全文获取类型
收费全文 | 7916篇 |
免费 | 703篇 |
国内免费 | 4篇 |
出版年
2023年 | 60篇 |
2022年 | 147篇 |
2021年 | 288篇 |
2020年 | 149篇 |
2019年 | 181篇 |
2018年 | 202篇 |
2017年 | 163篇 |
2016年 | 341篇 |
2015年 | 524篇 |
2014年 | 533篇 |
2013年 | 547篇 |
2012年 | 776篇 |
2011年 | 663篇 |
2010年 | 384篇 |
2009年 | 374篇 |
2008年 | 504篇 |
2007年 | 428篇 |
2006年 | 385篇 |
2005年 | 303篇 |
2004年 | 302篇 |
2003年 | 275篇 |
2002年 | 253篇 |
2001年 | 68篇 |
2000年 | 44篇 |
1999年 | 44篇 |
1998年 | 63篇 |
1997年 | 35篇 |
1996年 | 27篇 |
1995年 | 29篇 |
1994年 | 26篇 |
1993年 | 30篇 |
1992年 | 31篇 |
1991年 | 22篇 |
1990年 | 19篇 |
1989年 | 28篇 |
1988年 | 16篇 |
1987年 | 15篇 |
1986年 | 17篇 |
1985年 | 18篇 |
1984年 | 19篇 |
1983年 | 29篇 |
1982年 | 16篇 |
1981年 | 22篇 |
1980年 | 15篇 |
1979年 | 15篇 |
1978年 | 20篇 |
1977年 | 17篇 |
1976年 | 11篇 |
1975年 | 11篇 |
1969年 | 11篇 |
排序方式: 共有8623条查询结果,搜索用时 15 毫秒
71.
In a previous electron microscopic study of early fibrin polymers processed by freeze drying and rotatory shadowing, a large proportion of loosely constructed, frequently branching linear molecular chains was observed; their structural organization was inconsistent with a half-staggered double-stranded model for fibrin polymerization. These conflicting results prompted us to investigate the structure of early fibrin polymers prepared according to a large variety of methods currently used for electron microscopy of macromolecules. By use of a systematic random sampling procedure, fibrin polymers were photographically recorded. They were classified according to their morphological form, and the frequency of occurrence of each configuration was determined. Half-staggered double-stranded forms accounted for less than 1% of all types encountered. Interpretation of the structural organization manifested in the diverse polymer forms observed necessitated the construction of a new interlocked single-strand model for fibrin polymerization. The fibrin polymerization process combines simultaneous propagation of linear growth, branching, and lateral interlocking (leading to lateral association), resulting in the rapid formation of a fibrin network. The structural pattern developing during growth of fibrin polymers appears to be determined principally by the enzymatic mechanism and not solely by the intrinsic molecular structure of fibrinogen. The validity of the interlocked single-strand model was tested by selective fibrinopeptide-B-releasing experiments. Under such activation conditions, the polymer forms predicted according to this and the half-staggered double-strand models should differ; the structures observed were indeed consistent with the interlocked single-strand hypothesis. The compatibility of existing data with this model is discussed. 相似文献
72.
Cesar Cardona Julia Kornegay Carmen E. Posso Francisco Morales Hernando Ramirez 《Entomologia Experimentalis et Applicata》1990,56(2):197-206
Wild Phaseolus vulgaris L. accessions containing arcelin codominant alleles 1 through 5 were reconfirmed and characterized for resistance to the Mexican bean weevil, Zabrotes subfasciatus (Boheman) (Coleoptera: Bruchidae). Accession G 02771 (arcelin 5) had the highest level of antibiosis resistance, followed by G 12952 (arcelin 4), G 12882 (arcelin 1) and G 12866 (arcelin 2). Arcelin 3 accessions conferred the lowest levels of resistance. As the presence of arcelin is inherited as a single dominant gene, a backcross breeding program has been used to transfer resistance to the Mexican bean weevil from wild beans to bean cultivars using serological techniques to detect the presence of arcelin and replicated insect feeding tests to measure resistance levels. Progeny containing arcelin 1 showed resistance equal or superior to that of the resistant check. Arcelin 2-deerived lines had intermediate levels of resistance while no resistant progenies were obtained from crosses with arcelin 3 and 4 sources. Results are discussed in relation to the deployment of arcelin alleles in bean cultivars.
Valeurs comparées de 5 types d'arcéline dans l'obtention de lignées de Phaseolus vulgaris résistantes à Zabrotes subfasciatus
Résumé La résistance à Zabrotes subfasciatus est associée à la présence d'arcéline, une nouvelle protéine des graines, découverte chez quelques populations de Phaseolus vulgaris. 5 types d'arcéline, hérités comme allèles codominants ont été décrits dans la littérature. Nous avons reprécisé les différentes populations contenant différents types d'arcéline et caractérisé leurs résistances à Z. subfasciatus. La population G 02771, correspondant à l'arcéline 5, présente la résistance la plus élevée par antibiose, suivie de G 12952 (arcéline 4), G 12882 (arcéline 1) et G 12866 (arcéline 2). Les populations contenant l'arcéline 3 présentent le moins de résistance à Z. subfasciatus.Un programme de croisements en retour associé à des tests sérologiques pour déceler la présence d'arcéline chez les descendants jeunes et des expériences répétées d'alimentation par les insectes vec BC2F3 a été réalisé pour transférer la résistance de populations naturelles à des cultivars de haricots. Les lignées, provenant de croisements avec des populations sauvages avec de l'arcéline 1, ont été fortement résistantes à Z. subfasciatus. Les lignées contenant de l'arcéline 2 ont été considérées comme ayant une résistance intermédiaire. Les lignées avec arcélines 3 et 4 étaient sensibles. Les raisons de l'échec du transfert de la résistance élevée des parents contenant de l'arcéline 4, sont inconnues. On a constaté que la concentration de l'arcéline dans les lignées contenant cet allèle était très faible, tandis que la concentration en arcéline 1 restait remarquablement élevée. Les recherches sont poursuivies pour déterminer les raisons de l'absence de transfert de l'arcéline 4 chez les descendants contenant cet allèle. Quoi qu'il en soit, les caractéristiques agronomiques et les qualités des lignées résistantes (codées RAZ) ont été évaluées en vue d'une diffusion pour les programmes nationaux de recherche des pays de basses altitudes intertropicaux ou Zabrotes subfasciatus fait des dégâts importants.相似文献
73.
