全文获取类型
收费全文 | 22026篇 |
免费 | 381篇 |
国内免费 | 665篇 |
专业分类
23072篇 |
出版年
2023年 | 1164篇 |
2022年 | 811篇 |
2021年 | 562篇 |
2020年 | 364篇 |
2019年 | 744篇 |
2018年 | 705篇 |
2017年 | 638篇 |
2016年 | 207篇 |
2015年 | 230篇 |
2014年 | 369篇 |
2013年 | 402篇 |
2012年 | 178篇 |
2011年 | 1497篇 |
2010年 | 428篇 |
2009年 | 475篇 |
2008年 | 485篇 |
2007年 | 565篇 |
2006年 | 503篇 |
2005年 | 487篇 |
2004年 | 661篇 |
2003年 | 477篇 |
2002年 | 682篇 |
2001年 | 1063篇 |
2000年 | 1016篇 |
1999年 | 1062篇 |
1998年 | 1093篇 |
1997年 | 986篇 |
1996年 | 489篇 |
1995年 | 246篇 |
1994年 | 148篇 |
1993年 | 133篇 |
1992年 | 110篇 |
1991年 | 127篇 |
1990年 | 99篇 |
1989年 | 101篇 |
1988年 | 101篇 |
1987年 | 91篇 |
1985年 | 205篇 |
1984年 | 474篇 |
1983年 | 461篇 |
1982年 | 375篇 |
1981年 | 348篇 |
1980年 | 363篇 |
1979年 | 336篇 |
1978年 | 206篇 |
1977年 | 176篇 |
1976年 | 168篇 |
1975年 | 154篇 |
1974年 | 126篇 |
1973年 | 94篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
M. Couturier F. Lemonnier M. Conti M. Feneant-Thibault A. Lemonnier 《In vitro cellular & developmental biology. Plant》1990,26(1):29-32
Summary 2-Deoxy-d-glucose (2-DOG) uptake was tested in human fibroblast cultures in the presence and absence of vitamin E. Addition of 10 μg/ml
vitamin E to the culture medium significantly reduced this uptake for 2-DOG concentrations of 0.005, to 10 mmol/liter (P≤0.01). The decrease of 2-DOG uptake was inversely proportional to the rise in 2-DOG concentration (P≤0.01). The presence of vitamin E reduced by 71% the average cellular level of lipid peroxides (expressed as thiobarbituric
acid reactive substances) and caused a small but significant decrease in the cholesterol concentration (P≤0.01). These last results might explain the decrease in 2-DOG uptake observed in the presence of vitamin E. 相似文献
72.
We have previously reported that intralobular salivary duct cells contain an amiloride-sensitive Na+ conductance (probably located in the apical membranes). Since the amiloride-sensitive Na+ conductances in other tight epithelia have been reported to be controlled by extracellular (luminal) Na+, we decided to use whole-cell patch clamp techniques to investigate whether the Na+ conductance in salivary duct cells is also regulated by extracellular Na+. Using Na+-free pipette solutions, we observed that the whole-cell Na+ conductance increased when the extracellular Na+ was increased, whereas the whole-cell Na+ permeability, as defined in the Goldman equation, decreased. The dependency of the whole-cell Na+ conductance on extracellular Na+ could be described by the Michaelis-Menten equation with a K
m
of 47.3 mmol/1 and a maximum conductance (G
max) of 2.18 nS. To investigate whether this saturation of the Na+ conductance with increasing extracellular Na+ was due to a reduction in channel activity or to saturation of the single-channel current, we used fluctuation analysis of
the noise generated during the onset of blockade of the Na+ current with 200 μmol/l 6-chloro-3,5-diaminopyrazine-2-carboxamide. Using this technique, we estimated the single channel
conductance to be 4 pS when the channel was bathed symmetrically in 150 mmol/l Na+ solutions. We found that Na+ channel activity, defined as the open probability multiplied by the number of available channels, did not alter with increasing
extracellular Na+. On the other hand, the single-channel current saturated with increasing extracellular Na+ and, consequently, whole-cell Na+ permeability declined. In other words, the decline in Na+ permeability in salivary duct cells with increasing extracellular Na+ concentration is due simply to saturation of the single-channel Na+ conductance rather than to inactivation of channel activity.
Received: 27 July 1995/Revised: 7 December 1995 相似文献
73.
M. Torras-Llort R. Ferrer J.F. Soriano-García M. Moretó 《The Journal of membrane biology》1996,152(3):183-193
The properties of l-lysine transport in chicken jejunum have been studied in brush border membrane vesicles isolated from 6-wk-old birds. l-lysine uptake was found to occur within an osmotically active space with significant binding to the membrane. The vesicles
can accumulate l-lysine against a concentration gradient, by a membrane potential-sensitive mechanism. The kinetics of l-lysine transport were described by two saturable processes: first, a high affinity-transport system (K
mA= 2.4 ± 0.7 μmol/L) which recognizes cationic and also neutral amino acids with similar affinity in the presence or absence
of Na+ (l-methionine inhibition constant KiA, NaSCN = 21.0 ± 8.7 μmol/L and KSCN = 55.0 ± 8.4 μmol/L); second, a low-affinity transport mechanism (KmB= 164.0 ± 13.0 μmol/L) which also recognizes neutral amino acids. This latter system shows a higher affinity in the presence
of Na+ (KiB for l-methionine, NaSCN = 1.7 ± 0.3 and KSCN = 3.4 ± 0.9 mmol/L). l-lysine influx was significantly reduced with N-ethylmaleimide (0.5 mmol/L) treatment. Accelerative exchange of extravesicular labeled l-lysine was demonstrated in vesicles preloaded with 1 mmol/L l-lysine, l-arginine or l-methionine. Results support the view that l-lysine is transported in the chicken jejunum by two transport systems, A and B, with properties similar to those described
for systems b
0,+ and y+, respectively.
