全文获取类型
收费全文 | 130378篇 |
免费 | 3760篇 |
国内免费 | 830篇 |
专业分类
134968篇 |
出版年
2023年 | 235篇 |
2022年 | 495篇 |
2021年 | 1061篇 |
2020年 | 610篇 |
2019年 | 773篇 |
2018年 | 12550篇 |
2017年 | 11264篇 |
2016年 | 8517篇 |
2015年 | 2556篇 |
2014年 | 2479篇 |
2013年 | 2618篇 |
2012年 | 7443篇 |
2011年 | 15625篇 |
2010年 | 13730篇 |
2009年 | 9777篇 |
2008年 | 12021篇 |
2007年 | 13496篇 |
2006年 | 2368篇 |
2005年 | 2499篇 |
2004年 | 2788篇 |
2003年 | 2731篇 |
2002年 | 2375篇 |
2001年 | 591篇 |
2000年 | 395篇 |
1999年 | 364篇 |
1998年 | 420篇 |
1997年 | 293篇 |
1996年 | 254篇 |
1995年 | 209篇 |
1994年 | 220篇 |
1993年 | 248篇 |
1992年 | 236篇 |
1991年 | 240篇 |
1990年 | 196篇 |
1989年 | 173篇 |
1988年 | 191篇 |
1987年 | 176篇 |
1986年 | 128篇 |
1985年 | 129篇 |
1984年 | 172篇 |
1983年 | 141篇 |
1982年 | 155篇 |
1981年 | 145篇 |
1980年 | 134篇 |
1979年 | 103篇 |
1978年 | 107篇 |
1977年 | 99篇 |
1973年 | 83篇 |
1972年 | 310篇 |
1971年 | 331篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
Corti A 《Cellular and molecular neurobiology》2010,30(8):1163-1170
Chromogranin A (CgA) is an acidic glycoprotein belonging to a family of regulated secretory proteins stored in the dense core
granules of the adrenal medulla and of many other neuroendocrine cells and neurons. This protein is frequently used as a diagnostic
and prognostic serum marker for a range of neuroendocrine tumors. Circulating CgA is also increased in patients with other
diseases, including subpopulations of patients with non-neuroendocrine tumors, with important prognostic implications. A growing
body of evidence suggests that CgA is more than a diagnostic/prognostic marker for cancer patients. Indeed, results of in
vitro experiments and in vivo studies in animal models suggest that this protein and its fragments can affect several elements
of the tumor microenvironment, including fibroblasts and endothelial cells. In this article, recent findings implicating CgA
as a modulator of the tumor microenvironment and suggesting that abnormal secretion of CgA could play important roles in tumor
progression and response to therapy in cancer patients are reviewed and discussed. 相似文献
942.
NARC-1/PCSK9 and its natural mutants: zymogen cleavage and effects on the low density lipoprotein (LDL) receptor and LDL cholesterol 总被引:1,自引:0,他引:1
Benjannet S Rhainds D Essalmani R Mayne J Wickham L Jin W Asselin MC Hamelin J Varret M Allard D Trillard M Abifadel M Tebon A Attie AD Rader DJ Boileau C Brissette L Chrétien M Prat A Seidah NG 《The Journal of biological chemistry》2004,279(47):48865-48875
The discovery of autosomal dominant hypercholesterolemic patients with mutations in the PCSK9 gene, encoding the proprotein convertase NARC-1, resulting in the missense mutations suggested a role in low density lipoprotein (LDL) metabolism. We show that the endoplasmic reticulum-localized proNARC-1 to NARC-1 zymogen conversion is Ca2+-independent and that within the zymogen autocatalytic processing site SSVFAQ [downward arrow]SIP Val at P4 and Pro at P3' are critical. The S127R and D374Y mutations result in approximately 50-60% and > or =98% decrease in zymogen processing, respectively. In contrast, the double [D374Y + N157K], F216L, and R218S natural mutants resulted in normal zymogen processing. The cell surface LDL receptor (LDLR) levels are reduced by 35% in lymphoblasts of S127R patients. The LDLR levels are also reduced in stable HepG2 cells overexpressing NARC-1 or its natural mutant S127R, and this reduction is abrogated in the presence of 5 mm ammonium chloride, suggesting that overexpression of NARC-1 increases the turnover rate of the LDLR. Adenoviral expression of wild type human NARC-1 in mice resulted in a maximal approximately 9-fold increase in circulating LDL cholesterol, while in LDLR-/- mice a delayed approximately 2-fold increase in LDL cholesterol was observed. In conclusion, NARC-1 seems to affect both the level of LDLR and that of circulating apoB-containing lipoproteins in an LDLR-dependent and -independent fashion. 相似文献
943.
