首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1819篇
  免费   172篇
  国内免费   62篇
  2053篇
  2024年   7篇
  2023年   34篇
  2022年   51篇
  2021年   62篇
  2020年   81篇
  2019年   97篇
  2018年   88篇
  2017年   69篇
  2016年   82篇
  2015年   82篇
  2014年   105篇
  2013年   183篇
  2012年   81篇
  2011年   61篇
  2010年   49篇
  2009年   84篇
  2008年   82篇
  2007年   86篇
  2006年   72篇
  2005年   65篇
  2004年   54篇
  2003年   48篇
  2002年   54篇
  2001年   41篇
  2000年   29篇
  1999年   28篇
  1998年   31篇
  1997年   24篇
  1996年   24篇
  1995年   25篇
  1994年   24篇
  1993年   27篇
  1992年   13篇
  1991年   13篇
  1990年   14篇
  1989年   5篇
  1988年   7篇
  1987年   7篇
  1986年   5篇
  1985年   11篇
  1984年   10篇
  1983年   7篇
  1982年   7篇
  1981年   4篇
  1980年   6篇
  1979年   5篇
  1978年   2篇
  1976年   2篇
  1975年   2篇
  1973年   2篇
排序方式: 共有2053条查询结果,搜索用时 0 毫秒
51.
52.
Delayed fluorescence from Rhodopseudomonas viridis membrane fragments has been studied using a phosphoroscope employing single, short actinic flashes, under conditions of controlled redox potential and temperature. The emission spectrum shows that delayed fluorescence is emitted by the bulk, antenna bacteriochlorophyll. The energy for delayed fluorescence, however, must be stored in a reaction-center complex including the photooxidized form (P+) of the primary electron-donor (P) and the photoreduced form (X?) of the primary electron-acceptor. This is shown by the following observations: (1) Delayed luminescence is quenched (a) at low redox potentials which allow cytochromes to reduce P+ rapidly after the flash, (b) at higher redox potentials which, by oxidizing P chemically, prevent the photochemical formation of P+X?, and (c) upon transfer of an electron from X? to a secondary acceptor, Y. (2) Under conditions that prevent the reduction of P+ by cytochromes and the oxidation of X? by Y, the decay kinetics of delayed fluorescence are identical with those of P+X?, as measured from optical absorbance changes.The main decay route for P+X? under these conditions has a rate-constant of approximately 103 s?1. In contrast, a comparison of the intensities of delayed and prompt fluorescence indicates that the process in which P+X? returns energy to the bulk bacteriochlorophyll has a rate-constant of 3.7 s?1, at 295 °K and pH 7.8. The decay kinetics of P+X? and delayed fluorescence change little with temperature, whereas the intensity of delayed fluorescence increases with increasing temperature, having an activation energy of 12.5 kcal · mol?1. We conclude that the main decay route involves tunneling of an electron from X? to P+, without the promotion of P to an excited state. Delayed fluorescence requires such a promotion, followed by transfer of energy to the bulk bacteriochlorophyll, and this combination of events is rare. The activation energy, taken with potentiometric data, indicates that the photochemical conversion of PX to P+X? results in increases of both the energy and the entropy of the system, by 16.6 kcal · mol?1 and 8.8 cal · mol?1 · deg?1. The intensity of delayed fluorescence depends strongly on the pH; the origin of this effect remains unclear.  相似文献   
53.
54.
55.
We investigated the effect of adding an alkaline material (containing calcium carbonate and gypsum) on the immobilization of heavy metals (Cd, Cu, Pb, and Zn) in a paddy soil slightly contaminated with Cd and Zn under flooded and non-flooded conditions in the laboratory. Adding the alkaline material increased the soil pH and significantly decreased the exchangeable fraction of all of the metals, especially for Cd (>75% decrease) and Zn (ca. 90% decrease), under both flooded and non-flooded conditions. Drying the flooded soil samples increased the ratio of exchangeable fraction to the total fraction, particularly for Cd. The exchangeable fraction ratio was lower in the dried, previously flooded samples that contained the alkaline material than in the samples that did not contain the alkaline material, indicating that adding the alkaline material would be an effective way of immobilizing heavy metals during the oxidation of anoxic soils. These results show that the alkaline material can be used to immobilize heavy metals under both anoxic and oxic conditions, and that the effects of flooding and amending a paddy soil with alkaline material on the chemical forms will be different between heavy metals.  相似文献   
56.
