全文获取类型
收费全文 | 6418篇 |
免费 | 702篇 |
国内免费 | 5篇 |
出版年
2023年 | 30篇 |
2022年 | 71篇 |
2021年 | 166篇 |
2020年 | 103篇 |
2019年 | 124篇 |
2018年 | 147篇 |
2017年 | 132篇 |
2016年 | 211篇 |
2015年 | 383篇 |
2014年 | 376篇 |
2013年 | 385篇 |
2012年 | 562篇 |
2011年 | 484篇 |
2010年 | 322篇 |
2009年 | 278篇 |
2008年 | 373篇 |
2007年 | 356篇 |
2006年 | 300篇 |
2005年 | 308篇 |
2004年 | 228篇 |
2003年 | 225篇 |
2002年 | 180篇 |
2001年 | 97篇 |
2000年 | 98篇 |
1999年 | 82篇 |
1998年 | 67篇 |
1997年 | 30篇 |
1996年 | 36篇 |
1995年 | 40篇 |
1994年 | 26篇 |
1993年 | 32篇 |
1992年 | 65篇 |
1991年 | 59篇 |
1990年 | 45篇 |
1989年 | 37篇 |
1988年 | 48篇 |
1987年 | 41篇 |
1986年 | 32篇 |
1985年 | 49篇 |
1984年 | 40篇 |
1983年 | 33篇 |
1982年 | 38篇 |
1981年 | 34篇 |
1979年 | 33篇 |
1978年 | 25篇 |
1975年 | 25篇 |
1973年 | 24篇 |
1971年 | 21篇 |
1970年 | 23篇 |
1968年 | 25篇 |
排序方式: 共有7125条查询结果,搜索用时 0 毫秒
951.
Isolation of Strains Belonging to the Cosmopolitan Polynucleobacter necessarius Cluster from Freshwater Habitats Located in Three Climatic Zones 总被引:1,自引:0,他引:1
下载免费PDF全文
![点击此处可从《Applied microbiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Martin W. Hahn 《Applied microbiology》2003,69(9):5248-5254
More than 40 bacterial strains belonging to the cosmopolitan Polynucleobacter necessarius cluster (Betaproteobacteria) were isolated from a broad spectrum of freshwater habitats located in three climatic zones. Sequences affiliated with the freshwater P. necessarius cluster are among the most frequently detected in studies on bacterial diversity in freshwater ecosystems. Despite this frequent detection with culture-independent techniques and the cosmopolitan occurrence of members affiliated with this cluster, no isolates have been reported thus far. The isolated strains have been obtained from lakes, ponds, and rivers in central Europe, the People's Republic of China, and East Africa by use of the filtration-acclimatization method. The 16S rRNA gene sequences of the isolates are 98.8 to 100% identical to reference sequences obtained by various authors by use of culture-independent methods. The isolates, aerobic heterotrophs, grew on a wide range of standard complex media and formed visible colonies on agar plates. Thus, the previous lack of isolates cannot be explained by a lack of appropriate media. Most of the isolates possess, under a wide range of culture conditions, very small cells (<0.1 μm3), even when grown in medium containing high concentrations of organic substances. Thus, these strains are obligate ultramicrobacteria. The obtained strains have a C-shaped cell morphology which is very similar to that of recently isolated ultramicrobacterial Luna cluster strains (Actinobacteria) and the SAR11 cluster strains (Alphaproteobacteria). 相似文献
952.
