全文获取类型
收费全文 | 668篇 |
免费 | 63篇 |
出版年
2024年 | 1篇 |
2023年 | 11篇 |
2022年 | 15篇 |
2021年 | 30篇 |
2020年 | 28篇 |
2019年 | 25篇 |
2018年 | 18篇 |
2017年 | 22篇 |
2016年 | 27篇 |
2015年 | 32篇 |
2014年 | 45篇 |
2013年 | 46篇 |
2012年 | 69篇 |
2011年 | 51篇 |
2010年 | 38篇 |
2009年 | 39篇 |
2008年 | 39篇 |
2007年 | 42篇 |
2006年 | 31篇 |
2005年 | 28篇 |
2004年 | 20篇 |
2003年 | 17篇 |
2002年 | 15篇 |
2001年 | 3篇 |
2000年 | 3篇 |
1999年 | 2篇 |
1998年 | 10篇 |
1997年 | 6篇 |
1996年 | 5篇 |
1995年 | 2篇 |
1993年 | 3篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1984年 | 1篇 |
1981年 | 2篇 |
排序方式: 共有731条查询结果,搜索用时 31 毫秒
101.
We record here the occurrence of five exotic fish species viz Oreochromis mossambicus, Gambusia affinis, Osphronemus goramy, Xiphophorus maculatus and Poecilia reticulata in the Chalakudy River, part of the Western Ghats, a global biodiversity hotspot in Kerala, India. O. mossambicus was ubiquitous in occurrence with large shoals being encountered at all sampling sites spread along the downstream-upstream
gradient of the river, including at an altitude of 1050 m ASL. Osphronemus goramy was recorded from the downstream region of the river while Gambusia affinis was recorded from three sampling sites located downstream as well as midstream. Xiphophorus maculatus was collected from a second order stream flowing through a tea plantation at an altitude of 1050 m ASL. Samples of brooders
and early fry of Poecilia reticulata indicate that the fish has already established a breeding population in the river. Possible threats to the indigenous fish
fauna of the Chalakudy River as a result of the invasion and proliferation of these exotics is discussed. 相似文献
102.
Rapid prototyping of distributed systems can be achieved by integrating commercial off-the-shelf (COTS) components. With components
as the building blocks, it is important to predict the performance of the system based on the performance of individual components.
In this paper, performance prediction of a system consisting of a small number of components is investigated under different
inter-component communication patterns, and the number of threads provided by components. Based on the experimental results,
it can be inferred that the proposed composition rules provide a reasonably accurate prediction of the performance of a system
made out of these components.
相似文献
Barrett R. BryantEmail: |
103.
104.
Xiang Wang Yanhui Peng Jeremy W. Singer Anania Fessehaie Stephen L. Krebs Rajeev Arora 《Plant science》2009,177(6):607-617
Leaves of overwintering evergreen rhododendrons are typically exposed to freezing temperatures and high light during winters which can potentially result in photon flux exceeding that required for photochemistry. This excess energy, if not dissipated as heat or fluorescence, may cause photooxidative damage to PSII. The goal of this study is to compare the photoprotection strategies during seasonal cold acclimation (CA) in two Rhododendron species (R. catawbiense Michx. and R. ponticum L.) that are divergent in their leaf freezing tolerance and thermonastic behaviour (temperature-induced leaf movement). R. catawbiense exhibits thermonasty while R. ponticum does not. Differences in leaf freezing tolerance (LT50), photosynthesis, photoinhibition, early light-induced proteins (ELIPs) gene expression, and accumulation of antioxidant metabolites and enzymes during seasonal CA were investigated. During seasonal CA, maximum photosynthetic rate (Pmax) and maximum quantum efficiency of PSII (Fv/Fm) were significantly down-regulated. Compared with R. catawbiense, R. ponticum showed less photoinhibition and higher overall accumulation (in magnitude) of antioxidant systems while R. catawbiense exhibited more efficient up-regulation of ELIP expression and antioxidant system (i.e., greater efficiency of increasing this pool in winter months relative to the summer levels). The two species respond differently to winter conditions and have evolved strategies to avoid, reduce and/or tolerate photooxidative stress in winter. These include down-regulation of photosynthesis and up-regulation of ELIPs and antioxidant systems, together with specialized leaf anatomy and thermonasty behaviour. 相似文献
105.
