全文获取类型
收费全文 | 119092篇 |
免费 | 2045篇 |
国内免费 | 1830篇 |
专业分类
122967篇 |
出版年
2024年 | 56篇 |
2023年 | 324篇 |
2022年 | 660篇 |
2021年 | 1093篇 |
2020年 | 754篇 |
2019年 | 998篇 |
2018年 | 12630篇 |
2017年 | 11251篇 |
2016年 | 8452篇 |
2015年 | 2075篇 |
2014年 | 2039篇 |
2013年 | 2427篇 |
2012年 | 6325篇 |
2011年 | 14779篇 |
2010年 | 13067篇 |
2009年 | 9175篇 |
2008年 | 10961篇 |
2007年 | 12482篇 |
2006年 | 1452篇 |
2005年 | 1530篇 |
2004年 | 1822篇 |
2003年 | 1817篇 |
2002年 | 1477篇 |
2001年 | 738篇 |
2000年 | 625篇 |
1999年 | 403篇 |
1998年 | 229篇 |
1997年 | 203篇 |
1996年 | 201篇 |
1995年 | 180篇 |
1994年 | 156篇 |
1993年 | 144篇 |
1992年 | 180篇 |
1991年 | 199篇 |
1990年 | 142篇 |
1989年 | 112篇 |
1988年 | 122篇 |
1987年 | 120篇 |
1986年 | 79篇 |
1985年 | 110篇 |
1984年 | 73篇 |
1983年 | 85篇 |
1982年 | 42篇 |
1981年 | 37篇 |
1979年 | 53篇 |
1978年 | 41篇 |
1975年 | 40篇 |
1972年 | 266篇 |
1971年 | 295篇 |
1952年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
961.
Kazuhiro Nakaya 《Ichthyological Research》1988,34(4):431-442
Data on the individual variation and changes with growth in proportions and morphology are presented for the poorly known Apristurus longicephalus, and compared with those of other species. A. longicephalus is concluded to be a distinct species without synonyms, characterized by its long snout, widely separate nostrils, long caudal fin, short abdomen, very sparse teeth, and low number of monospondylous vertebrae. It is a species of small size, maturing at about 42 cm in total length. 相似文献
962.
Yifeng Miao Yongming Qiu Yuchang Lin Zengli Miao Jing Zhang Xiaojie Lu 《Molecular biology reports》2011,38(5):3235-3242
Pyruvate, an endogenous metabolite of glycolysis, is an anti-toxicity agent. Recent studies have suggested possible roles
for pyruvate in protecting CNS neurons from excitotoxic and metabolic insults. Utilizing cultures derived from embryonic rat
cortex, the studies presented in this paper indicate that an astroglia-mediated mechanism is involved in the neuroprotective
effects of pyruvate against glutamate toxicity. Glutamate-induced toxicity could be reversed by pyruvate in a mixed culture
of cortex cells. Importantly, in pure neuronal cultures from the same tissue, pyruvate failed to protect against glutamate
toxicity. Addition of astroglia to the pure neuronal cultures restores the ability of pyruvate to protect neurons from glutamate-induced
toxicity. Our results further suggest that pyruvate can induce glia to up-regulate the synthesis of glutathione (GSH), an
antioxidant that protects cells from toxins such as free radicals. Taken together, our data suggest that astroglia in mixed
cultures are essential for mediating the effects of pyruvate, revealing a novel mechanism by which pyruvate, an important
intermediate of tricarboxylic acid cycle in the body, may act to protect neurons from damage during insults such as brain
ischemia. 相似文献
963.
To determine the species of the yeast-like symbionts (YLS) in the brown planthoppers (BPH), Nilaparvata lugens, YLS were first isolated and purified by ultracentrifugation from the fat bodies of BPH, and then 18S rDNA and internal transcribed
spacer (ITS)–5.8S rDNA sequences of YLS were amplified with the different general primers for fungi. The results showed that
the two different 18S and ITS–5.8S rDNA sequences of YLS were obtained. One 2291-bp DNA sequence, which contained 18S and
ITS–5.8S rDNA, showed the high similarity to Cryptococcus and was named Cryp-Like symbiotes. Another 1248-bp DNA sequence, which contained a part of 18S and ITS–5.8S rDNA, showed the high similarity
to Pichia guilliermondii and was named Pichia-Like symbiotes. It was further proved that Cryp- and Pichia-Like symbiotes existed in BPH through nested PCR with specific primers for two symbiotes and in situ hybridization analysis
using digoxigenin-labeled probes. Our results showed that BPH harbored more than one species of eukaryotic YLS, which suggested
that diversity of fungal endosymbiotes may be occurred in planthoppers, just like bacterial endosymbiotes. 相似文献
964.
