全文获取类型
收费全文 | 50944篇 |
免费 | 23680篇 |
国内免费 | 12530篇 |
专业分类
87154篇 |
出版年
2025年 | 720篇 |
2024年 | 1819篇 |
2023年 | 1944篇 |
2022年 | 2997篇 |
2021年 | 3319篇 |
2020年 | 3113篇 |
2019年 | 3444篇 |
2018年 | 2509篇 |
2017年 | 2202篇 |
2016年 | 2465篇 |
2015年 | 3371篇 |
2014年 | 4612篇 |
2013年 | 4045篇 |
2012年 | 5614篇 |
2011年 | 5501篇 |
2010年 | 4297篇 |
2009年 | 4397篇 |
2008年 | 4653篇 |
2007年 | 4389篇 |
2006年 | 4002篇 |
2005年 | 3410篇 |
2004年 | 2610篇 |
2003年 | 2254篇 |
2002年 | 1956篇 |
2001年 | 1773篇 |
2000年 | 1680篇 |
1999年 | 1037篇 |
1998年 | 523篇 |
1997年 | 372篇 |
1996年 | 284篇 |
1995年 | 271篇 |
1994年 | 201篇 |
1993年 | 156篇 |
1992年 | 166篇 |
1991年 | 165篇 |
1990年 | 117篇 |
1989年 | 111篇 |
1988年 | 93篇 |
1987年 | 81篇 |
1986年 | 75篇 |
1985年 | 115篇 |
1984年 | 70篇 |
1983年 | 41篇 |
1982年 | 90篇 |
1981年 | 36篇 |
1980年 | 10篇 |
1978年 | 4篇 |
1959年 | 3篇 |
1954年 | 3篇 |
1950年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Based on nonlinear phenomena of biophoton emission observed in the past, an interference model concerning with the mechanism of interaction between living organisms and electromagnetic fields was raised. Caused by biological nonlinearly polarizable double layer, destructive interference of incoming and reflected waves establishes in the outside. As a consequence, in the inside constructive interference takes place at the same time. The interference patterns may play an important role in biological self organization and in biological functions. We investigate the boundary conditions necessary for explaining these non-linear optical effects in terms of the phase conjugation. It turns out that there are solutions of the Maxwell equations which satisfy destructive interference of biophotons in agreement with the experimental results. Necessary provisions are nonlinearly polarizable optically active double layers of distances which are small compared to the wavelength of light. In addition, they have to be able to move into the nodal planes of the impinging waves within a small time interval compared to the coherence time. These conditions are likely fulfilled in the optically dense, but ordered and optically excited, highly polarizable living matter. 相似文献
2.
HE Xiaolin LIU Xinqi ZENG Zonghao LI Hongmin WANG Miao ZHANG Ying WANG Dacheng 《中国科学C辑(英文版)》2000,43(1):39-46
BmK M4 is a neutral neurotoxin in the BmK toxin series.It is medially toxic and belongs to group III α-toxins.The purified sample was crystallized in rhombic space group P61.Using an X-ray diffraction technique,the crystal structure of BmK M4 was revealed by molecular replacement at 0.20 nm resolution.The model was refined.The final crystallographic R factor was 0.142 and the free R factor was 0.173.The root mean square deviation is 0.001 5 nm for the bond length and 1.753°for the bond angles.64 water molecules were added to the asymmetric unit.The refined structure showed an unusual non-prolyl cis peptide bond at residue 10.The structure was compared with group II α-toxin BmK M8 (an acidic,weak toxin).The potential structural implications of the cis peptide bond were discussed. 相似文献
3.
The selenium-containing phycocyanin from the selenium-rich algae (Spirulina platensis) has been crystallized in two crystal forms by the hanging-drop vapor diffusion techniques. A chromatographic procedure of gel filtration and anion exchange was used for purification. Form I crystal with space group P21 and cell parameters a =108.0 ?, b= 117.0 ?, c = 184.0?, β= 90.2° and 12(αβ) units in the asymmetric unit was obtained by using (NH4)2SO4 as precipitant. These crystals diffract up to 2.8 ?. Form II crystal obtained by using PEG4000 as precipitant belongs to space group P63 with unit cell constants a = 155.0 ?, c = 40.3 ?, γ =120.0° and one(αβ) unit in the asymmetric unit. The crystals diffract beyond 2.9 ?. The possible stacking forms of phycocyanin molecules in the first crystal form were discussed. 相似文献
4.
This paper gives a quantitative analysis on the non-CO2 emissions related to energy demand, energy activities and land use change of six scenarios with different development pattern in 2030 and 2050 based on IPAC emission model. The various mitigation technologies and policies are assessed to understand the corresponding non-CO2 emission reduction effect. The research shows that the future non-CO2 emissions of China will grow along with increasing energy demand, in which thermal power and transportation will be the major emission and mitigation sectors. During the cause of future social and economic development, the control and mitigation of non-CO2 emissions is a problem as challenging and pressing as that of CO2 emissions.This study indicates that the energy efficiency improvement, renewable energy, advanced nuclear power generation, fuel cell, coal-fired combined cycle, clean coal and motor vehicle emission control technologies will contribute to non-CO2 emissions control and mitigation. 相似文献
5.
