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   15593篇
  免费   1346篇
  国内免费   1904篇
  18843篇
  2024年   52篇
  2023年   281篇
  2022年   587篇
  2021年   905篇
  2020年   700篇
  2019年   812篇
  2018年   755篇
  2017年   555篇
  2016年   725篇
  2015年   1036篇
  2014年   1249篇
  2013年   1278篇
  2012年   1561篇
  2011年   1436篇
  2010年   906篇
  2009年   727篇
  2008年   808篇
  2007年   743篇
  2006年   604篇
  2005年   528篇
  2004年   385篇
  2003年   303篇
  2002年   282篇
  2001年   192篇
  2000年   181篇
  1999年   174篇
  1998年   120篇
  1997年   114篇
  1996年   117篇
  1995年   96篇
  1994年   89篇
  1993年   66篇
  1992年   86篇
  1991年   57篇
  1990年   57篇
  1989年   50篇
  1988年   35篇
  1987年   27篇
  1986年   28篇
  1985年   30篇
  1984年   15篇
  1983年   16篇
  1982年   13篇
  1981年   5篇
  1980年   4篇
  1979年   10篇
  1978年   4篇
  1976年   9篇
  1975年   8篇
  1971年   3篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Probabilistic Boolean networks (PBNs) are extensions of Boolean networks (BNs), and both have been widely used to model biological systems. In this paper, we study the long-range correlations of PBNs based on their corresponding Markov chains. PBN states are quantified by the deviation of their steady-state distributions. The results demonstrate that, compared with BNs, PBNs can exhibit these dynamics over a wider and higher noise range. In addition, the constituent BNs significantly impact the generation of 1/f dynamics of PBNs, and PBNs with homogeneous steady-state distributions tend to sustain the 1/f dynamics over a wider noise range.  相似文献   
992.
Tetrastigma hemsleyanum suspension cells were treated with four metal salts to screen suitable elicitors for the promotion of plant cell biomass and flavonoid production. The effects of calcium ions (Ca2+) on induction were also studied. It was found that the most effective elicitors were 50 μM of the heavy metal ion copper (Cu2+) and 100 μM of the rare earth element cerium (Ce3+). The maximal biomass levels under respective treatments over a 16-d culture period increased by 1.3- and 1.6-fold, and the total flavonoid content was 1.8- and 1.6-fold greater than the control, respectively. Reducing the exogenous Ca2+ concentration or adding Ca2+ antagonists (1 mM ethylene glycol-bis(2-aminoethylether)-N,N,N′,N-tetraacetc acid (EGTA) or 1 mM verapamil) strengthened inductive effects of metal elicitors and enhanced flavonoid production. However, 0.5 μM of the calcium ionophore A23187 showed contrary results. The increase in exogenous Ca2+ concentration in the presence of A23187 suppressed H2O2 bursts and peroxidase activity caused by metal elicitors. The results suggest that Ca2+ plays an inhibitory role in the plant cell response to metal elicitors. This suppression could have been caused by Ca2+ preventing the cells from absorbing metal ions and then easing the induction, or because the decrease of Ca2+ concentration worked as an induction signal. Therefore, reducing the Ca2+ concentration in culture medium, or adding Ca2+ antagonists could be used to improve flavonoid production and cell growth in combination with induction by metal elicitors during in vitro culture of T. hemsleyanum suspension cells.  相似文献   
993.
王秀梅  臧连生  林宝庆  杨军  陈鹏  阮长春 《生态学报》2013,33(20):6553-6559
榆紫叶甲赤眼蜂(Asynacta ambrostomae Liao)是榆紫叶甲卵期重要寄生蜂。在实验条件下对榆紫叶甲赤眼蜂生殖、寿命、性比等生物学特性进行观察研究。结果表明:每粒寄主卵平均出蜂量为2.92头,雌雄比为3.64:1,营两性生殖,孤雌产雄。不同营养条件对寿命有显著影响,其中成虫补充20%蜂蜜水和20%蔗糖水能显著增加雌雄虫寿命。组建了25℃条件下榆紫叶甲赤眼蜂成蜂补充不同营养的实验种群生命表,成虫补充20%蜂蜜水、20%蔗糖水、清水、对照组(不添加任何物质)的净生殖力R0分别为74.42、65.27、56.77和44.50。可见,成蜂补充营养能提高种群生殖力。研究结果将为榆紫叶甲赤眼蜂的开发利用提供理论依据。  相似文献   
994.
红松半同胞家系遗传变异分析及果材兼用优良家系选择   总被引:2,自引:0,他引:2  
以临江林业局闹枝林场内48个红松半同胞家系为材料,连续3年对其生长性状进行测定分析。方差分析结果表明不同家系间各性状差异均达极显著水平(P<0.01);家系间各指标表型变异系数变化范围为2.74%~51.39%;半同胞家系间各性状遗传力均超过0.97;48个半同胞家系树高、胸径和单株球果数量的一般配合力变化范围为(-1.82~1.70)、(-7.91~9.99)和(-14.85~15.15);不同树龄树高间、胸径间的相关系数达极显著水平(0.993 < r < 0.999,0.995 < r < 0.998);分别利用生长性状和结实性状对家系进行综合评价,以10%的入选率,初步选出5个生长性状优良的家系,入选家系树高和胸径平均值分别为8.02 m和21.95 cm,遗传增益分别为7.63%和46.29%。初步选出5个结实性状优良的家系,球果数量和千粒重平均值分别为30.26个和736.20 g,遗传增益分别为86.49%和1.46%。本研究中评价选出的优良家系可为红松良种登记提供材料,选择方法为其它果材兼用树种良种选育提供理论基础。  相似文献   
995.
He  Ziying  Peng  Yisheng  Guan  Dongsheng  Hu  Zhan  Chen  Yujuan  Lee  Shing Yip 《Plant and Soil》2018,429(1-2):425-436
Plant and Soil - A&nbsp;dominant species of steppe in northern China, Stipa grandis,&nbsp;is gradually being replaced by S. krylovii due to climate change events. The aim of this study was...  相似文献   
996.
He  Xue  Xiong  Chuhui  Liu  Aihua  Zhao  Wei  Xia  Xinghai  Peng  Shiqiao  Li  Chenyan  Zhou  Mi  Li  Yushu  Shi  Xiaoguang  Shan  Zhongyan  Teng  Weiping 《Biological trace element research》2018,184(1):196-205

