全文获取类型
收费全文 | 5027篇 |
免费 | 212篇 |
国内免费 | 13篇 |
专业分类
5252篇 |
出版年
2023年 | 55篇 |
2022年 | 142篇 |
2021年 | 296篇 |
2020年 | 125篇 |
2019年 | 170篇 |
2018年 | 189篇 |
2017年 | 149篇 |
2016年 | 211篇 |
2015年 | 253篇 |
2014年 | 285篇 |
2013年 | 412篇 |
2012年 | 358篇 |
2011年 | 365篇 |
2010年 | 204篇 |
2009年 | 174篇 |
2008年 | 195篇 |
2007年 | 189篇 |
2006年 | 175篇 |
2005年 | 147篇 |
2004年 | 116篇 |
2003年 | 86篇 |
2002年 | 92篇 |
2001年 | 56篇 |
2000年 | 62篇 |
1999年 | 47篇 |
1998年 | 29篇 |
1997年 | 14篇 |
1996年 | 20篇 |
1995年 | 17篇 |
1993年 | 16篇 |
1992年 | 31篇 |
1991年 | 47篇 |
1990年 | 41篇 |
1989年 | 33篇 |
1988年 | 28篇 |
1987年 | 42篇 |
1986年 | 30篇 |
1985年 | 36篇 |
1984年 | 28篇 |
1983年 | 20篇 |
1982年 | 17篇 |
1981年 | 13篇 |
1980年 | 13篇 |
1979年 | 23篇 |
1978年 | 24篇 |
1977年 | 13篇 |
1975年 | 14篇 |
1974年 | 13篇 |
1973年 | 20篇 |
1968年 | 12篇 |
排序方式: 共有5252条查询结果,搜索用时 15 毫秒
991.
G. Taret M. Sevilla V. Wornoayporn A. Islam S. Ahmad C. Caceres A. S. Robinson M. J. B. Vreysen 《Journal of Applied Entomology》2010,134(3):207-215
The codling moth Cydia pomonella (Linnaeus) (Lepidoptera: Tortricidae) is a serious pest of pome fruit worldwide and the sterile insect technique (SIT) provides an environmentally acceptable approach for its control. As the pest is present in both the southern and northern hemispheres it would be possible for a rearing facility in the northern hemisphere to supply sterile moths to an SIT programme in the southern hemisphere during the northern winter and vice versa. This could greatly improve the economics of moth production and the running costs of rearing facilities. However in order to develop this concept, it is important to assess if populations of codling moth from different geographical regions share mating compatibility. Twelve different laboratory and field populations from both hemispheres were sampled and field cage bisexual mating compatibility tests were carried out between selected combinations. The index of sexual isolation (ISI) and the female and male relative performance index (FRPI and MRPI, respectively) were calculated for each mating combination. In only two of the combinations was there a slight but significant deviation from random mating. There were also some significant differences in mating duration between the homotypic matings and the duration of a particular homotypic mating seemed to depend on the origin of the other population in the cage. It was concluded that there exist no barriers to mating between populations of codling moth from many parts of the world and that it would be feasible for sterile moths to be shipped from one rearing facility to SIT programmes in other parts of the world. 相似文献
992.
Florence Brian-Jaisson Maëlle Molmeret Ahmad Fahs Linda Guentas-Dombrowsky Gérald Culioli Yves Blache 《Biofouling》2016,32(5):547-560
This study investigated soluble (Sol-EPS), loosely bound (LB-EPS), and tightly bound extracellular polymeric substances (TB-EPS) harvested from biofilm and planktonic cultures of the marine bacterium Pseudoalteromonas ulvae TC14. The aim of the characterization (colorimetric methods, FTIR, GC-MS, NMR, HPGPC, and AFM analyses) was to identify new anti-biofilm compounds; activity was assessed using the BioFilm Ring Test®. A step-wise separation of EPS was designed, based on differences in water-solubility and acidity. An acidic fraction was isolated from TB-EPS, which strongly inhibited biofilm formation by marine bacterial strains in a concentration-dependent manner. The main constituents of this fraction were characterized as two glucan-like polysaccharides. An active poly(glutamyl-glutamate) fraction was also recovered from TB-EPS. The distribution of these key EPS components in Sol-EPS, LB-EPS, and TB-EPS was distinct and differed quantitatively in biofilm vs planktonic cultures. The anti-biofilm potential of the fractions emphasizes the putative antifouling role of EPS in the environment. 相似文献
993.
