全文获取类型
收费全文 | 21412篇 |
免费 | 1887篇 |
国内免费 | 2403篇 |
专业分类
25702篇 |
出版年
2024年 | 62篇 |
2023年 | 309篇 |
2022年 | 682篇 |
2021年 | 1058篇 |
2020年 | 831篇 |
2019年 | 1008篇 |
2018年 | 951篇 |
2017年 | 700篇 |
2016年 | 955篇 |
2015年 | 1427篇 |
2014年 | 1727篇 |
2013年 | 1739篇 |
2012年 | 2071篇 |
2011年 | 1841篇 |
2010年 | 1237篇 |
2009年 | 1042篇 |
2008年 | 1155篇 |
2007年 | 1024篇 |
2006年 | 951篇 |
2005年 | 809篇 |
2004年 | 672篇 |
2003年 | 685篇 |
2002年 | 538篇 |
2001年 | 336篇 |
2000年 | 274篇 |
1999年 | 268篇 |
1998年 | 188篇 |
1997年 | 148篇 |
1996年 | 149篇 |
1995年 | 134篇 |
1994年 | 127篇 |
1993年 | 80篇 |
1992年 | 105篇 |
1991年 | 85篇 |
1990年 | 79篇 |
1989年 | 52篇 |
1988年 | 34篇 |
1987年 | 29篇 |
1986年 | 39篇 |
1985年 | 32篇 |
1984年 | 16篇 |
1983年 | 11篇 |
1982年 | 9篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1969年 | 2篇 |
1968年 | 3篇 |
1966年 | 2篇 |
1962年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
在0.5%葡萄糖-Mandels盐培养液中添加终浓度为0.5%的L-山梨糖,可使里斯木霉(Trichoderma reesei C 30)和拟康氏木霉(Trichoderma pseudokoningii S38)的内切和外切β-1,4葡聚糖酶合成速率在培养的2天内提高4倍。与此同时β-葡萄糖苷酶的合成没有明显变化。L-山梨糖能明显抑制菌丝生长,但对葡萄糖的吸收没有影响,对菌丝分泌纤维素酶的机制影响不大。其对酶合成的促进可能主要是通过降低了菌丝体的生长速率。 相似文献
42.
本研究利用聚合酶链式反应技术,成功地克隆了枯草芽孢杆菌缺陷型原噬菌体PBSX阻遏基因及其温度敏感型等位基因。核苷酸序列分析发现,野生型及其温度敏感型阻遏基因之间的碱基变异较大,但却存在几乎完全相同的开放读框,尤其是开放读框orfⅠ,可能编码着113个氨基酸的阻遏蛋白,并且还推定了开放读框的启动区和核糖体结合位点。通过互补实验,证实了野生型阻遏基因的产物能够抑制温度诱导PBSX原噬菌体,表明克隆的基因有着正常的生物活性。 相似文献
43.
本文设计一种由胶原和高分子聚合物组成的新型生物一人工复合血管。其研制过程是将包绕有聚酯网的硅胶棒埋入羊的皮下组织,再将形成的经聚酯网为支架的胶原管经醛化处理。作者通过肉眼和SEM观察提出了研制生物——人工复合血管的要点:聚酯网网孔要合适,其与硅胶棒的间隙要恰当,理化处理方法更要选择好。 相似文献
44.
系统地研究了细胞色素c在多种氨基酸和多肽修饰电极上的电化学反应。并对影响加速细胞色素c电化学反应的因素进行了讨论。 相似文献
45.
无花果蛋白酶通过8%戊二醛活化载体,共价结合到聚苯乙烯阴离子交换树脂GM201上,固定化作用在pH7.7,酶浓度0.8mg/g树脂,4℃下进行6h。得到的固定化酶表观K_m值(酪蛋白,1.11×10~(-4)mol/L)小于溶液酶K_m值(1.96×10~(-4)mol/L);固定化酶活性在pH6~8保持稳定,溶液酶最适pH为7.2;固定化酶最适温度由溶液酶的50~60℃移至37℃;固定化酶25℃保持7d,重复水解酪蛋白7次后,保留83.3%活性。固定化酶对酪蛋白水解度达47.5%,对大豆球蛋白达11.6%。 相似文献
46.
将油桐尺蠖(Buzurasuppressaria)核多角体病毒晚期基因──多角体蛋白基因启动子及5′端编码区,以两种不同方式置于缺乏启动子的氯霉素乙酰基转移酶(CAT)基因上游,使其分别终止在不同翻译终止位点,其宿主菌具有明显不同的氯霉素抗性,最高达200mg/L LB培养基以上,表明昆虫病毒启动子能启动原核基因表达。对多角体蛋白基因启动子能在大肠杆菌中有效工作的原因进行了讨论。 相似文献
47.
