全文获取类型
收费全文 | 25511篇 |
免费 | 1880篇 |
国内免费 | 4篇 |
出版年
2023年 | 237篇 |
2022年 | 195篇 |
2021年 | 575篇 |
2020年 | 471篇 |
2019年 | 518篇 |
2018年 | 966篇 |
2017年 | 794篇 |
2016年 | 986篇 |
2015年 | 1103篇 |
2014年 | 1221篇 |
2013年 | 1707篇 |
2012年 | 2032篇 |
2011年 | 2113篇 |
2010年 | 1231篇 |
2009年 | 960篇 |
2008年 | 1529篇 |
2007年 | 1462篇 |
2006年 | 1383篇 |
2005年 | 1070篇 |
2004年 | 1036篇 |
2003年 | 938篇 |
2002年 | 832篇 |
2001年 | 599篇 |
2000年 | 668篇 |
1999年 | 411篇 |
1998年 | 219篇 |
1997年 | 155篇 |
1996年 | 143篇 |
1995年 | 133篇 |
1994年 | 119篇 |
1993年 | 114篇 |
1992年 | 150篇 |
1991年 | 141篇 |
1990年 | 108篇 |
1989年 | 88篇 |
1988年 | 88篇 |
1987年 | 74篇 |
1986年 | 65篇 |
1985年 | 71篇 |
1984年 | 65篇 |
1983年 | 49篇 |
1982年 | 60篇 |
1981年 | 49篇 |
1980年 | 43篇 |
1979年 | 51篇 |
1978年 | 43篇 |
1977年 | 31篇 |
1975年 | 38篇 |
1974年 | 31篇 |
1973年 | 30篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
1.
2.
A.C. Gonçalves T.D. Matos H.R. Simões-Teixeira M. Pimenta Machado M. Simão Ó.P. Dias M. Andrea G. Fialho H. Caria 《Gene》2014
Low-frequency sensorineural hearing loss (LFSNHL) is an unusual type of HL in which frequencies at 2000 Hz and below are predominantly affected. Most of the families with LFSNHL carry missense mutations in WFS1 gene, coding for wolframin. 相似文献
3.
C. Lu F.G. Acién Fernández E. Cañizares Guerrero D.O. Hall E. Molina Grima 《Journal of applied phycology》2002,14(5):331-342
The outdoor production of Monodus subterraneus wasstudied in bubble column and helical reactors, mainly analysing the influenceofdilution rate, air flow rate and solar irradiance on growth rate andbiochemicalcomposition. Photoinhibition and photo-oxidation phenomena were also analysed.The cultures were stressed at high solar irradiance and dissolved oxygenconcentrations. A clear relationship between stress of the cultures and thefluorescence from PSII measurements was observed, the Fv/Fm ratio being lowerinthe helical reactor than in the bubble column. Growth rate and biomassproductivity were both a function of the average irradiance and the Fv/Fmratio;maximum values of 0.040 h–1 and 0.54 gL–1 d–1 were measured. The influenceofphotoinhibition and average irradiance was modelled, the model also fitting theexperimental data reported by another author. The chlorophyll contenthyperbolically decreased, whereas the carotenoid content decreased linearlywiththe average irradiance. The higher the dilution rate the higher the protein andcarbohydrate content of the biomass, and the lower the lipid content. Theeicosapentaenoic acid (EPA) content ranged from 2.3 to 3.2% d.wt, the higherthe dilution rate, the lower EPA content, although the higher the EPAproportion. Maximum EPA productivity was only 9 mg L–1d–1, due to the stress to which the cultures wereexposed. 相似文献
4.
5.