In contrast to many studies on the habitat quality of road verges for butterflies in relation to management regimes, little
is known about whether road verges also function as corridors linking fragmented grassland habitats. We experimentally compared
movements of four model species, two small blues and two medium browns, with one habitat specialist and one habitat generalist
in each size and phylogenetic category. A total of 425 individual butterflies were caught and translocated to an experimental
arena with three 2 × 30 m grassland strips that approximated road verges; one with adult feeding resources, one sheltered
from the wind, and one without food and shelter. Movements in grassland strips were compared to movements in continuous grassland
habitat. Results indicated that (1) individuals did not use the low-quality strip, (2) only specialists used strips but not
in the same way according to their size and phylogenic category, and (3) strip use could not be predicted from habitat selection.
This finding supports the idea that corridors of intermediate quality are the most efficient to promote dispersal rates in
fragmented landscapes. Road verges cover 250,000 ha in Sweden, which is nearly the total amount of seminatural grasslands.
Our results suggest that, to benefit butterfly dispersal among grassland patches, road verges should be managed to create
a more favourable microclimate (e.g. sheltered from wind, high temperatures). 相似文献
944.
Lynn S. Dahleen Muthusamy Manoharan 《In vitro cellular & developmental biology. Plant》2007,43(6):493-506
Barley, an important member of the cereals, has been successfully transformed through various methods such as particle bombardment,
Agrobacterium tumefaciens, DNA uptake, and electroporation. Initially, the transformation in barley concentrated on developing protocols using marker
genes such as gus, bar, and hpt. Immature embryos and callus derived from immature embryos were targeted for transformation. Subsequently, genes of agronomic
and malting importance have been deployed in barley. Particle bombardment appears to be the preferred choice for barley transformation
in the majority of the reports, although Agrobacterium-mediated transformation is being used more often. The current review focuses on the challenges encountered in barley transformation
such as somaclonal variation, development of transformation systems for commercial cultivars, gene expression, stability and
inheritance, and gene flow. Newer markers such as the green fluorescent protein (gfp), firefly luciferase, and phosphomannose isomerase were found to be useful in the selection of transgenic plants. Tissue-specific
promoters such as those for B1-hordein and D-hordein genes, and spike-specific promoters, are increasingly used to drive gene
expression. The review also describes recent research on gene-tagging through transformation, insertion of disease resistance,
and abiotic stress resistance genes, transformation with genes for improved malting quality, nutrient content, feed quality,
and the production of feed enzymes and pharmaceutical compounds. 相似文献
945.
The red king crab (Paralithodes camtschaticus) was introduced in the Barents Sea in the 1960s and soon established a viable population. Proper management and exploitation
of the Barents Sea king crab stock require better understanding of the spatial dynamics at different scales. This study examines
the small-scale movement patterns of seven adult male crabs tracked for a period of up to one month from mid July to mid August
at 150 m depth in a semi-enclosed fjord on the Russo-Norwegian border. The crabs were tagged with acoustic transmitters and
their movements monitored with an acoustic positioning system. Low walking speeds (<0.01 m s−1) were most frequent but the crabs could move at a maximum speed of 0.15 m s−1 and walk an actual distance of up to 270 m over a period of one hour. However, the crabs usually moved within a relatively
restricted area with mean hourly longest rectilinear distance varying from 26 to 64 m. The crabs alternated between periods
of low and high activity, which could reflect feeding in and movements between food patches. The lack of a diel activity rhythm
may be due to high light levels during the polar summer night, or a chemically mediated food search strategy. 相似文献
946.
