全文获取类型
收费全文 | 3302篇 |
免费 | 195篇 |
出版年
2022年 | 5篇 |
2021年 | 42篇 |
2020年 | 25篇 |
2019年 | 24篇 |
2018年 | 47篇 |
2017年 | 47篇 |
2016年 | 58篇 |
2015年 | 107篇 |
2014年 | 132篇 |
2013年 | 200篇 |
2012年 | 204篇 |
2011年 | 227篇 |
2010年 | 147篇 |
2009年 | 145篇 |
2008年 | 224篇 |
2007年 | 228篇 |
2006年 | 191篇 |
2005年 | 180篇 |
2004年 | 190篇 |
2003年 | 193篇 |
2002年 | 207篇 |
2001年 | 56篇 |
2000年 | 51篇 |
1999年 | 56篇 |
1998年 | 41篇 |
1997年 | 31篇 |
1996年 | 24篇 |
1995年 | 31篇 |
1994年 | 27篇 |
1993年 | 41篇 |
1992年 | 22篇 |
1991年 | 35篇 |
1990年 | 26篇 |
1989年 | 25篇 |
1988年 | 19篇 |
1987年 | 23篇 |
1986年 | 17篇 |
1985年 | 19篇 |
1984年 | 22篇 |
1983年 | 12篇 |
1982年 | 15篇 |
1981年 | 6篇 |
1980年 | 16篇 |
1979年 | 8篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1975年 | 9篇 |
1974年 | 5篇 |
1973年 | 4篇 |
排序方式: 共有3497条查询结果,搜索用时 15 毫秒
81.
The unicellular red alga Cyanidium caldarium is tolerant to high levels of various metal ions. Cells of this alga cultured with divalent metal ions at 5 mM contained an elevated concentration of each metal, with the highest level for Zn followed by Mn > Ni > Cu. This order is in fair agreement with the toxicity levels reported previously, with the exception of Mn, which shows a toxicity level comparable to that of Ni. Transmission electron microscopy indicated the presence of electron-dense bodies in the algal cells, and elemental analysis by energy dispersive X-ray spectrometry showed high levels of Fe and P in these bodies. Accumulation of Zn was found in these particles in Zn-treated algal cells, whereas no such deposition was found for Cu, Ni, or Mn in cells treated with the respective metals. Although trapping of Zn in the intracellular bodies may contribute to reduction of metal activity in the cells, this effect can be overcome by high intracellular levels of Zn that result in a high degree of toxicity. The correlation between intracellular concentration and toxic levels of metal ions implies that the reduced incorporation of the metals is a major detoxification mechanism in this alga. 相似文献
82.
Effects of an experimental drought on soil emissions of carbon dioxide, methane, nitrous oxide, and nitric oxide in a moist tropical forest 总被引:8,自引:0,他引:8
Changes in precipitation in the Amazon Basin resulting from regional deforestation, global warming, and El Niño events may affect emissions of carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and nitric oxide (NO) from soils. Changes in soil emissions of radiatively important gases could have feedback implications for regional and global climates. Here we report results of a large‐scale (1 ha) throughfall exclusion experiment conducted in a mature evergreen forest near Santarém, Brazil. The exclusion manipulation lowered annual N2O emissions by >40% and increased rates of consumption of atmospheric CH4 by a factor of >4. No treatment effect has yet been detected for NO and CO2 fluxes. The responses of these microbial processes after three rainy seasons of the exclusion treatment are characteristic of a direct effect of soil aeration on denitrification, methanogenesis, and methanotrophy. An anticipated second phase response, in which drought‐induced plant mortality is followed by increased mineralization of C and N substrates from dead fine roots and by increased foraging of termites on dead coarse roots, has not yet been detected. Analyses of depth profiles of N2O and CO2 concentrations with a diffusivity model revealed that the top 25 cm soil is the site of most of the wet season production of N2O, whereas significant CO2 production occurs down to 100 cm in both seasons, and small production of CO2 occurs to at least 1100 cm depth. The diffusivity‐based estimates of CO2 production as a function of depth were strongly correlated with fine root biomass, indicating that trends in belowground C allocation may be inferred from monitoring and modeling profiles of H2O and CO2. 相似文献
83.
Emi Yoneyama Yoko Ishimoto-Negishi Yuzou Sano Ryo Funada Mitsuhiro Yamada Teruko Nakamura 《Biological Sciences in Space》2004,18(1):3-6
When the four-week-old woody stem of Prunus jamasakura was grown under simulated microgravity condition on a three-dimensional clinostat, it bent at growth, and width of its secondary xylem decreased due to the reduction of fiber cell numbers and a smaller microfibril angle in the secondary cell wall, as reported in our previous paper. Gravity induces the development of the secondary xylem that supports the stem upward against the action of gravity. In this study, morphological changes of the tissues and cells were microscopically observed. Disorder was found in the concentric structure of tissues that organize the stem. The radial arrangement of the cells was also disturbed in the secondary xylem, and in the secondary phloem secondary cell walls of the bast fiber cells were undeveloped. These findings suggest that differentiation and development of the secondary xylem and the bast fiber cells are strongly controlled by terrestrial gravity. These tissue and cells functions to support the stem under the action of gravity. Furthermore, clinorotation induced disorder in the straight joint of vessel elements and the lattice-like structure of radial parenchyma cells, which is responsible for water transportation and storage, respectively. Gravity is an essential factor for keeping the division and differentiation normal in woody stem. 相似文献
84.
