全文获取类型
收费全文 | 24350篇 |
免费 | 2036篇 |
国内免费 | 1768篇 |
学科分类
生物科学 | 28154篇 |
出版年
2024年 | 123篇 |
2023年 | 572篇 |
2022年 | 700篇 |
2021年 | 967篇 |
2020年 | 1037篇 |
2019年 | 1225篇 |
2018年 | 1026篇 |
2017年 | 928篇 |
2016年 | 932篇 |
2015年 | 1175篇 |
2014年 | 1454篇 |
2013年 | 2219篇 |
2012年 | 1022篇 |
2011年 | 1185篇 |
2010年 | 819篇 |
2009年 | 1275篇 |
2008年 | 1326篇 |
2007年 | 1308篇 |
2006年 | 1200篇 |
2005年 | 969篇 |
2004年 | 901篇 |
2003年 | 757篇 |
2002年 | 628篇 |
2001年 | 518篇 |
2000年 | 442篇 |
1999年 | 398篇 |
1998年 | 391篇 |
1997年 | 396篇 |
1996年 | 301篇 |
1995年 | 259篇 |
1994年 | 225篇 |
1993年 | 228篇 |
1992年 | 187篇 |
1991年 | 164篇 |
1990年 | 145篇 |
1989年 | 113篇 |
1988年 | 100篇 |
1987年 | 87篇 |
1986年 | 76篇 |
1985年 | 90篇 |
1984年 | 60篇 |
1983年 | 34篇 |
1982年 | 58篇 |
1981年 | 47篇 |
1980年 | 31篇 |
1979年 | 21篇 |
1978年 | 11篇 |
1977年 | 8篇 |
1976年 | 5篇 |
1975年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
Effect of lead stress on mineral content and growth of wheat (Triticum aestivum) and spinach (Spinacia oleracea) seedlings 总被引:1,自引:0,他引:1
Mostafa Lamhamdi Ouiam El Galiou Ahmed Bakrim Juan Carlos Nóvoa-Muñoz Manuel Arias-Estévez Ahmed Aarab René Lafont 《Saudi Journal of Biological Sciences》2013,20(1):29-36
Lead (Pb) is the most common heavy metal contaminant in the environment. Pb is not an essential element for plants, but they absorb it when it is present in their environment, especially in rural areas when the soil is polluted by automotive exhaust and in fields contaminated with fertilizers containing heavy metal impurities. To investigate lead effects on nutrient uptake and metabolism, two plant species, spinach (Spinacia oleracea) and wheat (Triticum aestivum), were grown under hydroponic conditions and stressed with lead nitrate, Pb(NO3)2, at three concentrations (1.5, 3, and 15 mM).Lead is accumulated in a dose-dependent manner in both plant species, which results in reduced growth and lower uptake of all mineral ions tested. Total amounts and concentrations of most mineral ions (Na, K, Ca, P, Mg, Fe, Cu and Zn) are reduced, although Mn concentrations are increased, as its uptake is reduced less relative to the whole plant’s growth. The deficiency of mineral nutrients correlates in a strong decrease in the contents of chlorophylls a and b and proline in both species, but these effects are less pronounced in spinach than in wheat. By contrast, the effects of lead on soluble proteins differ between species; they are reduced in wheat at all lead concentrations, whereas they are increased in spinach, where their value peaks at 3 mM Pb.The relative lead uptake by spinach and wheat, and the different susceptibility of these two species to lead treatment are discussed. 相似文献
82.
83.
84.
The function of bcl-2 in preventing cell death is well known, but the mechanisms whereby bcl-2 functions are not well characterized. One mechanism whereby bcl-2 is thought to function is by alleviating the effects of oxidative stress upon the cell. To examine whether Bcl-2 can protect cells against oxidative injury resulting from post-hypoxic reoxygenation (H/R), we subjected rat fibroblasts Rat-1 and their bcl-2 transfectants b5 to hypoxia (5% CO2, 95% N2) followed by reoxygenation (5% CO2, 95% air). The bcl-2 transfectants exhibited the cell viability superior to that of their parent non-transfectants upon treatment with reoxygenation after 24-, 48-, or 72-h hypoxia, but not upon normoxic serum-deprivation or upon serum-supplied hypoxic treatment alone. Thus bcl-2 transfection can prevent cell death of some types, which occurred during H/R but yet not appreciably until termination of hypoxia. The time-sequential events of H/R-induced cell death were shown to be executed via (1) reactive oxygen species (ROS) production at 1-12 h after H/R, (2) activation of caspases-1 and -3, at 1-3 h and 3-6 h after H/R, respectively, and (3) loss of mitochondrial membrane potential (DeltaPsi) at 3-12 h after H/R. These cell death-associated events were prevented entirely except caspase-1 activation by bcl-2 transfection, and were preceded by Bcl-2 upregulation which was executed as early as at 0-1 h after H/R for the bcl-2 transfectants but not their non-transfected counterpart cells. Thus upregulation of Bcl-2 proteins may play a role in prevention of H/R-induced diminishment of cell viability, but may be executed not yet during hypoxia itself and be actually operated as promptly as ready to go immediately after beginning of H/R, resulting in cytoprotection through blockage of either ROS generation, caspase-3 activation, or DeltaPsi decline. 相似文献
85.
