全文获取类型
收费全文 | 2031篇 |
免费 | 108篇 |
专业分类
2139篇 |
出版年
2023年 | 12篇 |
2022年 | 21篇 |
2021年 | 50篇 |
2020年 | 29篇 |
2019年 | 33篇 |
2018年 | 40篇 |
2017年 | 38篇 |
2016年 | 51篇 |
2015年 | 60篇 |
2014年 | 83篇 |
2013年 | 107篇 |
2012年 | 153篇 |
2011年 | 126篇 |
2010年 | 86篇 |
2009年 | 68篇 |
2008年 | 87篇 |
2007年 | 97篇 |
2006年 | 82篇 |
2005年 | 70篇 |
2004年 | 57篇 |
2003年 | 59篇 |
2002年 | 54篇 |
2001年 | 54篇 |
2000年 | 41篇 |
1999年 | 43篇 |
1998年 | 22篇 |
1997年 | 19篇 |
1996年 | 14篇 |
1995年 | 19篇 |
1994年 | 20篇 |
1993年 | 11篇 |
1992年 | 27篇 |
1991年 | 41篇 |
1990年 | 35篇 |
1989年 | 29篇 |
1988年 | 31篇 |
1987年 | 26篇 |
1986年 | 17篇 |
1985年 | 13篇 |
1984年 | 12篇 |
1983年 | 13篇 |
1982年 | 10篇 |
1981年 | 13篇 |
1979年 | 19篇 |
1978年 | 13篇 |
1973年 | 15篇 |
1972年 | 12篇 |
1971年 | 15篇 |
1970年 | 13篇 |
1969年 | 15篇 |
排序方式: 共有2139条查询结果,搜索用时 15 毫秒
991.
Andrew Maxwell Phineas Jones Mukund R. Shukla Abhishek Chattopadhyay Jerzy Zoń Praveen K. Saxena 《In vitro cellular & developmental biology. Plant》2013,49(6):765-772
The Poaceae includes some of the most important food, fiber, and bio-fuel crops. While there have been many studies investigating the function of phenylpropanoids in this family, most of our understanding is based on correlative data rather than experimental evidence. The current study was conducted to evaluate the roles of phenylpropanoids in the growth and development of Zea mays and to develop an experimental model for further investigations. Z. mays seedlings were grown in vitro with various concentrations of the competitive phenylalanine ammonia lyase inhibitor, 2-aminoindane-2-phosphonic acid (AIP). Ferulic acid, a downstream biosynthetic product, was added to determine if it could rescue the induced phenotypes. At lower concentrations of AIP, plants exhibited elongated roots and shoots, but at higher concentrations, growth was extremely stunted. At the cellular level, the epidermal cells of roots cultured with AIP exhibited a loss of intercellular adhesion and organization, and their cell walls were more readily degraded by enzymatic digestion. These characteristics were accompanied by significant reductions in primary cell wall autofluorescence, indicating that less ferulic acid and other phenolics were incorporated in the cell wall. The majority of these symptoms could be partially or entirely rescued by ferulic acid, providing further evidence that these differences were due to the inhibition of phenylpropanoid biosynthesis. This study provides experimental evidence supporting and expanding upon hypothesized functions of phenylpropanoids in the growth and development of Z. mays and provides an experimental system for further investigations in the Poaceae and other taxonomic groups. 相似文献
992.
Pandit A Morosinotto T Reus M Holzwarth AR Bassi R de Groot HJ 《Biochimica et biophysica acta》2011,1807(4):437-443
The light-harvesting complex II (LHCII) is the main component of the antenna system of plants and green algae and plays a major role in the capture of sun light for photosynthesis. The LHCII complexes have also been proposed to play a key role in the optimization of photosynthetic efficiency through the process of state 1-state 2 transitions and are involved in down-regulation of photosynthesis under excess light by energy dissipation through non-photochemical quenching (NPQ). We present here the first solid-state magic-angle spinning (MAS) NMR data of the major light-harvesting complex (LHCII) of Chlamydomonas reinhardtii, a eukaryotic green alga. We are able to identify nuclear spin clusters of the protein and of its associated chlorophyll pigments in 13C-13C dipolar homonuclear correlation spectra on a uniformly 13C-labeled sample. In particular, we were able to resolve several chlorophyll 131 carbon resonances that are sensitive to hydrogen bonding to the 131-keto carbonyl group. The data show that 13C NMR signals of the pigments and protein sites are well resolved, thus paving the way to study possible structural reorganization processes involved in light-harvesting regulation through MAS solid-state NMR. 相似文献
993.
