全文获取类型
收费全文 | 2811篇 |
免费 | 162篇 |
国内免费 | 4篇 |
出版年
2023年 | 9篇 |
2022年 | 14篇 |
2021年 | 36篇 |
2020年 | 30篇 |
2019年 | 47篇 |
2018年 | 70篇 |
2017年 | 46篇 |
2016年 | 76篇 |
2015年 | 131篇 |
2014年 | 152篇 |
2013年 | 193篇 |
2012年 | 248篇 |
2011年 | 246篇 |
2010年 | 161篇 |
2009年 | 121篇 |
2008年 | 193篇 |
2007年 | 192篇 |
2006年 | 132篇 |
2005年 | 148篇 |
2004年 | 156篇 |
2003年 | 85篇 |
2002年 | 87篇 |
2001年 | 70篇 |
2000年 | 74篇 |
1999年 | 59篇 |
1998年 | 15篇 |
1997年 | 16篇 |
1996年 | 16篇 |
1995年 | 21篇 |
1994年 | 10篇 |
1993年 | 7篇 |
1992年 | 16篇 |
1991年 | 20篇 |
1990年 | 10篇 |
1989年 | 8篇 |
1988年 | 8篇 |
1987年 | 9篇 |
1986年 | 4篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1980年 | 5篇 |
1979年 | 8篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1974年 | 2篇 |
1973年 | 3篇 |
1970年 | 1篇 |
1969年 | 4篇 |
1967年 | 1篇 |
1966年 | 1篇 |
排序方式: 共有2977条查询结果,搜索用时 15 毫秒
101.
102.
Chaga mushroom extract inhibits oxidative DNA damage in human lymphocytes as assessed by comet assay 总被引:7,自引:0,他引:7
The Chaga mushroom (Inonotus obliquus) is claimed to have beneficial properties for human health, such as anti-bacterial, anti-allergic, anti-inflammatory and antioxidant activities. The antioxidant effects of the mushroom may be partly explained by protection of cell components against free radicals. We evaluated the effect of aqueous Chaga mushroom extracts for their potential for protecting against oxidative damage to DNA in human lymphocytes. Cells were pretreated with various concentrations (10, 50, 100 and 500 microg/mL) of the extract for 1 h at 37 degrees C. Cells were then treated with 100 microM of H2O2 for 5 min as an oxidative stress. Evaluation of oxidative damage was performed using single-cell gel electrophoresis for DNA fragmentation (Comet assay). Using image analysis, the degree of DNA damage was evaluated as the DNA tail moment. Cells pretreated with Chaga extract showed over 40% reduction in DNA fragmentation compared with the positive control (100 micromol H2O2 treatment). Thus, Chaga mushroom treatment affords cellular protection against endogenous DNA damage produced by H2O2. 相似文献
103.
Smoking increases indices of free radical-mediated damage of DNA which are potential underlying processes in the pathogenesis of many diseases. In this study, we evaluated whether 8 weeks of green vegetable drink (Angelica keiskei based juice) supplementation to smokers can be protective against lymphocytic DNA damage. Twenty smokers were given 240 ml of commercially available green vegetable drink every day for 8 weeks. The DNA damage was determined using single cell gel electrophoresis (COMET assay) and the damage was quantified by measuring tail length (TL), tail moment (TM), and percent DNA in tail. Eight weeks of green vegetable drink consumption resulted in a significant in lymphocytes DNA damage in all three measurements; TL, TM and % DNA in tail. These results support the hypothesis that green vegetable drink exerts a cancer-protective effect via a decrease in oxidative damage to DNA in humans. 相似文献
104.
