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   7224篇
  免费   401篇
  国内免费   25篇
  7650篇
  2024年   11篇
  2023年   101篇
  2022年   266篇
  2021年   388篇
  2020年   342篇
  2019年   506篇
  2018年   394篇
  2017年   280篇
  2016年   361篇
  2015年   400篇
  2014年   484篇
  2013年   627篇
  2012年   570篇
  2011年   573篇
  2010年   300篇
  2009年   267篇
  2008年   299篇
  2007年   296篇
  2006年   257篇
  2005年   206篇
  2004年   161篇
  2003年   117篇
  2002年   110篇
  2001年   20篇
  2000年   20篇
  1999年   27篇
  1998年   22篇
  1997年   18篇
  1996年   17篇
  1995年   11篇
  1994年   10篇
  1993年   15篇
  1992年   9篇
  1991年   15篇
  1990年   6篇
  1989年   10篇
  1988年   4篇
  1987年   12篇
  1986年   13篇
  1985年   15篇
  1984年   13篇
  1983年   9篇
  1982年   8篇
  1981年   7篇
  1980年   6篇
  1974年   8篇
  1972年   3篇
  1971年   3篇
  1963年   3篇
  1962年   3篇
排序方式: 共有7650条查询结果,搜索用时 16 毫秒
91.
Water super absorbents are low cross-linked hydrophilic polymers that absorb water in amounts up to several hundred times their dry weight. In this study, the effect of adding these materials to the bed of a biofilter was investigated. Two equal size biofilters were used for this purpose. One of the biofilters was packed with a mixture of perlite and a commercial polyacrylamide based super absorbent (2.3% dry weight), and the other was packed with perlite to perform as a control. The biofilters were inoculated with a bacterial culture that was able to grow on n-hexane as the sole source of carbon and energy. Both biofilters removed up to 90% of the entering pollutants when using an inlet n-hexane concentration of 1 g/m3, and an air flow rate of 0.3 L/min (mass loading of 18.34 g/m3/h, and empty bed residence time of 3.27 min). The super absorbent had a positive effect on the performance of the biofilter. While the difference in the performance of the biofilters was marginal when frequent moistening was applied, the difference was considerable when moistening was less frequent.  相似文献   
92.
The hemolymph protein HP19 of the rice moth, Corcyra cephalonica, mediates the 20-hydroxyecdysone (20E)-dependent acid phosphatase (ACP) activity at a nongenomic level. Affinity-purified polyclonal antibody against HP19 (alphaHP19-IgG) was used in the present study to understand the role of HP19 during the postembryonic development of Corcyra. In the in vitro studies, HP19 action was blocked either by immuno-precipitation using alphaHP19-IgG, prior to its addition to the fat body culture or by the addition of the antibody directly to the culture, along with 20E and hemolymph containing HP19. The alphaHP19-IgG blocked the HP19-mediated 20E-dependent ACP activation. In the in vivo studies, the alphaHP19-IgG was injected into the fully developed last (final/Vth) instar larvae of Corcyra, to complex the HP19 in vivo, in order to block the action of HP19. The injection of alphaHP19-IgG resulted in defective development of larvae, which grew either into non-viable larvae or larval-pupal/pupal-adult intermediates relative to the effect of pre-immune IgG injected controls. The present study shows that HP19 plays an important role in controlling the metamorphosis of Corcyra by regulating the 20E-dependent ACP activity. Coupled with the earlier findings, the ecdysteroid hormone regulates this action at a nongenomic level.  相似文献   
93.
94.
The performance of encapsulated Saccharomyces cerevisiae CBS 8066 in anaerobic cultivation of glucose, in the presence and absence of furfural as well as in dilute-acid hydrolyzates, was investigated. The cultivation of encapsulated cells in 10 sequential batches in synthetic media resulted in linear increase of biomass up to 106 g/L of capsule volume, while the ethanol productivity remained constant at 5.15 (+/-0.17) g/L x h (for batches 6-10). The cells had average ethanol and glycerol yields of 0.464 and 0.056 g/g in these 10 batches. Addition of 5 g/L furfural decreased the ethanol productivity to a value of 1.31 (+/-0.10) g/L x h with the encapsulated cells, but it was stable in this range for five consecutive batches. On the other hand, the furfural decreased the ethanol yield to 0.41-0.42 g/g and increased the yield of acetic acid drastically up to 0.068 g/g. No significant lag phase was observed in any of these experiments. The encapsulated cells were also used to cultivate two different types of dilute-acid hydrolyzates. While the free cells were not able to ferment the hydrolyzates within at least 24 h, the encapsulated yeast successfully converted glucose and mannose in both of the hydrolyzates in less than 10 h with no significant lag phase. However, since the hydrolyzates were too toxic, the encapsulated cells lost their activity gradually in sequential batches.  相似文献   
95.
