全文获取类型
收费全文 | 619994篇 |
免费 | 70725篇 |
国内免费 | 449篇 |
专业分类
691168篇 |
出版年
2018年 | 5692篇 |
2016年 | 7441篇 |
2015年 | 9704篇 |
2014年 | 11438篇 |
2013年 | 16546篇 |
2012年 | 18297篇 |
2011年 | 19189篇 |
2010年 | 12939篇 |
2009年 | 11909篇 |
2008年 | 16854篇 |
2007年 | 17327篇 |
2006年 | 16313篇 |
2005年 | 15687篇 |
2004年 | 15230篇 |
2003年 | 15104篇 |
2002年 | 14575篇 |
2001年 | 29042篇 |
2000年 | 29181篇 |
1999年 | 22787篇 |
1998年 | 7643篇 |
1997年 | 8208篇 |
1996年 | 7868篇 |
1995年 | 7307篇 |
1994年 | 7199篇 |
1993年 | 7233篇 |
1992年 | 19105篇 |
1991年 | 18725篇 |
1990年 | 18272篇 |
1989年 | 17928篇 |
1988年 | 16482篇 |
1987年 | 15852篇 |
1986年 | 14535篇 |
1985年 | 14868篇 |
1984年 | 12206篇 |
1983年 | 10527篇 |
1982年 | 7855篇 |
1981年 | 7105篇 |
1980年 | 6747篇 |
1979年 | 11436篇 |
1978年 | 9019篇 |
1977年 | 8243篇 |
1976年 | 7915篇 |
1975年 | 8819篇 |
1974年 | 9221篇 |
1973年 | 9154篇 |
1972年 | 8166篇 |
1971年 | 7578篇 |
1970年 | 6476篇 |
1969年 | 6097篇 |
1968年 | 5660篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Quince (Cydonia oblonga Mill.), pear (Pyrus communis L.) and olive (Olea europaea L.) genotypes were evaluated for their tolerance to iron deficiency stress by growing young plants in three types of aerated nutrient solutions: (1) with iron, (2) without iron or (3) low in iron and with 10 mM bicarbonate. Plants were obtained either from rooted softwood cuttings or from germination of seeds. The degree of tolerance was evaluated with several indices: (1) the chlorophyll content, (2) the root Fe3+ reducing capacity and (3) the whole plant relative growth. Fifteen hours before Fe3+ reducing capacity determination, iron was applied to the roots of plants with iron-stress, since this method resulted in increasing the reductase activity. All quince and pear genotypes increased the root Fe3+ reducing capacity when grown in the treatments for iron-stress, in relation to control plants of the same genotypes. In olive cultivars, the Fe3+ reducing capacity was lower in the iron-stress treatments than in the control one. Studying the relationship between relative growth and chlorophyll content for each genotype under iron-stress, in relation to both indices in control plants, a classification of species and genotypes was established. According to that, most olive cultivars and some pear rootstocks and cultivars appear more iron-efficient than quince rootstocks. Our study shows that in some woody species, determining root Fe3+ reducing capacity is not the best method to establish tolerance to iron deficiency stress. 相似文献
962.
P D Ellis P S Marchetti P Strang J D Potter 《The Journal of biological chemistry》1988,263(21):10284-10288
The binding of cadmium to the calcium binding subunit of skeletal troponin (STnC) has been reinvestigated using direct binding methods and fluorescent derivatives. These data provide straightforward explanations of the observed titration behavior in the 113Cd NMR (Ellis, P.D., Strang, P., and Potter, J.D. (1984) J. Biol. Chem. 259, 10348-10356). Further, fluorescent derivatives of skeletal troponin C provide an excellent means of establishing a sequence assignment for the resonances observed in the 113Cd NMR. The results of these experiments demonstrate that sites I and II, the Ca2+ regulatory sites, can be assigned to resonances at -108.5 and -101.5 ppm, respectively. Sites III and IV, the structural sites, are assigned to resonances -112.8 and -106.8 ppm, respectively. These data are discussed in terms of recent structural findings and speculations. 相似文献
963.
D W Hill K J Cureton S C Grisham M A Collins 《European journal of applied physiology and occupational physiology》1987,56(2):206-211
The purpose of this study was to determine the effect of training on the rating of perceived exertion (RPE) at the ventilatory threshold. College students were assigned to either training (n = 17) or control (n = 10) groups. Trainers completed 18 interval training sessions (five X 5 min cycling at 90-100% VO2max) and 8 continuous training sessions (40 min running or cycling) in 6 weeks. Pre- and post-training, cardiorespiratory, metabolic, and perceptual variables were measured at the ventilatory threshold during graded exercise tests on a cycle ergometer. Ventilatory threshold was that point above which VE X VO2-1 increased abruptly relative to work rate. Post-training means of trained and control subjects were compared using analysis of covariance, with pre-training values as covariates. Following training, the adjusted means for the trained subjects were significantly greater (p less than 0.05) than for controls for VO2max (6%), and for work rate (20%), VO2 (23%), and %VO2max (13%) at the ventilatory threshold. However, adjusted means for RPE at the ventilatory threshold were not significantly different (2%). Both before and after training, exercise at the ventilatory threshold was perceived as 'somewhat hard' to 'hard' (RPE = 13-15) by both groups. The relationship between RPE and %VO2max was altered by training, with trained subjects having a lower RPE at a given %VO2max. It is concluded that RPE at the ventilatory threshold is not affected by training, despite that after training the ventilatory threshold occurs at a higher work rate and is associated with higher absolute and relative metabolic and cardiorespiratory demands. 相似文献
964.
