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   165698篇
  免费   5897篇
  国内免费   5639篇
  177234篇
  2024年   159篇
  2023年   1051篇
  2022年   2422篇
  2021年   4133篇
  2020年   2663篇
  2019年   3276篇
  2018年   14152篇
  2017年   12257篇
  2016年   10251篇
  2015年   5429篇
  2014年   5896篇
  2013年   6347篇
  2012年   11033篇
  2011年   18399篇
  2010年   15215篇
  2009年   11250篇
  2008年   13277篇
  2007年   14429篇
  2006年   3194篇
  2005年   2910篇
  2004年   2933篇
  2003年   2678篇
  2002年   2182篇
  2001年   1490篇
  2000年   1382篇
  1999年   1155篇
  1998年   676篇
  1997年   685篇
  1996年   676篇
  1995年   620篇
  1994年   552篇
  1993年   410篇
  1992年   589篇
  1991年   472篇
  1990年   414篇
  1989年   290篇
  1988年   262篇
  1987年   247篇
  1986年   167篇
  1985年   193篇
  1984年   116篇
  1983年   134篇
  1982年   73篇
  1981年   58篇
  1980年   38篇
  1979年   61篇
  1974年   38篇
  1973年   34篇
  1972年   276篇
  1971年   297篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
以现有乳酸菌信息为依据,利用汉字foxbase2.10进行数据处理,研制出一套乳酸菌微机检索软件。该软件的特点:检索速度快,途径灵活,增添、修改、删除、打印方便,各模块性能优良,实用性强。  相似文献   
952.
本文研究了球形芽孢杆菌(Bacillussphaericus)C3-41超氧化物歧化酶(SOD)的产生条件和部分特性。当C3-41菌株处于孢子囊中期时为产SOD酶高峰期,在30℃下的平板培养物及培养基起始pH为中性(pH7.0)时产生的SOD酶比活最高,经硫酸铵分级沉淀,DEAE-32离子交换层析和SephadexG-100凝胶过滤提纯了SOD酶。此酶属Mn-SOD,在25~35℃和pH5~9范围内较稳定,但在55℃下10min完全失活。  相似文献   
953.
介绍一种快速提取禽源性大肠埃希氏菌外膜蛋白的方法。该法是对Kapur等发表的方法的改进。全过程只需超速离心一次,比Kapur等的方法缩短了4h。所得样品可直接用于禽源性大肠埃希氏菌外膜蛋白模式的测定。  相似文献   
954.
在了解了产己酸细菌LII的发酵最适条件的基础上,进一步对己酸发酵过程中的生理及代谢特性作了较详细的分析。结果表明,在己酸发酵过程中除产生正常代谢产物己酸外,还产生丁酸、乙酸、戊酸等其它酸类和甲烷,氢气和CO等气体。用1—13C-乙酸钠和未标记的乙醇作为底物发酵,用色质联用仪(GC/MS)分析,结果表明:在己酸合成中碳的来源为乙醇和乙酸,中间代谢产物为丁酸。  相似文献   
955.
Pain is a multidimensional perception that includes unpleasant somatosensory and affective experiences; however, the underlying neural circuits that mediate different components of pain remain elusive. Although hyperactivity of basolateral amygdala glutamatergic (BLAGlu) neurons is required for the somatosensory and emotional processing of pain, the precise excitatory inputs to BLAGlu neurons and their roles in mediating different aspects of pain are unclear. Here, we identified two discrete glutamatergic neuronal circuits in male mice: a projection from the insular cortex glutamatergic (ICGlu) to BLAGlu neurons, which modulates both the somatosensory and affective components of pain, and a projection from the mediodorsal thalamic nucleus (MDGlu) to BLAGlu neurons, which modulates only the aversive-affective component of pain. Using whole-cell recording and fiber photometry, we found that neurons within the IC→BLA and MD→BLA pathways were activated in mice upon inflammatory pain induced by injection of complete Freund’s adjuvant (CFA) into their paws. Optical inhibition of the ICGlu→BLA pathway increased the nociceptive threshold and induced behavioral place preference in CFA mice. In contrast, optical inhibition of the MDGlu→BLA pathway did not affect the nociceptive threshold but still induced place preference in CFA mice. In normal mice, optical activation of the ICGlu→BLA pathway decreased the nociceptive threshold and induced place aversion, while optical activation of the MDGlu→BLA pathway only evoked aversion. Taken together, our results demonstrate that discrete ICGlu→BLA and MDGlu→BLA pathways are involved in modulating different components of pain, provide insights into its circuit basis, and better our understanding of pain perception.  相似文献   
956.