85.
Eri Hosaka Yoshinao Soma Yoko Kawa Hiroko Kaminaga Kayoko Osumi Shiho Ooka Hidenori Watabe Masaru Ito Fumiko Murakami Masako Mizoguchi 《Pigment cell & melanoma research》2004,17(2):150-157
To evaluate the etiologic role of ultraviolet (UV) radiation in acquired dermal melanocytosis (ADM), we investigated the effects of UVA and UVB irradiation on the development and differentiation of melanocytes in primary cultures of mouse neural crest cells (NCC) by counting the numbers of cells positive for KIT (the receptor for stem cell factor) and for the L ‐3,4‐dihydroxyphenylalanine (DOPA) oxidase reaction. No significant differences were found in the number of KIT‐ or DOPA‐positive cells between the UV‐irradiated cultures and the non‐irradiated cultures. We then examined the effects of UV light on KIT‐positive cell lines derived from mouse NCC cultures. Irradiation with UVA but not with UVB inhibited the tyrosinase activity in a tyrosinase‐positive cell line (NCCmelan5). Tyrosinase activity in the cells was markedly enhanced by treatment with α‐melanocyte‐stimulating hormone (α‐MSH), but that stimulation was inhibited by UVA or by UVB irradiation. Irradiation with UVA or UVB did not induce tyrosinase activity in a tyrosinase‐negative cell line (NCCmelb4). Levels of KIT expression in NCCmelan5 cells and in NCCmelb4 cells were significantly decreased after UV irradiation. Phosphorylation levels of extracellular signal‐regulated kinase 1/2 in cells stimulated with stem cell factor were also diminished after UV irradiation. These results suggest that UV irradiation does not stimulate but rather suppresses mouse NCC. Thus if UV irradiation is a causative factor for ADM lesions, it would not act directly on dermal melanocytes but may act in indirect manners, for instance, via the overproduction of melanogenic cytokines such as α‐MSH and/or endothelin‐1. 相似文献
86.
Tai Kaneshima Kazuki Toeda Takane Fujimori Makoto Nishizawa 《Bioscience, biotechnology, and biochemistry》2017,81(8):1461-1465
Various antimicrobial constituents of camu-camu fruit were isolated. Acylphloroglucinol (compound 1) and rhodomyrtone (compound 2) were isolated from the peel of camu-camu (Myrciaria dubia) fruit, while two other acylphloroglucinols (compounds 3 and 4) were obtained from camu-camu seeds. The structures of the isolated compounds were characterized by spectrophotometric methods. Compounds 1 and 4 were confirmed to be new acylphloroglucinols with different substituents at the C7 or C9 position of 2, and were named myrciarone A and B, respectively. Compound 3 was determined to be isomyrtucommulone B. This is the first report of the isolation of 3 from a natural resource. The antimicrobial activities of compounds 1, 3, and 4 were similar to those of 2, and the minimum inhibitory concentrations were either similar to or lower than that of kanamycin. These results suggest that the peel and seeds of camu-camu fruit could be utilized for therapeutic applications. 相似文献
87.
Naoki Nishizawa Yoko Kanematsu-Yamaki Masaaki Funata Hiroaki Nagai Ayako Shimizu Hisashi Fujita Junichi Sakamoto Shiro Takekawa Taiji Asami 《Bioorganic & medicinal chemistry letters》2017,27(20):4626-4629
Neuromedin U (NMU) mediates various physiological functions via NMUR1 and NMUR2 receptors. NMUR2 has been considered a promising treatment option for diabetes and obesity. Although NMU-8, a shorter peptide, has potent agonist activity for both receptors, it is metabolically unstable. Therefore, NMU-8 analogs modified with long-chain alkyl moieties via a linker were synthesized. An octadecanoyl analog (17) with amino acid substitutions [αMePhe19, Nle21, and Arg(Me)24] and a linker [Tra-γGlu-PEG(2)] dramatically increased NMUR2 selectivity, with retention of high agonist activity. Subcutaneous administration of 17 induced anorectic activity in C57BL/6J mice. Owing to its high metabolic stability, 17 would be useful in clarifying the physiological role and therapeutic application of NMU. 相似文献
88.
