全文获取类型
收费全文 | 2454篇 |
免费 | 212篇 |
出版年
2023年 | 3篇 |
2022年 | 17篇 |
2021年 | 49篇 |
2020年 | 17篇 |
2019年 | 20篇 |
2018年 | 32篇 |
2017年 | 35篇 |
2016年 | 76篇 |
2015年 | 116篇 |
2014年 | 130篇 |
2013年 | 145篇 |
2012年 | 218篇 |
2011年 | 205篇 |
2010年 | 152篇 |
2009年 | 131篇 |
2008年 | 141篇 |
2007年 | 157篇 |
2006年 | 137篇 |
2005年 | 123篇 |
2004年 | 147篇 |
2003年 | 124篇 |
2002年 | 129篇 |
2001年 | 31篇 |
2000年 | 20篇 |
1999年 | 31篇 |
1998年 | 39篇 |
1997年 | 25篇 |
1996年 | 19篇 |
1995年 | 20篇 |
1994年 | 20篇 |
1993年 | 22篇 |
1992年 | 18篇 |
1991年 | 12篇 |
1990年 | 11篇 |
1989年 | 16篇 |
1988年 | 7篇 |
1987年 | 7篇 |
1986年 | 9篇 |
1985年 | 4篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 7篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 5篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1973年 | 3篇 |
排序方式: 共有2666条查询结果,搜索用时 656 毫秒
991.
Polarized epithelial cells secrete proteins at either the apical or basolateral cell surface. A number of non-epithelial secretory
proteins also exhibit polarized secretion when they are expressed in polarized epithelial cells but it is difficult to predict
where foreign proteins will be secreted in epithelial cells. The question is of interest since secretory epithelia are considered
as target tissues for gene therapy protocols that aim to express therapeutic secretory proteins. In the parathyroid gland,
parathyroid hormone is processed by furin and co-stored with chromogranin A in secretory granules. To test the secretion of
these proteins in epithelial cells, they were expressed in MDCK cells. Chromogranin A and a secreted form of furin were secreted
apically while parathyroid hormone was secreted 60% basolaterally. However, in the presence of chromogranin A, the secretion
of parathyroid hormone was 65% apical, suggesting that chromogranin can act as a “sorting escort” (sorting chaperone) for
parathyroid hormone. Conversely, apically secreted furin did not affect the sorting of parathyroid hormone. The apical secretion
of chromogranin A was dependent on cholesterol, suggesting that this protein uses an established cellular sorting mechanism
for apical secretion. However, this sorting does not involve the N-terminal membrane-binding domain of chromogranin A. These
results suggest that foreign secretory proteins can be used as “sorting escorts” to direct secretory proteins to the apical
secretory pathway without altering the primary structure of the secreted protein. Such a system may be of use in the targeted
expression of secretory proteins from epithelial cells.
David V. Cohn—Deceased. 相似文献
992.
Schneider K Kuznetzov VK Sanzharova NI Kanter U Telikh KM Khlopuk MS 《Radiation and environmental biophysics》2008,47(2):241-252
Field-grown maize hybrids were assessed for variability in 137Cs accumulation in vegetative parts of young and mature maize shoots and grains during 2 years with contrasting climatic conditions.
Trials were carried out at different sites in the Tula region of Russia, which is characterized by a highly homogenous soil
classified as Luvic Chernozem according to FAO/UNESCO, and average contamination levels of about 509–564 Bq 137Cs kg−1 soil. In the first year, 19 hybrids were tested. The two hybrids with the highest and the two with the lowest 137Cs concentration ratios (C
r) were also tested in the second year, together with another 11 hybrids. All samples were additionally assessed for their
potassium content. In both investigation periods 137Cs accumulation in vegetative shoots and grains was found to vary up to more than twofold between hybrids. However, C
r values of those hybrids that showed a relatively low 137Cs accumulation in the first year were not necessarily low in the second year, and the ratio between the 137Cs C
r of low- and high-accumulating hybrids was much smaller than in the year before. In both vegetative shoots and grains the
variance caused by the different years was larger than the genotypic variance, thus indicating the limits of genotype selection
for this trait. Significant correlations were determined between the 40K and 137Cs C
r values in the same tissue, but for one hybrid indications for uncoupling of the two traits were found. Average Cs/K ratios
in young shoots, mature shoots and grains were 0.06, 0.05 and 0.02, respectively, indicating tissue- and stage-specific regulation
of accumulation within each plant. The findings are discussed with respect to new approaches towards a better understanding
of 137Cs accumulation and its potential reduction in plants.
