全文获取类型
收费全文 | 3474篇 |
免费 | 244篇 |
国内免费 | 7篇 |
专业分类
3725篇 |
出版年
2024年 | 6篇 |
2023年 | 33篇 |
2022年 | 87篇 |
2021年 | 146篇 |
2020年 | 95篇 |
2019年 | 129篇 |
2018年 | 124篇 |
2017年 | 103篇 |
2016年 | 142篇 |
2015年 | 206篇 |
2014年 | 203篇 |
2013年 | 264篇 |
2012年 | 301篇 |
2011年 | 276篇 |
2010年 | 155篇 |
2009年 | 130篇 |
2008年 | 159篇 |
2007年 | 156篇 |
2006年 | 168篇 |
2005年 | 124篇 |
2004年 | 117篇 |
2003年 | 85篇 |
2002年 | 73篇 |
2001年 | 31篇 |
2000年 | 28篇 |
1999年 | 33篇 |
1998年 | 23篇 |
1997年 | 20篇 |
1996年 | 18篇 |
1995年 | 9篇 |
1994年 | 14篇 |
1993年 | 14篇 |
1992年 | 19篇 |
1991年 | 14篇 |
1990年 | 12篇 |
1989年 | 17篇 |
1988年 | 24篇 |
1987年 | 15篇 |
1985年 | 9篇 |
1984年 | 16篇 |
1983年 | 14篇 |
1982年 | 8篇 |
1981年 | 10篇 |
1980年 | 6篇 |
1979年 | 6篇 |
1974年 | 6篇 |
1973年 | 9篇 |
1969年 | 10篇 |
1968年 | 6篇 |
1967年 | 6篇 |
排序方式: 共有3725条查询结果,搜索用时 4 毫秒
101.
The ability of the freshwater alga, Chlorella kessleri, to maintain a carbon concentrating mechanism when grown at acid pH was investigated. The alga grows over the pH range 4.0–9.0 and was found to take up bicarbonate and CO2 actively when grown at pH 6.0. However, when grown at acid pH (below 5.5), it does not have active CO2 uptake. The acidotolerant species maintained an internal pH of 6.1–7.5 over the external pH range 4.5–7.5, thus the pH difference between the cell interior and the external medium was large enough to allow for the diffusive uptake of CO2 at acid external pH. Mass spectrometric monitoring of O2 and CO2 fluxes by suspensions of C. kessleri, grown at acid pH, and maintained at pH 7.5 showed that the rates of O2 evolution did not exceed those of CO2 uptake. The final CO2 compensation concentrations of 14.0–17.7 µM reached by photosynthetic cells were above the CO2 equilibrium concentration in the external medium, indicating a lack of active CO2 uptake at acid pH. Chlorella kessleri accumulated CO2 with internal concentrations that were 9.9, 18.7 and 22.7‐fold that of the external medium for cells grown, respectively, at pH 4.5, 5.0 and 5.5. The ability of C. kessleri cells to accumulate high intracellular concentrations of inorganic carbon at acid pH would provide a sufficiently high concentration of CO2 at the active site of Rubisco thus allowing the alga to maintain growth rates similar to those at alkaline pH. 相似文献
102.
Silvana A. Rosú Omar J. Rimoldi Eduardo D. Prieto Lucrecia M. Curto José M. Delfino Nahuel A. Ramella M. Alejandra Tricerri 《PloS one》2015,10(5)
A number of naturally occurring mutations of human apolipoprotein A-I (apoA-I) have been associated with hereditary amyloidoses. The molecular mechanisms involved in amyloid-associated pathology remain largely unknown. Here we examined the effects of the Arg173Pro point mutation in apoA-I on the structure, stability, and aggregation propensity, as well as on the ability to bind to putative ligands. Our results indicate that the mutation induces a drastic loss of stability, and a lower efficiency to bind to phospholipid vesicles at physiological pH, which could determine the observed higher tendency to aggregate as pro-amyloidogenic complexes. Incubation under acidic conditions does not seem to induce significant desestabilization or aggregation tendency, neither does it contribute to the binding of the mutant to sodium dodecyl sulfate. While the binding to this detergent is higher for the mutant as compared to wt apoA-I, the interaction of the Arg173Pro variant with heparin depends on pH, being lower at pH 5.0 and higher than wt under physiological pH conditions. We suggest that binding to ligands as heparin or other glycosaminoglycans could be key events tuning the fine details of the interaction of apoA-I variants with the micro-environment, and probably eliciting the toxicity of these variants in hereditary amyloidoses. 相似文献
103.