Thiago B. Kirsten Marcella C. Galv?o Thiago M. Reis-Silva Nicolle Queiroz-Hazarbassanov Maria M. Bernardi 《PloS one》2015,10(3)
Sickness behavior is considered part of the specific beneficial adaptive behavioral and neuroimmune changes that occur in individuals in response to infectious/inflammatory processes. However, in dangerous and stressful situations, sickness behavior should be momentarily abrogated to prioritize survival behaviors, such as fight or flight. Taking this assumption into account, we experimentally induced sickness behavior in rats using lipopolysaccharides (LPS), an endotoxin that mimics infection by gram-negative bacteria, and then exposed these rats to a restraint stress challenge. Zinc has been shown to play a regulatory role in the immune and nervous systems. Therefore, the objective of this study was to examine the effects of zinc treatment on the sickness response of stress-challenged rats. We evaluated 22-kHz ultrasonic vocalizations, open-field behavior, tumor necrosis factor α (TNF-α), corticosterone, and brain-derived neurotrophic factor (BDNF) plasma levels. LPS administration induced sickness behavior in rats compared to controls, i.e., decreases in the distance traveled, average velocity, rearing frequency, self-grooming, and number of vocalizations, as well as an increase in the plasma levels of TNF-α, compared with controls after a stressor challenge. LPS also decreased BDNF expression but did not influence anxiety parameters. Zinc treatment was able to prevent sickness behavior in LPS-exposed rats after the stress challenge, restoring exploratory/motor behaviors, communication, and TNF-α levels similar to those of the control group. Thus, zinc treatment appears to be beneficial for sick animals when they are facing risky/stressful situations. 相似文献
6.
The accumulation of proline (Pro) in plants exposed to biotic/abiotic stress is a well-documented and conserved response in most vegetal species. Stress conditions induce the overproduction of reactive oxygen species which can lead to cellular damage. In vitro assays have shown that enzyme inactivation by hydroxyl radicals (·OH) can be avoided in presence of Pro, suggesting that this amino acid could act as an ·OH scavenger. We applied Density Functional Theory coupled with a polarizable continuum model to elucidate how Pro reacts with ·OH. In this work we suggest that Pro reacts favourably with ·OH by H–abstraction on the amine group. This reaction produces the spontaneous decarboxylation of Pro leading to the formation of pyrrolidin-1-yl. In turn, pyrrolidin-1-yl can easily be converted to Δ1-pyrroline, the substrate of the enzyme Δ1-pyrroline dehydrogenase, which produces γ-aminobutyric acid (GABA). GABA and Pro are frequently accumulated in stressed plants and several protective roles have been assigned to these molecules. Thereby we present an alternative non-enzymatic way to synthetize GABA under oxidative stress. Finally this work sheds light on a new beneficial role of Pro accumulation in the maintenance of photosynthetic activity. 相似文献
7.
In a pot experiment, burning or clipping ofTrifolium pratense plants colonized by the vesiculararbuscular mycorrhizal (VAM) fungusAcaulospora scrobiculata increased the number of spores in the substrate and the abundance of vesicles. The length of external mycelium and inoculum potential increased with clipping, but decreased with plant burning. Results suggest that the loss of photosynthesising tissue does not explain the effect of burning on VAM propagule activity. ei]R O D Dixon 相似文献
8.
9.
Treatment of nonmucoid Pseudomonas aeruginosa with gyrase inhibitors such as ciprofloxacin, norfloxacin, and ofloxacin, which target the A subunit of topoisomerase II,
resulted in 100% conversion to the mucoid phenotype. However, antibiotics that partially inhibited growth and macromolecular
synthesis (DNA, RNA, protein, or peptidoglycan) of nonmucoid isolates in a gluconate-limited chemostat culture system did
not promote conversion to mucoid subpopulations. An increase in resistance was observed in populations that expressed the
mucoid phenotype. Both mucoid conversion and antibiotic resistance were completely reversible when ciprofloxacin pressure
was withdrawn, but only partially reversible by the removal of norfloxacin and ofloxacin. Thus, these experiments indicate
that in the presence of some fluoroquinolones, a conditional response resulting in mucoid conversion and antibiotic resistance
may occur.
Received: 28 January 1997 / Accepted: 12 February 1997 相似文献