In the present study, we investigated the mechanism by which the antidiabetic drug phenformin increases insulin binding to its receptors in IM-9 human cultured lymphocytes. After a 24-hr preincubation, phenformin induced a twofold increase in specific 125I-insulin binding, and removal of phenformin was followed 6 hr later by a return in binding to control levels. This effect of phenformin on insulin binding was not a consequence of either inhibition of cell growth, changes in cellular cyclic adenosine monophosphate (AMP) levels, or changes in guanosine triphosphate (GTP) content. Since phenformin is known to inhibit various aspects of cellular energy metabolism, the relationship between 125I-insulin binding and energy metabolism in IM-9 cells was investigated. The phenformin-induced increase in insulin binding to IM-9 cells was related to a time- and dose-dependent decrease in ATP levels. Other agents that lowered ATP levels, including antimycin, dinitrophenol, and 2-deoxyglucose, also raised insulin binding. These studies indicated, therefore, that phenformin enhances insulin binding to receptors on IM-9 cells and that this effect on insulin receptors may be related to alterations in metabolic functions that are reflected by a lowering of ATP levels. 相似文献
965.
D Gloag 《BMJ (Clinical research ed.)》1982,284(6328):1509-1510
966.
Abstract: The enzymatic hydrolysis of UDP-galactose in rat and calf brain was studied. The hydrolysis occurs in two steps: The first is the conversion of UDP-galactose to galactose-1-phosphate catalyzed by nucleotide pyrophosphatase (EC 3.6.1.9), and the second is the conversion of the latter to free galactose by alkaline phosphatase (EC 3.1.3.1). The overall conversion has a pH optimum of 9.0, but there is considerable activity at pH 7.4, which is the optimum for UDP-galactose:ceramide galactosyltransferase in the synthesis of cerebrosides. Preparations from cytosol from calf brain cerebellum or stem that were enriched in UDP-galactose hydrolytic activity inhibit cerebroside synthesis under conditions optimal for the synthesis. Microsome-rich and nuclear debris fractions contain the highest apparent specific activity among the subcellular fractions studied. Hydrolysis of UDP-galactose occurs in all areas of brain, brainstem having the highest activity. The apparent specific activity in jimpy mouse brain homogenate is nearly twice as high as in the control brain homogenate. 相似文献
967.
R C Strange B T Chapman J D Johnston I A Nimmo I W Percy-Robb 《Biochimica et biophysica acta》1979,573(3):535-545
1. The subcellular distribution of conjugates of cholic acid and chenodeoxycholic acid between cytosol, nuclei, mitochondria and microsomes in rat liver has been determined. 2. The partition coefficients for the distribution of these bile acids between subcellular fractions and buffer have been measured and used to construct a compartmental model of the amounts of conjugated bile acids present in the different subcellular organelles in vivo. 3. This model indicates that a large percentage of the bile acid in the rat liver is found in the nuclear fraction; 42% of the cholic acid conjugates and 27% of the chenodeoxycholic acid conjugates. Substantial amounts of bile acid are also present in microsomes and mitochondria suggesting that published estimates of the amounts of bile acids in these fractions are underestimates. 4. The model also allows the amount of bile acid which is in free solution in cytosol to be determined; 10.9% of the cholic acid conjugates and 4.1% of the chenodeoxycholic acid conjugates in rat liver were present in this fraction. Knowlege of the amount of free bile acid allows possible roles of the cytosolic bile binding proteins to be assessed. 相似文献
968.
Eric J. Engstrom Ph. D. 《PSN》2003,1(1):53-61
Recent studies on Emil Kraepelin’s experimental work have focused on the relationship between the psychological experiment and his nosological delineation of dementia praecox and manic-depressive disorders. Interpreters have argued variously that the psychological experiment had either no influence on the nosology, or that it was the very precondition of the nosology, or that it biased the nosology in favor of organic disorders. This article calls into question this historiographic affiliation of experiment and nosology. It argues that by framing Kraepelin’s experimental research agenda solely in terms of his nosology, interpreters have overlooked other important personal, institutional, diagnostic, and professional motives governing that research. Kraepelin’s early career crisis, his frustrations working in overcrowded institutions, and his efforts to enhance the professional stature of psychiatry all influenced his experimental research agenda of amessende Individualpsychologie and the historical significance of that agenda in Kraepelinian psychiatry. 相似文献
969.
Dependence on blood acetate concentration of the metabolic effects of ethanol in perfused rat liver 总被引:3,自引:0,他引:3
D L Topping A M Snoswell G B Storer R C Fishlock R P Trimble 《Biochimica et biophysica acta》1984,800(1):103-105
Unprotected oligonucleotides and oligodeoxynucleotides terminated with an unhindered 5'-phosphate group react with nucleoside 5'- phosphorimidazolides in aqueous solution to give 'capped' pyrophosphates in at least 70% yield. If adenosine 5'- phosphorimidazolide is used as a substrate in the reaction, ligase intermediates are obtained as products. 相似文献
970.