Restriction fragment length analysis of PCR amplified 16S rDNA with AluI revealed the presence of a 265 bp fragment in all species of Bacillus with the exception of B. cereus and B. thuringiensis, which contains two restriction sites within this fragment which results in three smaller fragments totalling to 265 bp.
Some distant species of Bacillus with no evidence of this fragment could be delineated into other genera based on phenotypic and genotypic parameters. BLAST
search for homologous sequences of individual species revealed that it is a highly conserved region. Multiple alignment of
the fragment suggests that a region between 160 and 265 bp of the 265 bp fragment was a hypervariable region and were highly
species-specific. A set of primers was designed for amplification of this hypervariable region. Partial sequencing of the
hypervariable region within the 265 bp fragment seems an index for identification of Bacillus species. 相似文献
994.
Jennings ME Quick LN Soni A Davis RR Crosby K Ott CM Nickerson CA Wilson JW 《Journal of bacteriology》2011,193(9):2208-2217
Salmonella enterica serovar Typhimurium possesses a stimulon of genes that are differentially regulated in response to conditions of low fluid shear force that increase bacterial virulence and alter other phenotypes. In this study, we show that a previously uncharacterized member of this stimulon, ydcI or STM1625, encodes a highly conserved DNA binding protein with related homologs present in a range of gram-negative bacterial genera. Gene expression analysis shows that ydcI is expressed in different bacterial genera and is involved in its autoregulation in S. Typhimurium. We demonstrate that purified YdcI protein specifically binds a DNA probe consisting of its own promoter sequence. We constructed an S. Typhimurium ΔydcI mutant strain and show that this strain is more sensitive to both organic and inorganic acid stress than is an isogenic WT strain, and this defect is complemented in trans. Moreover, our data indicate that ydcI is part of the rpoS regulon related to stress resistance. The S. Typhimurium ΔydcI mutant was able to invade cultured cells to the same degree as the WT strain, but a strain in which ydcI expression is induced invaded cells at a level 2.8 times higher than that of the WT. In addition, induction of ydcI expression in S. Typhimurium resulted in the formation of a biofilm in stationary-phase cultures. These data indicate the ydcI gene encodes a conserved DNA binding protein involved with aspects of prokaryotic biology related to stress resistance and possibly virulence. 相似文献
995.
996.
Saxena P Bhatnagar A Nishad DK Tyagi R Rana P Ali S Mittal G 《Indian journal of experimental biology》2011,49(12):895-903
Incubation of BMG-1 cells with thallium chloride (201Tl) in the range of diagnostic dose did not show a smooth uptake curve and appeared to have an unsuspected deviation in initial phase. In the present study this unexpected phenomenon was explored, using commonly used radionuclides (viz., 201Tl and 131I). Comparison was made with technetium-99m pertechnetate (99mTcO4(-)) and technetium-99m labeled methoxyisobutylisonitrile (99mTc-MIBI) that are known to show conventional 2 phase graph representing inflow and outflow segments. Serial in vitro, ex-vivo and in vivo gamma scintigraphy as well as NMR spectroscopy experiments were conducted to corroborate the results. BMG-1 cells demonstrated a four-phase uptake pattern with 201Tl as compared to a conventional biphasic pattern with 99mTc-MIBI. Flow cytometry data however did not reveal any 201Tl induced cell injury. Further, mice tissue extracts injected with 201Tl also showed a transient depression in its uptake. Scintigraphy experiments in rabbits administered with diagnostic dose of 201Tl and 131I confirmed the in vitro and ex vivo findings. Further, proton NMR spectroscopy showed decrease in the level of choline at 3 h and 24 h in 201Tl treated animals as compared to control. Phosphoethanolamine peak firstly decreased at 3 h but reached normal level at 24 h time point. No significant change was observed in the level of betaine. This transient reduction in internalization of 201Tl and 131I may represent a hitherto unknown acute effect of low dose radiation, i.e., transient depression in Na+-K+ ATPase pump activity without any apparent evidence of cell damage, representing a transient cell membrane dysfunction. The phenomenon may present a mechanistical explanation of 'thyroid stunning' at cellular level and suggest that it may be more universal in nature than suspected till now. 相似文献
997.
