全文获取类型
收费全文 | 75519篇 |
免费 | 7455篇 |
国内免费 | 33篇 |
出版年
2021年 | 482篇 |
2019年 | 424篇 |
2018年 | 1091篇 |
2017年 | 1122篇 |
2016年 | 1257篇 |
2015年 | 1242篇 |
2014年 | 1509篇 |
2013年 | 2558篇 |
2012年 | 3806篇 |
2011年 | 4240篇 |
2010年 | 2317篇 |
2009年 | 1570篇 |
2008年 | 3503篇 |
2007年 | 3591篇 |
2006年 | 3369篇 |
2005年 | 3085篇 |
2004年 | 3003篇 |
2003年 | 2852篇 |
2002年 | 2853篇 |
2001年 | 3336篇 |
2000年 | 3550篇 |
1999年 | 2367篇 |
1998年 | 917篇 |
1997年 | 813篇 |
1996年 | 896篇 |
1995年 | 783篇 |
1994年 | 808篇 |
1993年 | 764篇 |
1992年 | 1492篇 |
1991年 | 1455篇 |
1990年 | 1417篇 |
1989年 | 1319篇 |
1988年 | 1269篇 |
1987年 | 1183篇 |
1986年 | 1022篇 |
1985年 | 1100篇 |
1984年 | 985篇 |
1983年 | 860篇 |
1982年 | 690篇 |
1981年 | 660篇 |
1980年 | 600篇 |
1979年 | 857篇 |
1978年 | 723篇 |
1977年 | 680篇 |
1976年 | 590篇 |
1975年 | 762篇 |
1974年 | 701篇 |
1973年 | 642篇 |
1972年 | 544篇 |
1971年 | 476篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
Purification and characterization of purple acid phosphatase from developing rat bone 总被引:1,自引:0,他引:1
Tartrate-resistant acid phosphatase active on nucleoside di- and triphosphate substrates was isolated from developing rat bone and purified 2500-fold. The enzyme concentration had a purple coloration and activity that was sensitive to reducing agents. Mild reducing agents such as ferrous ion and ascorbic acid caused loss of purple color and increased activity toward substrates severalfold; however, a strong reductant such as dithionite caused loss of both color and activity which were partially restored by addition of ferrous ion and ascorbic acid. Enzyme activity was homogeneous with protein during the final gel permeation steps of chromatography and gave an apparent molecular size of about 40,000 Da. Determination of iron in the most pure preparation revealed the presence of 1.3 atoms of iron per molecule of the tartrate-resistant enzyme E2. Other properties of the purified enzyme include a pI of approximately 9.5 and sensitivity to inhibition by ions of copper, zinc, fluoride, and molybdate. Antibody prepared to the pre-concanavalin A (Con A)-Sepharose purified enzyme reacted with all protein from the Con A step, but it did not react with tartrate-sensitive acid phosphatase from rat bone or with potato acid phosphatase. Purple acid phosphatase from rat bone has many properties that parallel the iron-containing purple acid phosphatases from rat spleen, bovine spleen, and pig uterine secretions. 相似文献
62.
63.
Strains of paramyxovirus type 1 (PMV-1) have been isolated from diseased racing pigeons in Sweden. One of these isolates was selected for studies of the pathogenicity and contagiousness in chickens. The same isolate was previously found to have a high intravenous pathogenicity index (IVPI) in 6 weeks old chickens. In three experiments it was found that the PMV-1 isolate was very pathogenic for 1 week old chickens but not pathogenic for 120 day old pullets inoculated intranasally and ocularly. Symptoms in the young chickens were similar to those seen in the neurotropic form of Newcastle disease. The mortality was high and the incubation period 5–11 days. The disease easily spread to young chickens kept in contact with diseased birds. The microscopic examination revealed an interstitial nonpurulent pneumonia and a nonpurulent encephalitis in the young chickens. In the pullets the only finding was a mild encephalitis. PMV-1 was recovered from all young chickens but not from the pullets. Both the chickens and the inoculated pullets developed antibodies to PMV-1. 相似文献
64.
Atrial natriuretic factor alters autonomic interactions in the control of heart rate in conscious rats 总被引:2,自引:0,他引:2
U Ackermann J Khanna T G Irizawa 《Canadian journal of physiology and pharmacology》1988,66(7):930-936
Regulation of heart rate was studied in rats receiving either i.v. saline at 64 microL/min or synthetic 28-residue rat atrial natriuretic peptide (ANF) at a dose sufficient to decrease mean arterial blood pressure by 10%. Autonomic influences were deduced from steady-state heart rate responses of each group to propranolol, atropine, or propranolol and atropine combined. A multiplicative model of heart rate control was used to derive quantitatively from the data the modulation of intrinsic heart rate by sympathetic and parasympathetic mechanisms. Animals receiving ANF showed a lower heart rate than control animals. This relative bradycardia was abolished by atropine. Blocking of sympathetic effects with propranolol had no effect on basal heart rate in either group, and atropinization led to significant increases in heart rate in both groups of rats. Mathematical analysis of the results showed that the bradycardia produced by ANF was due predominantly to a reduced intrinsic heart rate and to enhanced vagal inhibition of postganglionic sympathetic activity. Parasympathetic contribution to heart rate in the absence of sympathetic activity was negligible in control rats and small during ANF. We conclude that the major influences of ANF on heart rate control are a decrease of intrinsic heart rate and enhanced parasympathetic inhibition of postganglionic presynaptic sympathetic activity. 相似文献
65.
V T Kondakov A B Okulov M A Filippkin R Z Tandilava I U Artamonov O A Voronenko A N Sazonov 《Vestnik rentgenologii i radiologii》1991,(4):31-35
The paper is concerned with analysis of the results of intraoperative phlebotesticulography, performed in 50 patients with varicocele of degree I-II during Ivanissevich's operation. The effect of surgical intervention was shown to depend upon the quality of ligation of the testicular vein, some parts of which are anastomosed between themselves. The localization of this anastomosis is revealed by means of intraoperative phlebotesticulography, which permits increasing the results of surgical treatment and predicting a course of a postoperative period. 相似文献
66.
Background
Identifying essential genes in bacteria supports to identify potential drug targets and an understanding of minimal requirements for a synthetic cell. However, experimentally assaying the essentiality of their coding genes is resource intensive and not feasible for all bacterial organisms, in particular if they are infective. 相似文献67.
68.
69.
Summary Mutants of S. marcescens HY have been isolated which produce between five and one hundred times more exocellular nuclease than does the parental strain. These nuclease-superactive (nuc
su) mutants are highly pleiotropic: they produce more exocellular marcescin A and lipase than the wild-type and their ability to inactivate penicillin G is increased. Furthermore, all nuclease-superactive mutants if lysogenic for the heteroimmune phages Kappa and/or y show spontaneous induction rates for both prophages 10 to 200 fold greater than the corresponding wild-type. Nuc
su mutants of independent origin synthesize nuclease and marcescin A in approximately proportional amounts although the corresponding structural genes do not seem to be part of a single operon because some bacteriocin-superactive mutants were isolated which showed an increase of the synthesis of marcescin A only. Nuclease-defective (nuc) mutants are all of the non-pleiotropic type. Three hypotheses to explain the effects of the nuc
su mutation at the molecular level are discussed and some evidence in support of one of these hypotheses (gene-dosage effect) is presented in an accompanying paper (Timmis and Winkler, 1973). 相似文献
70.