全文获取类型
收费全文 | 93921篇 |
免费 | 332篇 |
国内免费 | 900篇 |
专业分类
95153篇 |
出版年
2023年 | 4篇 |
2022年 | 17篇 |
2021年 | 17篇 |
2020年 | 16篇 |
2019年 | 31篇 |
2018年 | 11866篇 |
2017年 | 10681篇 |
2016年 | 7486篇 |
2015年 | 643篇 |
2014年 | 333篇 |
2013年 | 364篇 |
2012年 | 4269篇 |
2011年 | 12886篇 |
2010年 | 12037篇 |
2009年 | 8249篇 |
2008年 | 9825篇 |
2007年 | 11419篇 |
2006年 | 307篇 |
2005年 | 544篇 |
2004年 | 993篇 |
2003年 | 1053篇 |
2002年 | 799篇 |
2001年 | 258篇 |
2000年 | 166篇 |
1999年 | 30篇 |
1998年 | 14篇 |
1997年 | 28篇 |
1996年 | 26篇 |
1995年 | 4篇 |
1994年 | 17篇 |
1993年 | 32篇 |
1992年 | 24篇 |
1991年 | 42篇 |
1990年 | 8篇 |
1989年 | 8篇 |
1988年 | 17篇 |
1987年 | 14篇 |
1984年 | 7篇 |
1983年 | 18篇 |
1982年 | 2篇 |
1980年 | 2篇 |
1975年 | 5篇 |
1972年 | 246篇 |
1971年 | 274篇 |
1970年 | 5篇 |
1965年 | 13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
221.
Anla Hu Li Li Chuanlai Hu Daoming Zhang Chen Wang Yan Jiang Meng Zhang Chunmei Liang Wenjun Chen Qingli Bo Qihong Zhao 《Biological trace element research》2018,182(1):21-28
Data on the effects of magnesium-zinc-calcium-vitamin D co-supplementation on hormonal profiles, biomarkers of inflammation, and oxidative stress among women with polycystic ovary syndrome (PCOS) are scarce. The objective of this study was to assess the effects of magnesium-zinc-calcium-vitamin D co-supplementation on hormonal profiles, biomarkers of inflammation, and oxidative stress in women with PCOS. Sixty PCOS women were randomized into two groups and treated with 100 mg magnesium, 4 mg zinc, 400 mg calcium plus 200 IU vitamin D supplements (n = 30), or placebo (n = 30) twice a day for 12 weeks. Hormonal profiles, biomarkers of inflammation, and oxidative stress were assessed at baseline and at end-of-treatment. After the 12-week intervention, compared with the placebo, magnesium-zinc-calcium-vitamin D co-supplementation resulted in significant reductions in hirsutism (?2.4 ± 1.2 vs. ?0.1 ± 0.4, P < 0.001), serum high sensitivity C-reactive protein (?0.7 ± 0.8 vs. +0.2 ± 1.8 mg/L, P < 0.001), and plasma malondialdehyde (?0.4 ± 0.3 vs. +0.2 ± 1.0 μmol/L, P = 0.01), and a significant increase in plasma total antioxidant capacity concentrations (+46.6 ± 66.5 vs. ?7.7 ± 130.1 mmol/L, P = 0.04). We failed to find any significant effect of magnesium-zinc-calcium-vitamin D co-supplementation on free androgen index, and other biomarkers of inflammation and oxidative stress. Overall, magnesium-zinc-calcium-vitamin D co-supplementation for 12 weeks among PCOS women had beneficial effects on hormonal profiles, biomarkers of inflammation, and oxidative stress. 相似文献
222.
Tropical rainforests are characterized by having high structural complexity, stratification, and species diversity. In Colombia, tropical rainforests are critically endangered with only 24% of their area remaining. Forest fragments are often valued based on the presence of vertebrate taxa despite that small habitat remnants may still harbor diverse invertebrate communities. We surveyed the ant fauna associated with rainforest fragments and their surrounding landscape elements (including mature forests, flooded forests, gallery forests, live fences, and pastures) in the Magdalena River watershed. Pitfall traps and litter samples were used to estimate ant richness and diversity, and to compare ant composition among landscape elements. We found 135 species from 42 genera, representing 16% of the species and 43% of the genera known for Colombia. Our surveys also uncovered 11 new ant records for the Colombian inter-Andean region and 2 new records for the country of Colombia: Mycocepurus curvispinosus (Mackay) and Rhopalothrix isthmica (Weber). The highest species richness was found in forest-covered sites, and richness and diversity was lower in the disturbed landscapes surrounding the forest patches. Species composition varied significantly between all habitat types, but was most similar between forest types suggesting that a loss of structural complexity has the greatest effect on ant communities. Across our study sites, ten species showed the greatest response to habitat type and could qualify as indicator taxa for this region. We conclude by discussing the value of conserving even small forests in this landscape due to their ability to retain high diversity of ants. 相似文献
223.