Ghada Baraket Khaled Chatti Olfa Saddoud Ahmed Ben Abdelkarim Messaoud Mars Mokhtar Trifi Amel Salhi Hannachi 《Plant Molecular Biology Reporter》2011,29(1):171-184
This study characterises the genetic variability of fig, Ficus carica L., using simple sequence repeat (SSR) and amplified fragment length polymorphism (AFLP) markers. It compares the efficiency
and utility of the two techniques in detecting variation and establishing genetic relationships among Tunisian fig cultivars.
Our results show that using both marker systems, the Tunisian fig germ plasm is characterised by having a large genetic diversity
at the deoxyribonucleic acid level, as most of AFLP bands were detected and all SSR markers were polymorphic. In fact, 351
(342 polymorphic) and 57 (57 polymorphic) bands were detected using AFLP and SSR primers, respectively. SSR markers were the
most polymorphic with an average polymorphic information content value of 0.94, while AFLP markers showed the highest effective
multiplex ratio (56.9) and marker index (45.2). The effective marker index was recorded highest (4.19) for AFLP markers and
lowest (0.70) for the SSR ones. Our results demonstrate that (1) independent as well as combined analyses of cluster analyses
of SSR and AFLP fragments showed that cultivars are clustered independently from their geographical origin, horticultural
classifications and tree sex; (2) the analysis of molecular variance allowed the partitioning of genetic variation within
and among fig groups and showed greater variation within groups and (3) AFLP and SSR markers datasets showed positive correlation.
This study suggests the SSR and AFLP markers are suitable for diversity analysis and cultivars fingerprinting. An understanding
of the genetic diversity and population structure of F. carica in Tunisia can also provide insight into the conservation and management of this species. 相似文献
965.
966.
Diana Stralberg D. Richard Cameron Mark D. Reynolds Catherine M. Hickey Kirk Klausmeyer Sylvia M. Busby Lynne E. Stenzel W. David Shuford Gary W. Page 《Biodiversity and Conservation》2011,20(1):19-40
Conservation of migratory shorebirds and waterfowl presents unique challenges due to extensive historic loss of wetland habitats, and current reliance on managed landscapes for wintering and migratory passage. We developed a spatially-explicit approach to estimate potential shorebird and waterfowl densities in California by integrating mapped habitat layers and statewide bird survey data with expert-based habitat rankings. Using these density estimates as inputs, we used the Marxan site-selection program to identify priority shorebird and waterfowl areas at the ecoregional level. We identified 3.7 million ha of habitat for shorebirds and waterfowl, of which 1.4 million ha would be required to conserve 50% of wintering populations. To achieve a conservation goal of 75%, more than twice as much habitat (3.1 million ha) would be necessary. Agricultural habitats comprised a substantial portion of priority areas, especially at the 75% level, suggesting that under current management conditions, large areas of agricultural land, much of it formerly wetland, are needed to provide the habitat availability and landscape connectivity required by shorebird and waterfowl populations. These habitats were found to be largely lacking recognized conservation status in California (96% un-conserved), with only slightly higher levels of conservation for priority shorebird and waterfowl areas. Freshwater habitats, including wetlands and ponds, were also found to have low levels of conservation (67% un-conserved), although priority shorebird and waterfowl areas had somewhat higher levels of conservation than the state as a whole. Conserving migratory waterfowl and shorebirds will require a diversity of conservation strategies executed at a variety of scales. Our modeled results are complementary with other approaches and can help prioritize areas for protection, restoration and other actions. Traditional habitat protection strategies such as conservation easements and fee acquisitions may be of limited utility for protecting and managing significant areas of agricultural lands. Instead, conservation strategies focused on incentive-based programs to support wildlife friendly management practices in agricultural settings may have greater utility and conservation effectiveness. 相似文献
967.
968.