Pyrolysis mass spectrometry (PyMS) is a rapid, simple, high-resolution analytical method based on thermal degradation of complex material in a vacuum and has been widely applied to the discrimination of closely related microbial strains. Leaf samples of six species and one variety of higher plants (Rosa multiflora, R. multiflora var. platyphylla, Sedum kamtschaticum, S. takesimense, S. sarmentosum, Hepatica insularis, and H. asiatica) were subjected to PyMS for spectral fingerprinting. Principal component analysis of PyMS data was not able to discriminate these plants in discrete clusters. However, canonical variate analysis of PyMS data separated these plants from one another. A hierarchical dendrogram based on canonical variate analysis was in agreement with the known taxonomy of the plants at the variety level. These results indicate that PyMS is able to discriminate higher plants based on taxonomic classification at the family, genus, species, and variety level.Abbreviations
ANNs
Artificial neural networks
-
CVA
Canonical variate analysis
-
GC/MS
Gas chromatography/mass spectrometry
-
PCA
Principal component analysis
-
PyMS
Pyrolysis mass spectrometry
-
UPGMA
Unweighted pair group method with arithmetic mean
Communicated by I.S. Chung 相似文献
105.
GERI-155 is a macrolide antibiotic containing two deoxyhexose molecules which has antimicrobial activities against gram-positive bacteria. The deoxyhexose biosynthetic gene cluster of GERI-155 from Streptomyces sp. GERI-155 genome has now been isolated. Four orf were identified and a putative orf, supposed to code for the dTDP-deoxyglucose epimerase gene, was designated as gerF. gerF was expressed in E. coli using recombinant expression vector pHJ3. The recombinant protein expressed in a soluble form. The enzyme was purified by Ni-affinity column using imidazole buffer as eluents. The molecular mass of the expressed protein correlated with the predicted mass (36,000 Da) deduced from the cloned gene sequence data. The purified enzyme produced maltol from dTDP-4-keto-6-deoxyglucose and it was confirmed that the expressed protein was dTDP-deoxyglucose epimerase catalyzing epimerization of C-3 and C-5 or C-3 of dTDP-4-keto-6-deoxyglucose. 相似文献
106.
Enhanced process monitoring of fed-batch penicillin cultivation using time-varying and multivariate statistical analysis 总被引:10,自引:0,他引:10
On-line monitoring of penicillin cultivation processes is crucial to the safe production of high-quality products. In the past, multiway principal component analysis (MPCA), a multivariate projection method, has been widely used to monitor batch and fed-batch processes. However, when MPCA is used for on-line batch monitoring, the future behavior of each new batch must be inferred up to the end of the batch operation at each time and the batch lengths must be equalized. This represents a major shortcoming because predicting the future observations without considering the dynamic relationships may distort the data information, leading to false alarms. In this paper, a new statistical batch monitoring approach based on variable-wise unfolding and time-varying score covariance structures is proposed in order to overcome the drawbacks of conventional MPCA and obtain better monitoring performance. The proposed method does not require prediction of the future values while the dynamic relations of data are preserved by using time-varying score covariance structures, and can be used to monitor batch processes in which the batch length varies. The proposed method was used to detect and identify faults in the fed-batch penicillin cultivation process, for four different fault scenarios. The simulation results clearly demonstrate the power and advantages of the proposed method in comparison to MPCA. 相似文献
107.
The most abundant root proteins of ginseng (Panax ginseng) have been detected and identified by comparative proteome analysis with cultured hairy root of ginseng. Four abundant proteins (28, 26, 21 and 20 kDa) of P. ginseng had isoforms with different pl values on two-dimensional gel electrophoresis (2DE). The results of N-terminal and internal amino acid sequencing, however, showed that all of them originate from a 28 kDa protein, known as ginseng major protein (GMP). The GMP gene was searched for in the expressed sequence tag database of P. ginseng and found to encode a 27.3 kDa protein having 238 amino acid residues. Analysis of the amino acid sequences indicates that GMP exhibits high sequence homology with plant RNases and RNase-like proteins. However, purified GMP had no RNase activity even though it has conserved amino acid residues known to be essential for active sites of RNase. The GMPs present in ginseng main root were not expressed in cultured hairy roots of ginseng. 2DE analysis showed that the amounts of GMPs in main roots change according to seasonal fluctuation. These results suggest that the GMPs are root-specific RNase-like proteins, which function as vegetative storage proteins of ginseng for survival in the natural environment. 相似文献
108.