Peter Redecker Michael R Kreutz Jürgen Bockmann Eckart D Gundelfinger Tobias M Boeckers 《The journal of histochemistry and cytochemistry》2003,51(6):809-819
Proteins of the presynaptic exocytic machinery have been found associated with the acrosome of male germ cells, suggesting that the sperm acrosome reaction and neurotransmission at chemical synapses may share some common mechanisms. To substantiate this hypothesis, we studied the expression and ultrastructural localization of prominent pre- and postsynaptic protein components in rat testis. The presynaptic membrane trafficking proteins SV2 and complexin, the vesicular amino acid transporters VGLUT and VIAAT, the postsynaptic scaffolding protein ProSAP/Shank, and the postsynaptic calcium-sensor protein caldendrin, could be identified in germ line cells. Immunogold electron microscopy revealed an association of these proteins with the acrosome. In addition, evidence was obtained for the expression of the plasmalemmal glutamate transporters GLT1 and GLAST in rat sperm. The novel finding that not only presynaptic proteins, which are believed to be involved in membrane fusion processes, but also postsynaptic elements are present at the acrosome sheds new light on its structural organization. Moreover, our data point to a possible role for neuroactive amino acids in reproductive physiology. 相似文献
953.
954.
955.
DNA prenatal diagnosis was successfully performed on a family with citrullinemia. The father carried the G324S mutation and the mother carried the IVS6–2A > G mutation in the argininosuccinate synthase gene. They had a previous child with citrullinemia who died in the week after birth owing to complicated hyperammonemia. The lost child turned out to be a compound heterozygote. DNA was extracted from the cultured amniotic cells after amniocentesis done at 18-week gestation. For the detection of the G324S mutation, the PCR and restriction fragment length polymorphism method was used, and for the IVS6–2A > G mutation, allele-specific PCR was performed. The fetus was found to carry G324S but not IVS6–2A > G, suggesting a heterozygote carrier. Pregnancy was continued and a healthy boy was born. Plasma amino acid analysis performed on the third day after birth was normal and the serial ammonia level was in the normal range. A molecular study on his genomic DNA after birth also agreed with the previous fetal DNA analysis. He is now 2-months old with normal growth and development. 相似文献
956.
Summary
In vitro plantlets of Phalaenopsis ‘Happy Valentine’, Neofinetia falcate Hu, Cymbidium kanran Makino, and Cymbidium goeringii Reichb. f. were grown under photoautotrophic [high photosynthetic photon flux (PPF), high CO2 concentration, and increased number of air exchanges] and heterotrophic (low PPF, low CO2 concentration, no air exchanges) culture conditions. After 40 d of culture, a significant difference in plantlet growth was
observed between the two cultures. Total fresh and dry mass were on average 1.5 times greater in photoautotrophic culture
than in heterotrophic culture. Higher net photosynthetic rates were also observed for Phalaenopsis in photoautotrophic culture. In photoautotrophic culture, little difference was observed in air temperature between the inside
and outside of the culture vessel, whereas in heterotrophic culture, air temperature inside the culture vessel was 1–2°C higher
than that outside the culture vessel. Relative humidity inside the culture vessel was remarkably different between the two
cultures: 83–85% in photoautotrophic culture and 97–99% in heterotrophic culture. These results indicated that growth and
net photosynthetic rate of in vitro orchid plantlets were susceptible to the culture environments such as PPF, CO2 concentration, relative humidity (RH), and the number of air exchanges, which would allow a more efficient micropropagation
system for these orchid plants. 相似文献
957.
Immunological probe of estrogen biosynthesis. Evidence for the 2 beta-hydroxylative pathway in aromatization of androgens 总被引:2,自引:0,他引:2
The terminal hydroxylation in placental estrogen biosynthesis from androgens is at the 2 beta position. The 2 beta-hydroxy-19-oxoandrogen derivative collapses nonenzymatically to estrogen and is therefore the proximate precursor of the female hormone. To establish the role of this pathway in biological aromatization, an immunological approach was employed in which an antibody was obtained which recognizes 2 beta-hydroxy-19-oxygenated androgens but not intermediates oxygenated at C-19 only. Binding of the 2 beta-hydroxy-19-oxo intermediate by the antibody stabilizes it so that its nonenzymatic transformation to estrogen is delayed and results in slower estrogen formation. When placental microsomes were incubated with [1,2-3H]androstenedione in the presence of the antibody antiserum, a 50% decrease in [3H]estradiol formation and 3H2O release was observed when compared with identical incubations containing normal rabbit serum alone. This inhibition is blocked when the antibody is inactivated by presaturation with 2 beta, 19-dihydroxyandrostenedione. Precipitation of immunoglobulins from the incubations followed by heating liberated the 2 beta-hydroxy-19-oxo intermediate (30%) from the antibody, and resulted in its nonenzymatic collapse to estrogen with concomitant release of 3H2O. Control normal rabbit serum or blocked antibody incubations did not show a similar increase in [3H]estradiol or 3H2O yields in the precipitate. Heat treatment (90 degrees C) of the antibody but not normal rabbit serum incubations resulted in a similar increase in [3H]estradiol and 3H2O yields. These results are consistent with the hypothesis that the final and rate-determining hydroxylation in aromatization of androgens is at the 2 beta position and that this pathway is the dominant, if not the sole, route of estrogen biosynthesis by placental aromatase. The antibody probe also permits the characterization of aromatization mechanisms in tissues other than the placenta. 相似文献
958.