Anaerobic Degradation of Propionate by a Mesophilic Acetogenic Bacterium in Coculture and Triculture with Different Methanogens 总被引:4,自引:2,他引:2 下载免费PDF全文
A mesophilic acetogenic bacterium (MPOB) oxidized propionate to acetate and CO2 in cocultures with the formate- and hydrogen-utilizing methanogens Methanospirillum hungatei and Methanobacterium formicicum. Propionate oxidation did not occur in cocultures with two Methanobrevibacter strains, which grew only with hydrogen. Tricultures consisting of MPOB, one of the Methanobrevibacter strains, and organisms which are able to convert formate into H2 plus CO2 (Desulfovibrio strain G11 or the homoacetogenic bacterium EE121) also degraded propionate. The MPOB, in the absence of methanogens, was able to couple propionate conversion to fumarate reduction. This propionate conversion was inhibited by hydrogen and by formate. Formate and hydrogen blocked the energetically unfavorable succinate oxidation to fumarate involved in propionate catabolism. Low formate and hydrogen concentrations are required for the syntrophic degradation of propionate by MPOB. In triculture with Methanospirillum hungatei and the aceticlastic Methanothrix soehngenii, propionate was degraded faster than in biculture with Methanospirillum hungatei, indicating that low acetate concentrations are favorable for propionate oxidation as well. 相似文献
48.
Cell density is known to modify the survival of mammalian cells exposed to elevated temperatures. We have examined the role that cell–cell contact plays in this phenomenon. The formation of cell–cell contact is carried out by cells' junctional complex, i.e., tight junctions, desmosomes, and gap junctions. Lack of formation of tight junctions and desmosomes, or their opening, could interfere with the functions and structures of cell membrane. Membrane damage is at least partially responsible for cell death at elevated temperatures. MDCK cells with high density plated in low calcium medium form confluent monolayers devoid of the formation of tight junctions and desmosomes but quickly assemble them after Ca2+ restoration. We used MDCK cells and the calcium switch technique to investigate effects of cell–cell contact and, independently, of cell density on hyperthermic cell killing. We found that MDCK cells that formed tight junctions and desmosomes were more resistant to hyperthermic treatment than those that did not. Blocking the formation pathway of tight junctions made cells sensitive to heat. Cells growing at lowdensity showed almost the same survival as did cells at high density in the absence of the formation of tight junctions and desmosomes. The results suggest that the formation of tight junctions and desmosomes play a more important role in determining hyperthermic response than does density per se. The formation of tight junctions and desmosomes appears to protect cells modestly against hyperthermic killing. © 1994 Wiley-Liss, Inc. 相似文献
49.
Water potentials for developing cladodes and fruits of a succulent plant, including xylem-versus-phloem implications for water movement 总被引:4,自引:1,他引:3
Nobel Park S.; Andrade Jose Luis; Wang Ning; North Gretchen B. 《Journal of experimental botany》1994,45(12):1801-1807
Developing cladodes had lower water potentials and developingfruits had higher water potentials than the underlying cladodesof the widely cultivated prickly pear cactus, Opuntia ficus-indica.The 0.06 MPa lower value in 4-week-old daughter cladodes indicateda typical water potential gradient from the underlying clad-odealong the xylem of 0.2 MPa m1; the 0.17 MPa highervalue in 4-week-old fruits, which decreased to 0.07 MPa by 10weeks, implicated the phloem as their supplier of water. Thephloem sap of the underlying cladodes had an osmotic pressureof only 0.90 to 0.98 MPa, so the phloem could supply a relativelydilute solution to the photosynthetically dependent fruits (daughtercladodes of O. ficus-indica are photosynthetically independentat 4 weeks). Although the water potentials were similar foradjacent tissues, the osmotic pressures were lower for the water-storagecompared with the photosynthetic tissue; the osmotic pressureswere higher for xylem sap from fruits, for which xylary flowapparently occurred toward the underlying cladodes, than fordaughter cladodes. The relative capacitance (change in relativewater content divided by change in tissue water potential) wasapproximately 0.71 MPa1 for the water-storage tissueand the photosynthetic tissue of both daughter cladodes andfruits at 4 weeks of age. When these organs approached maturityat 10 weeks, the relative capacitance increased about 40% fortheir water-storage tissue, but decreased 30% for their photosynthetictissue. As the plant water content decreases during drought,about twice as much water will thus be lost per unit volumeof the water-storage tissue compared with the photosynthetictissue of maturing fruits and cladodes. Key words: Opuntia ficus-indica, phloem, relative water content, water capacitance, water potential 相似文献
50.
Capsid assembly and involved function analysis of twelve core protein mutants of duck hepatitis B virus. 总被引:6,自引:6,他引:0 下载免费PDF全文
The roles of different regions of the duck hepatitis B virus (DHBV) core protein on viral capsid assembly and related functions were examined. Twelve deletion and insertion mutations which covered 80% of the DHBV C open reading frame were constructed and expressed in Escherichia coli. The N-terminal region (amino acids 3 to 66) of DHBV core protein was important for its tertiary structure and function in E. coli. The expressed core mutants without this region apparently inhibited E. coli growth. The results of transmission electron microscopy of E. coli thin sections, capsid agarose gel, and sucrose gradient sedimentation demonstrated that a few DHBV core mutants with insertion in the N terminus and deletion in the C terminus retained the ability to form core-like particles in E. coli. However, other mutations in most of N-terminal and central regions strongly inhibited the self-assembly ability of DHBV core protein in E. coli. In addition, the mutant with a C-terminal region deletion (amino acids 181 to 228) lost most of the nucleic acid-binding activity of the DHBV core protein. 相似文献