全文获取类型
收费全文 | 15082篇 |
免费 | 1127篇 |
国内免费 | 823篇 |
专业分类
17032篇 |
出版年
2024年 | 32篇 |
2023年 | 194篇 |
2022年 | 495篇 |
2021年 | 768篇 |
2020年 | 461篇 |
2019年 | 591篇 |
2018年 | 568篇 |
2017年 | 408篇 |
2016年 | 570篇 |
2015年 | 854篇 |
2014年 | 960篇 |
2013年 | 1077篇 |
2012年 | 1304篇 |
2011年 | 1193篇 |
2010年 | 747篇 |
2009年 | 659篇 |
2008年 | 741篇 |
2007年 | 693篇 |
2006年 | 591篇 |
2005年 | 520篇 |
2004年 | 458篇 |
2003年 | 370篇 |
2002年 | 329篇 |
2001年 | 308篇 |
2000年 | 255篇 |
1999年 | 230篇 |
1998年 | 148篇 |
1997年 | 147篇 |
1996年 | 150篇 |
1995年 | 110篇 |
1994年 | 111篇 |
1993年 | 81篇 |
1992年 | 134篇 |
1991年 | 102篇 |
1990年 | 78篇 |
1989年 | 78篇 |
1988年 | 63篇 |
1987年 | 71篇 |
1986年 | 64篇 |
1985年 | 50篇 |
1984年 | 48篇 |
1983年 | 41篇 |
1982年 | 24篇 |
1981年 | 13篇 |
1980年 | 16篇 |
1979年 | 19篇 |
1977年 | 14篇 |
1976年 | 11篇 |
1973年 | 10篇 |
1972年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Hao Yu Hongzhi Tang Xiongyu Zhu Yangyang Li Ping Xu 《Applied and environmental microbiology》2015,81(1):272-281
A newly isolated strain, SJY1, identified as Ochrobactrum sp., utilizes nicotine as a sole source of carbon, nitrogen, and energy. Strain SJY1 could efficiently degrade nicotine via a variant of the pyridine and pyrrolidine pathways (the VPP pathway), which highlights bacterial metabolic diversity in relation to nicotine degradation. A 97-kbp DNA fragment containing six nicotine degradation-related genes was obtained by gap closing from the genome sequence of strain SJY1. Three genes, designated vppB, vppD, and vppE, in the VPP pathway were cloned and heterologously expressed, and the related proteins were characterized. The vppB gene encodes a flavin-containing amine oxidase converting 6-hydroxynicotine to 6-hydroxy-N-methylmyosmine. Although VppB specifically catalyzes the dehydrogenation of 6-hydroxynicotine rather than nicotine, it shares higher amino acid sequence identity with nicotine oxidase (38%) from the pyrrolidine pathway than with its isoenzyme (6-hydroxy-l-nicotine oxidase, 24%) from the pyridine pathway. The vppD gene encodes an NADH-dependent flavin-containing monooxygenase, which catalyzes the hydroxylation of 6-hydroxy-3-succinoylpyridine to 2,5-dihydroxypyridine. VppD shows 62% amino acid sequence identity with the hydroxylase (HspB) from Pseudomonas putida strain S16, whereas the specific activity of VppD is ∼10-fold higher than that of HspB. VppE is responsible for the transformation of 2,5-dihydroxypyridine. Sequence alignment and phylogenetic analysis suggested that the VPP pathway, which evolved independently from nicotinic acid degradation, might have a closer relationship with the pyrrolidine pathway. The proteins and functional pathway identified here provide a sound basis for future studies aimed at a better understanding of molecular principles of nicotine degradation. 相似文献
993.
994.
Detecting spreading outbreaks in social networks with sensors is of great significance in applications. Inspired by the formation mechanism of humans’ physical sensations to external stimuli, we propose a new method to detect the influence of spreading by constructing excitable sensor networks. Exploiting the amplifying effect of excitable sensor networks, our method can better detect small-scale spreading processes. At the same time, it can also distinguish large-scale diffusion instances due to the self-inhibition effect of excitable elements. Through simulations of diverse spreading dynamics on typical real-world social networks (Facebook, coauthor, and email social networks), we find that the excitable sensor networks are capable of detecting and ranking spreading processes in a much wider range of influence than other commonly used sensor placement methods, such as random, targeted, acquaintance and distance strategies. In addition, we validate the efficacy of our method with diffusion data from a real-world online social system, Twitter. We find that our method can detect more spreading topics in practice. Our approach provides a new direction in spreading detection and should be useful for designing effective detection methods. 相似文献
995.
Similar or Totally Different: the Adjustment of the Twist Conformation Through Minor Structural Modification,and Dramatically Improved Performance for Dye‐Sensitized Solar Cell 下载免费PDF全文
Zhaofei Chai Mei Wu Manman Fang Sushu Wan Ting Xu Runli Tang Yujun Xie Anyi Mei Hongwei Han Qianqian Li Zhen Li 《Liver Transplantation》2015,5(18)
A novel approach for enhancing the performance of dye‐sensitized solar cells is presented. It is based on the analysis of five sensitizers by utilizing triarylamine as donor, thiophene benzothiadiazole as chromophore and substituted thienyl linked with cyanoacrylic acid as the anchoring group (LI‐80‐LI‐84). Accompanied with the increasing steric hindrance of the substituents on the thienyl isolation group, the conformation of the dyes, in particular the angle between the chromophore and the anchoring group, becomes more and more twisted. Surprisingly, sensitizers with poorer conjugation effects (the higher twisted conformation) achieve better photovoltaic performances, showing a contrary trend to the traditional donor‐(π‐spacer)‐acceptor dyes with a better co‐planarity. On the basis of the preceding fundamental comprehensions, an empirical method is successfully applied to a new phenyl‐based system (LI‐85 and LI‐86) to improve their performances. The systematical investigation indicates that the twisted structures can contribute to the ECB of the TiO2 film, electron lifetime and resistance at the TiO2/dye/electrolyte interface. Thereby, the efficiency of the initial LI‐80‐based cell has been dramatically improved to 2.45 times higher for LI‐86‐based cell, paving a new way for the design of better sensitizers with higher device performances. 相似文献
996.
