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   8336篇
  免费   671篇
  国内免费   3篇
  9010篇
  2023年   83篇
  2022年   145篇
  2021年   284篇
  2020年   230篇
  2019年   227篇
  2018年   340篇
  2017年   262篇
  2016年   396篇
  2015年   489篇
  2014年   512篇
  2013年   594篇
  2012年   694篇
  2011年   641篇
  2010年   393篇
  2009年   357篇
  2008年   416篇
  2007年   418篇
  2006年   395篇
  2005年   250篇
  2004年   262篇
  2003年   227篇
  2002年   198篇
  2001年   165篇
  2000年   159篇
  1999年   128篇
  1998年   63篇
  1997年   56篇
  1996年   51篇
  1995年   37篇
  1994年   38篇
  1993年   34篇
  1992年   56篇
  1991年   40篇
  1990年   25篇
  1989年   51篇
  1988年   28篇
  1987年   25篇
  1986年   17篇
  1985年   22篇
  1984年   31篇
  1983年   24篇
  1982年   21篇
  1981年   10篇
  1980年   9篇
  1979年   10篇
  1978年   14篇
  1975年   8篇
  1974年   12篇
  1971年   10篇
  1968年   7篇
排序方式: 共有9010条查询结果,搜索用时 15 毫秒
991.
This work reports the influence of the high acidity and high phenolic content in apple musts on the development of alcoholic and malolactic fermentations and on the final chemical and microbiological composition of the ciders. Four different musts were obtained by pressing several varieties and proportions of cider apples from the Basque Country (Northern Spain). Specially acidic and phenolic varieties were selected. Three musts were obtained in experimental stations and the fourth one, in a cider factory following usual procedures. The evolution of these musts was monitored during five months by measuring 18 parameters throughout eight samplings. In the most acidic of the three experimental musts, yeasts were added to complete the alcoholic fermentation. In the rest of the musts, alcoholic and malolactic fermentations took place spontaneously due to natural microflora and no chemical was added to control these processes. Malolactic fermentation (MLF) finished before alcoholic fermentation in the three tanks obtained in experimental stations, even in the most acidic and phenolic one (pH 3.18, 1.78 g tannic acid/l). After four months, these ciders maintained low levels of lactic acid bacteria (10(4)CFU/ml) and low content of acetic acid (<0.60 g/l). Both fermentations began simultaneously in the must obtained in the cider factory, but MLF finished 10 days after alcoholic fermentation. Subsequently, this must maintained a high population of lactic acid bacteria (>10(6)CFU/ml), causing a higher production of acetic acid (>1.00 g/l) than in the other ciders. These results show the possible advantages of MLF finishing before alcoholic fermentation.  相似文献   
992.
Vieira J  Teles E  Santos RA  Vieira CP 《Genetics》2008,180(1):483-491
In Prunus, the self-incompatibility (S-) locus region is <70 kb. Two genes-the S-RNase, which encodes the functional female recognition component, and the SFB gene, which encodes the pollen recognition component-must co-evolve as a genetic unit to maintain functional incompatibility. Therefore, recombination must be severely repressed at the S-locus. Levels of recombination at genes in the vicinity of the S-locus have not yet been rigorously tested; thus it is unknown whether recombination is also severely repressed at these loci. In this work, we looked at variability levels and patterns at the Prunus spinosa SLFL1 gene, which is physically close to the S-RNase gene. Our results suggest that the recombination level increases near the SLFL1 coding region. These findings are discussed in the context of theoretical models predicting an effect of linked weakly deleterious mutations on the relatedness of S-locus specificities. Moreover, we show that SLFL1 belongs to a gene family of at least five functional genes and that SLFL1 pseudogenes are frequently found in the S-locus region.  相似文献   
993.
Coumarin and its derivative 7-hydroxycoumarin (7-OHC) have antitumor and antimetastatic properties. The purpose of this study was to investigate the possible effects of these compounds on expression of the bcl-2 and Bax oncoproteins in two human lung cancer cell lines, A427 and Calu-1. The cells were cultured in vitro for 24 h in RPMI 1640 with 1.5% (v/v) ethanol, 1.0 mM ethanolic coumarin or 1.0 mM ethanolic 7-OHC. Viability was determined in each cell line by an MTT assay. Total protein was extracted from cell lysates and the bcl-2 and Bax oncoproteins were identified. Western blotting showed a decrease in bcl-2 and an increase in Bax in A427 cells cultured with coumarin or 7-OHC. Neither drug changed bcl-2 expression in Calu-1 cells compared to solvent controls, and Bax expression was only slightly increased by coumarin. We conclude that 7-OHC is a more potent inhibitor of cell proliferation than coumarin and has more marked effects on oncoprotein expression. Also, the A427 cell line was more sensitive to the drugs than Calu-1.  相似文献   
994.