Present study investigates relationships between total and bioaccessibility of trace elements (Cd, Co, Cr, Cu, Mn, NI, Pb, V, and Zn) concentrations in sediment and their bioaccumulation in species in Shadegan wetland in southwest of Iran. Bioavailability factor (BAF) and translocation factor (TF) were calculated in plants and trophic transfer factor (TTF) was determined in bird species. For this purpose, sampling of sediments, aquatic plants including Phragmites australis, Typha australis, Scripus maritimus and two bird species encircling Porphyrio porphyrio and globally threatened Marmaronetta angustirostris were carried out during winter 2009. Result of chemical analysis show that bioaccessibility concentrations of Mn (8.31 mg/kg), V (1.33 mg/kg), and Pb (1.03 mg/kg) are higher than other metals. The uptake trend of trace elements in plant decreases as root > stem > leaf. Accumulation levels of trace elements in different tissues of P. porphyrio and M. angustirostris are almost identical and considerable. Accumulation and toxicity of Cd in birds is more than plants. In addition, BAF of V, Pb, and Cr indicates high accumulation by plants and great pollution rate in the area of study. In S. maritimus TF for Mn, Cu, Pb, and V are high whereas in T. australis, Cu and Pb posses the highest TF. Also Cr, Co, Mn, Ni, and Zn have higher TF from stem to leaf than root to stem in P. australis. Finally, TTFs were compared in various bird species.  相似文献   
96.
97.
98.
Menthol is an organic compound with diverse medicinal and commercial applications, and is made either synthetically or through extraction from mint oils. The aim of the present study was to investigate menthol levels in selected menthol-producing species belonging to the Lamiaceae family, and to determine phylogenetic relationships of menthol dehydrogenase gene sequence among these species. Three genus of Lamiaceae, namely Mentha, Salvia, and Micromeria, were selected for phytochemical and phylogenetic analyses. After identification of each species based on menthol dehydrogenase gene in NCBI, BLAST software was used for the sequence alignment. MEGA4 software was used to draw phylogenetic tree for various species. Phytochemical analysis revealed that the highest and lowest amounts of both essential oil and menthol belonged to Mentha spicata and Micromeria hyssopifolia, respectively. The species Mentha spicata and Mentha piperita, which were assigned to one cluster in the dendrogram, contained the highest amounts of essential oil and menthol while Micromeria species, which was in the distinct cluster and placed in the farther evolutionary distance, contained the lowest amount of essential oil and menthol. Phylogenetic and phytochemistry analyses showed that essential oil and menthol contents of menthol-producing species are associated with menthol dehydrogenase gene sequence.  相似文献   
99.
Diabetes mellitus is associated with an increase in sialic acid concentration along with other complications. Sialic acid changes in NIDDM patients were investigated following bitter melon (55 ml/24 h) and rosiglitazone (4 mg/24 h) treatment. A total of 25 patients of both sexes were used in each experimental group. Patients following bitter melon treatment showed no significant difference of serum sialic acid (57.95±4.90 vs. 57.6±5.56 mg/dl, p=0.17) and serum glucose concentration (93.7±9.63 vs. 88.35±6.31 mg/dl, p=0.78) as compared to control subjects. However, the concentration of total cholesterol was significantly high in these patients as compared to control subjects (192±14.23 vs. 170.6±15.1 mg/dl, p<0.03) but within normal range (160–200 mg/dl), suggesting the significant hypoglycemic and lipid-lowering properties of bitter melon. The patients following rosiglitazone treatment showed a significant increase of serum sialic acid concentration (60.2±5.80 vs. 57.6±5.56 mg/dl, p=0.01) along with glucose (112±6.2 vs. 88.35±6.31 mg/dl, p<0.04) and total cholesterol concentration (216.45±20.2 vs. 170.6±15.1 mg/dl, p<0.01) as compared to control subjects. In addition six of the patients had retinopathy, two of whom were suffering also from myocardial infarction and they still had a higher serum sialic acid (61.05±1.20 mg/dl), glucose (187±2.11 mg/dl), total cholesterol (239.10±5.04 mg/dl) and triglyceride (183±4.14 mg/dl) concentration, indicating a poor response of these patients to rosiglitazone. Comparison of serum sialic acid concentration of patients, following bitter melon and rosiglitazone treatment revealed no significant difference but the study showed that bitter melon could be more effective in the management of diabetes and its related complications as compared to rosiglitazone.  相似文献   
100.
A group of 2-alkoxy-5-methoxyallylbenzene were designed, synthesised and evaluated as potential inhibitors of the soybean 15-lipoxygenase (SLO) on the basis of the eugenol and esteragol structures. Compound 4d showed the best half maximal inhibitory concentration (IC??) for SLO inhibition (IC???=?5.9?±?0.6 μM). All the compounds were docked in the SLO active site retrieved from the Research Collaboratory for Structural Bioinformatics (RCSB) Protein Data Bank (PDB entry: 1IK3) and showed that the allyl group of the synthetic compounds similar to the linoleic acid double bond, were oriented toward the Fe3+-OH moiety in the active site of the enzyme and this conformation was especially fixed by the hydrophobic interaction of the 2-alkoxy group with Leu?1?, Trp?1?, Val??? and Ile??2. It was concluded that the molecular volume and shape of the alkoxy moiety was a major factor in the inhibitory potency variation of the synthetic compounds.  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