Dominik Refardt Tobias Bergmiller Rolf Kümmerli 《Proceedings. Biological sciences / The Royal Society》2013,280(1759)
High relatedness among interacting individuals has generally been considered a precondition for the evolution of altruism. However, kin-selection theory also predicts the evolution of altruism when relatedness is low, as long as the cost of the altruistic act is minor compared with its benefit. Here, we demonstrate evidence for a low-cost altruistic act in bacteria. We investigated Escherichia coli responding to the attack of an obligately lytic phage by committing suicide in order to prevent parasite transmission to nearby relatives. We found that bacterial suicide provides large benefits to survivors at marginal costs to committers. The cost of suicide was low, because infected cells are moribund, rapidly dying upon phage infection, such that no more opportunity for reproduction remains. As a consequence of its marginal cost, host suicide was selectively favoured even when relatedness between committers and survivors approached zero. Altogether, our findings demonstrate that low-cost suicide can evolve with ease, represents an effective host-defence strategy, and seems to be widespread among microbes. Moreover, low-cost suicide might also occur in higher organisms as exemplified by infected social insect workers leaving the colony to die in isolation. 相似文献
959.
Jonathan Heubes Ingolf Kühn Konstantin König Rüdiger Wittig Georg Zizka Karen Hahn 《Journal of Biogeography》2011,38(12):2248-2258
Aim Africa is expected to face severe changes in climatic conditions. Our objectives are: (1) to model trends and the extent of future biome shifts that may occur by 2050, (2) to model a trend in tree cover change, while accounting for human impact, and (3) to evaluate uncertainty in future climate projections. Location West Africa. Methods We modelled the potential future spatial distribution of desert, grassland, savanna, deciduous and evergreen forest in West Africa using six bioclimatic models. Future tree cover change was analysed with generalized additive models (GAMs). We used climate data from 17 general circulation models (GCMs) and included human population density and fire intensity to model tree cover. Consensus projections were derived via weighted averages to: (1) reduce inter‐model variability, and (2) describe trends extracted from different GCM projections. Results The strongest predicted effect of climate change was on desert and grasslands, where the bioclimatic envelope of grassland is projected to expand into the desert by an area of 2 million km2. While savannas are predicted to contract in the south (by 54 ± 22 × 104 km2), deciduous and evergreen forest biomes are expected to expand (64 ± 13 × 104 km2 and 77 ± 26 × 104 km2). However, uncertainty due to different GCMs was particularly high for the grassland and the evergreen biome shift. Increasing tree cover (1–10%) was projected for large parts of Benin, Burkina Faso, Côte d’Ivoire, Ghana and Togo, but a decrease was projected for coastal areas (1–20%). Furthermore, human impact negatively affected tree cover and partly changed the direction of the projected change from increase to decrease. Main conclusions Considering climate change alone, the model results of potential vegetation (biomes) show a ‘greening’ trend by 2050. However, the modelled effects of human impact suggest future forest degradation. Thus, it is essential to consider both climate change and human impact in order to generate realistic future tree cover projections. 相似文献
960.