A simple and reliable continuous assay procedure for measurement of cellulase activity from several species using the new substrate resorufin-beta-D-cellobioside (Res-CB) has been developed. The product of enzyme reaction, resorufin, exhibits fluorescence emission at 585 nm with excitation at 571 nm and has a pK(a) of 5.8, which allows continuous measurement of fluorescence turnover at or near physiological pH values. The assay performed using purified cellulase from the microscopic fungus Trichoderma reesei has been shown to give the kinetic parameters K(m) of 112 microM and V(max) of 0.000673 micromol/mL/min. Methods for performing the assay using cellulases isolated from both live Arabidopsis thaliana plant and Aspergillus niger fungal species are presented. 相似文献
947.
948.
Rhamnolipids, produced by Pseudomonas aeruginosa, represent an important group of biosurfactants having various industrial, environmental, and medical applications. Current
methods for rhamnolipid quantification involve the use of strong hazardous acids/chemicals, indirect measurement of the concentration
of sugar moiety, or require the availability of expensive equipment (HPLC-MS). A safer, easier method that measures the whole
rhamnolipid molecules would significantly enhance strain selection, metabolic engineering, and process development for economical
rhamnolipid production. A semi-quantitative method was reported earlier to differentiate between the rhamnolipid-producing
and non-producing strains using agar plates containing methylene blue and cetyl trimethylammonium bromide (CTAB). In this
study, a rapid and simple method for rhamnolipid analysis was developed by systematically investigating the complexation of
rhamnolipids and methylene blue, with and without the presence of CTAB. The method relies on measuring the absorbance (at
638 nm) of the rhamnolipid−methylene blue complex that partitions into the chloroform phase. With P. aeruginosa fermentation samples, the applicability of this method was verified by comparison of the analysis results with those obtained
from the commonly used anthrone reaction technique. 相似文献
949.
There is an apparent allometric relationship between peak frequency of echolocation and body size in rhinophilids. However,
some rhinolophids deviate from this rule. To date this variation has been explained as a result of partitioning of communication
channels. An alternative hypothesis that food resource partitioning results in this divergence in expected frequencies was
tested by comparing prey selection between Rhinolophus macrotis Blyth, 1844 and Rhinolophus lepidus Blyth, 1844 in Yunnan Province, China. These two sympatric species are morphologically similar but acoustically divergent:
R. macrotis has an echolocation frequency significantly lower than that predicted by the allometric relationship, whereas that of R. lepidus agreed with expectations. Prey selection experiments, conducted in a flight tent, indicated that the dominant prey taxa of
R. macrotis were Lasiocampidae, Arctiidae and Noctuidae, whilst that of R. lepidus were Arctiidae, Noctuidae and Ichneumonidae. R. macrotis ate more earless moths and fewer eared moths than R. lepidus, and R. macrotis fed on larger prey in general and captured a wider size range than that captured by R. lepidus. These results confirmed the existence of finely tuned trophic niche differentiation and suggested that food resource partitioning
is one of the factors leading to lower peak frequency of calls in R. macrotis. 相似文献
950.
Xiaohuan Cheng Junfa Ding Fang Zheng Xin Zhou Chenling Xiong 《Molecular biology reports》2009,36(8):2053-2057
Familial hypercholesterolemia (FH) (OMIM 143890) is an autosomal dominantly inherited disease mainly caused by mutations of
the gene encoding the low density lipoprotein receptor (LDLR) and Apolipoprotein (Apo) B. First the common mutation R3500Q
in ApoB gene was determined using PCR/RFLP method. Then the LDLR gene was screened for mutations using Touch-down PCR, SSCP
and sequencing techniques. Furthermore, the secondary structure of the LDLR protein was predicted with ANTHEPROT5.0. The R3500Q
mutation was absent in these two families. A heterozygous p.W483X mutation of LDLR gene was identified in family A which caused
a premature stop codon, while a homozygous mutation p.A627T was found in family B. The predicted secondary structures of the
mutant LDLR were altered. We identified two known mutations (p.W483X, p.A627T) of the LDLR gene in two Chinese FH families
respectively. 相似文献