全文获取类型
收费全文 | 92989篇 |
免费 | 259篇 |
国内免费 | 887篇 |
专业分类
94135篇 |
出版年
2022年 | 2篇 |
2021年 | 1篇 |
2020年 | 5篇 |
2019年 | 4篇 |
2018年 | 11837篇 |
2017年 | 10670篇 |
2016年 | 7444篇 |
2015年 | 581篇 |
2014年 | 274篇 |
2013年 | 287篇 |
2012年 | 4194篇 |
2011年 | 12796篇 |
2010年 | 11991篇 |
2009年 | 8220篇 |
2008年 | 9771篇 |
2007年 | 11347篇 |
2006年 | 247篇 |
2005年 | 497篇 |
2004年 | 955篇 |
2003年 | 1011篇 |
2002年 | 771篇 |
2001年 | 254篇 |
2000年 | 158篇 |
1999年 | 23篇 |
1998年 | 7篇 |
1997年 | 21篇 |
1996年 | 9篇 |
1994年 | 6篇 |
1993年 | 29篇 |
1992年 | 20篇 |
1991年 | 37篇 |
1990年 | 7篇 |
1989年 | 7篇 |
1988年 | 17篇 |
1987年 | 13篇 |
1984年 | 7篇 |
1983年 | 17篇 |
1982年 | 3篇 |
1980年 | 1篇 |
1975年 | 5篇 |
1972年 | 246篇 |
1971年 | 274篇 |
1970年 | 5篇 |
1965年 | 13篇 |
1962年 | 24篇 |
1956年 | 5篇 |
1944年 | 12篇 |
1940年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Amino acids are constituents of proteins, precursors of many secondary metabolites and nitrogen carriers in plants. Transport across intracellular membranes and translocation of amino acids within the plant is mediated by membrane amino acid transporters. However, the amino acid transport in tea plant is rarely reported. In this study, six cationic amino acid transporter (CAT) family genes were cloned. Phylogenetic analysis categorized these CsCATs into four subgroups. These CsCATs all contain the 12–14 transmembrane domains and the conserved CAT motifs. Their expression was tissue-specific, with higher expression levels in root and stem and correlated to the abundances of key free amino acids such as Theanine. Some CsCATs expression responded to some abiotic stress conditions and to the exogenous application of theanine (Thea), glutamine or ethylamine hydrochloride, an ethylamine precursor for Thea biosynthesis. Our results indicated that the CsCATs expression is regulated by amino acid contents and is sensitive to abiotic stresses. These findings shed light on the mechanism of amino acid transport in tea plants. 相似文献
992.
Horned beetles are emerging models in the study of coevolution between novel morphologies and behavior. In Onthophagus beetles, large males use horns to fight other males in brood tunnels while small males with higher mobility sneak around the large males to gain access to females. Mating tactics have rarely been described in other dung beetle genera. We studied the horned dung beetle Sulcophanaeus velutinus that exhibits two parallel horns on the prothorax and one on the head. We put two males of different horn lengths, but similar mass, in observation chambers and found that the large male with longer horns won access to the female in physical competition. Speed tests in artificial tunnels show that locomotion is impeded in large males, suggesting an advantage in mobility for males with small horns. This work contributes to the limited existing evidence on the function of alternative morphologies in horned dung beetles taxa. 相似文献
993.
Ashim Paul Sourav Kumar Sourav Kalita Anup Kumar Ghosh Amal Chandra Mondal Bhubaneswar Mandal 《International journal of peptide research and therapeutics》2018,24(1):201-211
Aggregation of Amyloid β (Aβ) in the interneuronal spaces is a major etiopathological factor for onset and progression of Alzheimer’s disease (AD). Since the mechanism of aggregation is not fully understood, control and modulation of the aggregation process is a challenging task. Although, several strategies were developed for the past few decades, yet there is no proper therapeutics available. Herein, we report a peptide based pro-drug, termed as a conformational Pro-Drug peptide (PDp), which disrupts existing Aβ fibrils, but does not produce toxic soluble oligomers, through a series of spontaneous chemical reactions resulting in in situ generation of β-sheet destabilizing factors. Furthermore, PDp reduces Aβ mediated toxicity examined on an in vitro model consisting of the human neuroblastoma SH-SY5Y cells. PDp also disrupts fibrils originated from AD affected human cerebrospinal fluid. These findings will help to understand the process of amyloidogenesis better and also indicate a novel approach for therapeutically important peptide design. 相似文献
994.