Summary DBA/2 mice inoculated with either cells from the syngeneic P815 tumor or tumor cell membrane extracts develop T suppressor cells which suppress the in vitro generation of cytotoxic T lymphocytes with specificity for the tumor. A soluble suppressor factor with similar properties can be isolated from suppressor cell-enriched populations. It can be highly purified by appropriate immunoadsorption. Antisera to this suppressor factor raised in either DBA/2 or C57BL/6 mice can specifically absorb out suppressor factor and eliminate suppressor cells in the presence of complement. The in vivo effects of these antisera were tested for their ability to modulate the growth of P815 tumors in DBA/2 mice. It was found that the antiserum raised in syngeneic (DBA/2) but not allogeneic (C57BL/6) mice was able to significantly slow the rate of tumor growth and to prolong survival in treated mice. The antiserum was effective in this way only if it was administered early in the course of tumor growth. It was shown that this effect was not attributable to the presence in the serum of antibodies directed to antigens present on P815 cells, and it therefore appears to be due to interference with the function of T suppressor cells arising early in the immune response to the tumor cells. 相似文献
74.
P.J. Gardiner Julia L. Copas C. Schneider H.O.J. Collier 《Prostaglandins & other lipid mediators》1980,19(3):349-370
2-Decarboxy 2-hydroxymethyl prostaglandin E1 (TR4161) relaxed isolated guinea-pig trachea with about double and relaxed human isolated bronchial muscle with about one half the potency of PGE1. In conscious restrained cats an aerosol of TR4161 was about 100–1000 times less active than PGE1 in inducing tracheobronchial irritation. When given intravenously or by aerosol to the anaesthetised spontaneously breathing guinea-pig, TR4161 was approximately equipotent with PGE1 in inhibiting histamine-induced bronchoconstriction and in reducing basal inherent tone. The onset and duration of the bronchodilator effects of TR4161 administered intravenously, however, were significantly longer than those of PGE1. In conscious guinea-pigs, TR4161 by aerosol was approximately three times more potent than PGE1 in preventing histamine-induced convulsions, whereas only TR4161 was active in this test system when the test drugs were administered orally. These observations indicate that TR4161 might be therapeutically useful as a non-irritant prostaglandin bronchodilator in conditions of airway obstruction. 相似文献
75.
Julia Bell-Quint Trudy Forte Paul Graham 《Biochemical and biophysical research communications》1981,99(2):700-706
Cultured rat hepatocytes were used to demonstrate that the liver can synthesize two forms of apolipoprotein B. Separation of apolipoprotein B by disc gel electrophoresis indicated that hepatocyte low density lipoprotein contains predominantly apolipoprotein B with an apparent molecular weight of 345,000 ± 5,055. In contrast, the major apolipoprotein B component of hepatocyte very low density lipoprotein is a variant form with a molecular weight of 242,000 ± 2,720. Hepatocyte high density lipoprotein, unlike plasma HDL, also contains apolipoprotein B with an apparent molecular weight of 244,000 ± 2,742. Incorporation of [3H] leucine into hepatocyte apolipoprotein B components suggested de novo synthesis. 相似文献
76.
77.
A S Delk D P Nagle J C Rabinowitz K M Straub 《Biochemical and biophysical research communications》1979,86(2):244-251
The methyl carbon of ribothymidine in the tRNA of Streptococcus faecalis is derived from 5,10-methylenetetrahydrofolate, not S-adenosylmethionine. Isotope labeling experiments have shown that the reduction of the methylene carbon of the folate cofactor to the methyl carbon of the modified residue involves a mechanism in which hydrogen from solvent is incorporated into the methyl moiety. Although the identity of the reducing agent involved directly in this novel methylation remains to be established, data suggest that reduced flavin serves this function in vitro. 相似文献
78.
79.
A low molecular weight fraction from chitinase digested cell walls ofT. mentagrophytes containing both polysaccharide and peptide moieties was found to have immunological reactivity at both the cellular and humoral level. This fraction (UM2(a)) was further degraded by treatment with either a combination of pronase and carboxypeptidase A or with trypsin. Peptides were separated from the carbohydrate-rich fraction by ultrafiltration. The carbohydrate-rich fraction retained the ability to induce both immediate and delayed skin reactions in sensitized guinea pigs and to stimulate the proliferation of sensitized lymphocytesin vitro. The peptide moieties retained reactivity in that they caused delayed reactions and lymphocyte proliferation but were unable to induce immediate or Arthus reactions in sensitized animals. Tryptic peptides from UM2(a) were purified by ion exchange chromatography. A high proportion of these peptides demonstrated immunological activity at both the cellular and humoral level since they were capable of inducing delayed reactions and/or lymphocyte transformation, as well as being capable of blocking the complement fixation reaction between UM2 (a) and specific antiserum.This work was supported by grant number 6411 from the Medical Research Council of Canada.A. Kh. Al-Rammahy was supported by a scholarship from the Ministry of High Education, Iraq. 相似文献
80.