全文获取类型
收费全文 | 93815篇 |
免费 | 328篇 |
国内免费 | 884篇 |
专业分类
95027篇 |
出版年
2022年 | 7篇 |
2021年 | 13篇 |
2020年 | 11篇 |
2019年 | 16篇 |
2018年 | 11843篇 |
2017年 | 10681篇 |
2016年 | 7457篇 |
2015年 | 617篇 |
2014年 | 307篇 |
2013年 | 323篇 |
2012年 | 4262篇 |
2011年 | 12862篇 |
2010年 | 12027篇 |
2009年 | 8245篇 |
2008年 | 9837篇 |
2007年 | 11400篇 |
2006年 | 296篇 |
2005年 | 555篇 |
2004年 | 1016篇 |
2003年 | 1063篇 |
2002年 | 810篇 |
2001年 | 263篇 |
2000年 | 164篇 |
1999年 | 31篇 |
1998年 | 14篇 |
1997年 | 23篇 |
1996年 | 15篇 |
1994年 | 19篇 |
1993年 | 36篇 |
1992年 | 22篇 |
1991年 | 41篇 |
1990年 | 11篇 |
1989年 | 12篇 |
1988年 | 20篇 |
1987年 | 19篇 |
1985年 | 10篇 |
1984年 | 10篇 |
1983年 | 20篇 |
1982年 | 5篇 |
1980年 | 6篇 |
1979年 | 5篇 |
1975年 | 8篇 |
1972年 | 248篇 |
1971年 | 276篇 |
1970年 | 9篇 |
1965年 | 15篇 |
1962年 | 24篇 |
1956年 | 5篇 |
1944年 | 12篇 |
1940年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Ko Woon Lee Kyoungseon Min Kyungmoon Park Young Je Yoo 《Biotechnology and Bioprocess Engineering》2010,15(4):603-607
Biodiesel has been greatly interested as an alternative fuel and is produced by a transesterification reaction of oil with
alcohol. Recently, microbial lipases have been used for biodiesel production. Among the microbial lipase, immobilized Candida antartica lipase B (CALB) is the most widely used. However, CALB is unstable and shows low catalytic efficiency in the reaction media
because the reaction media contains a high concentration of methanol and the lipase is also inhibited by the by-product glycerol.
In this study, to overcome these limitations, we developed an amphiphilic matrix to immobilize CALB. The immobilized lipase
in an amphiphilic matrix with 80% ethyltrimethoxysilane (ETMS) in tetramethoxysilane (TMOS) and pretreated with oil showed
the highest specific activity and biodiesel conversion ratio; about 90% biodiesel conversion in 24 h at an initial molar ratio
of 1: 1 (oil: methanol) with stepwise methanol feeding in order to adjust the net molar ratio to be 1: 3. 相似文献
992.
Previous microdialysis studies performed in rats have revealed a decrease of striatal dopamine and glutamate induced by nitrogen
narcosis. We sought to establish the hypothetical role of the glutamatergic corticostriatal pathway because of the glutamate
deficiency which occurs in the basal ganglia in this hyperbaric syndrome. Retrodialysis with 1 mM of Saclofen and 100 mM of
KCl in the prefrontal cortex under normobaric conditions led to an increase in striatal levels of glutamate by 95.2% and no
changes in dopamine levels. Under 3 MPa of nitrogen and with the infusion, the rate of striatal glutamate decreased by 51.3%,
to a greater extent than under pressurised nitrogen alone (−23.8%). The rate of dopamine decreased, which also occurred under
pressurised nitrogen (−36.9 and −31.4%, respectively). In conclusion, the function of the corticostriatal pathway is affected
by nitrogen under pressure. This suggests that the nitrogen-induced break point seems to be located at the glutamatergic striatopetal
neurons. 相似文献
993.
Hélia Guerra Cardoso Maria Catarina Campos Maria Salomé Pais Augusto Peixe 《In vitro cellular & developmental biology. Plant》2010,46(6):499-508
Somatic embryogenesis induction from in vitro cultured stamens and carpels is highly dependent on explants’ inoculation at specific developmental stages. To establish
good correlations between measurable morphometric parameters of flowers or flower buds and developmental stages of micro-
and macrosporogenesis, this procedure is the easiest way to simplify the in vitro culture procedures. These correlations were established here for the most important Iberian grapevine cultivar, the “Aragonez”,
named “Tempranillo” in Spain and “Tinta Roriz” in the north of Portugal, and were based in floral buds and anther measurements.