Katharina Schneider was deceased. 相似文献
993.
994.
Fischer U Keller A Leidinger P Deutscher S Heisel S Urbschat S Lenhof HP Meese E 《Molecular cancer research : MCR》2008,6(4):576-584
To further understand the biological significance of amplifications for glioma development and recurrencies, we characterized amplicon frequency and size in low-grade glioma and amplicon stability in vivo in recurring glioblastoma. We developed a 12q13-21 amplicon-specific genomic microarray and a bioinformatics amplification prediction tool to analyze amplicon frequency, size, and maintenance in 40 glioma samples including 16 glioblastoma, 10 anaplastic astrocytoma, 7 astrocytoma WHO grade 2, and 7 pilocytic astrocytoma. Whereas previous studies reported two amplified subregions, we found a more complex situation with many amplified subregions. Analyzing 40 glioma, we found that all analyzed glioblastoma and the majority of pilocytic astrocytoma, grade 2 astrocytoma, and anaplastic astrocytoma showed at least one amplified subregion, indicating a much higher amplification frequency than previously suggested. Amplifications in low-grade glioma were smaller in size and displayed clearly different distribution patterns than amplifications in glioblastoma. One glioblastoma and its recurrencies revealed an amplified subregion of 5 Mb that was stable for 6 years. Expression analysis of the amplified region revealed 10 overexpressed genes (i.e., KUB3, CTDSP2, CDK4, OS-9, DCTN2, RAB3IP, FRS2, GAS41, MDM2, and RAP1B) that were consistently overexpressed in all cases that carried this amplification. Our data indicate that amplifications on 12q13-21 (a) are more frequent than previously thought and present in low-grade tumors and (b) are maintained as extended regions over long periods of time. 相似文献
995.
996.
997.
Christian Brümmer Nicolas Brüggemann Klaus Butterbach-Bahl Ulrike Falk Jörg Szarzynski Konrad Vielhauer Reiner Wassmann Hans Papen 《Ecosystems》2008,11(4):582-600
In a combined field and laboratory study in the southwest of Burkina Faso, we quantified soil-atmosphere N2O and NO exchange. N2O emissions were measured during two field campaigns throughout the growing seasons 2005 and 2006 at five different experimental
sites, that is, a natural savanna site and four agricultural sites planted with sorghum (n = 2), cotton and peanut. The agricultural fields were not irrigated and not fertilized. Although N2O exchange mostly fluctuated between −2 and 8 μg N2O–N m−2 h−1, peak N2O emissions of 10–35 μg N2O–N m−2 h−1 during the second half of June 2005, and up to 150 μg N2O–N m−2 h−1 at the onset of the rainy season 2006, were observed at the native savanna site, whereas the effect of the first rain event
on N2O emissions at the crop sites was low or even not detectable. Additionally, a fertilizer experiment was conducted at a sorghum
field that was divided into three plots receiving different amounts of N fertilizer (plot A: 140 kg N ha−1; plot B: 52.5 kg N ha−1; plot C: control). During the first 3 weeks after fertilization, only a minor increase in N2O emissions at the two fertilized plots was detected. After 24 days, however, N2O emission rates increased exponentially at plot A up to a mean of 80 μg N2O–N m−2 h−1, whereas daily mean values at plot B reached only 19 μg N2O–N m−2 h−1, whereas N2O flux rates at plot C remained unchanged. The calculated annual N2O emission of the nature reserve site amounted to 0.52 kg N2O–N ha−1 a−1 in 2005 and to 0.67 kg N2O–N ha−1 a−1 in 2006, whereas the calculated average annual N2O release of the crop sites was only 0.19 kg N2O–N ha−1 a−1 and 0.20 kg N2O–N ha−1 a−1 in 2005 and 2006, respectively. In a laboratory study, potential N2O and NO formation under different soil moisture regimes were determined. Single wetting of dry soil to medium soil water
content with subsequent drying caused the highest increase in N2O and NO emissions with maximum fluxes occurring 1 day after wetting. The stimulating effect lasted for 3–4 days. A weaker
stimulation of N2O and NO fluxes was detected during daily wetting of soil to medium water content, whereas no significant stimulating effect
of single or daily wetting to high soil water content (>67% WHCmax) was observed. This study demonstrates that the impact of land-use change in West African savanna on N trace gas emissions
is smaller—with the caveat that there could have been potentially higher N2O and NO emissions during the initial conversion—than the effect of timing and distribution of rainfall and of the likely
increase in nitrogen fertilization in the future. 相似文献
998.
