全文获取类型
收费全文 | 2344篇 |
免费 | 176篇 |
出版年
2021年 | 40篇 |
2020年 | 20篇 |
2019年 | 24篇 |
2018年 | 45篇 |
2017年 | 32篇 |
2016年 | 52篇 |
2015年 | 91篇 |
2014年 | 101篇 |
2013年 | 121篇 |
2012年 | 135篇 |
2011年 | 141篇 |
2010年 | 96篇 |
2009年 | 70篇 |
2008年 | 114篇 |
2007年 | 107篇 |
2006年 | 111篇 |
2005年 | 81篇 |
2004年 | 106篇 |
2003年 | 96篇 |
2002年 | 85篇 |
2001年 | 44篇 |
2000年 | 60篇 |
1999年 | 47篇 |
1998年 | 22篇 |
1997年 | 24篇 |
1996年 | 20篇 |
1995年 | 23篇 |
1994年 | 17篇 |
1993年 | 23篇 |
1992年 | 27篇 |
1991年 | 27篇 |
1990年 | 25篇 |
1989年 | 24篇 |
1988年 | 28篇 |
1987年 | 23篇 |
1986年 | 19篇 |
1985年 | 18篇 |
1984年 | 27篇 |
1983年 | 17篇 |
1982年 | 17篇 |
1981年 | 12篇 |
1980年 | 19篇 |
1979年 | 20篇 |
1978年 | 19篇 |
1977年 | 13篇 |
1976年 | 23篇 |
1975年 | 11篇 |
1974年 | 16篇 |
1973年 | 16篇 |
1970年 | 12篇 |
排序方式: 共有2520条查询结果,搜索用时 46 毫秒
51.
Effect of potassium on the water potential, the pressure potential, the osmotic potential and cell elongation in leaves of Phaseolus vulgaris 总被引:1,自引:0,他引:1
The effect of potassium on the water potential, the osmotic potential and the pressure potential in younger and older leaves of Phaseolus vulgaris grown in hydroponic culture was studied. Inadequate potassium supply resulted in an increase of the osmotic potential. In the older leaves the water potential was raised, in the younger leaves the pressure potential was depressed in the treatment insufficiently supplied with potassium as compared with leaves with an adequate potassium supply. Cell size of the younger leaves was smaller in the treatment with the low K+ supply in comparison with the leaves well supplied with K+ . Potassium had a beneficial effect on plant growth, especially on fresh matter production. The water status of leaves (water content, pressure potential, osmotic potential) responded more sensitively to potassium supply than dry matter production. Besides organic N and organic anions, K+ was the most abundant solute found in the press sap of the leaves. From the results it is concluded that K+ is indispensible for attaining an optimum potential (turgor) in young leaves which in turn has an impact on plant growth. 相似文献
52.
Konrad Heintze Karl-Uwe Petersen Peter Olles Sethna H. Saverymuttu John R. Wood 《The Journal of membrane biology》1979,45(1-2):43-59
Summary Fluid transport and net fluxes of Na, K, Cl and HCO3 by guinea pig gallbladder were investigatedin vitro. A perfused gallbladder preparation was devised to simultaneously study unidirectional fluxes of22Na and36Cl. The net Cl flux exceeded the net Na flux during fluid absorption in the presence of HCO3. This Cl excess was counter-balanced by a net HCO3 secretion: a HCO3–Cl exchange. PGE1 reversed the direction of fluid transport and abolished the net Cl flux. The magnitude of the HCO3 secretion remained unchanged, but shifted from a HCO3–Cl exchange to a net secretion of NaHCO3 and KHCO3. Furosemide inhibited both the HCO3–Cl exchange and HCO3 secretion after PGE1 without influencing fluid absorption. Ouabain inhibited the HCO3–Cl exchange as well as fluid absorption; only the effect on the HCO3 secretion was entirely reversible. Secreted HCO3 appeared not to be derived from metabolic sources since HCO3 secretion was abolished in a HCO3-free bathing medium. HCO3 secretion was also dependent on the Na concentration of the bathing fluid. Three lines of evidence are presented in favor of an active HCO3 secretion in guinea pig gallbladder. HCO3 is secreted against: (i) a chemical gradient, (ii) an electrical gradient and (iii) the direction of fluid movement under control conditions. 相似文献
53.
54.
Hyperammonemia interferes with normal brain function. The effect of ammonia on free and membrane-bound lysosomal enzymes and on mucopolysaccharide metabolism was studied in cultured rat brain cells (ROC-1, hybridoma between C6-astrocytoma and oligodendrocytes). Intralysosomal ammoniagenesis was achieved from urea by endocytosed Jackbean urease followed by incubation of the cultures with urea. The intralysosomal location of urease was evidenced by the protective effects of leupeptin and urea on the stability of intracellular urease. Ammonia formed from urea resulted in an increased secretion of lysosomal arylsulfatase-A (AS-A), but not of the membrane-bound lysosomal beta-glucosidase into the culture medium, thus intralysosomal AS-A activity decreased. Lysosomal, membrane-bound beta-glucosidase activity increased, presumably due to intralysosomal proteolytic protection following an increased lysosomal pH. Intralysosomal ammoniagenesis temporarily impaired 35SO4-glycosaminoglycan degradation of prelabeled cells. The results support the hypothesis that hyperammonemic states may interfere with lysosomal functions in vivo as well in cultured cells. 相似文献
55.
