全文获取类型
收费全文 | 997篇 |
免费 | 83篇 |
国内免费 | 2篇 |
出版年
2023年 | 4篇 |
2022年 | 11篇 |
2021年 | 28篇 |
2020年 | 8篇 |
2019年 | 17篇 |
2018年 | 25篇 |
2017年 | 25篇 |
2016年 | 27篇 |
2015年 | 43篇 |
2014年 | 55篇 |
2013年 | 71篇 |
2012年 | 67篇 |
2011年 | 61篇 |
2010年 | 52篇 |
2009年 | 45篇 |
2008年 | 57篇 |
2007年 | 45篇 |
2006年 | 50篇 |
2005年 | 44篇 |
2004年 | 30篇 |
2003年 | 40篇 |
2002年 | 42篇 |
2001年 | 22篇 |
2000年 | 21篇 |
1999年 | 12篇 |
1998年 | 14篇 |
1997年 | 7篇 |
1996年 | 9篇 |
1995年 | 5篇 |
1994年 | 7篇 |
1993年 | 6篇 |
1992年 | 12篇 |
1991年 | 15篇 |
1990年 | 6篇 |
1989年 | 8篇 |
1988年 | 9篇 |
1987年 | 9篇 |
1986年 | 6篇 |
1985年 | 15篇 |
1984年 | 10篇 |
1982年 | 3篇 |
1977年 | 4篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1971年 | 3篇 |
1970年 | 2篇 |
1969年 | 4篇 |
1968年 | 3篇 |
1967年 | 3篇 |
排序方式: 共有1082条查询结果,搜索用时 31 毫秒
991.
Regulation of chain length is essential to the proper functioning of prokaryotic and eukaryotic polysaccharides. Modulation of polymer size by substrate concentration is an attractive but unexplored control mechanism that has been suggested for many polysaccharides. The Streptococcus pneumoniae capsular polysaccharide is essential for virulence, and regulation of its size is critical for survival in different host environments. Synthesis of the type 3 capsule [-4)-beta-d-Glc-(1-3)-beta-d-GlcUA-(1-] from UDP-glucose (UDP-Glc) and UDP-glucuronic acid (UDP-GlcUA) is catalysed by the type 3 synthase, a processive beta-glycosyltransferase, and requires a UDP-Glc dehydrogenase for conversion of UDP-Glc to UDP-GlcUA. Strains containing mutant UDP-Glc dehydrogenases exhibited reduced levels of UDP-GlcUA, along with reductions in total capsule amount and polymer chain length. In both the parent and mutant strains, UDP-Glc levels far exceeded UDP-GlcUA levels, which were very low to undetectable in the absence of blocking synthase activity. The in vivo observations were consistent with in vitro conditions that effect chain termination and ejection of the polysaccharide from the synthase when one substrate is limiting. These data are the first to demonstrate modulation of polysaccharide chain length by substrate concentration and to enable a model for the underlying mechanism. Further, they may have implications for the control of chain length in both prokaryotic and eukaryotic polymers synthesized by similar mechanisms. 相似文献
992.
Gonçalves EM Ventura CA Yano T Rodrigues Macedo ML Genari SC 《Cell biology international》2006,30(6):485-494
Cisplatin is an antineoplastic agent used to treat solid tumours, such as ovarian, testicular and bladder tumours. However, studies in vitro and in vivo have shown that cisplatin is mutagenic, genotoxic and tumorigenic in other tissues and organs. In this work, we examined the effect of cisplatin on Vero cells, a fibroblast-like cell line. The morphological characteristics were investigated using phase contrast microscopy, scanning electron microscopy and the actin cytoskeleton was labelled with fluorescein isothiocyanate-phalloidin. Cell proliferation was assessed based on the growth curve. Cultured Vero cells treated with cisplatin showed behavioural and morphological alterations associated with cellular transformation. The transformed cells grew in multilayers and formed cellular aggregates. The proliferation and morphological characteristics of the transformed cells were very different from those of control ones. Since transformed Vero cells showed several characteristics related to neoplastic growth, these cells could be a useful model for studying tumour cells in vitro. 相似文献
993.
994.
995.
