首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   250959篇
  免费   17250篇
  国内免费   169篇
  268378篇
  2021年   2150篇
  2019年   2025篇
  2018年   3572篇
  2017年   3383篇
  2016年   4219篇
  2015年   4086篇
  2014年   5406篇
  2013年   7702篇
  2012年   9044篇
  2011年   9450篇
  2010年   6753篇
  2009年   5693篇
  2008年   8405篇
  2007年   8602篇
  2006年   8281篇
  2005年   7702篇
  2004年   7873篇
  2003年   7521篇
  2002年   7536篇
  2001年   10128篇
  2000年   9722篇
  1999年   7674篇
  1998年   2475篇
  1997年   2310篇
  1996年   2106篇
  1995年   1931篇
  1992年   5955篇
  1991年   6136篇
  1990年   6071篇
  1989年   5999篇
  1988年   5522篇
  1987年   5175篇
  1986年   4739篇
  1985年   5011篇
  1984年   4015篇
  1983年   3312篇
  1982年   2220篇
  1981年   1938篇
  1979年   3598篇
  1978年   2803篇
  1977年   2626篇
  1976年   2574篇
  1975年   3053篇
  1974年   3345篇
  1973年   3301篇
  1972年   2917篇
  1971年   2776篇
  1970年   2364篇
  1969年   2309篇
  1968年   2207篇
排序方式: 共有10000条查询结果,搜索用时 23 毫秒
31.
The gene encoding the circumsporozoite (CS) protein of the rodent malaria parasite Plasmodium berghei was cloned and characterized. A cDNA library made from P. berghei sporozoite RNA was screened with a monoclonal antibody for expression of CS protein epitopes. The resulting cDNA clone was used to isolate the CS protein gene from a lambda library containing parasite blood-stage DNA. The CS protein gene contains a central region encoding two types of tandemly repeated amino acid units, flanked by nonrepeated regions encoding amino- and carboxy-terminal signal and anchorlike sequences, respectively. One of the central repeated amino acid unit types contains the immunodominant epitopes.  相似文献   
32.
The authors studied the anabolic effect of peptide morphogen of the hydra undecapeptide on normal and regenerating rat liver. Ornithine decarboxylase (EC 4.1.1.17) activity served as a marker. Intraperitoneal injection of the peptide into intact animals stimulated ornithine decarboxylase activity in a dose-dependent manner. In partially hepatectomized rats the peptide stimulated ornithine decarboxylase activity in the dose of 20 micrograms/kg body weight while greater doses inhibited the enzyme activity.  相似文献   
33.
Peptide material has been first isolated from k-casein pepsin hydrolysate. Its subcutaneous injection to hungry animals induced high amplitude (250-350 mV) and high frequency (16-20 Hz) oscillations of electrical potentials usually observed in food satiety and cholecystokinin administration. The peptide reduced respiratory and to a lesser extent heart rate. Its effect is temporary eliminated by naloxone. According to an aminogram, the peptide is a fragment of para-k-casein. A neurotropic peptide effect is connected with satiety regulation and milk consumption in the postnatal period.  相似文献   
34.
Structure of the spectrin-actin binding site of erythrocyte protein 4.1   总被引:9,自引:0,他引:9  
The complete primary structure of the functional site of erythrocyte protein 4.1 involved in spectrin-actin associations has been determined. The sequence of this domain, which contains 67 amino acids and has a molecular mass of 8045 daltons, has been obtained by NH2-terminal sequence analysis of an 8-kDa chymotryptic peptide, three endoproteinase lysine C-cleaved peptides and two peptides obtained by Staphylococcus aureus protease V8 cleavage. All peptides including the 8-kDa domain peptide were purified by reverse-phase high performance liquid chromatography. Antibodies against two different synthetic peptides of the 8-kDa domain are able to inhibit the association between protein 4.1, spectrin, and F-actin, corroborating that the 8-kDa domain is responsible for the formation of a ternary complex. A computer search of the 8-kDa sequence with the National Biomedical Research Foundation database did not detect any significant homologies to known sequences. Protein 4.1 is not related to any known proteins and may represent a new protein superfamily.  相似文献   
35.
36.
