全文获取类型
收费全文 | 2347篇 |
免费 | 96篇 |
国内免费 | 83篇 |
专业分类
2526篇 |
出版年
2023年 | 20篇 |
2022年 | 32篇 |
2021年 | 46篇 |
2020年 | 43篇 |
2019年 | 69篇 |
2018年 | 56篇 |
2017年 | 49篇 |
2016年 | 43篇 |
2015年 | 28篇 |
2014年 | 98篇 |
2013年 | 112篇 |
2012年 | 62篇 |
2011年 | 80篇 |
2010年 | 56篇 |
2009年 | 95篇 |
2008年 | 92篇 |
2007年 | 95篇 |
2006年 | 86篇 |
2005年 | 114篇 |
2004年 | 92篇 |
2003年 | 76篇 |
2002年 | 72篇 |
2001年 | 61篇 |
2000年 | 57篇 |
1999年 | 47篇 |
1998年 | 50篇 |
1997年 | 51篇 |
1996年 | 40篇 |
1995年 | 36篇 |
1994年 | 46篇 |
1993年 | 45篇 |
1992年 | 48篇 |
1991年 | 33篇 |
1990年 | 33篇 |
1989年 | 37篇 |
1988年 | 48篇 |
1987年 | 45篇 |
1986年 | 32篇 |
1985年 | 54篇 |
1984年 | 60篇 |
1983年 | 30篇 |
1982年 | 55篇 |
1981年 | 34篇 |
1980年 | 20篇 |
1979年 | 20篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1976年 | 5篇 |
1974年 | 5篇 |
1973年 | 3篇 |
排序方式: 共有2526条查询结果,搜索用时 15 毫秒
91.
Nitrate reductase (EC 1.6.6.1–3; NR) activity was evaluated in nodulated lucerne ( Medicago sativa L. cv. Europe) grown aeroponically in both the presence and absence of applied nitrogen. Determination of in vivo NR activity was done with organ pieces in 0.1 M K+ -phosphate, pH 7.5, 0.1 M KNO3 and 1% n -propanol. NR activity was detected in all plant parts. Leaves accounted for 40% of the whole plant activity. Root activity was as high as leaf activity. Stem NR activity accounted for 14 to 20% of the total plant activity. NR activity was also detected in symbolically dependent plants grown without combined nitrogen. Nodule NR in symbolically dependent plants accounted for 17% of the tolal plant aclivity. When nitrate was present in the nulrienl medium, NR increased 5-fold as compared lo N2 -dependenl plants. Varying levels of nitrale (1.65 to 4 m M ) had no influence on leaf or stem activities. However, root NR activity seemed to be related to the nitrale concentration in the nulrient medium. Throughoul inilial vegelative growth, in vivo NR and nitrogenase (acelylene reduction) increased simultaneously. After shoot harvest, nitrogenase (acetylene reduction) aclivity drastically decreased with reduction of photosynthate supply, whereas NR increased in all organs, especially in N2 -dependenl plants. 相似文献
92.
微波杀菌过程中大肠杆菌与金黄色葡萄球菌细胞膜通透性的改变 总被引:4,自引:0,他引:4
对细胞膜通透性变化的研究是认识微波杀菌机理的途径之一。用荧光探针检测微波处理后细胞内Ca2 浓度的变化,可以精确地表征细胞膜通透性的改变。选用二乙酸荧光素(FDA)和Fluo-3/AM两种荧光染料,对大肠杆菌(Escherichiacoli)和金黄色葡萄球菌(Staphylococcus aureus)经微波处理后的酯酶活性及细胞膜通透性进行研究,结果表明大肠杆菌与金黄色葡萄球菌的胞内非特异性酯酶(NSE)活性及细胞膜通透性的变化情形有所不同。在50℃、55℃、60℃和65℃微波处理条件下,大肠杆菌细胞膜通透性分别增加了20.7%、28.1%、74.8%、89.8%,而金黄色葡萄球的增加不显著,分别比对照组提高了4.1%、6.0%、21.9%和19.7%。细胞膜通透性的改变与微生物致死率有一定的相关性,也可能是微波杀菌非热效应的表现之一。 相似文献
93.