Shereen Shawki Taghrid Gaafar Hadeel Erfan Engy El Khateeb Ahmad El Sheikhah Rabab El Hawary 《Microbiology and immunology》2015,59(6):348-356
Umbilical cord blood (UCB) is of great interest as a source of stem cells for use in cellular therapies. The immunomodulatory effect of mesenchymal stem cells (MSCs) originating from bone marrow, adipose tissue and amniotic membrane has previously been reported. In this study, MSCs were isolated from UCB with the aim of evaluating their immunomodulatory effects on proliferation of PB lymphocytes by two different techniques; namely, 5‐bromo‐2‐deoxyuridine ELISA and a carboxy fluorescein diacetate succinimidyl ester flow cytometric technique. MSCs were isolated from UCB, propagated until Passage four, and then characterized for cell surface markers by flow cytometry and ability to differentiate towards osteocytes and adipocytes. Immunosuppressive effects on PB lymphocytes were examined by co‐culturing mitomycin C‐treated UCB MSCs with mitogen‐stimulated lymphocytes for 72 hr. Thereafter, proliferation of lymphocytes was detected by CFSE flow cytometry and colorimetric ELISA. The titers of cytokines in cell culture supernatant were also assayed to clarify possible mechanisms of immunomodulation. UCB MSCs suppressed mitogen‐stimulated lymphocyte proliferation, which occurs via both cell‐cell contact and cytokine secretion. Titers of transforming growth factor beta and IL 10 increased, whereas that of IFN‐γ decreased in the supernatants of co‐cultures. Thus, UCB MSCs suppress the proliferation of mitogen‐stimulated lymphocytes. However further in vivo studies are required to fully evaluate the immunomodulatory effects of UCB MSCs. 相似文献
994.
Alimohammadian MH Darabi H Malekzadeh S Mahmoodzadeh-Niknam H Ajdary S Khamesipour A Bahonar A Mofarrah A 《Microbiology and immunology》2007,51(10):1003-1011
The immune responses of individuals exposed to Leishmania major were evaluated and compared with those of non-exposed volunteers. Forty-one patients with active lesion(s), 43 healed individuals, 15 vaccinees 1 month or 1 year post vaccination, and 15 non-exposed volunteers were studied. Leishmanin skin test (LST) response, proliferative response of lymphocyte (PRL) to L. major antigen, IFN-gamma and IL-4 production, and percentage of L. major-specific CD4+, CD8+ and CD16+/CD56+ cells in peripheral blood mononuclear cells were assessed. Data showed positive LST (>5 mm) in 92% of patients, 98% of healed, and 80% or 43% of vaccinees 1 month and 1 year post vaccination, respectively. Positive PRL (SI>2.5) was displayed in 90%, 84%, 46% and 7% of patients, healed, vaccinated (post 1 year) and non-exposed donors, respectively. The mean +/-S.E. of IFN-gamma was 924 +/- 149, 1,278 +/- 185, 470 +/- 282 or 258 +/- 82 pg/ml in patients, healed cases and vaccinees after 1 month or 1 year, respectively. Positive IFN-gamma responders (>300 pg/ml) were shown in 72% of patients, 81% of healed cases, 31% or 39% of vaccinees and 0% of non-exposed donors. A reduced percentage of CD4+ T-cells and an increased percentage of NK cells were found in exposed individuals compared to non-exposed donors. The data indicated that exposure to L. major modulates the proportion of CD4+ T cells and increases NK cells percentage. However, the cellular immune responses including induction of LST, and IFN-gamma production are increased in exposed individuals. 相似文献
995.
