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   1720篇
  免费   104篇
  国内免费   140篇
  2023年   26篇
  2022年   59篇
  2021年   71篇
  2020年   57篇
  2019年   80篇
  2018年   67篇
  2017年   52篇
  2016年   38篇
  2015年   43篇
  2014年   88篇
  2013年   201篇
  2012年   61篇
  2011年   107篇
  2010年   78篇
  2009年   66篇
  2008年   73篇
  2007年   101篇
  2006年   79篇
  2005年   70篇
  2004年   61篇
  2003年   52篇
  2002年   48篇
  2001年   45篇
  2000年   29篇
  1999年   35篇
  1998年   20篇
  1997年   16篇
  1996年   18篇
  1995年   13篇
  1994年   16篇
  1993年   14篇
  1992年   10篇
  1991年   14篇
  1990年   7篇
  1989年   5篇
  1988年   8篇
  1987年   4篇
  1986年   9篇
  1985年   18篇
  1984年   27篇
  1983年   8篇
  1982年   10篇
  1981年   11篇
  1980年   5篇
  1979年   9篇
  1977年   3篇
  1976年   5篇
  1975年   6篇
  1974年   7篇
  1973年   7篇
排序方式: 共有1964条查询结果,搜索用时 15 毫秒
991.
In vitro synthesis of cefminox by cell-free extracts of Streptomyces clavuligerus was investigated using alpha-ketoglutarate, L-ascorbic acid, FeSO(4), S-adenosyl-L-methionine, and 7alpha-demethoxycefminox as the substrates. The formation of cefminox was detected both by a biological assay with Proteus vulgaris GN 76/C-1 and by high performance liquid chromatography. Although the conversion rate of 7alpha-demethoxycefminox to cefminox was observed to be quite low, it still demonstrated the potential for an enzymatic process to replace the chemical steps which are currently in use for the production of cefminox.  相似文献   
992.
Purpose. To evaluate the outcome of corneal transplantation and the feasibility of tissue donation from donors with methanol intoxication. Methods. Four corneas from two methanol intoxication victims were procured and transplanted in four patients at our medical center. The outcome and graft survival were evaluated. Results. The recipients were between 12 and 49 years of age. Indications for transplantation were keratoconus (two patients), post-traumatic corneal perforation (one patient) and alkaline burn (one patient). The follow-up period ranged between 12 and 60 months. All grafts remained clear, and the postoperative visual acuity was 20/30 in keratoconus patients and 20/60 in others. Conclusions. The methanol intoxication victims appear to be suitable donors for corneal transplantation. To the best of our knowledge, this is the first study to report successful corneal transplantation from donors with methanol intoxication.  相似文献   
993.
The influence of varying concentrations (0, 1, 3, 4, 5, 6, 8, and 10 % v/v) of neem (Azadirachta indica) leaf extract on drought stressed (40 % field capacity) quinoa (Chenopodium quinoa Willd.) plants was assessed. During the current study two cultivars of quinoa (V7 and V9) were used. This study revealed that water stress adversely affects the fresh and dry weight of shoots and roots as well as chlorophyll pigments (a and b) of both quinoa cultivars. In contrast, drought stress enhanced glycinebetaine (GB), free proline, phenolic content, hydrogen peroxide (H2O2), activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) enzymes, and relative membrane permeability (RMP). However, application of neem leaf extract improved the accumulation of key osmoprotectants like proline, GB and activities of enzymatic antioxidants. Our findings showed 5 % neem leaf extract is an effective treatment in counteracting the oxidative damage caused by water stress, thereby improving overall plant growth. Of both cultivars of quinoa, the response of cv. V9 to stress as well as foliar applied neem was relatively more promising.  相似文献   
994.