Balbir Singh Kaith Rajeev Jindal Asim Kumar Jana Mithu Maiti 《Carbohydrate polymers》2009,78(4):987-996
Hybridization of the natural polymers with synthetic polymers is of great interest because of its application to biomedical and biodegradable materials. Synthesis of graft copolymers of methyl methacrylate (MMA) onto acetylated Saccharum spontaneum L. fiber using ferrous ammonium sulphate–potassium per sulphate (FAS–KPS) redox initiator under the influence of microwave radiation (MWR) was carried-out. Different reaction parameters such as time, initiator molar ratio, monomer concentration, microwave power, pH and solvent were optimized to get maximum graft yield (72.2%). On grafting, percentage crystallinity decreases rapidly with reduction in its stiffness and hardness. The graft copolymers thus formed were characterized by FTIR, SEM, XRD, TGA, DTA and DTG techniques. Moreover, graft copolymers have been found to be more moisture resistant and also showed higher chemical and thermal resistance. 相似文献
106.
Henri Gerken Emily S. Charlson Elisha M. Cicirelli Linda J. Kenney Rajeev Misra 《Molecular microbiology》2009,72(6):1408-1422
Analysis of suppressors that alleviate the acute envelope stress phenotype of a Δ bamB Δ degP strain of Escherichia coli identified a novel protein MzrA and pleiotropic envZ mutations. Genetic evidence shows that overexpression of MzrA – formerly known as YqjB and EcfM – modulates the activity of EnvZ/OmpR similarly to pleiotropic EnvZ mutants and alter porin expression. However, porin expression in strains devoid of MzrA or overexpressing it is still sensitive to medium osmolarity, pH and procaine, all of which modulate EnvZ/OmpR activities. Thus, MzrA appears to alter the output of the EnvZ/OmpR system but not its ability to receive and respond to various environmental signals. Localization and topology experiments indicate that MzrA is a type II membrane protein, with its N-terminus exposed in the cytoplasm and C-terminus in the periplasm. Bacterial two-hybrid experiments determined that MzrA specifically interacts with EnvZ but not with OmpR or the related membrane sensor kinase, CpxA. This and additional genetic and biochemical evidence suggest that the interaction of MzrA with EnvZ would either enhance EnvZ's kinase activity or reduce its phosphatase activity, thus elevating the steady state levels of OmpR∼P. Furthermore, our data show that MzrA links the two-component envelope stress response regulators, CpxA/CpxR and EnvZ/OmpR. 相似文献
107.
Junjun Wu Neal Green Rajeev Hotchandani Yonghan Hu Jeffrey Condon Adrian Huang Neelu Kaila Huan-Qiu Li Satenig Guler Wei Li Steve Y. Tam Qin Wang Jeffrey Pelker Suzana Marusic Sang Hsu J. Perry Hall Jean-Baptiste Telliez Junqing Cui Lih-Ling Lin 《Bioorganic & medicinal chemistry letters》2009,19(13):3485-3488
Tpl2 (cot/MAP3K8) is an upstream kinase of MEK in the ERK pathway. It plays an important role in Tumor Necrosis Factor-α (TNF-α) production and signaling. We have discovered that 8-halo-4-(3-chloro-4-fluoro-phenylamino)-6-[(1H-[1,2,3]triazol-4-ylmethyl)-amino]-quinoline-3-carbonitriles (4) are potent inhibitors of this enzyme. In order to improve the inhibition of TNF-α production in LPS-stimulated human blood, a series of analogs with a variety of substitutions around the triazole moiety were studied. We found that a cyclic amine group appended to the triazole ring could considerably enhance potency, aqueous solubility, and cell membrane permeability. Optimization of these cyclic amine groups led to the identification of 8-chloro-4-(3-chloro-4-fluorophenylamino)-6-((1-(1-ethylpiperidin-4-yl)-1H-1,2,3-triazol-4-yl)methylamino)quinoline-3-carbonitrile (34). In a LPS-stimulated rat inflammation model, compound 34 showed good efficacy in inhibiting TNF-α production. 相似文献
108.