Ahmad Khaskiye Jean-Pierre Sine Bernard Colas Didier Renaud 《Journal of neurochemistry》1990,54(3):828-833
The effects of denervation and direct electrical stimulation upon the activity and the molecular form distribution of butyrylcholinesterase (BuChE) were studied in fast-twitch posterior latissimus dorsi (PLD) and in slow-tonic anterior latissimus dorsi (ALD) muscles of newly hatched chicken. In PLD muscle, denervation performed at day 2 substantially reduced the rate of rapid decrease of BuChE specific activity which takes place during normal development, whereas in the case of ALD muscle little change was observed. Moreover, the asymmetric forms which were dramatically reduced in denervated PLD muscle were virtually absent in denervated ALD muscle at day 14. Denervated PLD and ALD muscles were stimulated from day 4 to day 14 of age. Two patterns of stimulation were applied, either 5-Hz frequency (slow rhythm) or 40-Hz frequency (fast rhythm). Both patterns of stimulation provided the same number of impulses per day (about 61,000). In PLD muscle, electrical stimulation almost totally prevented the postdenervation loss in asymmetric forms and led to a decrease in BuChE specific activity. In ALD muscle, electrical stimulation partially prevented the asymmetric form loss which occurs after denervation. This study emphasizes the role of evoked muscle activity in the regulation of BuChE asymmetric forms in the fast PLD muscle and the differential response of denervated slow and fast muscles to electrical stimulation. 相似文献
996.
Muhammad Irshad Sajjad Ahmad Arshid Pervez Mitsuhiro Inoue 《International journal of phytoremediation》2015,17(2):154-158
The objective of this research was to compare the potential of native plants for the phytoaccumulation of heavy metals (HM). Thirteen predominant plant species (including trees, bushes and grasses) namely Ricinus communis, Ipomoea carnea, Cannabis sativa, Parthenium hysterophorus, Acacia nilotica, Dalbergia sissoo, Acacia modesta, Solanum nigrum, Xanthium stromarium, Chenopodium album, Cynodon dactylon, Eleusine indica, and Dactyloctenium aegyptium were collected from the wastewater originated from Hattar industrial estate of Pakistan, Plants shoots and roots were analyzed for heavy metals / metalloid: Pb, Cr, Cd, Zn, Fe, Ni, and As. Among plant species, the accumulation potential for HM varied depending on the type of element. Regardless of the plant species, HM concentrations varied in the order of Fe > Zn > Cr > Pb > Ni > Cd > As. Tree species of R. communis, A. nilotica, A. modesta, and D. sissoo exhibited an enhanced concentrations of metals. Accumulation pattern of Fe, Pb, Cd, and As in plants could be related to the HM composition of soil and wastewater. Most of the species exhibited higher HM composition in the root as compared to shoot. The species that found with greater ability to absorb HM in the root, got higher HM concentrations in its shoot. Shoot tissue concentrations of HM were attained by the species as D. sissoo > A. modesta > A. nilotica > R. communis > I. carnea > C. album > E. indica > P. hysterophorus > S. nigrum > C. sativa > D. aegyptium > X. strumarium > C. dactylon. Based on results, tree plants were noticed as higher accumulators of HM in polluted soils. 相似文献
997.