Two main hypotheses have been proposed to explain the diversification of the Caatinga biota. The riverine barrier hypothesis (RBH) claims that the São Francisco River (SFR) is a major biogeographic barrier to gene flow. The Pleistocene climatic fluctuation hypothesis (PCH) states that gene flow, geographic genetic structure and demographic signatures on endemic Caatinga taxa were influenced by Quaternary climate fluctuation cycles. Herein, we analyse genetic diversity and structure, phylogeographic history, and diversification of a widespread Caatinga lizard (Cnemidophorus ocellifer) based on large geographical sampling for multiple loci to test the predictions derived from the RBH and PCH. We inferred two well‐delimited lineages (Northeast and Southwest) that have diverged along the Cerrado–Caatinga border during the Mid‐Late Miocene (6–14 Ma) despite the presence of gene flow. We reject both major hypotheses proposed to explain diversification in the Caatinga. Surprisingly, our results revealed a striking complex diversification pattern where the Northeast lineage originated as a founder effect from a few individuals located along the edge of the Southwest lineage that eventually expanded throughout the Caatinga. The Southwest lineage is more diverse, older and associated with the Cerrado–Caatinga boundaries. Finally, we suggest that C. ocellifer from the Caatinga is composed of two distinct species. Our data support speciation in the presence of gene flow and highlight the role of environmental gradients in the diversification process.  相似文献   
995.
Chromoblastomycosis (CBM) is an implantation mycosis mainly occurring in tropical and subtropical zones worldwide. If not diagnosed at early stages, patients with CBM require long-term therapy with systemic antifungals flanked by various physical treatment regimens. As in other neglected endemic mycoses, comparative clinical trials have not been performed for this disease; nowadays, therapy is mainly based on a few open trials and on expert opinions. Itraconazole, either as monotherapy or associated with other drugs, or with physical methods, is widely used. Recently, photodynamic therapy has been employed successfully in combination with antifungals in patients presenting with CBM. In the present paper, the most used therapeutic options against CBM are reviewed as well as the several factors that may have impact on the patient’s outcome.  相似文献   
996.
Many types of macroalgae contain a wide range of bioactive compounds that have antioxidant potential. However, in contrast to terrestrial plants, only a few studies have reported the antioxidant activity of seaweeds. Therefore, extracts from 26 marine macroalgae species from the south and southeast coasts of Brazil were evaluated for their antioxidant activity, using the 2,2-diphenyl-2-picrylhydrazyl hydrate (DPPH) method and β-carotene/linoleic acid assay, and their total phenolic contents, through Folin–Ciocalteu method. Padina gymnospora, Sargassum vulgare, and Osmundaria obtusiloba presented the highest values of total phenolic content. Using β-carotene bleaching assay, Colpomenia sinuosa, Dictyota sp., Dichotomaria marginata, Ganonema farinosum, and Spyridia clavata presented up to 65 % of antioxidant activity. Some of the extracts showed more than 60 % of inhibition of DPPH in the lowest concentration (0.01 mg/mL), including Amansia sp., Bostrychia tenella, Cryptonemia seminervis, Hypnea musciformis, Plocamium brasiliense (1), and S. clavata. Both Amansia sp., and C. seminervis presented the most relevant antioxidant potential, with percentage of inhibition greater than 70 % in the three tested concentrations. These two species were then analyzed by nuclear magnetic resonance spectroscopy (NMR) and were selected for guided fractionation bioassay. They both presented lipid compounds, fatty acids, esters of fatty acids, triglycerides, and sterols as major components. The fractionation of extracts revealed that the organic fractions were responsible for the antioxidant activity. The results obtained through this work indicate that the analyzed seaweeds are a promising source of compounds with high antioxidant potential.  相似文献   
997.