Autophagy is an intracellular degradation process for recycling macromolecules and organelles. It plays important roles in plant development and in response to nutritional demand, stress, and senescence. Organisms from yeast to plants contain many autophagy-associated genes (ATG). In this study, we found that a total of 33 ATG homologues exist in the rice [Oryza sativa L. (Os)] genome, which were classified into 13 ATG subfamilies. Six of them are alternatively spliced genes. Evolutional analysis showed that expansion of 10 OsATG homologues occurred via segmental duplication events and that the occurrence of these OsATG homologues within each subfamily was asynchronous. The Ka/Ks ratios suggested purifying selection for four duplicated OsATG homologues and positive selection for two. Calculating the dates of the duplication events indicated that all duplication events might have occurred after the origin of the grasses, from 21.43 to 66.77 million years ago. Semi-quantitative RT–PCR analysis and mining the digital expression database of rice showed that all 33 OsATG homologues could be detected in at least one cell type of the various tissues under normal or stress growth conditions, but their expression was tightly regulated. The 10 duplicated genes showed expression divergence. The expression of most OsATG homologues was regulated by at least one treatment, including hormones, abiotic and biotic stresses, and nutrient limitation. The identification of OsATG homologues showing constitutive expression or responses to environmental stimuli provides new insights for in-depth characterization of selected genes of importance in rice. 相似文献
86.
臭氧是氧气的同素异形体,在常温条件下它是一种有特殊臭味的淡蓝色气体。臭氧现已成为我国主要的一种环境污染物。臭氧的分布存在着明显的时间特异性和地区特异性,与气象条件、污染物的输送与扩散以及局地光化学反应强度等因素有着重要的关系。同时在人群流行病学和动物实验中发现了臭氧可以提高慢性阻塞性肺疾病、哮喘等呼吸系统疾病的发病率和病死率,同时提示了老年人、有呼吸系统疾病病史的人群和儿童都是易受臭氧影响的高危人群。国内外学者也对臭氧对机体损伤的机制进行了研究,主要集中在臭氧的致炎作用、氧化应激作用、造成机体气道高反应性和对细胞DNA的损伤等方面。本文主要就国内外对臭氧在时空分布、对机体呼吸系统影响和臭氧造成机体损伤的机制这3方面作一简要综述。 相似文献
87.
Sarcobatus vermiculatus (Hook) Torrey is a leaf succulent, sodium-accumulating shrub usually found in saline substrates of the Great Basin desert, Utah, USA. Laboratory experiments were conducted to determine the effect of salinity (0, 200, 400, 600, 800 and 1000 mM NaCl) and temperature (day/night: 5/15, 10/20, 15/25, 20/30, and 25/35°C) on seed germination. S. vermiculatus showed 100% germination in non-saline controls, at all thermoperiods. Percentage and rate of germination decreased with increases in salinity and few seeds germinated at even 1000 mM NaCl. High salinity exposure caused the loss of viability at higher temperature regimes, while some recovery was recorded in low salinity treatments. 相似文献
88.