Adsorption of cellulase on solids resulting from pretreatment of poplar wood by ammonia fiber expansion (AFEX), ammonia recycled percolation (ARP), controlled pH, dilute acid (DA), flowthrough (FT), lime, and sulfur dioxide (SO2) and pure Avicel glucan was measured at 4°C, as were adsorption and desorption of cellulase and adsorption of β‐glucosidase for lignin left after enzymatic digestion of the solids from these pretreatments. From this, Langmuir adsorption parameters, cellulose accessibility to cellulase, and the effectiveness of cellulase adsorbed on poplar solids were estimated, and the effect of delignification on cellulase effectiveness was determined. Furthermore, Avicel hydrolysis inhibition by enzymatic and acid lignin of poplar solids was studied. Flowthrough pretreated solids showed the highest maximum cellulase adsorption capacity (σsolids = 195 mg/g solid) followed by dilute acid (σsolids = 170.0 mg/g solid) and lime pretreated solids (σsolids = 150.8 mg/g solid), whereas controlled pH pretreated solids had the lowest (σsolids = 56 mg/g solid). Lime pretreated solids also had the highest cellulose accessibility (σcellulose = 241 mg/g cellulose) followed by FT and DA. AFEX lignin had the lowest cellulase adsorption capacity (σlignin = 57 mg/g lignin) followed by dilute acid lignin (σlignin = 74 mg/g lignin). AFEX lignin also had the lowest β‐glucosidase capacity (σlignin = 66.6 mg/g lignin), while lignin from SO2 (σlignin = 320 mg/g lignin) followed by dilute acid had the highest (301 mg/g lignin). Furthermore, SO2 followed by dilute acid pretreated solids gave the highest cellulase effectiveness, but delignification enhanced cellulase effectiveness more for high pH than low pH pretreatments, suggesting that lignin impedes access of enzymes to xylan more than to glucan, which in turn affects glucan accessibility. In addition, lignin from enzymatic digestion of AFEX and dilute acid pretreated solids inhibited Avicel hydrolysis less than ARP and flowthrough lignin, whereas acid lignin from unpretreated poplar inhibited enzymes the most. Irreversible binding of cellulase to lignin varied with pretreatment type and desorption method. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009 相似文献
109.
Thaidra Gaufin Melissa Pattison Rajeev Gautam Crystal Stoulig Jason Dufour Jeanne MacFarland Daniel Mandell Coty Tatum Matthew H. Marx Ruy M. Ribeiro David Montefiori Cristian Apetrei Ivona Pandrea 《Journal of virology》2009,83(20):10347-10357
Simian immunodeficiency virus (SIV)-infected African nonhuman primates do not progress to AIDS in spite of high and persistent viral loads (VLs). Some authors consider the high viral replication observed in chronic natural SIV infections to be due to lower anti-SIV antibody titers than those in rhesus macaques, suggesting a role of antibodies in controlling viral replication. We therefore investigated the impact of antibody responses on the outcome of acute and chronic SIVagm replication in African green monkeys (AGMs). Nine AGMs were infected with SIVagm.sab. Four AGMs were infused with 50 mg/kg of body weight anti-CD20 (rituximab; a gift from Genentech) every 21 days, starting from day −7 postinfection up to 184 days. The remaining AGMs were used as controls and received SIVagm only. Rituximab-treated AGMs were successfully depleted of CD20 cells in peripheral blood, lymph nodes (LNs), and intestine, as shown by the dynamics of CD20+ and CD79a+ cells. There was no significant difference in VLs between CD20-depleted AGMs and control monkeys: peak VLs ranged from 107 to 108 copies/ml; set-point values were 104 to 105 SIV RNA copies/ml. Levels of acute mucosal CD4+ T-cell depletion were similar for treated and nontreated animals. SIVagm seroconversion was delayed for the CD20-depleted AGMs compared to results for the controls. There was a significant difference in both the timing and magnitude of neutralizing antibody responses for CD20-depleted AGMs compared to results for controls. CD20 depletion significantly altered the histological structure of the germinal centers in the LNs and Peyer''s patches. Our results, although obtained with a limited number of animals, suggest that humoral immune responses play only a minor role in the control of SIV viral replication during acute and chronic SIV infection in natural hosts.In marked contrast to pathogenic human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) infections of humans and macaques, which are characterized by the constant progression to AIDS in a variable time frame (26), African monkey species naturally infected with SIV are generally spared from any signs of disease (reviewed in references 53 and 71).There are currently three animal models of SIV infection in natural hosts: SIVagm infection of African green monkeys (AGMs), SIVsmm infection of sooty mangabeys, and