Received: 14 August 1995/Revised: 2 April 1996 相似文献
74.
We studied the characteristics of the basal and antidiuretic hormone (arginine vasotocin, AVT)-activated whole cell currents
of an aldosterone-treated distal nephron cell line (A6) at two different cytosolic Ca2+ concentrations ([Ca2+]
c
, 2 and 30 nm). A6 cells were cultured on a permeable support filter for 10 ∼ 14 days in media with supplemental aldosterone (1 μm). At 30 nm [Ca2+]
c
, basal conductances mainly consisted of Cl− conductances, which were sensitive to 5-nitro-2-(3-phenylpropylamino)-benzoate. Reduction of [Ca2+]
c
to 2 nm abolished the basal Cl− conductance. AVT evoked Cl− conductances at 2 as well as 30 nm [Ca2+]
c
. In addition to Cl− conductances, AVT induced benzamil-insensitive nonselective cation (NSC) conductances. This action on NSC conductances was
observed at 30 nm [Ca2+]
c
but not at 2 nm [Ca2+]
c
. Thus, cytosolic Ca2+ regulates NSC and Cl− conductances in a distal nephron cell line (A6) in response to AVT. Keeping [Ca2+]
c
at an adequate level seems likely to be an important requirement for AVT regulation of ion conductances in aldosterone-treated
A6 cells.
Received: 6 May 1996/Revised: 28 June 1996 相似文献
75.
Received: 18 April 1996/Revised: 26 June 1996 相似文献
76.
Tenshuk A. Kadima Susan E. Jensen Michael A. Pickard 《Journal of industrial microbiology & biotechnology》1995,14(1):35-40
Summary -(l--Aminoadipyl)-l-cysteinyl-d-valine (ACV)-synthetase fromStreptomyces clavuligerus was studied under conditions that enabled the reuse of the enzyme. Coupling of ACV-synthetase to DEAE-Trisacryl and aminopropyl-glass resulted in an immobilized enzyme product of little or no catalytic activity. However, an enzyme reactor was designed by physical confinement of partially-purified ACV-synthetase in an ultrafiltration cell. This system was stimulated by phosphoenolpyruvate at lower concentrations of ATP, an effect not observed with purified enzyme. Up to 30% conversion of the limiting substrate, cysteine, to ACV occurred under semi-continuous conditions. Reaction products were investigated as potential inhibitors: AMP was the most inhibitory, but only when used at concentrations in excess of those produced in reaction mixtures. Under a nitrogen atmosphere, both product and enzyme stabilities were greatly improved and the enzyme retained 45–46% of its initial activity after five uses at room temperature during a 24-h period. Extrapolations based on these data suggest that 1.3 g partially purified enzyme (0.13 U g–1) would be capable of producing 411 mg of ACV in a 1-L reaction mixture in this period. 相似文献
77.
本文引用等效模型,对激光消融过程进行了推导与计算,和实验结果比较表明,所得公式与实验结果能较好相符,可用于描述紫外激光消融过程。 相似文献
78.
Energetic constraints and foraging efficiency 总被引:2,自引:1,他引:1
Previous research considers foraging options that differ interms of their gross rate of gain b and rate of energy expenditurec. This research argues that maximizing efficiency b/c willmaximize net energetic gain when there is an upper limit onthe amount of energy that can be assimilated. This analysisdoes not include the expenditure during the time for which theanimal is unable to forage because of this constraint. Whenthis expenditure is included, maximizing efficiency is no longeroptimal. Instead the best feeding option is the one with thehighest value of b/(c c1), where c, is the metabolicrate when the animal is not foraging. 相似文献
79.
80.
The severity of the incidence of the fungal disease, potato scab, varies with different soil groups at the same soil pH. At a soil pH of 5.3, potato scab is easily controlled in soils of western Hokkaido (soil group A) by simply decreasing soil pH, but in soils from eastern Hokkaido (soil group B) it is not so easily controlled. The difference appears to be due to higher levels and exchangeable aluminium in Group A.Addition of sufficient aluminium or ferrous sulfate to a group B soil decreased the incidence of potato scab in a field experiment. Higher levels of aluminium sulfate depressed crop production. It is concluded that aluminium ions control the incidence of potato scab in acid soils. It is suggested that, in soils with low exchange acidity Y1, potato scab can be controlled by adding sufficient aluminium to increase their exchange acidity Y1 to above 7–8. 相似文献