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   1496篇
  免费   101篇
  国内免费   2篇
  1599篇
  2024年   2篇
  2023年   20篇
  2022年   35篇
  2021年   46篇
  2020年   48篇
  2019年   49篇
  2018年   49篇
  2017年   44篇
  2016年   70篇
  2015年   95篇
  2014年   90篇
  2013年   116篇
  2012年   142篇
  2011年   125篇
  2010年   62篇
  2009年   60篇
  2008年   91篇
  2007年   79篇
  2006年   55篇
  2005年   56篇
  2004年   45篇
  2003年   43篇
  2002年   32篇
  2001年   27篇
  2000年   12篇
  1999年   12篇
  1998年   5篇
  1997年   9篇
  1996年   5篇
  1995年   5篇
  1994年   1篇
  1993年   2篇
  1992年   10篇
  1991年   7篇
  1990年   4篇
  1989年   4篇
  1988年   8篇
  1987年   2篇
  1986年   4篇
  1985年   6篇
  1984年   2篇
  1982年   1篇
  1981年   2篇
  1978年   1篇
  1977年   6篇
  1976年   4篇
  1975年   3篇
  1974年   1篇
  1967年   2篇
排序方式: 共有1599条查询结果,搜索用时 15 毫秒
991.
992.

Objective

To assess the physical activity levels of pregnant women and to examine the characteristics associated with the practice of exercise and the activities of daily living during pregnancy.

Methods

For this cross-sectional study, 1,279 women were recruited within 72 hours postpartum. They were interviewed about their socio-demographic data and obstetric history and were administered self-report questionnaires about exercise and daily physical activities during pregnancy. Data on the current pregnancy, labor, delivery, and newborn outcomes were collected from participants’ medical records. To analyze factors related to the practice of exercise, we used the student t-test, X², and odds ratio (OR), with a corresponding 95% confident interval (CI), followed by a multiple logistic regression. The significance level was 5%.

Results

Compared to the pre-pregnancy period, the prevalence of physical activity among participants was lower throughout pregnancy (20.1%) (p = 0.01). Half of the women interrupted practicing physical exercise due to pregnancy. The lowest prevalence of exercise was observed in the first (13.6%) and third trimesters (13.4%). Less than half of women received exercise guidance during prenatal care meetings (47.4%). Walking was the most commonly reported exercise, followed by water aerobics. Factors positively associated with exercise practice were higher educational level (OR= 1.82; CI 95% 1.28–2.60), primiparity (OR=1.49; CI 95% 1.07–2.07), exercising before pregnancy (OR= 6.45; CI 95% 4.64–8.96), and exercise guidance during prenatal care (OR=2.54; CI 95% 1.80–3.57). Mildly intense exercise and domestic activities were most frequently reported among pregnant women. There were no differences in maternal and perinatal outcomes between active and sedentary pregnant women.