Gharipour Mojgan Ouguerram Khadija Nazih El-Hassane Salehi Mansoor Behmanesh Mehrdad Razavi Rouzbeh Gharipour Amin Diantkhah Minoo Sadeghi Masoumeh 《Molecular biology reports》2019,46(6):5685-5693
Molecular Biology Reports - Metabolic syndrome (MetS) results from the interaction between environmental and genetic factors. Several previous studies considered the role of selenium in developing... 相似文献
104.
Ayman F. Omar 《Journal of Plant Interactions》2016,11(1):101-107
Typical symptoms of phytoplasma such as whitening of the leaves, shortening of the stolons on Bermuda grass, variegated leaves, yellows, stunting, little leaves and yellows on Giant reed, Cooba and sand olive shrub were observed in Qassim province, Saudi Arabia, during the autumn season of 2015. When tested for phytoplasma by universal primers P1/P7 followed by R16mF2/R16mR2, products of approximately 1400?bp (as expected) were amplified from 16 plants with symptoms but not from symptomless plants. Based on sequencing, phylogenetic analysis and virtual restriction fragment length polymorphism patterns of the 16S rDNA F2nR2 fragments of seven Qassim phytoplasma isolates, bermuda grass isolates 170, 175 and 177, giant reed isolate 180, sand olive isolates 181 and 182 and cooba isolate 185, the associated phytoplasma was identified as a member of ‘Candidatus Phytoplasma cynodontis’ which belong to the 16SrXIV-A subgroup. The 16S rDNA gene sequences of seven Qassim phytoplasma isolates exhibited over 99.2% identity with members of ‘Ca. Phytoplasma cynodontis’ group of phytoplasmas. This is the first report of characterization of ‘Ca. phytoplasma cynodonties’ (16SrXIV) associated with Cynodon dactylon in Saudi Arabia and its new hosts, Dodonaea angustifolia, Arundo donax and Acacia salicia. 相似文献
105.
HER2+ mCRC patients with exon 20 R784G substitution mutation do not respond to the cetuximab therapy
Ehsan Gharib Reza Salmanipour Ehsan Nazemalhosseini Mojarad Mohammad Yaghoob Taleghani Saharnaz Sarlak Mona Malekzade-Moghani Parinaz Nasri Nasrabadi Mohammad Amin Meiary Hamid Asadzadeh Aghdaei Mohammad Reza Zali 《Journal of cellular physiology》2019,234(8):13137-13144
The human epidermal growth factor 2 (HER2) gene undergoes various mutations that could alter its activity or respond to the antibody therapies. Cetuximab, a known anti-EGFR monoclonal antibody (mAB), is widely administered in metastatic colorectal cancer (mCRC) cases. Here we identified mCRC patients who did not respond to cetuximab (500 mg/m2, q2w) after fluoropyrimidine/oxaliplatin regimen failure. Tumor samples were examined with immunohistochemistry for protein distribution, polymerase chain reaction (PCR) sequencing for mutation detection and real-time PCR for mRNA expression pattern analysis between cetuximab sensitive and resistance patients. The conformational differences of normal and mutated protein structures were predicted by bioinformatics analysis. The 5-year survival rates of target groups were estimated using the Kaplan–Meier method. Immunohistochemistry showed that all cases had high level of HER2 protein. No K-Ras or B-Raf mutation was observed among the study population; however, cetuximab resistance patients harbored a somatic mutation R784G at the exon 20 region of HER2 coding sequence. According to bioinformatics analysis, this mutation caused a notable misfold in protein conformation. Meanwhile, survival analysis showed R784G mutated mCRC patients had shortened survival rate compared with the mCRC cases with wild-type HER2. Collectively, these data report a new mechanism of resistance to cetuximab and might be applicable in modifying new therapeutic strategies for HER2 involved cancers. 相似文献
106.