Valerie Vreeland Suzanne R. Morse Robert H. Robichaux Kathleen L. Miller Sui-Sheng T. Hua Watson M. Laetsch 《Planta》1989,177(4):435-446
Carbohydrate-hybridization probes (Vreeland and Laetsch, 1989, Planta (177, 423–434) were used to localize the homogalacturonan (pectate) component of pectins in the cell walls of leaves and soybean root nodules. Leaves of two species of the dicotyledon Dubautia were compared; these species contain much pectin but differ in their tissue water relations with respect to their cell-wall properties. Maturation of the primary cell walls in nodules was studied in the Bradyrhizobium japonicum-Glycine max symbiosis. Probe labelling was based on the divalent-cation-mediated association between pectate in tissue sections and fluorescein-conjugated pectate fragments. Pectate was also labelled by mixed-dimer formation with fluorescent polyguluronate derived from alginate. The specificity of the probe for unesterified polygalacturonate was indicated by increased cell-wall labelling after chemical or enzymatic deesterification of tissue sections, in contrast to elimination of labelling by chemical esterification. Postfixation of tissue sections improved retention of soluble pectate. Pectate differences were found in the leaves among cell types, in degree of esterification, and between plant species. The cell walls of soybean nodules were strongly labelled by the pectate probe in nodules one week and three weeks after infection. Pectate was more highly esterified in the central infected zone than in the surrouding cortex. Within the infected zone, walls of uninfected cells and infected cells were similarly labelled by the pectate probe. The results indicate that the pectate molecular probe provides detailed information on pectate distribution at the cellular level for investigations of cell-wall structure, development and physiology.Abbreviations EDTA
ethylenedinitrilotetraacetic acid (ethylenediaminetetraacetic acid)
- NMR
nuclear magnetic resonance spectroscopy
- TTB
1,3,5-triazido-2,4,6-trinitrobenene 相似文献
94.
该试验以抗寒性不同的5年生软枣猕猴桃品种‘魁绿’、‘丰绿’和‘96-6’为试验材料,测定分析自然越冬期间枝条组织解剖结构、细胞膜透性和内源激素GA3、ABA、IAA含量水平的变化,以明确不同品种软枣猕猴桃越冬期间对低温的适应能力及其内在生理机制,为软枣猕猴桃抗寒品种的选育及鉴别提供理论依据。结果表明:(1)软枣猕猴桃枝条木质部比率大小依次为‘魁绿’>‘96-6’>‘丰绿’,皮层比率依次为‘丰绿’>96-6’>魁绿’,二者表现趋势相反,且‘魁绿’枝条的解剖结构与其他两品种差异显著。(2)在自然越冬期间,各品种软枣猕猴桃枝条相对电导率和MDA含量随温度变化均呈先升后降的趋势,在温度最低的1月份达到最高,其中‘魁绿’枝条的相对电导率和MDA含量变化幅度最小,而‘丰绿’受低温影响较大。(3)在11月份至2月份,随自然温度的变化,各品种软枣猕猴桃枝条的GA3和IAA含量均呈先降低后升高的变化趋势,ABA含量则呈先升后降的趋势,且都在1月份分别达到最低或最高值。研究发现,在自然越冬期间,低温对3个软枣猕猴桃品种的组织解剖结构、膜透性及内源激素含量均都造成较大的影响,品种‘魁绿’枝条的木质部比率最大、皮层比率最小,相对电导率和MDA含量变化幅度最小,内源激素含量GA3和ABA/GA3变化趋势较小,故其抗寒性比‘丰绿’和‘96-6’更强。 相似文献
95.
96.
The permeability of biological membranes to boric acid was investigated using the giant internodal cells of the charophyte
alga Chara corallina (Klein ex Will. Esk. R.D. Wood). The advantage of this system is that it is possible to distinguish between membrane transport
of boron (B) and complexing of B by plant cell walls. Influx of B was found to be rapid, with equilibrium between the intracellular
and extracellular phases being established after approximately 24 h when the external concentration was 50 μM. The intracellular
concentration at equilibrium was 55 μM, which is consistent with passive distribution of B across the membrane along with
a small amount of internal complexation. Efflux of B occurred with a similar half-time to influx, approximately 3 h, which
indicates that the intracellular B was not tightly complexed. The concentration dependence of short-term influx measured with
10B-enriched boric acid was biphasic. This was tentatively attributed to the operation of two separate transport systems, a
facilitated system that saturates at 5 μM, and a linear component due to simple diffusion of B through the membrane. V
max and K
m for the facilitated transport system were 135 pmol m−2 s−1 and 2 μM, respectively. The permeability coefficient for boric acid in the Chara plasmalemma estimated from the slope of the linear influx component was 4.4 × 10−7 cm s−1 which is an order of magnitude lower than computed from the ether:water partition coefficient for B.
Received: 14 August 2000 / Accepted: 16 September 2000 相似文献
97.