田耀加  梁广文  曾玲  曹流 《生态学报》2012,32(19):6187-6194
不同的作物生境可为田间自然天敌提供不同的栖息场所及猎物或寄主,从而影响天敌自身种群的维持及繁衍。在秋收后的玉米地布局不同的作物生境(玉米秸秆生境、叶菜类生境、甘薯生境及杂草带生境),采用目测法和扫网法调查研究其天敌群落组成及时序动态,通过聚类分析探讨了不同作物生境对玉米地自然天敌冬季保育作用的差异性。结果表明,玉米秸秆生境天敌数量最大,共收集天敌555头,约为叶菜类(甘蓝、菜心、油麦菜)生境的2倍,为杂草带和甘薯生境的4.5倍;天敌种类以蜘蛛类群为主,其中甘蓝、甘薯和杂草带生境蜘蛛个体数量占天敌总量的80%以上,而玉米秸秆生境天敌优势种为稻红瓢虫,其种群发生量达30头/100株以上。天敌物种数、多样性指数大小为玉米秸秆生境>叶菜类生境≥杂草带生境>甘薯生境,天敌群落优势度集中性指数与之相反。系统聚类可将6种不同作物生境聚为3类,玉米秸秆生境为一类,叶菜类生境为一类,杂草带和甘薯生境为一类。研究发现,玉米秸秆生境对玉米地天敌的冬季保育作用最佳。  相似文献   
957.
Ageing severely affects the chromosome segregation process in human oocytes resulting in aneuploidy, infertility and developmental disorders. A considerable amount of segregation errors in humans are introduced at the second meiotic division. We have here compared the chromosome segregation process in young adult and aged female mice during the second meiotic division. More than half of the oocytes in aged mice displayed chromosome segregation irregularities at anaphase II, resulting in dramatically increased level of aneuploidy in haploid gametes, from 4% in young adult mice to 30% in aged mice. We find that the post‐metaphase II process that efficiently corrects aberrant kinetochore‐microtubule attachments in oocytes in young adult mice is approximately 10‐fold less efficient in aged mice, in particular affecting chromosomes that show small inter‐centromere distances at the metaphase II stage in aged mice. Our results reveal that post‐metaphase II processes have critical impact on age‐dependent aneuploidy in mammalian eggs.  相似文献   
958.
This study aimed to explore the clinical practice of phospholipid metabolic pathways in COVID-19. In this study, 48 COVID-19 patients and 17 healthy controls were included. Patients were divided into mild (n=40) and severe (n=8) according to their severity. Phospholipid metabolites, TCA circulating metabolites, eicosanoid metabolites, and closely associated enzymes and transfer proteins were detected in the plasma of all individuals using metabolomics and proteomics assays, respectively. 30 of the 33 metabolites found differed significantly (P<0.05) between patients and healthy controls (P<0.05), with D-dimmer significantly correlated with all of the lysophospholipid metabolites (LysoPE, LysoPC, LysoPI and LPA). In particular, we found that phosphatidylinositol (PI) and phosphatidylcholine (PC) could identify patients from healthy controls (AUC 0.771 and 0.745, respectively) and that the severity of the patients could be determined (AUC 0.663 and 0.809, respectively). The last measurement before discharge also revealed significant changes in both PI and PC. For the first time, our study explores the significance of the phospholipid metabolic system in COVID-19 patients. Based on molecular pathway mechanisms, three important phospholipid pathways related to Ceramide-Malate acid (Cer-SM), Lysophospholipid (LPs), and membrane function were established. Clinical values discovered included the role of Cer in maintaining the inflammatory internal environment, the modulation of procoagulant LPA by upstream fibrinolytic metabolites, and the role of PI and PC in predicting disease aggravation.  相似文献   
959.
The pandemic of COVID-19 is the biggest public health crisis in 21st Century. Besides the acute symptoms after infection, patients and society are also being challenged by the long-term health complications associated with COVID-19, commonly known as long COVID. While health professionals work hard to find proper treatments, large amount of knowledge has been accumulated in recent years. In order to deal with long COVID efficiently, it is important for people to keep up with current progresses and take proactive actions on long COVID. For this purpose, this review will first introduce the general background of long COVID, and then discuss its risk factors, diagnostic indicators and management strategies. This review will serve as a useful resource for people to understand and prepare for long COVID that will be with us in the foreseeable future.  相似文献   
960.
Phosphorus is an essential macronutrient for plant growth and development. Root system architecture (RSA) affects a plant's ability to obtain phosphate, the major form of phosphorus that plants uptake. In this review, I first consider the relationship between RSA and plant phosphorus-acquisition efficiency, describe how external phosphorus conditions both induce and impose changes in the RSA of major crops and of the model plant Arabidopsis, and discuss whether shoot phosphorus status affects RSA and whether there is a universal root developmental response across all plant species. I then summarize the current understanding of the molecular mechanisms governing root developmental responses to phosphorus deficiency. I also explore the possible reasons for the inconsistent results reported by different research groups and comment on the relevance of some studies performed under laboratory conditions to what occurs in natural environments.  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