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   17782篇
  免费   1370篇
  国内免费   1344篇
  20496篇
  2024年   40篇
  2023年   194篇
  2022年   504篇
  2021年   858篇
  2020年   567篇
  2019年   711篇
  2018年   637篇
  2017年   491篇
  2016年   726篇
  2015年   1120篇
  2014年   1281篇
  2013年   1400篇
  2012年   1683篇
  2011年   1461篇
  2010年   916篇
  2009年   802篇
  2008年   1038篇
  2007年   820篇
  2006年   746篇
  2005年   626篇
  2004年   501篇
  2003年   428篇
  2002年   338篇
  2001年   306篇
  2000年   250篇
  1999年   234篇
  1998年   162篇
  1997年   134篇
  1996年   141篇
  1995年   136篇
  1994年   120篇
  1993年   97篇
  1992年   146篇
  1991年   126篇
  1990年   90篇
  1989年   113篇
  1988年   65篇
  1987年   56篇
  1986年   70篇
  1985年   75篇
  1984年   23篇
  1983年   32篇
  1982年   31篇
  1981年   13篇
  1980年   32篇
  1979年   21篇
  1978年   20篇
  1977年   19篇
  1974年   11篇
  1972年   15篇
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
171.
Histamine was immobilized on Sepharose CL‐6B (Sepharose) for use as a ligand of hydrophobic charge induction chromatography (HCIC) of proteins. Lysozyme adsorption onto Histamine‐Sepharose (HA‐S) was studied by adsorption equilibrium and calorimetry to uncover the thermodynamic mechanism of the protein binding. In both the experiments, the influence of salt (ammonium sulfate and sodium sulfate) was examined. Adsorption isotherms showed that HA‐S exhibited a high salt tolerance in lysozyme adsorption. This property was well explained by the combined contributions of hydrophobic interaction and aromatic stacking. The isotherms were well fitted to the Langmuir equation, and the equilibrium parameters for lysozyme adsorption were obtained. In addition, thermodynamic parameters (ΔHads, ΔSads, and ΔGads) for the adsorption were obtained by isothermal titration calorimetry by titrating lysozyme solutions into the adsorbent suspension. Furthermore, free histamine was titrated into lysozyme solution in the same salt‐buffers. Compared with the binding of lysozyme to free histamine, lysozyme adsorption onto HA‐S was characterized by a less favorable ΔGads and an unfavorable ΔSads because histamine was covalently attached to Sepharose via a three‐carbon‐chain spacer. Consequently, the immobilized histamine could only associate with the residues on the protein surface rather than those in the hydrophobic pocket, causing a less favorable orientation between histamine and lysozyme. Further comparison of thermodynamic parameters indicated that the unfavorable ΔSads was offset by a favorable ΔHads, thus exhibiting typical enthalpy‐entropy compensation. Moreover, thermodynamic analyses indicated the importance of the dehydration of lysozyme molecule and HA‐S during the adsorption and a substantial conformational change of the protein during adsorption. The results have provided clear insights into the adsorption mechanisms of lysozyme onto the new HCIC material. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2010  相似文献   
172.
A novel diamino-substituted hypocrellin derived from hypocrellin B (HB) was synthesized by a mild method. The red absorption of the resulting product was significantly enhanced relative to the parent hypocrellins and any other hypocrellin derivatives, and the active oxygen species generating abilities were enhanced distinctly, which will remarkably improve its photodynamic therapy effectiveness.  相似文献   
173.
Zhang H  Xia Y  Wang G  Shen Z 《Planta》2008,227(2):465-475
The effects of excess copper (Cu) on the accumulation of hydrogen peroxide (H2O2) and antioxidant enzyme activities in roots of the Cu accumulator Elsholtzia haichowensis Sun were investigated. Copper at 100 and 300 μM significantly increased the concentrations of malondialdehyde and H2O2, and the activities of catalase (E.C. 1.11.1.6), ascorbate peroxidase (E.C. 1.11.1.11), guaiacol peroxidase (GPOD, E.C. 1.11.1.7) and superoxide dismutase (SOD, E.C. 1.15.1.1). Isoenzyme pattern and inhibitor studies showed that, among SOD isoforms, only copper–zinc superoxide dismutase (CuZn–SOD) increased. Excess Cu greatly increased the accumulation of superoxide anion (O2 ·−) and H2O2 in E. haichowensis roots. This study also provides the first cytochemical evidence of an accumulation of H2O2 in the root cell walls as a consequence of Cu treatments. Experiments with diphenyleneiodonium as an inhibitor of NADPH oxidase, 1,2-dihydroxybenzene-3,5-disulphonic acid as an O2 ·− scavenger, and N-N-diethyldithiocarbamate as an inhibitor of SOD showed that the source of H2O2 in the cell walls could partially be NADPH oxidase. The enzyme can use cytosolic NADPH to produce O2 ·−, which rapidly dismutates to H2O2 by SOD. Apoplastic GPOD and CuZn–SOD activities were induced in roots of E. haichowensis with 100 μM Cu suggesting that these two antioxidant enzymes may be responsible for H2O2 accumulation in the root apoplast.  相似文献   
174.
The neural cell adhesion molecule (NCAM) plays a pivotal role in the development and maintenance of the nervous system via homophilic (NCAM–NCAM) and heterophilic (NCAM-other molecules) interactions. Many synthetic peptides have been engineered to mimic these interactions and induce NCAM-downstream signaling pathways. Such NCAM mimetics have displayed neuritogenic and neuroprotective properties, as well as synaptic modulation in vitro and in vivo. Furthermore, they have been used successfully in preclinical studies to treat neurological disorders including stroke, traumatic brain injury and Alzheimer’s disease. This review focuses on recent progress in the development of NCAM mimetic peptides, in particular, on establishing C3, plannexin, and FGL as therapeutic candidates for neurological disorders.  相似文献   
175.