Cold and carbon dioxide used as multi-hurdle preservation do not induce appearance of viable but non-culturable Listeria monocytogenes 总被引:1,自引:0,他引:1
AIMS: To study whether the exposure to cold (4 degrees C) and carbon dioxide which results in the elongation of Listeria cells, induces a viable but nonculturable (VBNC) state. METHODS AND RESULTS: When cold and CO2 stressed L. monocytogenes were observed under a fluorescence microscope, using the LIVE/DEAD BacLight bacteria viability kit (Molecular Probes, Eugene, OR, USA), the healthy, mildly injured, and the putative VBNC cells accounted for 31.0% of the stressed cell population. By using the selective plate count, 31.4% of the same stressed cell population was found to be healthy and mildly injured (putative VBNC cells not included). If there were VBNC state cells present, we should have observed a significant difference between the above two numbers. In fact, there was no significant difference between the results obtained from those two methods. CONCLUSIONS: There were no VBNC state cells observed in the stressed cell population. We conclude that cold and CO2 do not induce L. monocytogenes to enter a VBNC state. SIGNIFICANCE AND IMPACT OF THE STUDY: Cold and modified atmospheres are widely used in fresh muscle food and fruit preservation. Whether they would induce L. monocytogenes into a VBNC state is of a great concern for microbial food safety. 相似文献
89.
MicroRNA (miR) plays an integral role in cardiovascular diseases. M-iR-423-5p is aberrantly expressed in patients with myocardial infarction and heart failure. The aim of the present study was to study the roles and mechanisms of miR-423-5p in hypoxia/reoxygenation (H/R) mediated cardiomyocytes injury. H9C2 cells were transfected with negative control, miR-423-5p mimic, and inhibitor for 48 hr, followed by exposed to H/R condition. Cell apoptosis rate, caspase 3/7 activities, Bax and cleaved-caspase 3 (c-caspase 3) protein levels were assayed by flow cytometry, Caspase-Glo 3/7 Assay kit, western blot analysis, respectively. Furthermore, the mitochondrial membrane potential, adenosine triphosphate (ATP) content, reactive oxygen species (ROS) production, and Drp1 expression were also investigated. Furthermore, the dual-luciferase reporter assay was used to evaluate the relationship between miR-423-5p and Myb-related protein B (MYBL2). The roles of miR-423-5p in wnt/β-catenin were assessed by western blot analysis. The results revealed that H/R triggered miR-423-5p expression. Overexpression of miR-423-5p promoted cardiomyocyte apoptosis, enhanced the activities of caspase 3/7, upregulated the expression of Bax and c-caspase 3. miR-423-5p upregulation caused the loss of mitochondrial membrane potential and the reduction of ATP content, the augment of ROS production and Drp1 expression. However, the opposite trends were observed upon suppression of miR-423-5p. In addition, miR-423-5p could target the 3′ untranslated region of MYBL2. miR-423-5p depletion led to the activation of the wnt/β-catenin signaling pathway via targeting MYBL2. Knockdown of MYBL2 was obviously reversed the roles of miR-423-5p in apoptosis and mitochondrial dysfunction. Taken together, miR-423-5p suppression reduced H/R-induced cardiomyocytes injury through activation of the wnt/β-catenin signaling pathway via targeting MYBL2 in cardiomyocytes. 相似文献
90.
Md. Ejaz Ahmed Mohd. Moshahid Khan Hayate Javed Kumar Vaibhav Andleeb Khan Rizwana Tabassum Mohammad Ashafaq Farah Islam Mohammed M. Safhi Fakhrul Islam 《Neurochemistry international》2013
Alzheimer’s disease (AD) is a progressive neurodegenerative disorder resulting in cognitive decline and enhancement of oxidative loads in the brain. Flavonoids have been considered to exert human health benefits by anti-oxidant and anti-inflammatory properties. The present study is aimed to elucidate the neuroprotective effect of catechin hydrate (CH), a natural flavanoid with potential antioxidant and anti-inflammatory properties, on intracerebroventricular streptozotocin (ICV-STZ) induced neuronal loss and memory impairment. To test this hypothesis, male Wistar rats were pretreated with CH (10 and 20 mg/kg bwt) orally once daily for 21 days and then bilaterally injected with ICV-STZ (3 mg/kg bwt), while sham group rats receive the same volume of vehicle. After 2 weeks of ICV-STZ infusion, rats were tested for cognitive performance using Morris water maze (MWM) test and then sacrifice for biochemical and histopathological assays. CH was found to be successful in upregulating the antioxidant status and prevented the memory loss. The expression of choline acetyl transferase (ChAT) was decreased in ICV-STZ group and CH pretreatment increases the expression of ChAT. Moreover, inflammatory mediators like TNF-α, IL-1β levels and expression of iNOS were significantly attenuated by CH pretreatment. The study suggests that CH is effective in preventing memory loss, ameliorating the oxidative stress and might be beneficial for the treatment of sporadic dementia of Alzheimer’s type (SDAT). 相似文献