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   1863篇
  免费   158篇
  国内免费   95篇
  2024年   9篇
  2023年   32篇
  2022年   47篇
  2021年   62篇
  2020年   65篇
  2019年   73篇
  2018年   92篇
  2017年   63篇
  2016年   57篇
  2015年   53篇
  2014年   105篇
  2013年   147篇
  2012年   87篇
  2011年   90篇
  2010年   68篇
  2009年   70篇
  2008年   74篇
  2007年   99篇
  2006年   92篇
  2005年   67篇
  2004年   67篇
  2003年   64篇
  2002年   47篇
  2001年   36篇
  2000年   43篇
  1999年   32篇
  1998年   37篇
  1997年   42篇
  1996年   31篇
  1995年   31篇
  1994年   22篇
  1993年   28篇
  1992年   30篇
  1991年   21篇
  1990年   13篇
  1989年   14篇
  1988年   12篇
  1987年   10篇
  1986年   7篇
  1985年   11篇
  1984年   15篇
  1983年   5篇
  1982年   9篇
  1981年   8篇
  1980年   7篇
  1979年   3篇
  1978年   5篇
  1977年   6篇
  1976年   3篇
  1974年   3篇
排序方式: 共有2116条查询结果,搜索用时 15 毫秒
991.
992.
Potato pulp is a high-volume co-processing product resulting from industrial potato starch manufacturing. Potato pulp is particularly rich in pectin, notably galactan branched rhamnogalacturonan I polysaccharides, which are highly bifidogenic when solubilized. The objective of the present study was to characterize and compare four homogalacturonan degrading enzymes capable of catalyzing the required solubilization of these pectinaceous polysaccharides from potato pulp in a 1 min reaction. An additional purpose was to assess the influence of the pH and the potential buffer chelating effects on the release of these polysaccharides from the potato pulp. The pH and temperature optima of two selected pectin lyases from Emericella nidulans (formerly known as Aspergillus nidulans) and Aspergillus niger were determined to 8.6 and 4.0, respectively, at ≥100 °C within 1 min of reaction. The optima for the two selected polygalacturonases from E. nidulans and Aspergillus aculeatus were determined to pH 4.4 and 46 °C, and pH 3.7 and ≥80 °C, respectively. The polygalacturonase from A. aculeatus was 4-42 times more heat-resistant at 50 °C than the other enzymes. The difference in pH optima of the pectin lyases and the exceptional thermal stabilities of some of the enzymes are proposed to be related to specific amino acid substitutions, stabilizing hydrogen bonding and structural traits of the enzymes. The KM and Vmax values ranged from 0.3-0.6 g/L and 0.5-250.5 U/mg protein, respectively. Phosphate buffer induced release of a higher amount of dry matter than Tris-acetate buffer at pH 6, indicating a chelating effect of the phosphate. Moreover, the phosphate had a higher chelating effect at pH 6 than at pH 4. The optimal conditions for a high yield of polysaccharides from potato pulp were therefore: 1% (w/w) potato pulp treated with 1% (w/w) enzyme/substrate (E/S) pectin lyase from E. nidulans and 1% (w/w) E/S polygalacturonase from A. aculeatus at pH 6.0 and 60 °C for 1 min.  相似文献   
993.
Lipolysis is the biochemical pathway responsible for the catabolism of triacylglycerol (TAG) stored in cellular lipid droplets. The hydrolytic cleavage of TAG generates non-esterified fatty acids, which are subsequently used as energy substrates, essential precursors for lipid and membrane synthesis, or mediators in cell signaling processes. Consistent with its central importance in lipid and energy homeostasis, lipolysis occurs in essentially all tissues and cell types, it is most abundant, however, in white and brown adipose tissue. Over the last 5years, important enzymes and regulatory protein factors involved in lipolysis have been identified. These include an essential TAG hydrolase named adipose triglyceride lipase (ATGL) [annotated as patatin-like phospholipase domain-containing protein A2], the ATGL activator comparative gene identification-58 [annotated as α/β hydrolase containing protein 5], and the ATGL inhibitor G0/G1 switch gene 2. Together with the established hormone-sensitive lipase [annotated as lipase E] and monoglyceride lipase, these proteins constitute the basic "lipolytic machinery". Additionally, a large number of hormonal signaling pathways and lipid droplet-associated protein factors regulate substrate access and the activity of the "lipolysome". This review summarizes the current knowledge concerning the enzymes and regulatory processes governing lipolysis of fat stores in adipose and non-adipose tissues. Special emphasis will be given to ATGL, its regulation, and physiological function.  相似文献   
994.