不同光质对中华植生藻的影响   总被引:10,自引:0,他引:10  
光质是调节控制植物代谢的基本因素之一,它对植物的生长、形态结构、光合作用和物质代谢都具有一定的调控作用。大多数生物合成过程能通过改变光质进行调节。中华植生藻是作者分离培养出的一个具异形胞丝状的固氮蓝藻新种,它具有生长快,易培养,成本低,蛋白质含量高及营养丰富等优点1,是一个具有较好应用前景的优良藻株。为进一步了解它对光质的反应特征,本研究在不同光质条件下观察了中华植生藻的生理生化特征。    相似文献   
176.
One particular strategy to render anticancer therapies efficient consists of converting the patient's own tumor cells into therapeutic vaccines, via the induction of immunogenic cell death (ICD). One of the hallmarks of ICD dwells in the active release of ATP by cells committed to undergo, but not yet having succumbed to, apoptosis. We observed that the knockdown of essential autophagy-related genes (ATG3, ATG5, ATG7 and BECN1) abolishes the pre-apoptotic secretion of ATP by several human and murine cancer cell lines undergoing ICD. Accordingly, autophagy-competent, but not autophagy-deficient, tumor cells treated with ICD inducers in vitro could induce a tumor-specific immune response in vivo. Cancer cell lines stably depleted of ATG5 or ATG7 normally generate tumors in vivo, and such autophagy-deficient neoplasms, upon systemic treatment with ICD inducers, exhibit the same levels of apoptosis (as monitored by nuclear shrinkage and caspase-3 activation) and necrosis (as determined by following the kinetics of HMGB1 release) as their autophagy-proficient counterparts. However, autophagy-incompetent cancers fail to release ATP, to recruit immune effectors into the tumor bed and to respond to chemotherapy in conditions in which autophagy-competent tumors do so. The intratumoral administration of ecto-ATPase inhibitors increases extracellular ATP concentrations, re-establishes the therapy-induced recruitment of dendritic cells and T cells into the tumor bed, and restores the chemotherapeutic response of autophagy-deficient cancers. Altogether, these results suggest that autophagy-incompetent tumor cells escape from chemotherapy-induced (and perhaps natural?) immunosurveillance because they are unable to release ATP.  相似文献   
177.
Chen RF  Shen RF  Gu P  Dong XY  DU CW  Ma JF 《Annals of botany》2006,98(2):389-395
BACKGROUND AND AIMS: Rice (Oryza sativa) is an aquatic plant with a characteristic of forming iron plaque on its root surfaces. It is considered to be the most Al-tolerant species among the cereal crops. The objective of this study was to determine the effects of root surface iron plaque on Al translocation, accumulation and the change of physiological responses under Al stress in rice in the presence of iron plaque. METHODS: The japonica variety rice, Koshihikari, was used in this study and was grown hydroponically in a growth chamber. Iron plaque was induced by exposing the rice roots to 30 mg L(-1) ferrous iron either as Fe(II)-EDTA in nutrient solution (6 d, Method I) or as FeSO(4) in water solution (12 h, Method II). Organic acid in root exudates was retained in the anion-exchange resin and eluted with 2 m HCl, then analysed by high-performance liquid chromatography (HPLC) after proper pre-treatment. Fe and Al in iron plaque were extracted with DCB (dithionite-citrate-bicarbonate) solution. KEY RESULTS AND CONCLUSIONS: Both methods (I and II) could induce the formation of iron plaque on rice root surfaces. The amounts of DCB-extractable Fe and Al on root surfaces were much higher in the presence of iron plaque than in the absence of iron plaque. Al contents in root tips were significantly decreased with iron plaque; translocation of Al from roots to shoots was significantly reduced with iron plaque. Al-induced secretion of citrate was observed and iron plaque could greatly depress this citrate secretion. These results suggested that iron plaque on rice root surfaces can be a sink to sequester Al onto the root surfaces and Fe ions can pre-saturate Al-binding sites in root tips, which protects the rice root tips from suffering Al stress to a certain extent.  相似文献   
178.
179.
Aspergillusoryzae2336经两次紫外诱变后筛选出突变菌株UV21012-1。该菌株曲酸产量比出发菌株提高了1.68倍;遗传性状稳定,传代5次曲酸产量基本不下降,而且发酵周期也比出发菌株明显缩短。  相似文献   
180.
目的首次报道中国北方1例由Fonsecaea monophora所致着色芽生菌病。方法患者男性,58岁,主因"右腕部皮损伴瘙痒8~9a"就诊。对皮损脓液直接镜检,皮损组织病理检查,真菌培养,并对培养获得菌株进行形态学,分子生物学鉴定等实验室研究。结果脓液直接镜检可见多个圆形、厚壁、棕色硬壳细胞,皮损组织病理表现为慢性肉芽肿样改变,并可见硬壳小体。真菌培养可见暗棕色,橄榄色至黑色菌落生长,生长速度较慢,镜下可见枝孢型和喙枝孢型产孢。ITS区序列分析鉴定为Fonsecaea monophora。依据临床及实验室检查确诊该例为Fonsecaea monophora所致的着色芽生菌病。口服伊曲康唑200mg,1次/d;特比萘芬250mg,1次/d。治疗3个月后,皮损消退痊愈。结论 Fonsecaea monophora感染也可见于我国北方地区的着色芽生菌病患者,而ITS区序列分析是该菌种鉴定的重要手段,伊曲康唑联合特比萘芬治疗本例患者显示较好疗效。  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