Elzbieta Sucajtys-Szulc Alicja Debska-Slizien Boleslaw Rutkowski Ryszard Milczarek Iwona Pelikant-Malecka Tomasz Sledzinski Julian Swierczynski Marek Szolkiewicz 《Molecular and cellular biochemistry》2018,439(1-2):11-18
Little is known about the effects of coffee that are not related to the presence of caffeine. The aim of the study was to analyse changes in kidney function and nucleotide metabolism related to high intake of decaffeinated coffee. Mice consumed decaffeinated coffee extract for two weeks. Activities of AMP deaminase, ecto5′-nucleotidase, adenosine deaminase, purine nucleoside phosphorylase were measured in kidney cortex and medulla by analysis of conversion of substrates into products using HPLC. Concentration of nucleotides in kidney cortex, kidney medulla and serum were estimated by HPLC. Activity of ecto5′-nucleotidase increased from 0.032 ± 0.006 to 0.049 ± 0.014 nmol/mg tissue/min in kidney cortex of mice administered high-dose decaffeinated coffee (HDC) together with increase in cortex adenosine concentration and decrease in plasma creatinine concentration. HDC leads to increased activity of ecto5′-nucleotidase in kidney cortex that translates to increase in concentration of adenosine. Surprisingly this caused improved kidney excretion function. 相似文献
995.
Xiangchang Zeng Wenjing Zong Qing Gao Siyu Chen Lulu Chen Guirong Zeng Weihua Huang Zhenyu Li Chang Zeng Yuanyuan Xie Xiaohui Li Bo Xiao Dongsheng-Ouyang Kai Hu 《Neurochemical research》2018,43(7):1328-1338
Abnormal dendritic sprouting and synaptic remodelling are important pathological features of temporal lobe epilepsy. BC1 RNA is a translation repressor involved in the regulation of the dendritic protein synthesis and mRNA transport, which is essential for dendritic development and plasticity. The expression alteration of BC1 RNA in the pilocarpine induced epilepsy model remains unknown. It is unclear if the interactions between BC1 RNA and eukaryotic initiation factor 4A (eIF4A) exists in this model. The purpose of this study was to investigate the expression changes of BC1 RNA and its interactions with eIF4A post-status epilepticus (SE). Chloride lithium and pilocarpine were used to induce the SE rat model. Either a whole brain or hippocampus tissues were collected at different time points after SE. The expression patterns of BC1 was detected by qPCR and in situ hybridization. The levels of eIF4AI/II protein expression were analyzed via western blotting and immunohistochemistry. The BC1 RNA-eIF4AI/II interaction was determined by electrophoretic mobility shift assay (EMSA). We found that the BC1 RNA levels decreased in hippocampus 3d, 1w and 2w post-SE before the levels recovered. The eIF4AI/II began to rise 3d post-SE and reached the maximum level 1w post-SE. After 1w post-SE the levels decreased in the hippocampal CA1, CA3 and DG subregions. EMSA analysis showed that BC1 RNA specifically interacted with the eIF4AI/II. The BC1 RNA-eIF4AI/II complex reduced to the lowest level 1w post-SE. Our results suggested that BC1 has a negative regulatory correlation with eIF4AI/II, where BC1 RNA could be involved in epileptogenesis by regulating dendritic protein synthesis. 相似文献
996.