Apoptosis occurs in many autoimmune diseases. Excess iodine induces thyrocyte apoptosis and increases the incidence and prevalence of autoimmune thyroiditis (AIT). However, the sequence of events between the appearance of thyrocyte apoptosis and the occurrence of thyroiditis remains uncharacterized. Furthermore, few studies have investigated the role of macrophage phagocytosis in the development of AIT. Therefore, we evaluated the relationship between apoptosis and inflammatory infiltration in NOD.H-2h4 mouse thyroids by comparing the sequence of events in tissue samples. We also investigated the role of macrophages by comparing macrophage phagocytosis function in BALB/c, C57BL/6, and NOD.H-2h4 mice treated with different levels of iodine. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assays and thyroid inflammatory scores revealed that apoptosis (2 weeks) occurred before inflammatory infiltration (4 weeks). Phosphatidylserine (PS) expression on the extracellular surface of the cell membrane and double-stranded DNA fragments associated with apoptosis appeared at 2 and 8 weeks, respectively. Additionally, although apoptosis was enhanced in the thyroids of mice supplemented with excess iodine (0.05 ± 0.12 vs 1.63 ± 0.82% for BALB/c, 0.09 ± 0.14 vs 1.51 ± 0.34% for C57BL/6, and 0.07 ± 1.11 vs 4.72 ± 0.62% for NOD.H-2h4 mice), only NOD.H-2h4 mouse thyroids presented with inflammation. Furthermore, macrophages from NOD.H-2h4 mice (44.46 ± 1.79%) exhibited decreased phagocytotic activity relative to that in BALB/c (54.21 ± 4.58%) and C57BL/6 (58.96 ± 4.04%) mice. There were no differences in phagocytosis function between NOD.H-2h4 mice supplemented with excess iodine or left untreated (24.50 ± 2.66 vs 21.71 ± 1.79%, p = 0.06). In conclusion, deficiencies in the apoptosis clearance of macrophages in NOD.H-2h4 mice may constitute an early pathogenic mechanism in AIT that is not influenced by iodine intake.

  相似文献   
997.

Objective

To compare the analgesic effects of patient-controlled intravenous analgesia (PCA) with hydromorphone and sufentanil after thoracic surgery on postoperative pulmonary complications (PPCs).

Methods

A total of 142 patients who were scheduled for thoracic surgery were randomly allocated to receive PCA with hydromorphone (group A: experimental group): hydromorphone 0.2?mg/kg?+?dezocine 0.5?mg/kg?+?ramosetron 0.6?mg diluted with normal saline to 200?mL; or with sufentanil (group B: control group): sufentanil 3.0μg/kg?+?dezocine 0.5?mg/kg?+?ramosetron 0.6?mg diluted with normal saline to 200?mL. The parameters of intravenous analgesia pump were set as background dose 4?ml/h, PCA dose 1?mL, locking time 15?min. Pain NRS (numerical rating scale), Ramsay sedation score, nausea or vomiting score were evaluated at 0?h, 6?h, 12?h, 24?h, 48?h after operation. The cases of PPCs (atelectasis, pulmonary infection, respiratory failure), CRP (C-reaction protein) and inflammatory cells (white cell count and percentage of neutrophils) and blood gas analysis at 12?h after operation, length of ICU and postoperative stay were recorded for each patient.