997.
Phenylephrine enhances glutamate release in the medial prefrontal cortex through interaction with N‐type Ca2+ channels and release machinery 下载免费PDF全文
Fei Luo Si‐hai Li Hua Tang Wei‐ke Deng Yu Zhang Ying Liu 《Journal of neurochemistry》2015,132(1):38-50
α1‐adrenoceptors (α1‐ARs) stimulation has been found to enhance excitatory processes in many brain regions. A recent study in our laboratory showed that α1‐ARs stimulation enhances glutamatergic transmission via both pre‐ and post‐synaptic mechanisms in layer V/VI pyramidal cells of the rat medial prefrontal cortex (mPFC). However, a number of pre‐synaptic mechanisms may contribute to α1‐ARs‐induced enhancement of glutamate release. In this study, we blocked the possible post‐synaptic action mediated by α1‐ARs to investigate how α1‐ARs activation regulates pre‐synaptic glutamate release in layer V/VI pyramidal neurons of mPFC. We found that the α1‐ARs agonist phenylephrine (Phe) induced a significant enhancement of glutamatergic transmission. The Phe‐induced potentiation was mediated by enhancing pre‐synaptic glutamate release probability and increasing the number of release vesicles via a protein kinase C‐dependent pathway. The mechanisms of Phe‐induced potentiation included interaction with both glutamate release machinery and N‐type Ca2+ channels, probably via a pre‐synaptic Gq/phospholipase C/protein kinase C pathway. Our results may provide a cellular and molecular mechanism that helps explain α1‐ARs‐mediated influence on PFC cognitive functions.
998.
Nidogen‐1 is a common target of microRNAs MiR‐192/215 in the pathogenesis of Hirschsprung's disease 下载免费PDF全文
Dongmei Zhu Hua Xie Hongxing Li Peng Cai Hairong Zhu Chao Xu Pingfa Chen Ankur Sharan Yankai Xia Weibing Tang 《Journal of neurochemistry》2015,134(1):39-46
Recent studies have emphasized the important role of microRNA (miRNA) clusters and common target genes in disease progression. Despite the known involvement of the miR‐192/215 family in many human diseases, its biological role in Hirschsprung disease (HSCR) remains undefined. In this study, we explored the role of the miR‐192/215 family in the pathogenesis of HSCR. Quantitative real‐time PCR and western blotting measured relative expression levels of miRNAs, mRNAs, and proteins in 80 HSCR patients and 77 normal colon tissues. Targets were evaluated by dual‐luciferase reporter assays, and the functional effects of miR‐192/215 on human 293T and SH‐SY5Y cells were detected by the Transwell assay, CCK8 assay and flow cytometry. MiR‐192/215 was significantly down‐regulated in HSCR tissue samples, and their knockdown inhibited cell migration and proliferation in the human 293T and SH‐SY5Y cell lines. Nidogen 1 (NID1) was confirmed as a common target gene of miR‐192/215 by dual‐luciferase reporter gene assay and its expression was inversely correlated with that of miR‐192/215 in tissue samples and cell lines. Silencing of NID1 could rescue the extent of the suppressing effects by miR‐192/215 inhibitor. The down‐regulation of miR‐192/215 may contribute to HSCR development by targeting NID1.
999.
Combined effects of girdling and leaf removal on fluorescence characteristic of Alhagi sparsifolia leaf senescence 下载免费PDF全文
Plant senescence is largely influenced by carbohydrate content. In order to investigate the impact of carbohydrate content on leaf senescence and photosystem II (PSII) during the senescence process, phloem girdling (PG), leaf removal (LR) and a combination of phloem girdling and leaf removal (GR) were performed on Alhagi sparsifolia (Fabaceae) at the end of the growing season. The results showed that during senescence, leaf soluble sugar content, starch content, the energy absorbed by the unit reaction centre (ABS/RC) increased; whereas, leaf photosynthetic rate, photosynthetic pigment content, maximum photochemical efficiency (φPo) and energy used by the acceptor site in electron transfer (ETo/RC) decreased. The degree of change was PG> GR> CK (control)> LR. The results of the present work implied that phloem girdling (PG) significantly accelerated leaf senescence, and that single leaf removal (LR) slightly delayed leaf senescence; although leaf removal significantly delayed the senescence process on the girdled leaf (GR). Natural or delayed senescence only slightly inhibited the acceptor site of PSII and did not damage the donor site of PSII. On the other hand, induced senescence not only damaged the donor site of PSII (e.g. oxygen‐evolving complex), but also significantly inhibited the acceptor site of PSII. In addition, leaf senescence led to an increase in the energy absorbed by the unit reaction centre (ABS/RC), which subsequently resulted in increasing excitation pressure in the reaction centre (DIo/RC), as well as additional saved Car for absorbing residual light energy and quenching reactive oxygen species during senescence. 相似文献
1000.