Amin Soltangheisi Paul J. A. Withers Paulo Sergio Pavinato Maurício Roberto Cherubin Raffaella Rossetto Janaina Braga Do Carmo Gustavo Casoni da Rocha Luiz Antonio Martinelli 《Global Change Biology Bioenergy》2019,11(12):1444-1455
Phosphorus (P) use in global food and bioenergy production needs to become more efficient and sustainable to reduce environmental impacts and conserve a finite and critical resource (Carpenter & Bennett, Environmental Research Letters, 2011, 6, 014009; Springmann et al., Nature, 2018, 562, 519). Sugarcane is one crop with a large P footprint because production is centered on P‐fixing soils with low P availability (Roy et al., Nature Plants, 2016, 2, 16043; Withers et al., Scientific Reports, 2018, 8, 2537). As global demand for processed sugar and bioethanol continues to increase, we advocate that improving P efficiency could become a key sustainability goal for the sugarcane industry. Here, we applied the 5R global P stewardship framework (Withers et al., Ambio, 2015, 44, 193) to identify more sustainable options to manage P in Brazilian sugarcane production. We show that current inputs of P fertilizer to the current crop area could be reduced by over 305 Gg, or 63%, over the next three decades by reducing unnecessary P fertilizer use, better utilization of recyclable bioresources and redesigning recommendation systems. Adoption of these 5R options would save the sugarcane industry in Brazil 528 US$ million and help safeguard global food and energy security. 相似文献
107.
108.
Rojas P Serrano-García N Medina-Campos ON Pedraza-Chaverri J Ogren SO Rojas C 《Neurochemistry international》2011,59(5):628-636
EGb761 is a well-defined mixture of active compounds extracted from Ginkgo biloba leaves. This extract is used clinically due to its neuroprotective effects, exerted probably via its potent antioxidant or free radical scavenger action. Previous studies suggest that oxidative stress, via free radical production, may play an important role in depression and animal models for depression-like behavior. Preclinical studies have suggested that antioxidants may have antidepressants properties. The aim of this study was to investigate the antidepressant-like of EGb761 due to its antioxidant role against oxidative stress induced in the forced swimming test, the most widely used preclinical model for assessing antidepressant-like behavior. Male BALB/c mice were pretreated with EGb761 (10 mg/kg, ip) daily for 17 days followed by the forced swimming test and spontaneous locomotor activity. Animals were sacrificed to evaluate lipid peroxidation, different antioxidant enzyme activities, serotonin and dopamine content in midbrain, hippocampus and prefrontal cortex. EGb761 significantly decreased the immobility time (39%) in the forced swimming test. This antidepressant-like effect of EGb761 was associated with a reduction in lipid peroxidation and superoxide radical production (indicated by a downregulation of Mn-superoxide dismutase activity), both of which are indicators of oxidative stress. The protective effect of EGb761 is not related to excitatory or inhibitory effects in locomotor activity, and was also associated with the modulation of serotonergic and dopaminergic neurotransmission. It is suggested that EGb761 produces an antidepressant-like effect, and that an antioxidant effect against oxidative stress may be partly responsible for its observed neuroprotective effects. 相似文献
109.
Mesarwi Omar A. Sharma Ellora V. Jun Jonathan C. Polotsky Vsevolod Y. 《Sleep and biological rhythms》2015,13(1):2-17
Sleep and Biological Rhythms - It has recently become clear that obstructive sleep apnea (OSA) is an independent risk factor for the development of metabolic syndrome, a disorder of defective... 相似文献
110.
Nobre V Serufo JC Carvalho Odos S Mendonça CL Santos SG Mota EM Gomes D Braga E Antunes CM Lenzi HL Lambertucci JR 《Memórias do Instituto Oswaldo Cruz》2004,99(7):717-720
The association between worm infections and bacterial diseases has only recently been emphasized. This study examined the effect of experimental Angiostrongylus costaricensis infection on endogenous intestinal flora of Swiss Webster mice. Eight mice aging six weeks were selected for this experiment. Four were infected with A. costaricensis and the other four were used as controls. Twenty eight days after the worm infection, all mice in both groups were sacrificed and samples of the contents of the ileum and colon were obtained and cultured for aerobic and anaerobic bacteria. In the mice infected with A. costaricensis there was a significant increase in the number of bacteria of the endogenous intestinal flora, accompanied by a decrease in the number of Peptostreptococcus spp. This alteration in the intestinal flora of mice infected by the nematode may help to understand some bacterial infections described in humans. 相似文献