Naoki Nishizawa Ayumu Niida Yusuke Adachi Yasushi Masuda Satoshi Kumano Kotaro Yokoyama Tomoko Asakawa Hideki Hirabayashi Nobuyuki Amano Shiro Takekawa Tetsuya Ohtaki Taiji Asami 《Bioorganic & medicinal chemistry letters》2017,27(16):3829-3832
The gastrointestinal peptide, peptide YY3–36 (PYY3–36) and its shorter peptide analogues have been reported to reduce appetite by activating the neuropeptide Y2 receptor (Y2R), which is associated with obesity and other metabolic diseases. A 14-amino acid PYY analogue, Ac-[d-Pro24,Cha27,28,36,Aib31]PYY(23–36) (3), showed high binding affinity and agonist activity for the Y2R, similar to that of PYY3–36, but had weak anorectic activity upon continuous administration in lean mice. Three amino acid substitutions [Pya(4)26, Aib28, Lys30], which contributed to the decreased hydrophobicity of 3, efficiently increased its anorectic activity. The compound containing these three amino acids, Ac-[d-Pro24,Pya(4)26,Cha27,36,Aib28,31,Lys30]PYY(23–36) (22), exerted more potent and durable food intake suppression than that by PYY3–36 in lean mice, as well as excellent Y2R agonist activity (EC50: 0.20 nM) and good subcutaneous bioavailability (66.6%). The 11-day continuous administration of 22 at 1 mg/kg/day successfully produced antiobese and antidiabetic effects, with more than 20% body weight loss in obese and Type 2 diabetes ob/ob model mice. 相似文献
89.
Dongsu Choi Yoko Watanabe Robert D. Guy Tetsuto Sugai Hiroto Toda Takayoshi Koike 《Acta Physiologiae Plantarum》2017,39(3):71
Key message
The black locust is adapted to elevated [CO 2 ] through changes in nitrogen allocation characteristics in leaves.Abstract
The black locust (Robinia pseudoacacia L.) is an invasive woody legume within Japan. This prolific species has a high photosynthetic rate and growth rate, and undergoes symbiosis with N2-fixing micro-organisms. To determine the effect of elevated CO2 concentration [CO2] on its photosynthetic characteristics, we studied the chlorophyll (Chl) and leaf nitrogen (N) content, and the leaf structure and N allocation patterns in the leaves and acetylene reduction activity after four growing seasons, in R. pseudoacacia. Our specimens were grown at ambient [CO2] (370 μmol mol?1) and at elevated [CO2] (500 μmol mol?1), using a free air CO2 enrichment (FACE) system. Net photosynthetic rate at growth [CO2] (A growth) and acetylene reduction activity were significantly higher, but maximum carboxylation rate of RuBisCo (V cmax), maximum rate of electron transport driving RUBP regeneration (J max), net photosynthetic rate under enhanced CO2 concentration and light saturation (A max), the N concentration in leaf, and in leaf mass per unit area (LMA) and ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCo) content were significantly lower grown at elevated [CO2] than at ambient [CO2]. We also found that RuBisCo/N were less at elevated [CO2], whereas Chl/N increased significantly. Allocation characteristics from N in leaves to photosynthetic proteins, NL (Light-harvesting complex: LHC, photosystem I and II: PSI and PSII) and other proteins also changed. When R. pseudoacacia was grown at elevated [CO2], the N allocation to RuBisCo (NR) decreased to a greater extent but NL and N remaining increased relative to specimens grown at ambient [CO2]. We suggest that N remobilization from RuBisCo is more efficient than from proteins of electron transport (NE), and from NL. These physiological responses of the black locust are significant as being an adaptation strategy to global environmental changes.90.
Yoko Marutani Yasuo Yamauchi Mari Higashiyama Akihito Miyoshi Seiji Akimoto Kanako Inoue Ken-ichi Ikeda Masaharu Mizutani Yukihiro Sugimoto 《Photosynthesis research》2017,131(1):41-50
Light and temperature affect state transitions through changes in the plastoquinone (PQ) redox state in photosynthetic organisms. We demonstrated that light and/or heat treatment induced preferential photosystem (PS) I excitation by binding light-harvesting complex II (LHCII) proteins. The photosystem of wheat was in state 1 after dark overnight treatment, wherein PQ was oxidized and most of LHCII was not bound to PSI. At the onset of the light treatment [25 °C in the light (100 µmol photons m?2 s?1)], two major LHCIIs, Lhcb1 and Lhcb2 were phosphorylated, and the PSI–LHCII supercomplex formed within 5 min, which coincided with an increase in the PQ oxidation rate. Heat treatment at 40 °C of light-adapted wheat led to further LHCII protein phosphorylation of, resultant cyclic electron flow promotion, which was accompanied by ultrafast excitation of PSI and structural changes of thylakoid membranes, thereby protecting PSII from heat damage. These results suggest that LHCIIs are required for the functionality of wheat plant PSI, as it keeps PQ oxidized by regulating photochemical electron flow, thereby helping acclimation to environmental changes. 相似文献