Selenium (Se) is an essential micronutrient, and low Se intake in Se‐deficient areas plays roles in an endemic osteochondropathy characterized by chondronecrosis in growth plate and articular cartilage. However, the biological activities of Se on cartilage are largely unknown. In this study, we examined the effects of Se on chondrogenic cell ATDC5 and the possible mechanisms involved. We demonstrated that Se stimulated ATDC5 cell proliferation under serum deprivation but not routine culture. Furthermore, Se promoted G1‐phase cell cycle progression along with induction of cyclin D1 expression at the mRNA and protein level. Moreover, Se increased intracellular ATP content and decreased intracellular superoxide anion concentration without affecting intracellular redox status as estimated by ratio of the reduced and oxidized glutathione. In addition, suppression of intracellular ATP synthesis by glycolysis inhibitor or mitochondrial uncoupler both abrogated Se‐mediated cyclin D1 induction. These findings suggest Se stimulates proliferation of chondrogenic cell ATDC5 through acceleration of cell cycle progression accompanied with cyclin D1 induction by enhancement of intracellular ATP content. This novel finding provides evidence for a role of Se in cartilage formation and degenerative processes and further supports the relationship between Se status and cartilage function that may lead to better utilization of Se for cartilage homeostasis. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   
57.
A new type of pH-sensitive liposomes (fliposomes) was designed based on the amphiphiles that are able to perform a pH-triggered conformational flip (flipids). This flip disrupts the liposome membrane and causes rapid release of the liposome cargo, specifically in response to lowered pH. The flipids (1) and (2) are equipped with a trans-2-aminocyclohexanol conformational switch. pH-sensitive fliposomes containing one or both of these flipids, as well as POPC and PEG ceramide, were constructed and characterized. These compositions were stable at 4°C and pH 7.4 for several months. Fliposomes loaded with ANTS/DPX performed an unusually quick content release within a few seconds at pH below 8.5 (in case of 2) and 6.0 (in case of 1). This difference in pH sensitivity demonstrates a potential for the custom design of flipids by variation of the amino group to target areas with specific pH values. The pH titration curves for the fliposome leakage parallel the curves for the acid-induced conformational flip of 1 and 2 studied by 1H NMR. A plausible mechanism of pH sensitivity starts with an acid-triggered conformational flip of 1 or 2, which changes the molecular size and shape, shortens the lipid tails, and perturbs the liposome membrane, resulting in the content leakage.  相似文献   
58.
59.
60.
In animals, the Sep15 protein participates in disease resistance, growth, and development, but the function of its plant homologues remains unclear. Here, the function of maize Sep15 was analysed by characterization of two independent Sep15‐like loss‐of‐function mutants. In the absence of ZmSep15‐like, seedling tolerance to both water and salinity stress was compromised. The mutants experienced a heightened level of endoplasmic reticulum stress, and over‐accumulated reactive oxygen species, resulting in leaf necrosis. Characterization of Arabidopsis thaliana atsep15 mutant as well as like with ectopic expression of ZmSep15‐like indicated that ZmSep15‐like contributed to tolerance of both osmotic and salinity stress. ZmSep15‐like interacted physically with UDP‐glucose: glycoprotein glucosyltransferase1 (UGGT1). When the interaction was disrupted, the response to both osmotic and salinity stresses was impaired in maize or Arabidopsis. Co‐expressing ZmUGGT1 and ZmUGGT2 enhanced the tolerance of Athaliana to both stressors, indicating a functional interaction between them. Together, the data indicated that plants Sep15‐like proteins promote osmotic and salinity stress resistance by influencing endoplasmic reticulum stress response and reactive oxygen species level.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号