Results

Data of 136 patients were analyzed. Compared with group B (4[IQR:2,2]), the pain NRS in group A (2[IQR:4,4]) was significantly lower at 6?h after operation (P?=?0.000). The CRP in group A (69.79?±?32.13?mg/L) were lower than group B (76.76?±?43.42?mg/L) after operation, but the difference was not significant (P?=?0.427). No difference of nausea or vomiting was found between group A (7.3%) and group B (5.8%) postoperatively (P?=?0.999). The PPCs were happened in 11 patients in group A (16.2%) and 22 patients in group B (32.4%) and the difference between two groups was significant (P?=?0.027). Seven patients in group A (10.3%) and eighteen patients in group B (26.5%) had clinical evidence of pneumonia and the difference between two groups was significant (P?=?0.014). The length of ICU and postoperative stay in group A were 2.73?h and 1.82?days less than group B respectively but the differences were not significant (P?=?0.234, P?=?0.186 respectively).

Conclusion

Compared with sufentanil, hydromorphone may provide better postoperative analgesic effect with less pulmonary complications for patients undergoing thoracic surgery, and it may accelerate patients’ rehabilitation.

Trial registration

Randomized Controlled Trials ChiCTR1800014282c. Registered 3 January 2018.
  相似文献   
998.
DNA damage response (DDR) is essential for maintaining genome stability and protecting cells from tumorigenesis. Ubiquitin and ubiquitin-like modifications play an important role in DDR, from signaling DNA damage to mediating DNA repair. In this report, we found that the E3 ligase ring finger protein 126 (RNF126) was recruited to UV laser micro-irradiation-induced stripes in a RNF8-dependent manner. RNF126 directly interacted with and ubiquitinated another E3 ligase, RNF168. Overexpression of wild type RNF126, but not catalytically-inactive mutant RNF126 (CC229/232AA), diminished ubiquitination of H2A histone family member X (H2AX), and subsequent bleomycin-induced focus formation of total ubiquitin FK2, TP53-binding protein 1 (53BP1), and receptor-associated protein 80 (RAP80). Interestingly, both RNF126 overexpression and RNF126 downregulation compromised homologous recombination (HR)-mediated repair of DNA double-strand breaks (DSBs). Taken together, our findings demonstrate that RNF126 negatively regulates RNF168 function in DDR and its appropriate cellular expression levels are essential for HR-mediated DSB repair.  相似文献   
999.
Calycosin, a bioactive component derived from Astragali Radix (AR; Huang Qi), has been shown to have an effect of anti‐allergic dermatitis with unknown mechanism. This study aims to investigate the mechanism of calycosin related to tight junctions (TJs) and HIF‐1α both in FITC‐induced mice allergic contact dermatitis and in IL‐1β stimulated HaCaT keratinocytes. Th2 cytokines (IL‐4, IL‐5 and IL‐13) were detected by ELISA. The epithelial TJ proteins (occludin, CLDN1 and ZO‐1), initiative key cytokines (TSLP and IL‐33) and HIF‐1α were assessed by Western blot, real‐time PCR, immunohistochemistry or immunofluorescence. Herein, we have demonstrated that allergic inflammation and the Th2 cytokines in ACD mice were reduced significantly by calycosin treatment. Meanwhile, calycosin obviously decreased the expression of HIF‐1α and repaired TJs both in vivo and in vitro. In HaCaT keratinocytes, we noted that IL‐1β induced the deterioration of TJs, as well as the increased levels of TSLP and IL‐33, which could be reversed by silencing HIF‐1α. In addition, administration of 2‐methoxyestradiolin (2‐ME), a HIF‐1α inhibitor,significantly repaired the TJs and alleviated the allergic inflammation in vivo. Furthermore, TJs were destroyed by DMOG or by overexpressing HIF‐1α in HaCaT keratinocytes, and simultaneously, calycosin down‐regulated the expression of HIF‐1α and repaired the TJs in this process. These results revealed that calycosin may act as a potential anti‐allergy and barrier‐repair agent via regulating HIF‐1α in AD and suggested that HIF‐1α and TJs might be possible therapy targets for allergic dermatitis.  相似文献   
1000.
Side‐chain engineering is an important strategy for optimizing photovoltaic properties of organic photovoltaic materials. In this work, the effect of alkylsilyl side‐chain structure on the photovoltaic properties of medium bandgap conjugated polymer donors is studied by synthesizing four new polymers J70 , J72 , J73 , and J74 on the basis of highly efficient polymer donor J71 by changing alkyl substituents of the alkylsilyl side chains of the polymers. And the photovoltaic properties of the five polymers are studied by fabricating polymer solar cells (PSCs) with the polymers as donor and an n‐type organic semiconductor (n‐OS) m‐ITIC as acceptor. It is found that the shorter and linear alkylsilyl side chain could afford ordered molecular packing, stronger absorption coefficient, higher charge carrier mobility, thus results in higher Jsc and fill factor values in the corresponding PSCs. While the polymers with longer or branched alkyl substituents in the trialkylsilyl group show lower‐lying highest occupied molecular orbital energy levels which leads to higher Voc of the PSCs. The PSCs based on J70 :m‐ITIC and J71 :m‐ITIC achieve power conversion efficiency (PCE) of 11.62 and 12.05%, respectively, which are among the top values of the PSCs reported in the literatures so far.  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