Emilie Ventura Richard Durant Audrey Jaussent Marie-Christine Picot Marion Morena Stéphanie Badiou Anne-Marie Dupuy Claude Jeandel Jean-Paul Cristol 《Free radical biology & medicine》2009,46(6):737-744
Oxidative stress is commonly observed in the elderly and could be involved in age-related diseases. However, the determinants of superoxide anion overproduction are not clearly understood. Superoxide anion production was evaluated using a lucigenin-based chemiluminescence method in 478 elderly subjects (304 women, 174 men; 79.5 ± 7.1 years). Homocysteine (HCy) metabolism (homocysteinemia, vitamin B12, plasma, and erythrocyte folates), inflammation (CRP, fibrinogen, α-1 acid glycoprotein), lipid parameters (total cholesterol, triglycerides, HDL and LDL cholesterol), and nutritional parameters (albumin, transthyretin) were determined. The results show that HCy levels (p < 0.001) and superoxide anion production (p = 0.04) increase with aging, but CRP does not. Highest HCy (> 20 μM) (OR 1.83 (1.09–3.07), p = 0.02) and CRP over 5 mg/L (adjusted OR 2.01 (1.15–3.51), p = 0.01) are the main determinants in superoxide anion production in the elderly. These clinical data are confirmed in an in vitro study using THP-1 monocyte-like cells. Incubation with HCy thiolactone (HTL) (0–200 μM) and LPS (0–20 ng/ml) dramatically enhances NADPH oxidase expression and activation. Moreover, a synergic action was evidenced for low concentrations of HTL (20 μM) and LPS (5 ng). Taken together, the clinical data and in vitro experiments support the hypothesis that moderate homocysteinemia and low-grade inflammation synergically enhance NADPH oxidase activity in the elderly. 相似文献
996.
In order to evaluate the need for using scale acidification to remove carbonates prior to stable isotope analysis, we compared
acidified and non-acidified scales of six freshwater fish species (perch, roach, rudd, pike, tench and bream) with contrasting
mineral content in their scales. Fish samples were taken from six lakes with variable trophic conditions, ranging from oligotrophic
to hypertrophic, and differing in CO2 concentrations. The scale mineral content of the six species studied ranged between 31.8 and 61.3% dry weight (DW) in tench
and perch, respectively. The elemental composition was characterised by high amounts of phosphorus, varying from 4.5 to 9.1%
DW. The mineral fraction was dominated by apatite (range 24.4–49.2% DW), carbonates constituted a very small proportion of
the total carbon content (average ± SD: 5.5 ± 1.7%). The average effect of acidification was very small for all species (average ± SD:
0.181 ± 0.122 and −0.208 ± 0.243 for carbon and nitrogen, respectively), albeit significant for five out of the six species
(excepting tench that had the lowest mineral content). Linear regression slopes between acidified and untreated scales did
not differ significantly from one for almost all the species and isotopes. The effects of acidification on the two isotopes
were correlated with the relative carbonate content as well as with the CO2 concentration for carbon and total phosphorus for nitrogen. We conclude that the need for scale acidification depends on
the different species and on the system studied, although in most cases the acidification effect will be biologically irrelevant.
However, dual analysis of acidified and untreated scales may provide useful information on differences in stable isotope composition
of dissolved inorganic carbon and on phytoplankton carbon fractionation generated by varying levels of CO2 availability. 相似文献
997.
BUALUANG FAIYUE CHENNIAPPAN VIJAYALAKSHMI SHAFQAT NAWAZ YASUO NAGATO SHIN TAKETA MASAHIKO ICHII MOHAMMED J. AL‐AZZAWI TIMOTHY J. FLOWERS 《Plant, cell & environment》2010,33(5):687-701
An apoplastic pathway, the so‐called bypass flow, is important for Na+ uptake in rice (Oryza sativa L.) under saline conditions; however, the precise site of entry is not yet known. We report the results of our test of the hypothesis that bypass flow of Na+ in rice occurs at the site where lateral roots emerge from the main roots. We investigated Na+ uptake and bypass flow in lateral rootless mutants (lrt1, lrt2), a crown rootless mutant (crl1), their wild types (Oochikara, Nipponbare and Taichung 65, respectively) and in seedlings of rice cv. IR36. The results showed that shoot Na+ concentration in lrt1, lrt2 and crl1 was lower (by 20–23%) than that of their wild types. In contrast, the bypass flow quantified using trisodium‐8‐hydroxy‐1,3,6‐pyrenetrisulphonic acid (PTS) was significantly increased in the mutants, from an average of 1.1% in the wild types to 3.2% in the mutants. Similarly, bypass flow in shoots of IR36 where the number of lateral and crown roots had been reduced through physical and hormonal manipulations was dramatically increased (from 5.6 to 12.5%) as compared to the controls. The results suggest that the path of bypass flow in rice is not at the sites of lateral root emergence. 相似文献
998.
