching coefficient of the miniximum fluorescence yield
- PS II
apparent absorption cross-section of PS II
- arr
average interval between exciton arrival to the PS II reaction center (ms)
- rem
average interval between electron turnover during photosynthesis in the PS II reaction center (ms)
- d
the probability that an exciton is non-radiatively dissipated in the reaction center
- T
the probability that an exciton in the antenna is transferred to the reaction center
- t
the probability that an exciton is transferred back from the reaction center to the antenna 相似文献
107.
Capacity and electric resistance of lipid membranes composed of lecithin and cholesterol were determined. The components were chosen for the study because they were present in biological membranes. Capacitance of the lecithin and cholesterol membranes amounts to 0.38 and 0.61 microF/cm(2), and resistance to 1.44(10(4)and 2.12(10(6)Omega cm(2), respectively. A 1:1 complex appears as a result of lecithin-cholesterol membrane formation. Parameters of the membrane formed of the lecithin-cholesterol complex were determined: surface concentration (Gamma(3)), capacitance (C(3)), and conductance (R;(3)(-1), as well as the stability constant (K) of the complex. The mean values of those magnitudes are as follows: 4.265(10(-6)mol/m(2), 0.54 microF/cm(2), 1.381(10(-6)Omega(-1)cm(-2)and 3.748(10(7), respectively. 相似文献
108.
Wiesław Malinka Piotr Świątek Małgorzata Śliwińska Bogumiła Szponar Andrzej Gamian Zbigniew Karczmarzyk Andrzej Fruziński 《Bioorganic & medicinal chemistry》2013,21(17):5282-5291
In this paper we describe synthesis, structures and some physicochemical properties of 20 isothiazolopyridines 8–13 substituted differently into an isothiazole ring as well as their in vitro antibacterial assays against Mycobacterium tuberculosis H37Rv, Mycobacterium fortuitum PCM 672 and Propionibacterium acnes PCM 2400. Compound 13a was found to be the most active derivative against M. tuberculosis H37Rv, demonstrating 100% growth inhibition of microorganisms in the primary screen (minimum inhibitory concentration [MIC] 6.25 μg/mL). Nineteen of the prepared compounds were evaluated against M. fortuitum PCM 672 and P. acnes PCM 2400 and only compounds 9 and 12d exhibited excellent activity against individual strains of microorganisms with MIC90 <1 μg/mL. The inhibitory action of the remaining isothiazolopyridines towards the tested strains of the microorganism was low, absent, or a non-linear correlation prohibited accurate determination of MIC values. Unexpectedly, seven of the remaining isothiazolopyridines tested against M. fortuitum and P. acnes stimulated growth of the microorganisms in the range 10–50% or even more (10b) under experimental conditions. 相似文献
109.
110.
Rodziewicz-Motowidło S Juszczyk P Kołodziejczyk AS Sikorska E Skwierawska A Oleszczuk M Grzonka Z 《Biopolymers》2007,87(1):23-39
The beta-amyloid (Abeta) is the major peptide constituent of neuritic plaques in Alzheimer's disease (AD) and its aggregation is believed to play a central role in the pathogenesis of the disease. Naturally occurring mutations resulting in changes in the Abeta sequence (pos. 21-23) are associated with familial AD-like diseases with extensive cerebrovascular pathology. It was proved that the mutations alter the aggregation ability of Abeta and its neurotoxicity. Among five mutations at positions 21-23 there are two mutations with distinct clinical characteristics and potentially distinct pathogenic mechanism-the Italian (E22K) and the Flemish (A21G) mutations. In our studies we have examined the structures of the 11-28 fragment of the Italian and Flemish Abeta variants. The fragment was chosen because it has been shown to be the most important for amyloid fibril formation. The detailed structure of both variants Abeta(11-28) was determined using CD, 2D NMR, and molecular dynamics techniques under water-SDS micelle conditions. The NMR analysis revealed two distinct sets of proton resonances for the peptides. The studies of both peptides pointed out the existence of well-defined alpha-helical conformation in the Italian mutant, whereas the Flemish was found to be unstructured with the possibility of a bent structure in the central part of the peptide. 相似文献