The anther length, with a correlation coefficient of 0.90, proved to be the best morphometric parameter to follow microsporogenesis
evolution. A correlation between micro- and macrosporogenesis evolutionary stages was also positively established, allowing
the use of morphometric parameters for tracking ovule evolution as well. Carpels in several evolutionary stages were in vitro cultured to evaluate the aging effect on the capacity for somatic embryogenesis induction. Explants inoculated in the earliest
stages of macrosporogenesis presented the best results. Media culture formulations were also tested for ovary culture, with
the best results being achieved with a 5:1 auxin/cytokinin ratio. 相似文献
994.
995.
Yang Guo Qiaojuan Yan Zhengqiang Jiang Chao Teng Xinlei Wang 《Journal of industrial microbiology & biotechnology》2010,37(11):1137-1143
The aim of this study is to investigate production of l-lactic acid from sucrose and corncob hydrolysate by the newly isolated R. oryzae GY18. R. oryzae GY18 was capable of utilizing sucrose as a sole source, producing 97.5 g l−1
l-lactic acid from 120 g l−1 sucrose. In addition, the strain was also efficiently able to utilize glucose and/or xylose to produce high yields of l-lactic acid. It was capable of producing up to 115 and 54.2 g l−1 lactic acid with yields of up to 0.81 g g−1 glucose and 0.90 g g−1 xylose, respectively. Corncob hydrolysates obtained by dilute acid hydrolysis and enzymatic hydrolysis of the cellulose-enriched
residue were used for lactic acid production by R. oryzae GY18. A yield of 355 g lactic acid per kg corncobs was obtained after 72 h incubation. Therefore, sucrose and corncobs could
serve as potential sources of raw materials for efficient production of lactic acid by R. oryzae GY18. 相似文献
996.
R. Bruce Medhurst Mark S. Wipfli Chris Binckley Karl Polivka Paul F. Hessburg R. Brion Salter 《Hydrobiologia》2010,641(1):71-83
Effects of forest management on stream communities have been widely documented, but the role that climate plays in the disturbance
outcomes is not understood. In order to determine whether the effect of disturbance from forest management on headwater stream
communities varies by climate, we evaluated benthic macroinvertebrate communities in 24 headwater streams that differed in
forest management (logged-roaded vs. unlogged-unroaded, hereafter logged and unlogged) within two ecological sub-regions (wet
versus dry) within the eastern Cascade Range, Washington, USA. In both ecoregions, total macroinvertebrate density was highest
at logged sites (P = 0.001) with gathering-collectors and shredders dominating. Total taxonomic richness and diversity did not differ between
ecoregions or forest management types. Shredder densities were positively correlated with total deciduous and Sitka alder
(Alnus sinuata) riparian cover. Further, differences in shredder density between logged and unlogged sites were greater in the wet ecoregion
(logging × ecoregion interaction; P = 0.006) suggesting that differences in post-logging forest succession between ecoregions were responsible for differences
in shredder abundance. Headwater stream benthic community structure was influenced by logging and regional differences in
climate. Future development of ecoregional classification models at the subbasin scale, and use of functional metrics in addition
to structural metrics, may allow for more accurate assessments of anthropogenic disturbances in mountainous regions where
mosaics of localized differences in climate are common. 相似文献
997.
Lihong Wang Hao Li Zhen Liu Xiaobo Xu Huaijing Wang Zhenzhong Li 《Cellular and molecular neurobiology》2010,30(1):43-49
The neuropeptide-immunoreactive (IR) and neurofilament-IR neurons are two major phenotypical classes in dorsal root ganglion
(DRG). Targets of neuronal innervation play a vital role in regulating the survival and differentiation of innervating neurotrophin-responsive
neurons. Monosialoganglioside (GM1) has been considered to have a neurotrophic factor-like activity. Both GM1 and target skeletal
muscle (SKM) cells are essential for the maintenance of the function of neurons. However, whether target SKM cells and GM1,
alone or associated, generate neuropeptide or neurofilament expression remains unclear. The aim of the present study is to
investigate the effects of GM1 or/and SKM on DRG neuronal phenotypes. DRG neurons containing the neuropeptide substance P
(SP) and neurofilament 200 (NF-200) were quantified using immunofluorescent labeling in cultures of DRG, which was dissected
out at times before (at embryonic days 12.5, E12.5) and after (at E19.5) sensory neurons contact peripheral targets in vivo.