SIVmnd-1 and SIVmnd-2 infection of mandrills (53, 71). SIV infection in natural hosts is characterized by the following: (i) active viral replication, with set-point viral loads (VLs) similar to or even higher than those found in pathogenic infections (44-46, 49, 50, 52, 61-63); (ii) transient depletion of peripheral CD4+ T cells during primary infection, which rebound to preinfection levels during chronic infection (12, 30, 44-46, 49, 62); (iii) significant CD4+ T-cell depletion in the intestine, which can be partially restored during chronic infection in spite of significant viral replication (21, 48); (iv) low levels of CD4+ CCR5+ cells in blood and tissues (47); (v) transient and moderate increases in immune activation and T-cell proliferation during acute infection, with a return to baseline levels during the chronic phase (44-46, 49, 50, 52, 61-63), as a result of an anti-inflammatory milieu which is rapidly established after infection (14, 30); and (vi) no significant increase in CD4+ T-cell apoptosis during either acute or chronic infection (37, 48), thus avoiding enteropathy and microbial translocation, which control excessive immune activation and prevent disease progression by allowing CD4+ T-cell recovery in the presence of high VLs (21, 48). Hence, the current view is that the main reason behind the lack of disease progression in natural African hosts lies in a better adaptation of the host in response to the highly replicating virus. A better understanding of the mechanisms underlying the lack of disease in natural hosts for SIV infection may provide important clues for understanding the pathogenesis of HIV infection (53, 71).To date, it is still unknown whether or not immune responses are responsible for the lack of disease progression in natural hosts, since data are scarce. Studies of cellular immune responses are significantly more limited than is the case with pathogenic infection, and although not always in agreement (3, 13, 28, 29, 73, 76), their convergence point is that cellular immune responses are not essentially superior to those observed in pathogenic infections (3, 13, 28, 29, 73, 76). This observation is not surprising in the context of the high viral replication in natural hosts. Data are even scarcer on the role of humoral immune responses in the control of disease progression in natural hosts. However, several studies reported that anti-SIV antibody titers are lower in SIV infections of natural hosts, with a lack of anti-Gag responses being characteristic of natural SIV infections in African nonhuman primates (1, 6, 24, 25, 42, 43, 71). Because the viral replication in SIVagm-infected AGMs is of the same magnitude or higher than that in pathogenic infections of rhesus macaques (RMs), it has been hypothesized that these high VLs may be a consequence of the lower antibody titers. Moreover, a recent study has also shown that B cells in lymph nodes (LNs) of AGMs are activated at an earlier time point than is the case for SIVmac251-infected RMs, which implies that humoral immune responses may be important in controlling SIV replication in the natural hosts (9). Conversely, it has been shown that passively transferring immunoglobulins from animals naturally infected with SIVagm prior to infection with a low dose of SIVagm did not prevent infection in AGMs (42, 60), which is in striking contrast to results in studies of pathogenic infections, which convincingly demonstrated with animal models that intravenously administered or topically applied antibodies can protect macaques against intravenous or mucosal simian-human immunodeficiency virus challenge (34-36, 54, 72).Previous CD20+ B-cell-depletion studies during pathogenic RM infections have indicated that humoral immune responses may be important for controlling both the postpeak VL and disease progression (38, 57). However, these studies used strains that are highly resistant to neutralization (SIVmac251 and SIVmac239), making it difficult to assess the role that antibodies have in controlling SIV replication and disease progression. Moreover, our recent results suggested a limited impact of humoral immune responses in controlling replication of a neutralization-sensitive SIVsmm strain in rhesus macaques (18).To investigate the effect that CD20+ B cells and antibodies have on SIV replication in natural hosts, we have depleted CD20+ B cells in vivo in AGMs infected with SIVagm.sab92018. We assessed the impact of humoral immune responses on the control of viral replication and other immunological parameters, and we report that ablating humoral immune responses in SIVagm-infected AGMs does not significantly alter the course of virus replication or disease progression. 相似文献
110.
Raman R Rajanikanth V Palaniappan RU Lin YP He H McDonough SP Sharma Y Chang YF 《PloS one》2010,5(12):e14377