DANIEL  V.; GAFF  D. F. 《Annals of botany》1980,45(2):163-171
Significant changes in sulphydryl (‘SH’) and disulphide(‘SS’) levels during air-drying in leaves of ‘resurrection’plants (whose protoplasm survives dehydration) stemmed mainlyfrom protein turnover effects. No significant changes were foundin the SH, SS levels in leaves of the desiccation sensitivespecies Sporobolus pyramidalis following air-drying. The three tolerant species studied differed in the directionof change. Some data were consistent with Levitt's SH, SS hypothesis:increases in protein-SS levels in Sporobolus stapfianus (desiccationtolerant) were consistent with a stabilization of new proteinby SS bonds; lower reactivity of protein-SH in the tolerantspecies Talbotia elegans (which on the other hand has decreasedprotein-SS) is consistent with a second mechanism of decreasingprotein denaturation proposed in Levitt's hypothesis. Evidence of some conversion of SH to SS in the soluble proteinsof Xerophyta viscosa (a tolerant species) would on Levitt'shypothesis indicate an injurious process. Some degree of proteindenaturation might be indicated by partial inactivation of thesoluble enzyme ribulose bisphosphate carboxylase in this species,and loss of some soluble isoenzymes (peroxidase and alkalinephosphatase). An apparent lack of SH conversion to SS in thesensitive species Sporobolus pyramidalis was not consistentwith the SH, SS hypothesis. Resurrection plants, Sporobolus pyramidalis, Sporobolus stapfianus, Talbotia elegans, Xerophyta viscosa, drought resistance, desiccation tolerance, protein turnover, sulphydryl groups  相似文献   
37.
With the use of a patch-clamp technique in the whole-cell configuration, we studied the effects of pinacidil and its fluorine derivatives on A-type potassium current (I A) through the membrane of pyramidal neurons of the rat hippocampus. Hydrogen peroxide (10 mM) exerted no influence on the rate of inactivation ofI A; therefore, this current is probably mediated by Shal Kv4.2 potassium channels. Pinacidil demonstrated the properties of a weakI A blocker: in the 500 μM concentration it blocked about 45% of the current, while 50 μM of pinacidil fluorine derivatives were capable of blocking up to 30% ofI A. The effects of pinacidil and its derivatives showed no dependence on the stimulating potential. A similar pattern of the effects of pinacidil fluorine derivatives, which are an order of magnitude stronger than those of pinacidil itself, allows us to suppose that the imine nitrogen of the tested compounds is significantly more involved in the molecular interaction with the site of an A-type potassium channel than the pyridine nitrogen.  相似文献   
38.
The mechanisms of hemodynamic responses to orthostatic stresses and orthostatic stability (OS) of cosmonauts were studied before and after short-and long-term spaceflights (SFs) using orthostatic tests, as well as before, during, and after SFs using ultrasonic methods in tests with exposure to lower body negative pressure (LBNP). The capacitance and distensibility of the veins of the lower extremities were studied using occlusive air plethysmography before, during, and after SFs of different durations. A stay in microgravity has been proved to result in detraining of, mainly, the vascular mechanisms of compensating orthostatic perturbations. It has been established that the decrease in OS under the influence of microgravity is determined by a reduction of the vasoconstrictive ability of large blood vessels of the lower extremities; an increase in venous distensibility and capacitance of the legs; and an impairment of blood flow regulation, which leads to a cerebral blood flow deficit in orthostatic stresses, and of the initial individual OS before the flight. The results of preflight studies of hemodynamics by ultrasonic methods at LBNP and the data of orthostatic tests before SFs make it possible to predict the degree of decrease of OS after an SF proceeding in the normal mode. At the same time, the data of ultrasonic blood flow examination provide more a accurate estimation of OS and make it possible to assess the physiological reserves of hemodynamic regulation and to reveal the loss of regulation capacity even in cases where integrated indices (heart rate and blood pressure) are within the normal ranges.  相似文献   
39.
40.
Jan Scheirs  Luc De Bruyn 《Oikos》2002,97(1):139-144
The role of top-down forces in host choice evolution of phytophagous arthropods is the subject of a vividly animated debate. Empirical evidence for the evolutionary role of top-down forces comes from studies showing that phytophagous arthropods prefer hosts that entail enemy-free space. The aim of this paper is to draw the attention of plant–arthropod researchers to the potentially, temporally variable nature of third trophic level effects. We show that this aspect is largely neglected in studies on enemy-free space, despite the fact that relative enemy impact varies seasonally among plants in at least some studies. We conclude that rigorous testing of the enemy-free space hypothesis can only be performed when within and between season variation in higher trophic level effects is taken into account.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号