Major objective of this study was to explore the protective effect of the methanolic extract of Chenopodium album against carbon tetrachloride induced hepatotoxicity in rats. Chenopodium album has locally been used for multiple medicinal proposes. Methanolic extract of Chenopodium album (whole plant) was prepared with Soxhlet extractor and rotatory evaporator. Antioxidant activity of Chenopodium album was determined by DPPH free radical scavenging assay. Thirty Wister (albino) rats (150–200 g) were divided into six groups for the evaluation of hepatoprotective potential of different concentrations of Chenopodium album against carbon tetrachloride (1:1 CCl4: Olive oil) under the controlled laboratory conditions. Group-I rats were administrated with olive oil (Normal control), Group-II with CCl4 only, Group-III with Silymarin (positive control), Group-IV with Chenopodium album (100 mg/kg), Group-V with Chenopodium album (200 mg/kg) and Group-VI rats with Chenopodium album (300 mg/kg) for the period of 28 days. Serum was taken to determine the levels of alanine transaminase, aspartate transaminase, alkaline phosphatase, cholesterol, triglyceride, creatinine and urea in the blood. Formalin stored tissues were examined for histopathological analysis. DPPH assay showed that Chenopodium album has the potential for reduction of oxidative stress. Chenopodium album minimized the levels of ALT (70 ± 8.68 U/L, 68.75 ± 8.38 U/L & 73.5 ± 10.28 U/L), AST (219.5 ± 19.16 U/L, 140.75 ± 13.35 U/L & 221.25 ± 13.33 U/L) and ALP (289.5 ± 28.21 U/L, 258 ± 11.12 U/L & 248.25 ± 4.03 U/L) at different concentrations (100 mg/kg, 200 mg/kg, 300 mg/kg respectively). Chenopodium album enhanced triglyceride level (64.75 ± 12.66 mg/dl at 200 mg/kg) as compared to CCl4 treated group (33.25 ± 1.26 mg/dl). Carbon tetrachloride elevated urea level (43.25 ± 6.6) was decreased by high dose of Chenopodium album (18 ± 8.17). Moreover, Chenopodium album also improved WBC level (9.69 × 103 /Cu.mr & 10.59 × 103 /Cu.mr at low and medium doses respectively), RBCs level (6.97 × 103 /Cu.mr) and hemoglobin level (13.95 G/dL, 13.467 G/dL & 14.05 G/dL at low, medium and high doses). In vivo study of Chenopodium album methanolic extract demonstrates the potential for protection of liver and after pre-clinical studies the plant can be used as a safe alternative of commercially available hepatoprotective medicines.  相似文献   
995.
甘蔗是糖类加工产业主要的经济作物,甘蔗叶作为广西特色瑶药,在广西民间及瑶族地区具有悠久的药用历史。该课题组前期发现甘蔗叶乙酸乙酯部位具有抗肿瘤活性,为进一步明确其乙酸乙酯部位的化学成分,,该文采用硅胶柱色谱、Sephadex LH-20柱色谱、制备型高效液相色谱等多种分离纯化的方法对甘蔗叶乙酸乙酯部位进行研究。结果表明:从甘蔗叶乙酸乙酯萃取部位分离且鉴定了20个化合物,分别为原儿茶醛(1)、3,4-二羟基-苯甲酸甲酯(2)、3, 4-二羟基苯甲酸(3)、3-羟基-4-甲氧基苯甲酸(4)、对羟基苯甲酸(5)、对羟基苯甲醛(6)、对羟基肉桂酸(7)、丁香酸(8)、3, 5-二甲氧基对苯二酚(9)、1-hydroxy-benzoyl-4-O-α-L-rhamnopyranoside(10)、对羟基苯甲酸-β-D-吡喃葡萄糖酯苷(11)、槲皮素(12)、小麦黄素(13)、异柽柳素(14)、异鼠李素(15)、5, 3'', 4''-三羟基-7-甲氧基二氢黄酮(16)、7-O-甲基圣草酚(17)、[(E)-4-(1S,3R,4R)-1-hydroxy-4,5,5-trimethyl-7-oxabicyclo [4.1.0]heptan-1-yl]but-1-en-3-o-ne(18)、blumenol A(19)和胸腺嘧啶脱氧核苷(20)。其中,化合物1-4、6、9-11、13-16、18和20均为首次从甘蔗叶中分离得到。该研究结果为日后甘蔗叶乙酸乙酯部分的进一步开发提供了依据  相似文献   
996.
997.