Xiuyan Ren Fang Liu Zhilong Bao Chunling Zhang Xiaojing Wu Lei Chen Ruoxue Liu Hansong Dong 《Plant Molecular Biology Reporter》2008,26(3):225-240
HrpNEa is a harpin protein from Erwinia amylovora, a bacterial pathogen that causes fire blight in rosaceous plants. Treating plants with HrpNEa stimulates ethylene and abscisic acid (ABA) to induce plant growth and drought tolerance, respectively. Herein, we report
that both growth hormones cooperate to mediate the role of HrpNEa in promoting root growth of Arabidopsis thaliana seedlings. Root growth is promoted coordinately with elevation in levels of ABA and ethylene subsequent to soaking of germinating
seeds of wild-type (WT) Arabidopsis in a solution of HrpNEa. However, these responses are arrested by inhibiting WT roots from synthesizing ethylene as well as sensing of ABA and ethylene.
The effects of HrpNEa on roots are also nullified in ethylene-insensitive etr1-1 and ein5-1 mutants and in the ABA-insensitive mutant abi2-1 of Arabidopsis. These results provide evidence for presence of a relationship between root growth enhancement and signaling by ABA and ethylene
in response to HrpNEa. Nevertheless, when HrpNEa is applied to leaves, ethylene signaling is active in the absence of ABA signaling to promote plant growth. This suggests
the presence of a different signaling mechanism in leaves from that in roots.
X. Ren and F. Liu contributed equally to this study and are regarded as joint first authors 相似文献
6.
Hong F Zhou J Liu C Yang F Wu C Zheng L Yang P 《Biological trace element research》2005,105(1-3):269-279
The effects of nano-TiO2 (rutile) on the photochemical reaction of chloroplasts of spinach were studied. The results showed that when spinach was
treated with 0.25% nano-TiO2, the Hill reaction, such as the reduction rate of FeCy, and the rate of evolution oxygen of chloroplasts was accelerated
and noncyclic photophosphorylation (nc-PSP) activity of chloroplasts was higher than cyclic photophosphorylation (c-PSP) activity,
the chloroplast coupling was improved and activities of Mg2+-ATPase and chloroplast coupling factor I (CF1)-ATPase on the thylakoid membranes were obviously activated. It suggested that photosynthesis promoted by nano-TiO2 might be related to activation of photochemical reaction of chloroplasts of spinach. 相似文献
7.
8.
Lei Peng-Cheng Takashi Yoshiike Hitoshi Yaguchi Hideoki Ogawa MD PhD 《Mycopathologia》1993,122(2):89-93
Defense mechanisms againstSporothrix schenckii were studied using mouse models. After an intracutaneous injection of the yeast form ofS. schenckii to the dorsal skin of the congenitally athymic nude and normal heterozygote littermate mice, nodules were formed. They regressed and disappeared in 10 weeks in the case of normal mice. On the other hand, nodules and then ulceration developed progressively in nude mice until all animals expired by dissemination of microorganisms at the 11th week of inoculation. Histopathologically the migrated cells were similar in both the normal and the nude mice, particularly during the early phase (within 24 h), with infiltration by PMNs being predominant. Fragmentation ofS. schenckii commenced early during the 12–24 h stage of inoculation in the normal mice, while such fragmentation was scarce in nude mice even though numerous PMNs accumulated. Microscopic observations in the early stages (within 24 h of inoculation) suggested that the lack of killing activity by PMNs in nude mice contributes more to the impaired defense than the lack of macrophage activation by T-cells. 相似文献
9.
以DCC为催化剂,采用低温化学催化法合成生物素化白藜芦醇.产物采用Resourse RPC反相色谱柱分离、纯化,并且运用液质联用进行分析验证.MTT法测定产物体外对Hep G2、MCF-7肿瘤细胞的抑制作用.实验结果显示:通过HPLC-MS、紫外扫描分析鉴定证实,分离得到的产物确实是生物素化白藜芦醇,其保留了原有的体外抗肿瘤活性.此种化学合成方法反应条件温和,容易控制,操作方便.合成得到的白藜芦醇的生物素化衍生物,为下一步探讨白藜芦醇的结合蛋白提供简单有效的分子工具. 相似文献
10.
Mitochondrial Ca2+ has been considered a trigger for the release of cytochrome c, which is a critical and early event in the induction of cell apoptosis, although the molecular mechanism underlying this effect is still not fully understood. Here we investigate the interaction between cytochrome c and cardiolipin and the effect of Ca2+ on this interaction using electrochemical methods. Experimental results revealed that modification of cardiolipin onto the surface of a pyrolytic graphite electrode could lead to a rapid direct electron transfer of cytochrome c through the electrostatic interaction between the protein and the cardiolipin. Addition of Ca2+ to the test solution containing cytochrome c could cause the decrease of the redox peaks of the protein, and the peaks could be recovered when Ca2+ was chelated by ethylenediaminetetraacetate. The cardiolipin-cytochrome c interaction and the Ca2+ effect were also investigated with the variation of the charges of lipids, buffer solutions, reaction time, and valencies of cations for comparison. 相似文献