林坤  杨敏丽  李建涛  剡根姣 《昆虫知识》2011,48(4):1103-1106
室内人工大量继代饲养小菜蛾Plutella xylostella L.,是开展小菜蛾各种研究的基础.本文介绍用三月慢青菜萝卜苗法饲养小菜蛾的方法.在(24±1)℃、相对湿度60%~70%、光周期L:D=14:10条件下,小菜蛾能连续继代健康繁衍,卵的孵化率和蛹的羽化率可达90%以上.该方法简单实用,成本低,管理方便,可...  相似文献   
995.
Plant secondary metabolites are known to facilitate interactions with a variety of beneficial and detrimental organisms, yet the contribution of specific metabolites to interactions with fungal pathogens is poorly understood. Here we show that, with respect to aliphatic glucosinolate‐derived isothiocyanates, toxicity against the pathogenic ascomycete Sclerotinia sclerotiorum depends on side chain structure. Genes associated with the formation of the secondary metabolites camalexin and glucosinolate were induced in Arabidopsis thaliana leaves challenged with the necrotrophic pathogen S. sclerotiorum. Unlike S. sclerotiorum, the closely related ascomycete Botrytis cinerea was not identified to induce genes associated with aliphatic glucosinolate biosynthesis in pathogen‐challenged leaves. Mutant plant lines deficient in camalexin, indole, or aliphatic glucosinolate biosynthesis were hypersusceptible to S. sclerotiorum, among them the myb28 mutant, which has a regulatory defect resulting in decreased production of long‐chained aliphatic glucosinolates. The antimicrobial activity of aliphatic glucosinolate‐derived isothiocyanates was dependent on side chain elongation and modification, with 8‐methylsulfinyloctyl isothiocyanate being most toxic to S. sclerotiorum. This information is important for microbial associations with cruciferous host plants and for metabolic engineering of pathogen defenses in cruciferous plants that produce short‐chained aliphatic glucosinolates.  相似文献   
996.
The provenance of white sharks (Carcharodon carcharias) in the Mediterranean is both a conundrum and an important conservation issue. Considering this species's propensity for natal philopatry, any evidence that the Mediterranean stock has little or no contemporary immigration from the Atlantic would suggest that it is extraordinarily vulnerable. To address this issue we sequenced the mitochondrial control region of four rare Mediterranean white sharks. Unexpectedly, the juvenile sequences were identical although collected at different locations and times, showing little genetic differentiation from Indo-Pacific lineages, but strong separation from geographically closer Atlantic/western Indian Ocean haplotypes. Historical long-distance dispersal (probably a consequence of navigational error during past climatic oscillations) and potential founder effects are invoked to explain the anomalous relationships of this isolated 'sink' population, highlighting the present vulnerability of its nursery grounds.  相似文献   
997.
研究了一类含时滞的Harrison型捕食者-食饵模型在随机扰动环境下的动力学行为.对于非时滞和时滞模型分别给出了局部和全局稳定性条件.通过白噪声分别对食饵人口增长率的和捕食者人口死亡率进行随机扰动,构建相应的随机时滞微分方程模型讨论环境噪声对其作用的动力学行为.在一定条件下,随机时滞模型存在随机最终有界的唯一全局正解且解的二阶均值是有界的.最后通过数值模拟对给出的分析结果进行了验证.  相似文献   
998.