Aloke K. Bera Nancy W. Y. Ho Aftab Khan Miroslav Sedlak 《Journal of industrial microbiology & biotechnology》2011,38(5):617-626
Robust microorganisms are necessary for economical bioethanol production. However, such organisms must be able to effectively
ferment both hexose and pentose sugars present in lignocellulosic hydrolysate to ethanol. Wild type Saccharomyces cerevisiae can rapidly ferment hexose, but cannot ferment pentose sugars. Considerable efforts were made to genetically engineer S. cerevisiae to ferment xylose. Our genetically engineered S cerevisiae yeast, 424A(LNH-ST), expresses NADPH/NADH xylose reductase (XR) that prefer NADPH and NAD+-dependent xylitol dehydrogenase (XD) from Pichia stipitis, and overexpresses endogenous xylulokinase (XK). This strain is able to ferment glucose and xylose, as well as other hexose
sugars, to ethanol. However, the preference for different cofactors by XR and XD might lead to redox imbalance, xylitol excretion,
and thus might reduce ethanol yield and productivity. In the present study, genes responsible for the conversion of xylose
to xylulose with different cofactor specificity (1) XR from N. crassa (NADPH-dependent) and C. parapsilosis (NADH-dependent), and (2) mutant XD from P. stipitis (containing three mutations D207A/I208R/F209S) were overexpressed in wild type yeast. To increase the NADPH pool, the fungal
GAPDH enzyme from Kluyveromyces lactis was overexpressed in the 424A(LNH-ST) strain. Four pentose phosphate pathway (PPP) genes, TKL1, TAL1, RKI1 and RPE1 from S. cerevisiae, were also overexpressed in 424A(LNH-ST). Overexpression of GAPDH lowered xylitol production by more than 40%. However, other
strains carrying different combinations of XR and XD, as well as new strains containing the overexpressed PPP genes, did not
yield any significant improvement in xylose fermentation. 相似文献
969.
Ding Haixia Zhang Jingsong Jiang Lei Dong Hairong Wang Jun Xiao Hang Chen Weixian 《Cell biochemistry and biophysics》2011,59(1):39-47
Extracellular domains of the transmembrane glycoprotein, neuropilin-1 (Np1), specifically bind an array of factors and co-receptors
including class-3 semaphorins (Sema3a), vascular endothelial growth factor (VEGF), hepatocyte growth factor, platelet-derived
growth factor BB, transforming growth factor-β 1 (TGF-β1), and fibroblast growth factor2 (FGF2). Np1 may have a role in immune
response, tumor cell growth, and angiogenesis, but its relative expression in comparison to its co-primary receptors, VEGF
and Sema3a, is not known. In this study we determined the mRNA expression of Np1 and its co-receptors, VEGF and Sema3a, and
the ratio of VEGF/Sema3a in different human and rodent cell lines. Expression of Np1, VEGF and Sema3a is very low in cells
derived from normal tissues, but these proteins are highly expressed in tumor-derived cells. Furthermore, the ratio of VEGF/Sema3a
is highly variable in different tumor cells. The elevated mRNA expression of Np1 and its putative receptors in tumor cells
suggests a role for these proteins in tumor cell migration and angiogenesis. As different tumor cells exhibit varying VEGF/Sema3a
ratios, it appears that cancer cells show differential response to angiogenic factors. These results bring to light the individual
variation among the cancer-related genes, Np1, VEGF, and Sema3a, and provide an important impetus for the possible personalized
therapeutic approaches for cancer patients. 相似文献
970.
Barton HJ 《Biological trace element research》2011,143(2):637-658
The population of about 300 6-year-old preschool children was studied for lead (Pb) and cadmium (Cd) in deciduous teeth, scalp
hair, and capillary blood. Zinc (Zn) content in hair was also measured due to its possible interferences with the elements
studied. The Pb and Cd contents were measured by graphite furnace atomic absorption spectrometry with Zeeman background correction,
and Zn, by flame method. Metal levels found were comparable to those in European countries. Positive correlations between
Pb and Cd in the same and different tissues were widespread (r = 0.23–0.68). A positive correlation was found between Pb in teeth and blood (r = 0.65, p < 0.001). Pb blood threshold 100 μg L−1 referred to 2.6 μg g−1 Pb in teeth. Predictability of this Pb teeth limit was 14% vs. 5% for Pb blood. Strong negative correlation was found between
hair Pb and Zn (r = −0.68, p < 0.001). Enhanced Cd and Zn levels in hair were associated with the household drinking water hardness. The associations
with some other environmental and behavioral factors were also presented and discussed. The results proved the advantage of
analysis of primary teeth for Pb exposure screening of preschool children. 相似文献