Background
Extracting biological information from high-density Affymetrix arrays is a multi-step process that begins with the accurate annotation of microarray probes. Shortfalls in the original Affymetrix probe annotation have been described; however, few studies have provided rigorous solutions for routine data analysis. 相似文献224.
Roberto Gamarra Emma Ortúñez Pablo Galán Cela Virginia Guadaño 《Plant Systematics and Evolution》2012,298(3):597-607
Seed micromorphology of 24 taxa of the genera Anacamptis and Orchis was examined under light and scanning electron microscopy. Seed qualitative characters appear very useful at the supraspecific
level in the subtribe Orchidinae. Based on our observations, the sculpturing of the periclinal walls of the medial testa cells,
the seed shape and several features of the anticlinal walls of the apical cells showed variability between the studied taxa,
with special relevance of the first character. According to this factor, we found one type of seed for Anacamptis and two for Orchis. The taxonomic value of seed coat characters is compared with the last taxonomic proposals for the genera Anacamptis and Orchis. 相似文献
225.
Biosynthesis of flavonoid derivatives requires enzyme(s) having high reactivity as well as regioselectivity. We have synthesized
3-O-kaempferol from naringenin using two enzymes. The first reaction, in which naringenin is converted to kaempferol, is mediated
by flavonol synthase (FLS). An FLS (PFLS) with strong catalytic activity was cloned and characterized from the genome sequence of the poplar (Populus deltoides). PFLS consists of a 1,008 bp ORF encoding a 38 kDa protein. PFLS was expressed in Escherichia coli with a glutathione-S-transferase (GST) tagging. The purified recombinant PFLS was characterized. Catalytically, it was more efficient than the
previously characterized FLSs. A mixture of two E. coli transformants harboring either PFLS or ROMT9 (a kaempferol 3-O-methyltransferase) converted naringenin into 3-O-methylkaempferol. 相似文献
226.
Several chromosome types have been recognized in Citrus and related genera by chromomycin A3 (CMA) banding patterns and fluorescent in situ hybridization (FISH). They can be used to characterize cultivars and species
or as markers in hybridization and backcrossing experiments. In the present work, characterization of six cultivars of P. trifoliata (“Barnes”, “Fawcett”, “Flying Dragon”, “Pomeroy”, “Rubidoux”, “USDA”) and one P. trifoliata × C. limonia hybrid was performed by sequential analyses of CMA banding and FISH using 5S and 45S rDNA as probes. All six cultivars showed
a similar CMA+ banding pattern with the karyotype formula 4B + 8D + 6F. The capital letters indicate chromosomal types: B, a chromosome
with one telomeric and one proximal band; D, with only one telomeric band; F, without bands. In situ hybridization labeling
was also similar among cultivars. Three chromosome pairs displayed a closely linked set of 5S and 45S rDNA sites, two of them
co-located with the proximal band of the B type chromosomes (B/5S-45S) and the third one co-located with the terminal band
of a D pair (D/5S-45S). The B/5S-45S chromosome has never been found in any citrus accessions investigated so far. Therefore,
this B chromosome can be used as a marker to recognize the intergeneric Poncirus × Citrus hybrids. The intergeneric hybrid analyzed here displayed the karyotype formula 4B + 8D + 6F, with two chromosome types B/5S-45S
and two D/5S-45S. The karyotype formula and the presence of two B/5S-45S chromosomes clearly indicate that the plant investigated
is a symmetric hybrid. It also demonstrates the suitability of karyotype analyses to differentiate zygotic embryos or somatic
cell fusions involving trifoliate orange germplasm.
During the submission of this paper, we analyzed 25 other citrus cultivars with the same methodology and we found that the
chromosome marker reported here can indeed distinguish Poncirus trifoliata from grapefruits, pummelos, and one variegated access of Citrus, besides the previously reported access of limes, limons, citrons, and sweet-oranges. However, among 14 mandarin cultivars,
two of them displayed a single B/5S-45S chromosome, whereas in Citrus hystrix D.C., a far related species belonging to the Papeda subgenus, this chromosome type was found in homozygosis. Since these two mandarin cultivars are probably of hybrid origin,
we assume that for almost all commercial cultivars and species of the subgenus Citrus this B type chromosome is a useful genetic marker. 相似文献
227.