During nuclear export, Gle1 (the nuclear-pore-associated mRNA export factor) activates the DEAD-box protein Dbp5 to remodel exported mRNA-protein complexes on the cytoplasmic face of the nuclear pore complex. In this issue, Bolger et al. (2008) now report additional roles for Gle1 in translation initiation and termination. 相似文献
999.
Ecophysiological investigations on the salinity acclimation of the cyanobacterium Prochlorothrix hollandica SAG 10.89 led to significantly revised salinity tolerance limits. Besides potential effects of cultivation techniques, clear
ion composition effects mainly explain formerly described hypersensitivity to NaCl-mediated salinity and lack of osmolyte
detection. An extraordinarily broad plasticity of cellular chlorophyll a/b ratios occurred with variations of NaCl-induced salinity. Photosynthesis characteristics, pigment regulation, respiration,
and biomass yield in growth medium with field-like ion composition indicated generally reduced acclimation pressure. A simultaneously
significant increase in osmolyte (sucrose) accumulation indicated more efficient osmotic acclimation. Minor growth inhibition
up to salinities of 10 practical salinity units enlarged the potential habitat of P. hollandica but at the most to about 300,000 km2 in the Baltic Sea. This supports probable observations of Prochlorothrix sp. in phytoplankton assemblages of open waters in Baltic Sea-monitoring studies. Brackish habitats differ from so far known
habitats of Prochlorothrix spp. in turbidity, productivity, and plankton composition. Adjusted physiological features dispel fundamental doubts on the
establishment of filamentous prochlorophytes in brackish waters. 相似文献
1000.
In microcosm experiments, we simultaneously tested the effects of increased numbers of deposit-feeding macrofauna (chironomids, oligochaetes and cladocerans) on the standing stock, activities and interactions of heterotrophic bacteria, viruses, and bacterivorous protozoa (heterotrophic nanoflagellates and ciliates) in the aerobic layer of a silty littoral freshwater sediment. On average, bacterial secondary production was stimulated between 11 and 29% by all macrofaunal groups compared to control experiments without macrofauna addition. Bacterial standing stock increased significantly by 8 and 13% in case of chironomids and cladocerans, respectively. Oligochaetes and chironomids produced significant negative effects on viral abundance while the results with cladocerans were inconsistent. The addition of oligochaetes and chironomids resulted in a significant decrease by on average 68 and 32% of viral decay rates, respectively, used as a measure of viral production. The calculated contribution of virus-induced lysis to benthic bacterial mortality was low, with 2.8 to 11.8% of bacterial secondary production, and decreased by 39 to 81% after the addition of macrofauna compared to the control. The abundances of heterotrophic nanoflagellates were significantly reduced by 20% by all tested macrofauna groups, while ciliates showed inconsistent results. The importance of heterotrophic nanoflagellate grazing on benthic bacteria was very low (<1% of bacterial secondary production) and was further reduced by elevated numbers of macrofauna. Thus, the selected deposit feeding macrofauna groups seem to have several direct and indirect and partly antagonistic effects on the benthic bacterial compartment through the enhancement of bacterial production and the reduction of virus-induced cell lysis and protozoan grazing. 相似文献