Pankaj Kumar Joshi Saurabh Chandra Saxena Sandeep Arora 《Acta Physiologiae Plantarum》2011,33(3):811-822
Present study characterizes the anti-oxidative defense potential of four Brassica juncea varieties, Pusa Jaikisan, Varuna, RLM-198, and CS-52, differing in their ability to withstand salinity stress. 7-day-old
seedlings raised in MS medium supplemented with 0, 50, 100, and 150 mM NaCl were used to monitor changes in the growth profile,
level of stress marker molecules, and activities of important antioxidant enzymes. Increasing NaCl concentration resulted
in a significant (P ≤ 0.05) reduction of shoot fresh and dry mass and vigor index in all the varieties tested. Maximum reduction in growth was
recorded for RLM-198 while CS-52 maintained better growth characteristics. Varuna and RLM-198 exhibited a limited increase
in superoxide dismutase, ascorbate peroxidase, and total peroxidase activity under increasing salinity. These varieties also
recorded maximum salt stress-induced damage in terms of increased lipid peroxidation, H2O2 content, and electrolyte leakage. On the other hand, CS-52 recorded maximum proline accumulation with minimum levels of H2O2, electrolyte leakage, and malondialdehyde contents. With increasing salinity stress, CS-52 recorded maximal increase in the
activity of antioxidant enzymes. However, catalase activity did not correlate with alterations in H2O2 levels under stress. Interestingly, a lower superoxide dismutase:ascorbate peroxidase ratio in CS-52 correlated with stress
tolerance trait, while a comparatively higher superoxide dismutase:ascorbate peroxidase ratio in RLM-198 marked the susceptible
nature of the variety. Our results propose that superoxide dismutase:ascorbate peroxidase ratio is the critical factor, determining
the degree of stress tolerance in Brassica juncea. 相似文献
998.
Rheological properties of binary (AgarGelA and AgarGelB) and ternary (AgarGelAB and GelABAgar) co-hydrogels of agar (polysaccharide) with gelatin A and gelatin B (proteins) were studied to investigate their differential viscoelastic behavior. Two sets of rheological experiments, isochronal temperature and isothermal frequency sweep, were performed and the storage modulii, G' was measured which could be correlated to the gel strengths. Two separate peaks at 70°C and 35°C, corresponding to melting temperatures of agar and gelatin gels respectively, were obtained when derivative of G' with respect to temperature, dG'/dT was plotted against temperature which clearly showed the presence of two separate networks of gelatin and agar interconnected to each other. The results revealed that AgarGelAB was the strongest and AgarGelA was the weakest gel among all the gels studied. In order to see the effect of gel microstructure on drug encapsulation and release behavior, a model drug salbutamol was encapsulated in various gel matrices and the release of the same was seen in phosphate buffer pH 7.4, in simulated gastric fluid pH 1.2 (SGF) and in simulated intestinal fluid pH 6.8 (SIF) media. The drug release behavior universally followed sigmoidal kinetics invariant of gel composition. It is concluded that the hydrogel microstructure influenced the release behavior and best release, in all the three media, could be found with binary gel, AgarGelB, and ternary gel, AgarGelAB. Finally, microstructure of these gels is proposed. 相似文献
999.
Chitosan scaffolds containing silicon dioxide and zirconia nano particles for bone tissue engineering 总被引:1,自引:0,他引:1
Pattnaik S Nethala S Tripathi A Saravanan S Moorthi A Selvamurugan N 《International journal of biological macromolecules》2011,49(5):1167-1172
A scaffold harboring the desired features such as biodegradation, biocompatibility, porous structure could serve as template for bone tissue engineering. In the present study, chitosan (CS), nano-scaled silicon dioxide (Si) and zirconia (Zr) were combined by freeze drying technique to fabricate a bio-composite scaffold. The bio-composite scaffold (CS/Si/Zr) was characterized by SEM, XRD and FT-IR studies. The scaffold possessed a porous nature with pore dimensions suitable for cell infiltration and colonization. The presence of zirconia in the CS/Si/Zr scaffold decreased swelling and increased biodegradation, protein adsorption and bio-mineralization properties. The CS/Si/Zr scaffold was also found to be non-toxic to rat osteoprogenitor cells. Thus, we suggest that CS/Si/Zr bio-composite scaffold is a potential candidate to be used for bone tissue engineering. 相似文献
1000.