Pan Wu Man Wang Ting Zhang Xiaxiu Tong Taihe Xiang 《Plant Cell, Tissue and Organ Culture》2018,132(3):433-447
The influence of light quality and cytokinin content in media on growth, development, photosynthetic pigments and secondary metabolite content of Myrtus communis L. was evaluated in an in vitro culture. Various treatments with light emitting diodes (LEDs): 100% blue (B), a mix of 70% red and 30% blue (RB) and 100% red were applied and compared with a traditional fluorescent lamp as control. Axillary shoots were incubated on Murashige and Skoog medium with 30 g dm?3 sucrose, 0.5% BioAgar, 0.5 μM 1-naphthaleneacetic acid and different concentrations of 6-benzyladenine (BA): 1, 2.5 and 5 µM. Cultures were maintained for 6 weeks in 23/21?±?1 °C (day/night), 80% relative humidity and 16/8 h photoperiod; photosynthetic photon flux density (PPFD) was 35 µmol m?2 s?1 in all treatments. Light spectra and BA content in media affected biometrical and phytochemical M. communis properties. Red LEDs and 5 µM BA resulted in the highest multiplication rate. The highest shoots were obtained under red LEDs, but with the lowest concentration of cytokinin in media. Fresh weight was greatest on LEDs containing blue light in the spectrum (B and RB); moreover, 5 µM BA increased dry weight. Photosynthetic pigment levels were lower under LED light compared to control lamps. Phenolic acids and flavonoids were identified in M. communis leaf extracts. Myricetin was the major constituent with highest concentration under red LEDs and highest BA level. 相似文献
997.
Mangroves provide valuable ecosystem services for the wellbeing of coastal communities. Assessment and valuation of these mangroves services are increasingly advocated in development and conservation decision-making. Translating the values of services into more explicit monetary terms requires understanding of stakeholder activities, socio-economic context and local organizational structure to effectively support decision-making. Based on a survey of 100 households of three villages of Sundarban in Bangladesh, mangroves services to local communities were identified and their economic values estimated. The households perceived 18 mangroves services, of which capture fisheries, fuel energy, storm protection, habitat for fish breeding and nursery grounds and aesthetic enjoyment were ranked the most important. For provisioning services, households obtained important monetary benefits annually from capture fishery (US$ 976 per ha), fuel energy (US$ 80 per ha), honey (US$ 53 per ha) and fodder (US$ 26 per ha). The average annual willingness to pay for storm protection, erosion control and habitat for fish breeding and nursery services were estimated, respectively, as US$ 13 per ha, US$ 2 per ha and US$ 9 per ha. However, unsustainable exploitation and salinity intrusion impacted the services provided by mangroves. This study provides an important insight into the services and values of mangroves for local welfare, and thus can inform policy for protection and better use of mangrove resources. 相似文献
998.
Ivan Muñoz Cristina Rodríguez Dominique Gillet Bruno M. Moerschbacher 《The International Journal of Life Cycle Assessment》2018,23(5):1151-1160
Purpose
The aim of this article is to present the first life cycle assessment of chitosan production based on data from two real producers located in India and Europe. The goal of the life cycle assessment (LCA) was to understand the main hot spots in the two supply chains, which are substantially different in terms of raw materials and production locations.Methods
The LCA is based on consequential modelling principles, whereby allocation is avoided by means of substitution, and market mixes include only flexible, i.e. non-constrained suppliers. The product system is cradle to gate and includes the production of raw materials, namely waste shells from snow crab and shrimp in Canada and India, respectively, the processing of these in China and India and the manufacture of chitosan in Europe and India. Primary data for chitin and chitosan production were obtained from the actual producers, whereas raw material acquisition as well as waste management activities were based on literature sources. The effects of indirect land use change (iLUC) were also included. Impact assessment was carried out at midpoint level by means of the recommended methods in the International Life Cycle Data (ILCD) handbook.Results and discussion
In the Indian supply chain, the production of chemicals (HCl and NaOH) appears as an important hot spot. The use of shrimp shells as raw material affects the market for animal feed, resulting in a credit in many impact indicators, especially in water use. The use of protein waste as fertilizer is also an important source of greenhouse-gas and ammonia emissions. In the European supply chain, energy use is the key driver for environmental impacts, namely heat production based on coal in China and electricity production in China and Europe. The use of crab shells as raw material avoids the composting process they would be otherwise subject to, leading to a saving in composting emissions, especially ammonia. In the Indian supply chain, the effect of iLUC is relevant, whereas in the European one, it is negligible.Conclusions
Even though we assessed two products from the same family, the results show that they have very different environmental profiles, reflecting their substantially different supply chains in terms of raw material (shrimp shells vs. crab shells), production locations (locally produced vs. a global supply chain involving three continents) and the different applications (general-purpose chitosan vs. chitosan for the medical sector).999.
Marieke ten Hoeve Sander Bruun Irina Naroznova Camilla Lemming Jakob Magid Lars S. Jensen Charlotte Scheutz 《The International Journal of Life Cycle Assessment》2018,23(10):1950-1965