【目的】通过探究特异腐质霉角质酶-OMP25融合蛋白(HiC-OMP25)在不同大肠杆菌(Escherichia coli)菌株中的表达情况、底物降解情况、热稳定性及宿主菌细胞膜通透性与细胞表面疏水性,揭示表达HiC-OMP25时不同宿主菌的差异性,并进一步提高HiC-OMP25在大肠杆菌中的表达量。【方法】分别在E.coli BL21(DE3)及E.coli C43(DE3)中表达HiC-OMP25,并测定其对对硝基苯丁酸酯(4-nitrophenol butyrate,pNPB)、聚丙烯酸乙酯(polyethyl acrylate,PEA)的降解效果、50℃稳定性;测定表达HiC-OMP25时宿主菌的细胞膜通透性及细胞表面疏水性变化;共表达伴侣蛋白提高HiC-OMP25在E.coli C43(DE3)中的表达量。【结果】HiC-OMP25在E.coli BL21(DE3)与E.coli C43(DE3)中均成功表达并降解pNPB,但前者对PEA的降解效果及50 ℃稳定性均低于后者。同时,表达HiC-OMP25显著增强了E.coli BL21(DE3)的细胞膜通透性及细胞表面疏水性。HiC-OMP25与巯基氧化酶(Erv1p)、二硫键异构酶(DsbC)在E.coli C43(DE3)中共表达时,其表达量为原始菌株的2.14倍,且对pNPB及PEA均有良好的降解效果。【结论】异源表达时,HiC-OMP25在E.coli C43(DE3)中正确折叠,而在E.coli BL21(DE3)中未完全正确折叠;通过共表达伴侣蛋白提高了HiC-OMP25在E.coli C43(DE3)中的表达量,为以后HiC-OMP25的工业化生产及应用奠定了基础。 相似文献
98.
Apical root meristems and segments of root elongation zone were sampled from 4- to 5-day-old Zea mays L. seedlings. The vacuolar ATPase and pyrophosphatase, the tonoplast marker enzymes, and the tonoplast -, -, and -aquaporins were visualized by means of indirect immunofluorescent microscopy with the use of the respective antibodies. Following cell plasmolysis (700 mM mannitol, 2.5 h), the vacuolar ATPase and pyrophosphatase were detected in cell wall pores where plasmodesmata remained detached from the plasmolyzed protoplasts. This finding provides further evidence for existence of the vacuolar symplast in the elongation zone of maize root, which may ensure intercellular continuity of plant tissues. The pulsed NMR method was used to study the self-diffusion of water molecules. The diffusive decay in the root elongation zone was nonexponential, and it was transformed to three exponential terms with characteristic coefficients of self-diffusion; two of these coefficients (D
2 and D
3) characterize the water self-diffusion in the cytoplasmic and vacuolar symplasts of root, respectively. The root apical meristem was also investigated with NMR technique by virtue of paramagnetic doping of the apoplast. This approach allowed selective studying of water diffusion within the symplast compartments. Partial dehydration with PEG-6000, 12 and 20%, for 2.5 h and chemical stressors (ABA and salicylic acid, 0.1 mM, 24 h) were applied to modify water permeability of plasmodesmata and tonoplast aquaporins. The transcellular water permeability increased in the root meristem under the action of all stress factors. In the root elongation zone exposed to partial dehydration, the water exchange in the apoplast became the dominant component. Other stress factors affected water relations in different manners. ABA elevated the water permeability of the vacuolar symplast, in contrast to salicylic acid that decreased water conductance of both the cytoplasmic and vacuolar symplasts. 相似文献
99.
蔡海芳 《中国微生态学杂志》2018,30(4)
目的 观察双歧杆菌三联活菌散对儿童轮状病毒性肠炎(RVE)患者肠道菌群和肠黏膜通透性的影响。方法 选择2016年1月至2017年8月于丽水市中心医院儿科门诊治疗的RVE患儿94例,随机分为观察组和对照组各47例。两组患儿均予饮食调整、补液及纠正电解质紊乱和酸碱失衡等治疗。观察组患儿加用双歧杆菌三联活菌散1.0 g/次,3次/d,温水冲服。观察两组患儿治疗前后肠道菌群(双歧杆菌、乳杆菌和大肠埃希菌)数量和肠黏膜通透性指标[内毒素(ETX)和D-乳酸]的变化,并比较其临床疗效。结果 治疗72 h后,观察组患儿双歧杆菌和乳杆菌数量明显上升,大肠埃希菌数量明显下降(P0.05)。治疗后观察组患儿双歧杆菌、乳杆菌数量较对照组更高,大肠埃希菌数量较对照组更低(P<0.05)。两组患儿治疗后血清ETX和D-乳酸水平均明显下降(P<0.05),且观察组下降幅度更大(P<0.05);同时观察组患儿总有效率高于对照组(χ2=4.11,P<0.05)。结论 双歧杆菌三联活菌散治疗儿童RVE的疗效确切,其作用机制可能与其能增加双歧杆菌和乳杆菌等有益菌的数量,减少大肠埃希菌等致病菌的数量,改善患儿肠道微生态状况;并与其能降低肠黏膜通透性和减少肠液的分泌渗出相关。 相似文献
100.