Ozaki S Imai H Iwakiri T Sato T Shimoda K Nakayama T Hamada H 《Biotechnology letters》2012,34(3):475-481
A glucosyltransferase (GT) of Phytolacca americana (PaGT3) was expressed in Escherichia coli and purified for the synthesis of two O-β-glucoside products of trans-resveratrol. The reaction was moderately regioselective with a ratio of 4′-O-β-glucoside: 3-O-β-glucoside at 10:3. We used not only the purified enzyme but also the E. coli cells containing the PaGT3 gene for the synthesis of glycoconjugates. E. coli cell cultures also have other advantages, such as a shorter incubation time compared with cultured plant cells, no need for
the addition of exogenous glucosyl donor compounds such as UDP-glucose, and almost complete conversion of the aglycone to
the glucoside products. Furthermore, a homology model of PaGT3 and mutagenesis studies suggested that His-20 would be a catalytically
important residue. 相似文献
228.
Ozu M Dorr RA Teresa Politi M Parisi M Toriano R 《European biophysics journal : EBJ》2011,40(6):737-746
This work studies water permeability properties of human aquaporin 1 (hAQP1) expressed in Xenopus laevis oocyte membranes, applying a technique where cellular content is replaced with a known medium, with the possibility of measuring
intracellular pressure. Consequences on water transport—produced by well-known anisotonic gradients and by the intracellular
effect of probable aquaporin inhibitors—were tested. In this way, the specific intracellular inhibition of hAQP1 by the diuretic
drug furosemide was demonstrated. In addition, experiments imposing anisotonic mannitol gradients with a constant ionic strength
showed that the relationship between water flux and the applied mannitol gradient deflects from a perfect osmometer response
when the gradient is higher than 150 mosmol kgW−1. These results would indicate that the passage of water molecules through hAQP1 may have a maximum rate. As a whole, this
work demonstrates the technical advantage of controlling both intracellular pressure and medium composition in order to study
biophysical properties of hAQP1, and contributes information on water channel behavior under osmotic challenges and the discovery
of new inhibitors. 相似文献
229.
Manjula Senthilkumaran Michaela E. Johnson Larisa Bobrovskaya 《Neurochemical research》2016,41(7):1612-1624
In this study we investigated the effects of insulin-induced hypoglycaemia on tyrosine hydroxylase (TH) protein and TH phosphorylation in the adrenal gland, C1 cell group, locus coeruleus (LC) and midbrain dopaminergic cell groups that are thought to play a role in response to hypoglycaemia and compared the effects of different concentrations of insulin in rats. Insulin (1 and 10 U/kg) treatment caused similar reductions in blood glucose concentration (from 7.5–9 to 2–3 mmol/L); however, plasma adrenaline concentration was increased 20–30 fold in response to 10 U/kg insulin and only 14 fold following 1 U/kg. Time course studies (at 10 U/kg insulin) revealed that in the adrenal gland, Ser31 phosphorylation was increased between 30 and 90 min (4–5 fold), implying that TH was activated to increase catecholamine synthesis in adrenal medulla to replenish the stores. In the brain, Ser19 phosphorylation was limited to certain dopaminergic groups in the midbrain, while Ser31 phosphorylation was increased in most catecholaminergic regions at 60 min (1.3–2 fold), suggesting that Ser31 phosphorylation may be an important mechanism to maintain catecholamine synthesis in the brain. Comparing the effects of 1 and 10 U/kg insulin revealed that Ser31 phosphorylation was increased to similar extent in the adrenal gland and C1 cell group in response to both doses whereas Ser31 and Ser19 phosphorylation were only increased in response to 1 U/kg insulin in LC and in response to 10 U/kg insulin in most midbrain regions. Thus, the adrenal gland and some catecholaminergic brain regions become activated in response to insulin administration and brain catecholamines may be important for initiation of physiological defences against insulin-induced hypoglycaemia. 相似文献
230.
Görel Nyman Stina Marntell Anna Edner Pia Funkquist Karin Morgan Göran Hedenstierna 《Acta veterinaria Scandinavica》2009,51(1):22