全文获取类型
收费全文 | 3544篇 |
免费 | 250篇 |
国内免费 | 278篇 |
专业分类
4072篇 |
出版年
2024年 | 8篇 |
2023年 | 42篇 |
2022年 | 109篇 |
2021年 | 199篇 |
2020年 | 126篇 |
2019年 | 154篇 |
2018年 | 161篇 |
2017年 | 104篇 |
2016年 | 137篇 |
2015年 | 194篇 |
2014年 | 268篇 |
2013年 | 264篇 |
2012年 | 348篇 |
2011年 | 294篇 |
2010年 | 165篇 |
2009年 | 144篇 |
2008年 | 189篇 |
2007年 | 143篇 |
2006年 | 136篇 |
2005年 | 107篇 |
2004年 | 84篇 |
2003年 | 97篇 |
2002年 | 78篇 |
2001年 | 41篇 |
2000年 | 58篇 |
1999年 | 62篇 |
1998年 | 34篇 |
1997年 | 30篇 |
1996年 | 23篇 |
1995年 | 30篇 |
1994年 | 26篇 |
1993年 | 22篇 |
1992年 | 32篇 |
1991年 | 24篇 |
1990年 | 23篇 |
1989年 | 19篇 |
1988年 | 15篇 |
1987年 | 14篇 |
1986年 | 9篇 |
1985年 | 9篇 |
1984年 | 7篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1976年 | 3篇 |
1975年 | 3篇 |
1969年 | 3篇 |
1966年 | 2篇 |
1951年 | 2篇 |
排序方式: 共有4072条查询结果,搜索用时 0 毫秒
21.
河南省冬小麦需水量的时空变化及影响因素 总被引:3,自引:0,他引:3
基于河南省17个气象监测站点1961-2012年逐旬气象数据,采用FAO推荐的Penman-Monteith公式计算参考作物需水量,结合“全国灌溉试验资料数据库”中冬小麦生育期、作物系数等数据计算冬小麦生育期需水量,利用时间序列分析法和灰色关联分析法,分析河南省冬小麦需水量近51年来的时空分布特征、变化规律及其主要影响因素.结果表明: 1961-2011年,河南省冬小麦平均需水量在345~492 mm,卢氏站冬小麦平均需水量最低,孟津站最高.大部分站点1980-1989年冬小麦平均需水量最低,1961-1969年冬小麦平均需水量最高.17个站点中,新乡、栾川、开封、西峡、南阳、信阳、固始7站冬小麦需水量随年份呈上升趋势,其余10个站点需水量均呈下降趋势;豫北需水量高于豫南,豫西地区需水量跨度较大.河南省17站冬小麦生育期日平均高温、日平均低温均随年份呈上升趋势,生育期日平均风速、日平均相对湿度、日照时数基本呈下降趋势.河南省冬小麦生育期日平均高温、日照时数是影响冬小麦需水量的主要因素,日平均相对湿度对其影响最小. 相似文献
22.
23.
24.
25.
Hydrogen sulfide (H2S) is the most undesirable inorganic gas in biogas from anaerobic digestion (AD). However, H2S production in AD is complex and understanding of its processes is still limited. This study performed six controlled batch anaerobic co-digestion experiments to investigate H2S production. Materials were obtained from four field anaerobic digester systems and co-digestion feedstocks from agroindustry. An additional precipitation experiment was conducted to further examine H2S production dynamics. Digesters containing highly soluble, carbohydrate-based wastes had a high H2S final specific production (FSP) value. Additionally, the FSP values were negatively correlated with the initial Fe(II):S ratios in the digester liquid of the batch tests. The precipitation experiment indicated that iron sulfide precipitation was preferred in the presence of an anaerobic community. The H2S production as a time series was successfully modeled using a generalized additive model (R2 > 0.82). This study revealed that sulfate, phosphorus, and iron concentrations are important predictors and potential inhibitors of H2S production in AD. Further examination of real-time H2S modeling in AD is warranted. 相似文献
26.
27.