Baragli A  Lanfranco F  Allasia S  Granata R  Ghigo E 《Peptides》2011,32(11):2323-2332
Acylated ghrelin (AG) is a 28 amino acid gastric peptide a natural ligand for the growth hormone secretagogue (GHS) receptor type 1a (GHS-R1a), endowed with GH-secreting and orexigenic properties. Besides, ghrelin exerts several peripheral metabolic actions, including modulation of glucose homeostasis and stimulation of adipogenesis. Notably, AG administration causes hyperglycemia in rodents as in humans. Ghrelin pleiotropy is supported by a widespread expression of the ghrelin gene, of GHS-R1a and other unknown ghrelin binding sites. The existence of alternative receptors for AG, of several natural ligands for GHS-R1a and of acylation-independent ghrelin non-neuroendocrine activities, suggests that there might be a complex ‘ghrelin system’ not yet completely explored. Moreover, the patho-physiological implications of unacylated ghrelin (UAG), and obestatin (Ob), the other two ghrelin gene-derived peptides, need to be clarified. Within the next few years, we may better understand the ‘ghrelin system’, where we might envisage clinical applications.  相似文献   
999.
Pluripotency and their neural crest origin make dental pulp stem cells (DPSCs) an attractive donor source for neuronal cell replacement. Despite recent encouraging results in this field, little is known about the integration of transplanted DPSC derived neuronal pecursors into the central nervous system. To address this issue, neuronally predifferentiated DPSCs, labeled with a vital cell dye Vybrant DiD were introduced into postnatal rat brain. DPSCs were transplanted into the cerebrospinal fluid of 3-day-old male Wistar rats. Cortical lesion was induced by touching a cold (−60 °C) metal stamp to the calvaria over the forelimb motor cortex. Four weeks later cell localization was detected by fluorescent microscopy and neuronal cell markers were studied by immunohistochemistry. To investigate electrophysiological properties of engrafted, fluorescently labeled DPSCs, 300 μm-thick horizontal brain slices were prepared and the presence of voltage-dependent sodium and potassium channels were recorded by patch clamping.Predifferentiated donor DPSCs injected into the cerebrospinal fluid of newborn rats migrated as single cells into a variety of brain regions. Most of the cells were localized in the normal neural progenitor zones of the brain, the subventricular zone (SVZ), subgranular zone (SGZ) and subcallosal zone (SCZ). Immunohistochemical analysis revealed that transplanted DPSCs expressed the early neuronal marker N-tubulin, the neuronal specific intermediate filament protein NF-M, the postmitotic neuronal marker NeuN, and glial GFAP. Moreover, the cells displayed TTX sensitive voltage dependent (VD) sodium currents (INa) and TEA sensitive delayed rectifier potassium currents (KDR). Four weeks after injury, fluorescently labeled cells were detected in the lesioned cortex. Neurospecific marker expression was increased in DPSCs found in the area of the cortical lesions compared to that in fluorescent cells of uninjured brain. TTX sensitive VD sodium currents and TEA sensitive KDR significantly increased in labeled cells of the cortically injured area. In conclusion, our data demonstrate that engrafted DPSC-derived cells integrate into the host brain and show neuronal properties not only by expressing neuron-specific markers but also by exhibiting voltage dependent sodium and potassium channels. This proof of concept study reveals that predifferentiated hDPSCs may serve as useful sources of neuro- and gliogenesis in vivo, especially when the brain is injured.  相似文献   
1000.
Stem cell markers are utilized to isolate or identify stem cells. So far, many stem-cell-specific markers have been described, although some of them turned out to be not as specific as it was originally proposed. In this study, we sought to search for a specific stem cell marker that would be phenotypically helpful, characteristically specific, economically affordable and easy to use. Because organelles are one of the major characteristics of eukaryotic cells, we asked the question of whether organelle characteristics might be a useful tool for stem cell characterization. We studied distribution and characteristics of the endoplasmic reticulum, the mitochondria and the Golgi apparatus in human dental-pulp-derived mesenchymal stem cells before and during osteogenic differentiation. Although it was not possible to find a useful macromolecular marker for stem cell characterization, we found that during osteogenic differentiation, the stem cells changed their Golgi characteristics and displayed a unique in vivo pattern. We analysed these unique Golgi structures and proposed a potential osteogenic differentiation marker for human dental-pulp-derived mesenchymal stem cells. This pattern may be used in the evaluation of osteogenic differentiation.  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