全文获取类型
收费全文 | 3481篇 |
免费 | 418篇 |
国内免费 | 517篇 |
专业分类
4416篇 |
出版年
2024年 | 23篇 |
2023年 | 43篇 |
2022年 | 52篇 |
2021年 | 74篇 |
2020年 | 108篇 |
2019年 | 163篇 |
2018年 | 155篇 |
2017年 | 139篇 |
2016年 | 151篇 |
2015年 | 144篇 |
2014年 | 147篇 |
2013年 | 205篇 |
2012年 | 159篇 |
2011年 | 189篇 |
2010年 | 130篇 |
2009年 | 177篇 |
2008年 | 209篇 |
2007年 | 196篇 |
2006年 | 194篇 |
2005年 | 172篇 |
2004年 | 120篇 |
2003年 | 151篇 |
2002年 | 109篇 |
2001年 | 112篇 |
2000年 | 97篇 |
1999年 | 89篇 |
1998年 | 70篇 |
1997年 | 96篇 |
1996年 | 64篇 |
1995年 | 75篇 |
1994年 | 53篇 |
1993年 | 34篇 |
1992年 | 41篇 |
1991年 | 40篇 |
1990年 | 27篇 |
1989年 | 26篇 |
1988年 | 24篇 |
1987年 | 17篇 |
1986年 | 16篇 |
1985年 | 33篇 |
1984年 | 46篇 |
1983年 | 36篇 |
1982年 | 48篇 |
1981年 | 27篇 |
1980年 | 25篇 |
1979年 | 27篇 |
1978年 | 12篇 |
1977年 | 30篇 |
1976年 | 14篇 |
1975年 | 12篇 |
排序方式: 共有4416条查询结果,搜索用时 0 毫秒
41.
Samer Al-Samir Maximilian Prill Claudiu T. Supuran Gerolf Gros Volker Endeward 《Journal of enzyme inhibition and medicinal chemistry》2021,36(1):1602
We have studied the CO2 permeability of the erythrocyte membrane of the rat using a mass spectrometric method that employs 18 O-labelled CO2. The method yields, in addition, the intraerythrocytic carbonic anhydrase activity and the membrane HCO3− permeability. For normal rat erythrocytes, we find at 37 °C a CO2 permeability of 0.078 ± 0.015 cm/s, an intracellular carbonic anhydrase activity of 64,100, and a bicarbonate permeability of 2.1 × 10−3 cm/s. We studied whether the rat erythrocyte membrane possesses protein CO2 channels similar to the human red cell membrane by applying the potential CO2 channel inhibitors pCMBS, Dibac, phloretin, and DIDS. Phloretin and DIDS were able to reduce the CO2 permeability by up to 50%. Since these effects cannot be attributed to the lipid part of the membrane, we conclude that the rat erythrocyte membrane is equipped with protein CO2 channels that are responsible for at least 50% of its CO2 permeability. 相似文献
42.
43.
Meta-analysis: the past, present and future 总被引:1,自引:0,他引:1
44.
Hilde De Boeck Frank G. Loontiens Halina Lis Nathan Sharon 《Archives of biochemistry and biophysics》1984,234(1):297-304
Erythrina cristagalli agglutinin, a dimeric lectin [J. L. Iglesias, et al. (1982) Eur. J. Biochem.123, 247–252] was shown by equilibrium dialysis to be bivalent for 4-methylumbelliferyl-β-d-galactoside. Upon binding to the lectin, this ligand showed a difference absorption spectrum with two maxima (at 322 and 336 nm) of equal intensity (Δ? = 1.2 × 103m?1 cm?1). A similar spectrum with a comparable value of Δ? was obtained with 4-methylumbelliferyl-N-acetyl-β-d-galactosaminide. Binding of methyl-α-d-galactoside, lactose, and N-acetyllactosamine all produced small but equally intense protein difference spectra with a maximum (Δ? = 2.8 × 102 M?1 cm?1) at 291.6 nm. Upon binding of N-dansyl-d-galactosamine to the lectin, there was a fivefold increase in fluorescence intensity of this ligand. The association constant for N-dansyl-d-galactosamine was caused by a very favorable ΔS° of the dansyl group without affecting the strictly carbohydrate-specific character of binding. N-Dansyl-d-galactosamine was employed as a fluorescent indicator ligand in substitution titrations. This involved the use of simple carbohydrates, N-acetyllactosamine, and oligosaccharides which occur in the carbohydrate units of N-glycoproteins; the latter were Gal(β → 4)GlcNAc(β1 → 2)Man, Gal(β1 → 4)GlcNAc(β1 → 6)Man, and Gal(β1 → 4)GlcNAc(β1 → 6)[Gal(β1 → 4)GlcNAc(β1 → 2)]Man. The titrations were performed at two temperatures to determine the thermodynamic parameters. In the series N-acetyl-d-galactosamine, methyl-α-d-galactoside, and lactose, ?ΔH° increased from 24 to 41 kJ mol?1; it increased further for N-acetyllactosamine and then remained unchanged for the N-acetyllactosamine-containing oligosaccharides (55 ± 1 kJ mol?1). This indicated that the site specifically accommodated the disaccharide structure with an important contribution of the 2-acetamido group in the penultimate sugar. Beyond this, no additional contacts seemed to be formed. This conclusion also followed from considerations of ΔS° values which became more unfavorable in the above series (?23 to ?101 ± 4 J mol?1 K?1); the most negative value of ΔS° was observed with N-acetyllactosamine and the three N-acetyllactosamine-containing oligosaccharides. 相似文献
45.