DRG neurons were cultured in absence or presence of GM1 or/and SKM cells. In this experiment, we found that: (1) GM1 promoted
expression of SP and NF-200 in E12.5 DRG cultures; (2) SKM cells promoted expression of NF-200 but not SP in E12.5 DRG cultures;
(3) GM1 and target SKM cells had additive effects on expression of SP and NF-200 in E12.5 DRG cultures; and (4) SKM or/and
GM1 did not have effects on expression of SP and NF-200 in E19.5 DRG cultures. These results suggested that GM1 could influence
DRG, two major neuronal phenotypes, before sensory neurons contact peripheral targets in vivo. Target SKM cells could only
influence neurofilament-expressed neuronal phenotype before sensory neurons contact peripheral targets in vivo. GM1 and SKM
cells had the additive effects on two major DRG neuronal classes, which express neuropeptide or neurofilament when DRG cells
were harvested before sensory neurons contact peripheral targets in vivo. These results offered new clues for a better understanding
of the association of GM1 or/and SKM with neuronal phenotypes. 相似文献
998.
Subramanian Mohan Raj Chelladurai Rathnasingh Woo-Chel Jung Edwardraja Selvakumar Sunghoon Park 《Biotechnology and Bioprocess Engineering》2010,15(1):131-138
3-Hydroxypropionic acid (3-HP), a versatile and valuable platform chemical, has diverse industrial applications; but its biological
production from glycerol is often limited by the capability of the enzyme aldehyde dehydrogenase (ALDH) to convert an intermediary
compound, 3-hydroxypropionaldehyde (3-HPA), to 3-HP. In this study, we report a new ALDH, PuuC, from Klebsiella pneumoniae DSM 2026, that efficiently converts 3-HPA to 3-HP. The identified gene puuC was cloned, expressed in Escherichia coli, purified, and characterized for its properties. The recombinant enzyme with a molecular weight of 53.8 kDa exhibited broad
substrate specificity for various aliphatic aldehydes, especially C2–C5 aldehydes. NAD+ was the preferred coenzyme for the oxidation of most aliphatic and aromatic aldehydes tested. The optimum pH and temperature
for PuuC activity were pH 8.0 and 45°C. The K
m values for 3-HPA and NAD+ were 0.48 and 0.09 mM, respectively. The activity of PuuC was enhanced in the presence of reducing agents such as 2-mercaptoethanol
or dithiothreitol, while several metal ions, particularly Hg2+, Ag+, and Cu2+ inhibited its activity. The predicted structure of PuuC indicated the presence of K191 and E194 in close proximity to the
glycine motif, suggesting that PuuC belongs to class 2 ALDHs. 相似文献
999.
Nakul Mandal Steffen Heegaard Jan Ulrik Prause Bent Honoré Henrik Vorum 《Biological procedures online》2010,12(1):56-88
The intention of this review is to provide an overview of current methodologies employed in the rapidly developing field of
ocular proteomics with emphasis on sample preparation, two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) and mass
spectrometry (MS). Appropriate sample preparation for the diverse range of cells and tissues of the eye is essential to ensure
reliable results. Current methods of protein staining for 2D-PAGE, protein labelling for two-dimensional difference gel electrophoresis,
gel-based expression analysis and protein identification by MS are summarised. The uses of gel-free MS-based strategies (MuDPIT,
iTRAQ, ICAT and SILAC) are also discussed. Proteomic technologies promise to shed new light onto ocular disease processes
that could lead to the discovery of strong novel biomarkers and therapeutic targets useful in many ophthalmic conditions. 相似文献
1000.
Patrick Hoehn Ingolf Steffan-Dewenter Teja Tscharntke 《Biodiversity and Conservation》2010,19(8):2189-2200
Due to increasing human modification of tropical landscapes, the relative importance of natural habitats and agricultural
systems has become a major conservation topic to counteract global species loss. We investigated the contribution of tropical
primary forest, cacao agroforestry systems of varying management practices and openland to the temporal and spatial variation
of diversity of native bee communities in the herb layer (Apidae, Hymenoptera) in Sulawesi (Indonesia). Local bee density
and diversity were highest in openland, followed by agroforestry systems and were lowest in primary forests, revealing the
importance of herbaceous food resources in the understorey. In contrast, highest regional bee richness was found in agroforestry
systems, because of high community dissimilarity. Multidimensional scaling supported these findings with openland habitats
showing more compactly clustered bee species communities than agroforestry habitats. In conclusion, the herb associated bee
community profited from the opening of the landscape as a result of agricultural activities, while agroforestry systems increased
bee species richness especially on a regional scale due to high management diversity. 相似文献