Conclusion

The findings indicate that promoting physical activity remains a priority in public health policy, and women of childbearing age, especially those planning a pregnancy, should be encouraged to adopt an exercise routine or maintain an active lifestyle during pregnancy in order to avoid sedentary- and obesity-associated risks.  相似文献   
993.
Recent studies have investigated the composition of compounds that cover the cuticle in social insects, but few studies have focused on solitary bees. Cuticular hydrocarbons may provide a tool for chemotaxonomy, and perhaps they can be used as a complement to morphology and genetic characters in phylogenetic studies. Orchid bees (Tribe Euglossini) are a highly diverse group of Neotropical bees with more than 200 species. Here, the cuticular hydrocarbons of 17 species were identified and statistical analysis revealed 108 compounds, which allowed for the taxonomic classification according to the genera. The most significant compounds discriminating the four genera were (Z)-9-pentacosene, (Z,Z)-pentatriacontene-3, (Z)-9-tricosene, and (Z)-9-heptacosene. The analyses demonstrated the potential use of CHCs to identify different species.  相似文献   
994.
Axon guidance is required for the establishment of brain circuits. Whether much of the molecular basis of axon guidance is known from animal models, the molecular machinery coordinating axon growth and pathfinding in humans remains to be elucidated. The use of induced pluripotent stem cells (iPSC) from human donors has revolutionized in vitro studies of the human brain. iPSC can be differentiated into neuronal stem cells which can be used to generate neural tissue-like cultures, known as neurospheres, that reproduce, in many aspects, the cell types and molecules present in the brain. Here, we analyzed quantitative changes in the proteome of neurospheres during differentiation. Relative quantification was performed at early time points during differentiation using iTRAQ-based labeling and LC-MS/MS analysis. We identified 6438 proteins, from which 433 were downregulated and 479 were upregulated during differentiation. We show that human neurospheres have a molecular profile that correlates to the fetal brain. During differentiation, upregulated pathways are related to neuronal development and differentiation, cell adhesion, and axonal guidance whereas cell proliferation pathways were downregulated. We developed a functional assay to check for neurite outgrowth in neurospheres and confirmed that neurite outgrowth potential is increased after 10 days of differentiation and is enhanced by increasing cyclic AMP levels. The proteins identified here represent a resource to monitor neurosphere differentiation and coupled to the neurite outgrowth assay can be used to functionally explore neurological disorders using human neurospheres as a model.  相似文献   
995.
The insecticidal Cry11Aa and Cyt1Aa proteins are produced by Bacillus thuringiensis as crystal inclusions. They work synergistically inducing high toxicity against mosquito larvae. It was proposed that these crystal inclusions are rapidly solubilized and activated in the gut lumen, followed by pore formation in midgut cells killing the larvae. In addition, Cyt1Aa functions as a Cry11Aa binding receptor, inducing Cry11Aa oligomerization and membrane insertion. Here, we used fluorescent labeled crystals, protoxins or activated toxins for in vivo localization at nano-scale resolution. We show that after larvae were fed solubilized proteins, these proteins were not accumulated inside the gut and larvae were not killed. In contrast, if larvae were fed soluble non-toxic mutant proteins, these proteins were found inside the gut bound to gut-microvilli. Only feeding with crystal inclusions resulted in high larval mortality, suggesting that they have a role for an optimal intoxication process. At the macroscopic level, Cry11Aa completely degraded the gastric caeca structure and, in the presence of Cyt1Aa, this effect was observed at lower toxin-concentrations and at shorter periods. The labeled Cry11Aa crystal protein, after midgut processing, binds to the gastric caeca and posterior midgut regions, and also to anterior and medium regions where it is internalized in ordered “net like” structures, leading finally to cell break down. During synergism both Cry11Aa and Cyt1Aa toxins showed a dynamic layered array at the surface of apical microvilli, where Cry11Aa is localized in the lower layer closer to the cell cytoplasm, and Cyt1Aa is layered over Cry11Aa. This array depends on the pore formation activity of Cry11Aa, since the non-toxic mutant Cry11Aa-E97A, which is unable to oligomerize, inverted this array. Internalization of Cry11Aa was also observed during synergism. These data indicate that the mechanism of action of Cry11Aa is more complex than previously anticipated, and may involve additional steps besides pore-formation activity.  相似文献   
996.
Immune cells are bioenergetically expensive during activation, which requires tightly regulated control of metabolic pathways. Both low and high glycemic conditions can modulate immune function. States of undernourishment depress the immune system, and in the same way, excessive intake of nutrients, such as an obesity state, compromise its functioning. Multicellular organisms depend on two mechanisms to survive: the regulation and ability to store energy to prevent starvation and the ability to fight against infection. Synergic interactions between metabolism and immunity affect many systems underpinning human health. In a chronic way, the breakdown of glycemic homeostasis in the body can influence cells of the immune system and consequently contribute to the onset of diseases such as type II diabetes, obesity, Alzheimer's, and fat and lean mass loss. On the contrary, exercise, recognized as a primary strategy to control hyperglycemic disorders, also induces a coordinated immune-neuro-endocrine response that acutely modulates cardiovascular, respiratory, and muscle functions and the immune response to exercise is widely dependent on the intensity and volume that may affect an immunodepressive state. These altered immune responses induced by exercise are modulated through the “stress hormones” adrenaline and cortisol, which are a threat to leukocyte metabolism. In this context, carbohydrates appear to have a positive acute response as a strategy to prevent depression of the immune system by maintaining plasma glucose concentrations to meet the energy demand from all systems involved during strenuous exercises. Therefore, herein, we discuss the mechanisms through which exercise may promotes changes on glycemic homeostasis in the metabolism and how it affects immune cell functions under higher or lower glucose conditions.  相似文献   
997.
Current predictions about the responses of species to climate change strongly rely on projecting altered environmental conditions on their distributions. In this study, we investigated the effects of future climate change scenarios on the potential distribution of 10 species of scorpions in north‐eastern Brazil in the context of their degree of specialisation to closed (Atlantic and Amazon Forests) and open (Caatinga and Cerrado) habitats. Scorpion species were classified as habitat specialists or generalists according to the IndVal index, and present and future species distribution models were prepared using minimum volume ellipsoids. According to IndVal, four species were classified as closed‐forest specialists (Ananteris mauryi, Tityus brazilae, Tityus pusillus and Tityus neglectus), four as open‐forest specialists (Jaguajir agamemnon, Jaguajir rochae, Physoctonus debilis and Bothriurus rochai), and two as generalists (Tityus stigmurus and Bothriurus asper). All species presented a drastic reduction in potential distribution, ranging from 44% to 72%, when compared with their current distribution. In addition, we found a reduction in scorpion species richness under future climate change scenarios. This finding has implications for scorpion conservation. Further, the results show that climate change may impact the composition of scorpion assemblages in north‐eastern Brazil, revealing important implications for human–scorpion interactions.  相似文献   
998.
999.
Abstract

The protease from Aspergillus tamarii Kita UCP1279 extraction by aqueous two-phase PEG-Citrate (ATPS) systems, using a factorial design 24, was investigated. Then, the variables studied were polyethylene glycol (PEG) molar mass (MPEG), concentrations of PEG (CPEG) and citrate (CCIT), and pH. The responses analyzed were the partition coefficient (K), activity yield (Y) and purification factor (PF). The thermodynamic parameters of the ATPS partition were estimated as a function of temperature. ATPS was able to pre-purify the protease (PF = 1.6) and obtained 84% activity yield. The thermodynamic parameters ΔG°m (?10.89?kJ mol?1), ΔHm (?5.0?kJ?mol?1) and partition ΔSm (19.74?J mol?1 K?1) showed that the preferential migration of almost all protein contaminants of the crude extract to the salt-rich phase, while the preferred protease was the PEG rich phase. The extracted enzyme presents optimum temperature and pH at range of 40–50?°C and 9.0–11.0, respectively. Moreover, the enzyme was identified as serine protease based on inhibition profile. ATPS showed the satisfactory performance as the first step for Aspergillus tamarii Kita UCP1279 protease pre-purification.  相似文献   
1000.
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