Wheat germ lipase is a cereal lipase which is a monomeric protein. In the present study we sought to structurally characterize this protein along with equilibrium unfolding in solution. Conformational changes occurring in the protein with varying pH, were monitored by circular dichroism (CD) spectroscopy, fluorescence emission spectroscopy, binding of hydrophobic dye, 1-anilino 8-naphthalenesulfonic acid (ANS) and dynamic light scattering (DLS). Our study showed that acid denaturation of lipase lead to characterization of multiple monomeric intermediates. Native protein at pH 7.0 showed far-UV spectrum indicating mixed structure with both alpha and beta-type of characteristics. Activity of lipase was found to fall on either sides of pH 7.0–8.0. Acid-unfolded state was characterized at pH 4.0 with residual secondary structure, disrupted tertiary spectrum and red-shifted fluorescence spectrum with decreased intensity. Further decrease in pH lead to formation of secondary structure and acid-induced molten globule state was found to be stabilized at pH 1.4, with exposed tryptophan residues and hydrophobic patches. Notably, interesting finding of this study was characterization of acid-induced state at pH 0.8 with higher secondary structure content than native lipase, regain in tertiary spectrum and induction of compact conformation. Although enzymatically inactive, acid-induced state at pH 0.8 was found to be structurally more stable than native lipase, as shown by chemical and thermal denaturation profiles. 相似文献
998.
A comparative study of DNA complexation with Mg(II) and Ca(II) in aqueous solution: major and minor grooves bindings 下载免费PDF全文
Although structural differences for the Mg-DNA and Ca-DNA complexes are provided in the solid state, such comparative study in aqueous solution has been less investigated. The aim of this study was to examine the bindings of Mg and Ca cations with calf thymus DNA in aqueous solution at physiological pH, using constant concentration of DNA (1.25 or 12.5 mM) and various concentrations of metal ions (2 microM-650 microM). Capillary electrophoresis, UV-visible, and Fourier transform infrared spectroscopic methods were used to determine the cation-binding modes, the binding constants, and DNA structural variations in aqueous solution. Direct Ca-PO(2) binding was evident by major spectral changes (shifting and splitting) of the backbone PO(2) asymmetric stretching at 1222 cm(-1) with K = 4.80 x 10(5) M(-1), whereas an indirect Mg-phosphate interaction occurred (due to the lack of shifting and splitting of the phosphate band at 1222 cm(-1)) with K = 5.6 x 10(4) M(-1). The metal-base bindings were directly for the Mg with K = 3.20 x 10(5) M(-1) and indirectly for the Ca cation with K = 3.0 x 10(4) M(-1). Both major and minor groove bindings were observed with no alteration of the B-DNA conformation. 相似文献
999.
Md. Shimul Bhuia Raihan Chowdhury Fatema Akter Sonia Hossam Kamli Ahmad Shaikh Heba A. S. El-Nashar Mohamed El-Shazly Muhammad Torequl Islam 《化学与生物多样性》2023,20(9):e202300847
With the increasing prevalence of cancer and the toxic side effects of synthetic drugs, natural products are being developed as promising therapeutic approaches. Gracillin is a naturally occurring triterpenoid steroidal saponin with several therapeutic activities. It is obtained as a major compound from different Dioscorea species. This review was designated to summarize the research progress on the anti-cancer activities of gracillin focusing on the underlying cellular and molecular mechanisms, as well as its pharmacokinetic features. The data were collected (up to date as of May 1, 2023) from various reliable and authentic literatures comprising PubMed, Springer Link, Scopus, Wiley Online, Web of Science, ScienceDirect, and Google Scholar. The findings demonstrated that gracillin displays promising anticancer effects through various molecular mechanisms, including anti-inflammatory effects, apoptotic cell death, induction of oxidative stress, cytotoxicity, induction of genotoxicity, cell cycle arrest, anti-proliferative effect, autophagy, inhibition of glycolysis, and blocking of cancer cell migration. Additionally, this review highlighted the pharmacokinetic features of gracillin, indicating its lower oral bioavailability. As a conclusion, it can be proposed that gracillin could serve as a hopeful chemotherapeutic agent. However, further extensive clinical research is recommended to establish its safety, efficacy, and therapeutic potential in cancer treatment. 相似文献
1000.