每种粮食的储粮环境中都存在有其特征性挥发物,储粮害虫的发生对储粮环境中挥发物具有重要影响。本文简述了顶空式固相微萃取法、浸入式固相微萃取法、电子鼻检测法等储粮挥发物的提取方法及其优缺点,对小麦 Triticum aestivum L.、稻谷 Olyza sativa L.、玉米 Zea mays L.和燕麦 Avena sativa L.等主要储藏粮食种类的挥发性化合物成分、粮食挥发物对昆虫行为反应的影响、主要储粮害虫(赤拟谷盗 Tribolium castaneum、锈赤扁谷盗 Cryptolestes ferrugineus 和象虫 Sitophilus spp.)发生与粮食挥发物的关系等研究进展进行综述,探讨了储粮环境挥发性化合物与储粮害虫关系未来的研究方向,以期对今后储粮害虫生态防治研究与应用提供参考信息。  相似文献   
998.
The Rad6/Rad18-dependent monoubiquitination of PCNA plays a crucial role in regulating replication past DNA damage in eukaryotic cells. We show here that in human cell-free extracts, efficient PCNA monoubiquitination requires both the synthesis of relatively long DNA tracts and polymerase idling or stalling at sites of DNA modification or DNA secondary structures. This dual dependency suggests a dynamic process in which, following initiation, the DNA synthesizing complex undergoes modifications that make it competent as a mediator for the activation of the Rad6/Rad18 pathway.  相似文献   
999.
博落回散能抑菌消炎、改善畜禽肠道内环境,兼具营养和药用双重作用,已经成为一类新型饲料添加剂。酶制剂是目前已在养殖业广泛使用的饲料添加剂,它能提高动物对饲料的消化利用率、提高饲料转化率、降低饲喂成本。博落回散能否和酶制剂同时使用目前尚未有相关的研究报道。本文研究了不同浓度博落回散溶液对8种常用饲用酶制剂活性的影响,实验结果表明,在正常使用浓度(30~50 mg/L)范围内,博落回散不会明显影响淀粉酶、纤维素酶、果胶酶、甘露聚糖酶和木聚糖酶这5种糖基水解酶的活性;对植酸酶、酸性蛋白酶和脂肪酶这3种非糖基水解酶的活性影响不大,这说明博落回散能和酶制剂同时添加到饲料中。本研究为兽用博落回散饲料添加剂在养殖行业的广泛应用提供了实验支持。  相似文献   
1000.
the present study the was done to evaluate chemopreventive and chemotherapeutic anti-tumor potential of some Egyptian plant extract (moringa, graviola, ginger garden cress and artemisinin) against 7,12-dimethylbenz(a)anthracene (DMBA)-induced mammary carcinogenesis in Swiss albino mice. chemopreventive and chemotherapeutic evaluation was assessed by monitoring the tumor incidence and tumor volume as well as by analyzing the status of (a) biochemical markers (maspin, survivin, livin, caveolin-1, osteopontin and Fucosyltransferase 4 gene expressions), oxidative stress related profile including; total antioxidant capacity (TAC), glutathione reductase (GR) activity, glutathione-s-transferase (GST) activity assay, superoxide dismutase (SOD) activity, catalase (CAT) activity and lipid peroxidation (MDA), renal and hepatic toxicity markers (urea, creatinine, alanine transaminase (alt) activity, aspartate aminotransferase (ast) activity, alkaline phosphatase (ALP) Activity and γ-Glutamyltransferase (GGT) activity also study of (b) biophysical markers (trace and heavy metals (lead (Pb), cadmium (Cd), chromium (Cr), nickel (Ni), iron (Fe), selenium (Se), copper (Cu) and zinc (Zn)), dielectric properties and body water distribution) finally (c) histopathological examination oral administration of increasing dose of moringa, graviola, ginger garden cress and artemisinin extracts, respectively significantly prevented the tumor incidence and tumor volume as well as brought back the status of the above mentioned biochemical and biophysical variables. Histopathological changes also confirmed the formation of tumor tubules and neovascularization after the treatment. Overall, these results suggest that treatment with moringa, graviola, ginger garden cress and artemisinin extracts provided antioxidant defense with strong chemopreventive and chemotherapeutic activity against DMBA-induced mammary tumors.  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