Viral particles are biological machines that have evolved to package, protect, and deliver the viral genome into the host via regulated conformational changes of virions. We have developed a procedure to modify lysine residues with S-methylthioacetimidate across the pH range from 5.5 to 8.5. Lysine residues that are not completely modified are involved in tertiary or quaternary structural interactions, and their extent of modification can be quantified as a function of pH. This procedure was applied to the pH-dependent structural transitions of brome mosaic virus (BMV). As the reaction pH increases from 5.5 to 8.5, the average number of modified lysine residues in the BMV capsid protein increases from 6 to 12, correlating well with the known pH-dependent swelling behavior of BMV virions. The extent of reaction of each of the capsid protein's lysine residues has been quantified at eight pH values using coupled liquid chromatography-tandem mass spectrometry. Each lysine can be assigned to one of three structural classes identified by inspection of the BMV virion crystal structure. Several lysine residues display reactivity that indicates their involvement in dynamic interactions that are not obvious in the crystal structure. The influence of several capsid protein mutants on the pH-dependent structural transition of BMV has also been investigated. Mutant H75Q exhibits an altered swelling transition accompanying solution pH increases. The H75Q capsids show increased reactivity at lysine residues 64 and 130, residues distal from the dimer interface occupied by H75, across the entire pH range. 相似文献
28.
Cancer-related genes harbored in the loss regions containing a high frequency of hepatocellular carcinoma (HCC) were selected.Related information was gathered and the coding single nucleotide polymorphism (cSNP) sequences were obtained from the single nucleotide polymorphism (SNP) database.The appropriate primers and oligonucleotide probes were then designed in accordance with the SNP sites,and subsequently,the gene chips for detecting SNPs were constructed.Genomic DNA was extracted from blood samples of healthy controls and from patients with HBV infection.The sequences,including the SNPs,were amplified via polymerase chain reaction (PCR) and labeled using digoxigenin deoxyuridine tri-phosphate (Dig-dUTP).The labeled products were then hybridized with the SNP chips.Results confirmed that the differences in allele frequencies of three SNPs EGFL3 (rs947345),Caspase9 (rs2308950),and E2F2 (rs3218171) were distinct between HBV-infected patients and controls,suggesting that these SNPs ocuring in high frequency in HBV-infected individuals may be associated with susceptibility to HCC. 相似文献
29.
The monocyte locomotion inhibitory factor (MLIF) is an anti-inflammatory oligopeptide produced by Entamoeba histolytica. Among its different effects, it inhibits locomotion of human monocytes, hence its original name. The carboxyl-terminal end group Cys-Asn-Ser is the pharmacophore of anti-inflammatory peptide Met-Gln-Cys-Asn-Ser. In this study, the N-terminal of Cys-Asn-Ser was modified. With the aim to enhance the antioxidant ability and penetrability of Cys-Asn-Ser, we designed and synthesized two tetrapeptides Tyr-Cys-Asn-Ser and His-Cys-Asn-Ser. The neuroprotective effects of Tyr-Cys-Asn-Ser and His-Cys-Asn-Ser on focal ischemia reperfusion were investigated, and their pharmacological activities compared with Cys-Asn-Ser were studied. In order to study the mechanism of neuroprotective effect of these peptides, the level of oxidative stress markers malondialdehyde (MDA) and superoxide dismutase (SOD) and pro-inflammatory factors interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), and myeloperoxidase (MPO) were detected in brain tissue homogenate. 相似文献
30.
Li Y Tian C Yang Y Yan Y Ni Y Wei Y Pleasure SJ Zhao C 《Genesis (New York, N.Y. : 2000)》2011,49(12):919-926
The hippocampus is crucial for higher brain functions, such as learning, memory, and emotion. Many diseases like epilepsy and Down's syndrome are associated with abnormalities in early hippocampal development. In addition, adult dentate neurogenesis is thought to be defective in several classes of psychiatric disorders. However, the mechanisms regulating hippocampal development and adult neurogenesis remain unclear. One of the limitations to studying these processes is the scarcity of available specific mouse tools. Here, we report an inducible transgenic Cre mouse line, Frizzled 9‐CreER?, in which tamoxifen administration induces Cre recombinant. Our data show that Cre is expressed in the developing hippocampal primordium, confined to the granule cell layer at P20 and further limited to the subgranular zone in the adult dentate gyrus. Cre recombinase shows very high activity in all of these regions. Thus, this transgenic line will be a powerful tool in understanding the mechanisms of hippocampal development, adult neurogenesis, and associated diseases. genesis 49:919–926, 2011. © 2011 Wiley Periodicals, Inc. 相似文献