The stability constants of La(3+), Sm(3+) and Ho(3+) complexes with 13- and 14-membered macrocycles having methylcarboxylate (trita and teta) or methylphosphonate (tritp and tetp) arms were determined. All the ligands were labelled with (153)Sm and (166)Ho in order to evaluate the effect of the macrocyclic cavity size and type of appended arms on their in vitro and in vivo behaviour. The radiolabelling efficiency was found to be higher than 98% for all the complexes, except for those of tetp. All radiocomplexes studied are hydrophilic with an overall negative charge and low plasmatic protein binding. Good in vitro stability in physiological media and human serum was found for all complexes, except the (153)Sm/(166)Ho-teta, which are unstable in phosphate buffer (pH 7.4). In vitro hydroxyapatite (HA) adsorption studies indicated that (153)Sm/(166)Ho-tritp complexes bind to HA having the (166)Ho complex the highest degree of adsorption (>80%, 10 mg). Biodistribution studies in mice demonstrated that (153)Sm/(166)Ho-trita complexes have a fast tissue clearance with more than 95% of the injected activity excreted after 2 h, value that is comparable to the corresponding dota complexes. In contrast, the (153)Sm-teta complex has a significantly lower total excretion. (153)Sm/(166)Ho-tritp complexes are retained by the bone, particularly (166)Ho-tritp that has 5-6% (% I.D./g) bone uptake and also a high rate of total excretion. Thus, these studies support the potential interest of (153)Sm/(166)Ho-trita complexes for therapy when conjugated to a biomolecule and the potential usefulness of the (166)Ho-tritp complex in bone pain palliation.  相似文献   
998.
A case of Scedosporium apiospermum keratitis is reported in a 65-year-old farmer referred for treatment of an extensive corneal ulcer in the left eye. Direct examination of scrapes revealed abundant filamentous septate hyphae; all cultures were consistently positive for the same fungus, identified later as Scedosporium apiospermum. The patient successfully responded to treatment with amphotericin B.  相似文献   
999.
The present study investigated the underlying mechanism associated with the hypocholesterolemic activity of beta-carotene by examining its effects on the serum lipid profile, fecal cholesterol excretion, and gene expression of the major receptors, enzymes, and transporters involved in cholesterol metabolism. Female Fischer rats were divided into three groups and were fed either a control or a hypercholesterolemic diet supplemented or not supplemented with 0.2 % beta-carotene. After 6 weeks of feeding, blood, livers, and feces were collected for analysis, and quantitative real-time polymerase chain reaction (qRT-PCR) was performed. Dietary supplementation with 0.2 % beta-carotene decreased serum total cholesterol, non-HDL cholesterol, the atherogenic index, and hepatic total lipid and cholesterol contents. These changes were accompanied by an increase in the total lipid and cholesterol contents excreted in the feces. The qRT-PCR analyses demonstrated that the hypercholesterolemic diet promoted a decrease in the gene expression of sterol regulatory element-binding protein 2, 3-hydroxy-3-methylglutaryl CoA reductase, and low-density lipoprotein receptor and an increase in the gene expression of peroxisome proliferator-activated receptor α and cholesterol-7a-hydroxylase. The expression of these genes and gene expression of ATP-binding cassette subfamily G transporters 5and 8 were unaffected by beta-carotene supplementation. In conclusion, the decrease in serum cholesterol and the elevation of fecal cholesterol obtained following beta-carotene administration indicate that this substance may decrease cholesterol absorption in the intestine and increase cholesterol excretion into the feces without a direct effect on the expression of cholesterol metabolism genes.  相似文献   
1000.
We studied the influence of metabolites of -aminobutyric shunt of the tricarboxylic acid cycle on the activities of aconitate hydratase (EC.4.2.1.3) as well as NAD- and NADP-specific isocitrate dehydrogenases (EC.1.1.1.41 and EC.1.1.1.42, respectively) using purified enzyme preparations from pea leaves (Pisum sativum L.) and maize scutellum (Zea mays L.). -Aminobutyric acid and succinate proved to have no significant effect on these enzymes, while 0.1–0.2 mM glutamate markedly activated NADP isocitrate dehydrogenase from both sources, especially at unsaturating concentrations of the substrate. Succinic semialdehyde stimulated the activities of aconitate hydratase and NADP isocitrate dehydrogenase. The obtained data point to a similar pattern of the effect of intermediates of -aminobutyric shunt on the studied enzymatic activities for both photosynthetic tissues (pea leaves) and those with acidifying, transport, and digestive functions (maize scutellum). However, the absence of pronounced control effects of most metabolites on the studied enzymes allows us to assign them to a relatively inert pool of metabolites.  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