Yoo DS 《Bioelectromagnetics》2004,25(7):492-497
The dielectric properties of various cancers, namely brain tumor, breast cancer, gastric carcinoma, and colon cancer, were measured in the frequency range of 500 MHz to 5 GHz. Cancers were cultivated applying the xenograft model of growing human cancerous tissues using the specific pathogen free, homo inbred mouse (a nude mouse). The complex permittivity was measured using an open-ended coaxial probe (HP85070B) and a computer controlled network analyzer (HP8510C). For the measurement of the dielectric properties, a total of 58 xenografted specimens was used. The results showed that measured values of complex permittivity for all four cancerous tissues were similar, with little variations over the frequency range used. It might be agreed that components and characteristics of different cancerous tissues would be similar despite their different occurrences in the human body. It is necessary to investigate this result further. 相似文献
109.
The activities of antioxidant enzymes in response to oxidative stresses and hormones in paraquat-tolerant Rehmannia glutinosa plants 总被引:1,自引:0,他引:1
Choi DG Yoo NH Yu CY de Los Reyes B Yun SJ 《Journal of biochemistry and molecular biology》2004,37(5):618-624
All members of R. glutinosa show the unique characteristic of intrinsic tolerance to paraquat (PQ). Antioxidant enzymes have been proposed to be the primary mechanism of PQ resistance in several plant species. Therefore, the antioxidant enzyme systems of R. glutinosa were evaluated by comparatively analyzing cellular antioxidant enzyme levels, and their responses of oxidative stresses and hormones. The levels of ascorbate peroxidase (APX), glutathione reductase (GR), non-specific peroxidase (POX), and superoxide dismutase (SOD) were 7.3-, 4.9-, 2.7- and 1.6-fold higher in PQ-tolerant R. glutinosa than in PQ-susceptible soybeans. However, the activity of catalase (CAT) was about 12-fold higher in the soybeans. The activities of antioxidant enzymes reduced after PQ treatment in the two species, with the exception of POX and SOD in R. glutinosa, which increased by about 40 %. Interestingly, the activities of APX, SOD and POX in R. glutinosa, relative to those in soybeans, were further increased by 49, 67 and 93 % after PQ treatment. The considerably higher intrinsic levels, and increases in the relative activities of antioxidant enzymes in R. glutinosa under oxidative stress support the possible role of these enzymes in the PQ tolerance of R. glutinosa. However, the relatively lower levels of SOD versus PQ tolerance, and the mixed responses of antioxidant enzymes to stresses and hormones, suggest a possible alternative mechanism(s) for PQ tolerance in R. glutinosa. 相似文献
110.
Kim SU Lee KM Park SK Yoo KY Noh DY Choe KJ Ahn SH Hirvonen A Kang D 《Journal of biochemistry and molecular biology》2004,37(5):582-585
To evaluate the potential association between the GSTP1 genotype and the development of breast cancer, a hospital based case-control study was conducted on Korean women. The study population consisted of 171 histologically confirmed incident breast cancer cases and 171 age-matched controls with no present or previous history of cancer. PCR-RFLP was used for the GSTP1 genotyping and statistical evaluations were performed using an unconditional logistic regression model. Postmenopausal women with the GSTP1 Val allele were found to have a reduced risk of breast cancer (OR = 0.3, 95 % CI = 0.10-0.74). A significant interaction was observed between the GSTP1 genotype and alcohol consumption (p for interaction = 0.01); compared with never-drinking women with Ile/Ile genotype, ever-drinking women with the GSTP1 Val allele had almost a three-fold risk of breast cancer (OR = 2.9, 95 % CI = 1.05-7.85), whereas never-drinking women with Val allele had half this risk (OR = 0.5, 95 % CI = 0.27-0.93). Our findings suggest that the GSTP1 polymorphism influences individual susceptibility to breast cancer in the Korean women and this effect may be modified by alcohol consumption. 相似文献