Hans Holtschmidt Konrad Sandhoff Werner Fürst Hae Young Kwon Doris Schnabel Kunihiko Suzuki 《FEBS letters》1991,280(2):267-270
The organization of 14 exons covering 97% of the cDNA sequence of human cerebroside sulfate activator protein precursor has been determined from two overlapping EMBL-4 human genomic clones extending over 17kb. All exons and exon/intron splice junctions and five introns were sequenced. Exon 8 consists of only 9 bp and is involved in alternative splicing which generates three different mRNAs of cerebroside sulfate activator precursor. 相似文献
56.
Uptake and Storage of Choline by Rat Brain: Influence of Dietary Choline Supplementation 总被引:1,自引:1,他引:0
In order to elucidate the regulation of the levels of free choline in the brain, we investigated the influence of chronic and acute choline administration on choline levels in blood, CSF, and brain of the rat and on net movements of choline into and out of the brain as calculated from the arteriovenous differences of choline across the brain. Dietary choline supplementation led to an increase in plasma choline levels of 50% and to an increase in the net release of choline from the brain as compared to a matched group of animals which were kept on a standard diet and exhibited identical arterial plasma levels. Moreover, the choline concentration in the CSF and brain tissue was doubled. In the same rats, the injection of 60 mg/kg choline chloride did not lead to an additional increase of the brain choline levels, whereas in control animals choline injection caused a significant increase; however, this increase in no case surpassed the levels caused by chronic choline supplementation. The net uptake of choline after acute choline administration was strongly reduced in the high-choline group (from 418 to 158 nmol/g). Both diet groups metabolized the bulk (greater than 96%) of newly taken up choline rapidly. The results indicate that choline supplementation markedly attenuates the rise of free choline in the brain that is observed after acute choline administration. The rapid metabolic choline clearance was not reduced by dietary choline load. We conclude that the brain is protected from excess choline by rapid metabolism, as well as by adaptive, diet-induced changes of the net uptake and release of choline. 相似文献
57.
Jürgen Bachmann Monika Feldmer Ursula Ganten Günter Stock Detlev Ganten 《The Journal of steroid biochemistry and molecular biology》1991,40(4-6):511-515
The prevalence of hypertension in men is higher than in women and the onset of this disease is earlier in male than in female subjects. In spontaneously hypertensive rats, males also have higher blood pressures than females. Evidence from epidemiological, physiological, molecular biological and morphological studies concerning this sexual dimorphism is reviewed. We demonstrate that the gonadal steroids testosterone and estrogen have important effects on the gene regulation of the renin-angiotensin system. This may in part contribute to the sexual dimorphism in blood pressure control. The direct effect of steroid hormones on genes related to hypertension provides a suitable paradigm to improve our understanding of molecular and cellular mechanisms of cardiovascular control. 相似文献
58.
Primary cultured neurons were fractionated using sucrose density gradients. The activities of four sialyltransferases (GM3, GD3, GD1a, and GT1a synthase) involved in ganglioside biosynthesis were assayed in the collected fractions. The distribution of GM3 synthase coincided with that of mannosidase II, an enzyme assumed to be a cis-Golgi marker. Both enzymes were mainly associated with the more dense fraction. GD1a and GT1a synthase activities, on the other hand, were mainly recovered in the less dense fraction. Moreover, they were colocalized with thiamine pyrophosphatase, an enzyme assumed to be a marker of the late Golgi (trans-Golgi and trans-Golgi network). GD3 synthase activity was equally distributed between both fractions. These results are integrated in a model of ganglioside biosynthesis. 相似文献
59.
Cells obtained from bronchoalveolar lavage, or neutrophils of peripheral blood of dog, were incubated with the canine surfactant-associated protein A (SP-A). A significant decrease of the production of Superoxide anion was observed after subsequent stimulation with phorbol-12-myristate-13-acetate (PMA) as measured by the lucigenin-dependent chemiluminesence (CL). Several other proteins used for control experiments did not decrease lucigenin-dependent CL, indicating a specific effect of SP-A on phagocytes. Treatment of SP-A with collagenase prior to incubation with neutrophils destroyed the depleting effect on oxygen radical production of PMA-stimulated cells. We propose that SP-A acts as a regulatory factor of the respitatory burst of alveolar macrophages and neutrophils in the lungs. The inhibitory effect of SP-A is down-regulated by collagenase released from stimulated alveolar macrophages. 相似文献
60.
Purification and properties of acetyl-CoA:L-glutamate N-acetyltransferase from human liver. 总被引:1,自引:1,他引:0 下载免费PDF全文
Acetyl-CoA:L-glutamate N-acetyltransferase (amino acid acetyltransferase, EC 2.3.1.1) was isolated from human liver mitochondria by precipitation with (NH4)2SO4 and chromatography on hydroxyapatite, DEAE-cellulose and Sephacryl 300. This gave a 360-fold purification. The molecular weight was estimated to be approx. 190 000. The kinetic properties in the absence of arginine are compatible with a rapid-equilibrium random Bi Bi mechanism. The estimated constants are: for the substrates Km,acetyl-CoA 4.4 mM, Ki,acetyl-CoA 4.7 mM, Km,glutamate 8.1 mM, Ki,glutamate 8.8 mM; for the products, Ki,acetylglutamate 0.28 mM, Ki,CoA 5.6 mM. The rate constant for the forward direction is 1.24s-1. If in vivo the constants are of the same order of magnitude as in vitro, the synthesis of N-acetylglutamate, an obligate activator of the first step of urea synthesis, can be expected to occur in the mitochondrion under conditions where the amino acid acetyltransferase is not saturated by its substrates. The regulation of the first step of urea synthesis could thus depend mainly on the intramitochondrial substrate and perhaps product concentrations of amino acid acetyltransferase. 相似文献