Bonaventura Majolo Raffaella Ventura Gabriele Schino 《International journal of primatology》2010,31(5):736-750
Cords and Aureli (2000) proposed that relationship quality can be described by three components, i.e., value, security, and compatibility, based
on the benefits social partners receive from their relationship and on how likely it is for them to maintain a stable friendly
relationship over time. We aimed to examine whether this 3-components structure can describe social relationships and to investigate
the degree of asymmetry of social relationships. We used the Japanese macaque as our model species, as group members may form
strong social relationships with one another. We collected 359.2 h of focal samples, on 21 monkeys, and ran a principal component
analysis (PCA) on dyadic scores for 9 variables measuring social relationships, e.g., grooming or aggression. This PCA gave
3 components. However, a second PCA run on scores per individual, i.e. for each variable and dyad we obtained 1 score for
the actor and 1 for the receiver of behavior, gave a 5-component structure. We interpreted this result as evidence of the
asymmetric nature of social relationships, in terms of the frequency and type of behavior exchanged between social partners,
in the Japanese macaque. This interpretation was confirmed by the low correlation coefficients between behaviors given and
received within a dyad. Overall these results provide mixed support for the 3-components structure predicted by Cords and
Aureli (2000). They also show that social relationships may be largely asymmetric and that this asymmetry should be considered in studies
on social behavior. 相似文献
999.
Glória Maria de Farias Viégas Aquije Poliana Belisário Zorzal David Shaun Buss José Aires Ventura Patricia Machado Bueno Fernandes Antonio Alberto Ribeiro Fernandes 《Plant cell reports》2010,29(10):1109-1117
Fusariosis, caused by the fungus Fusarium subglutinans f. sp. ananas (Syn. F. guttiforme), is one of the main phytosanitary threats to pineapple (Ananas comosus var. comosus). Identification of plant cell responses to pathogens is important in understanding the plant–pathogen relationship and establishing
strategies to improve and select resistant cultivars. Studies of the structural properties and phenolic content of cell walls
in resistant (Vitoria) and susceptible (Perola) pineapple cultivars, related to resistance to the fungus, were performed.
The non-chlorophyll base of physiologically mature leaves was inoculated with a conidia suspension. Analyses were performed
post-inoculation by light, atomic force, scanning and transmission electron microscopy, and measurement of cell wall-bound
phenolic compounds. Non-inoculated leaves were used as controls to define the constitutive tissue characteristics. Analyses
indicated that morphological differences, such as cell wall thickness, cicatrization process and lignification, were related
to resistance to the pathogen. Atomic force microscopy indicated a considerable difference in the mechanical properties of
the resistant and susceptible cultivars, with more structural integrity, associated with higher levels of cell wall-bound
phenolics, found in the resistant cultivar. p-Coumaric and ferulic acids were shown to be the major phenolics bound to the cell walls and were found in higher amounts
in the resistant cultivar. Leaves of the resistant cultivar had reduced fungal penetration and a faster and more effective
cicatrization response compared to the susceptible cultivar. 相似文献
1000.
The present study demonstrates that morphine (10(-6) and 10(-5) M), methionine-enkephalin or leucine-enkephalin (10(-10), 10(-8), and 10(-6) M) were able to inhibit significantly, in a dose-dependent manner, both the sarcolemmal Ca2+-dependent ATPase and the ouabain-sensitive Na+-K+ ATPase activities. The inhibitory action of these opioids on the two ATPases was not antagonized by preincubation with naloxone (10(-6) M). Naloxone alone (10(-8), 10(-6) and 10(-5) M) did not affect both the sarcolemmal Ca2+-dependent ATPase and the ouabain-sensitive Na+-K+ ATPase activities. Heat-denatured methionine-enkephalin (10(-6) M) or leucine-enkephalin (10(-6) M) also unaffected both the ATPases. The possibility is also discussed that opioid peptides may regulate myocardial contractility by modulating the movement of ions across the heart sarcolemma. 相似文献