Angiostatin production in cultivation of recombinant Pichia pastoris fed with mixed carbon sources 总被引:4,自引:0,他引:4
A recombinant strain of Pichia pastoris with a phenotype of MutS was used to produce angiostatin. Due to the low methanol consumption rate of this strain, both methanol and glycerol feedings, that produced oscillation in dissolved O2 concentration, were used during the expression phase to improve cell growth and angiostatin expression. However, enhanced cell growth led to nitrogen limitation that suppressed further production of angiostatin, but addition of ammonia allowed angiostatin concentration to reach 108 mg l–1 after an expression period of 96 h. The ratio of consumed glycerol to methanol of 1.5:1 (w/w) in the expression phase suggested that methanol played an important role in the metabolism of carbon sources. 相似文献
46.
UV-B辐射引起的绿豆幼苗叶片Rubisco量降低与H2O2含量升高有关 总被引:6,自引:0,他引:6
为了探讨UV-B辐射引起的Rubisco含量降低的可能机制,研究了两个绿豆品种(秦豆-20和中绿-1)幼苗在UV-B辐射下叶片Rubisco含量、蛋白水解酶活性和H2O2含量的变化.结果表明:UV-B辐射显著加速了两个绿豆品种幼苗叶片H2O2含量和蛋白水解酶活性上升,使Rubiscco含量下降.秦豆-20品种在UV-B辐射下H2O2含量和蛋白水解酶活性的上升程度明显大于中绿-1,相应其Rubisco含量的下降程度也大于中绿-1.抗坏血酸处理能明显降低UV-B辐射下两品种幼苗叶片H2O2含量,同时明显抑制蛋白水解酶活性的上升及Rubisco含量的下降.结果说明UV-B辐射诱导Rubisco含量的降低可能通过提高H2O2水平从而加强蛋白水解酶系统的活化而加速了Rubisco的降解. 相似文献
47.
MOSIX is a cluster management system that supports preemptive process migration. This paper presents the MOSIX Direct File System Access (DFSA), a provision that can improve the performance of cluster file systems by allowing a migrated process to directly access files in its current location. This capability, when combined with an appropriate file system, could substantially increase the I/O performance and reduce the network congestion by migrating an I/O intensive process to a file server rather than the traditional way of bringing the file's data to the process. DFSA is suitable for clusters that manage a pool of shared disks among multiple machines. With DFSA, it is possible to migrate parallel processes from a client node to file servers for parallel access to different files. Any consistent file system can be adjusted to work with DFSA. To test its performance, we developed the MOSIX File-System (MFS) which allows consistent parallel operations on different files. The paper describes DFSA and presents the performance of MFS with and without DFSA. 相似文献
48.
The antigenic O-polysaccharide of the lipopolysaccharide produced by Actinobacillus pleuropneumoniae serotype 14 was shown by chemical analysis and 1D and 2D nuclear magnetic resonance methods to be a high-molecular-mass polymer of a repeating disaccharide unit composed of a chain of (1-->5)-linked beta-D-galactofuranose (beta-D-Galf) residues substituted at their O-2 positions by alpha-D-galactopyranose residues (D-Galp) (1:1): [formula: see text]. 相似文献
49.
温度和水分对草甸草原土壤氧化亚氮产生速率的调控 总被引:4,自引:0,他引:4
通过试验室模拟试验,研究了调控草甸草原黑钙土N2O产生速率的因素.结果表明,土壤水分对草甸草原土壤N2O产生速率有重要影响(F=6.149,P<0.01).但相关性分析表明,草甸草原土壤含水量与N2O的产生速率间线性相关不显著.就土壤N2O的产生速率而言,温度的影响远低于土壤水分的影响(F=2.275,P=0.106).土壤水分和温度对不同生长阶段的草甸草原土壤N2O产生速率的调控作用主要表现为阶段性的多阶多项式关系:y=ax3+bx2+cx+d,其中a、b、c、d是常数. 相似文献
50.
I. I. Pottosin P. R. Andjus D. Vučelić G. N. Berestovsky 《The Journal of membrane biology》1993,136(2):113-124
We studied the effects of H2O/D2O substitution on the permeation and gating of the large conductance Ca2+-activated K+ channels inChara gymnophylla droplet membrane using the patchclamp technique. The selectivity sequence of the channel was: K+>Rb+≫Li+, Na+, Cs+ and Cl−. The conductance of this channel in symmetric 100mm KCl was found to be 130 pS. The single channel conductance was decreased by 15% in D2O as compared to H2O. The blockade of channel conductance by cytosolic Ca2+ weakened in D2O as a result of a decrease in zero voltage Ca2+ binding affinity by a factor of 1.4. Voltage-dependent channel gating was affected by D2O primarily due to the change in Ca2+ binding to the channel during the activation step. The Hill coefficient for Ca2+ binding was 3 in D2O and around 1 in H2O. The values of the Ca2+ binding constant in the open channel conformation were 0.6 and 6 μm in H2O and D2O, respectively, while the binding in the closed conformation was much less affected by D2O. The H2O/D2O substitution did not produce a significant change in the slope of channel voltage dependence but caused a shift as large
as 60 mV with 1mm internal Ca2+. 相似文献