全文获取类型
收费全文 | 725篇 |
免费 | 47篇 |
出版年
2023年 | 5篇 |
2022年 | 9篇 |
2021年 | 13篇 |
2020年 | 20篇 |
2019年 | 17篇 |
2018年 | 29篇 |
2017年 | 25篇 |
2016年 | 37篇 |
2015年 | 50篇 |
2014年 | 35篇 |
2013年 | 51篇 |
2012年 | 53篇 |
2011年 | 44篇 |
2010年 | 23篇 |
2009年 | 32篇 |
2008年 | 37篇 |
2007年 | 22篇 |
2006年 | 21篇 |
2005年 | 19篇 |
2004年 | 21篇 |
2003年 | 21篇 |
2002年 | 17篇 |
2001年 | 15篇 |
2000年 | 19篇 |
1999年 | 17篇 |
1998年 | 6篇 |
1997年 | 13篇 |
1996年 | 9篇 |
1995年 | 6篇 |
1994年 | 12篇 |
1993年 | 4篇 |
1992年 | 4篇 |
1991年 | 9篇 |
1990年 | 4篇 |
1989年 | 6篇 |
1986年 | 1篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 4篇 |
1975年 | 6篇 |
1974年 | 5篇 |
1973年 | 3篇 |
1972年 | 3篇 |
1971年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有772条查询结果,搜索用时 125 毫秒
1.
2.
C P Dietrich J F de Paiva C T Moraes H K Takahashi M A Porcionatto H B Nader 《Biochimica et biophysica acta》1985,843(1-2):1-7
The isolation, some structural features, physicochemical properties and pharmacological activities of a heparin from Anomalocardia brasiliana are reported. It is shown that the mollusc heparin is very similar to those present in mammalian tissues with regard to chemical composition, physicochemical properties, pharmacological activities and susceptibility to heparinase and heparitinase II from Flavobacterium heparinum, as well as to the types of products formed by the action of these enzymes. Three significant quantitative differences were observed for the mollusc heparin when compared with the ones from mammalian origin, namely, a higher degree of binding with antithrombin III (45%), higher molecular weight (27-43 kDa) and higher anticoagulant activity (320 I.U./mg). The possible biological role of heparin is discussed in view of the present findings. 相似文献
3.
4.
We studied the changes in functional residual capacity (FRC), thoracoabdominal volume (Vw), and chest wall configuration in five normal subjects seated in an aircraft flying parabolic trajectories resulting in 20-s periods of microgravity. We measured vital capacity (VC), inspiratory capacity, and tidal volume by integrating airflow at the mouth and changes in rib cage and abdominal volume (delta Vrc and delta Vab, respectively, where delta Vrc + delta Vab = delta Vw) using induction plethysmography. During microgravity (0 Gz) FRC decreased by 413 +/- 70 (SE) ml and VC by 0.37 liter. The decrease in Vw did not differ from that in FRC and was entirely the result of reduction of Vab, the Vrc showing no significant change. During tidal breathing the abdominal contribution (delta Vab/delta Vw) increased from 0.39 +/- 0.08 at 1 Gz to 0.57 +/- 0.08 at 0 Gz. During brief periods of hypergravity (approximately 1.8 Gz) all changes were opposite in sign and relatively smaller. Limited data during "roller coaster" flight patterns suggested that, in contrast to configurational changes, the temporal pattern of breathing was uninfluenced by changes in Gz. We conclude that at the onset of weightlessness there are substantial changes in lung volume and thoracoabdominal configuration. Abdominal contribution to tidal excursions increases but the temporal pattern of breathing is unchanged. 相似文献
5.
Paiva R. P. Munita C. J. S. Cunha I. I. L. Romano J. Alonso C. D. 《Biological trace element research》1990,(1):231-242
Two nuclear techniques, Energy-Dispersive X-Ray Fluorescence Analysis (EDXRF) and Instrumental Neutron Activation Analysis (INAA), were used to analyze aerosol samples collected in the city of São Paulo, Brazil. Na, Cl, Mn, V, Al, Sm, Mo, W, La, As, Br, Sb, K, Ba, Se, Th, Cr, Rb, Ca, Fe, Ce, and Sc were determined by INAA, and Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Ga, As, Se, Br, Rb, Sr, Hg, and Pb were determined by EDXRF. A preliminary identification of the main source of the atmospheric aerosol was performed based on enrichment factor and correlation coefficient calculations.
相似文献6.
Xóchttl Castañeda Víctor Ortíz Betania Allen Cecilia García Mauricio Hernández-Avila 《Culture, medicine and psychiatry》1996,20(2):229-247
The central topic of the article is the divided world of female commercial sex workers (FCSW) in Mexico City. Fourteen focus group sessions were conducted with 133 FCSW from varying socio-economic levels and types of work site, as well as seven individual interviews.FCSW live in a constant double bind, as mother and prostitute, and come into daily contact with society's double standard for women. Reactions include justifying sex work as a better paying employment opportunity for women, as a necessary evil, and as a type of social service, while at the same time hiding their profession from their families. FCSW also live out an archetypal female ambivalence, their selves divided between the mother/saint and the traitor/prostitute.This article defines elements which should be taken into account in culturally appropriate programs for prevention of HIV/AIDS transmission, especially the importance which FCSW give to their role as mothers and promotion of the condom as a physical and symbolic barrier between professional and private life. 相似文献
7.
8.
Lignin is a major structural polymer of secondarily thickended plant vascular tissue and fibres, imparting mechanical strength to stems and trunks and hydrophobicity to conducting vessels. Constitutive expression of a lucerne caffeic acid 3-O-methyltransferase antisense RNA in transgenic tobacco leads to a significant reduction in lignin content, particularly in the younger parts of the stems, without apparent alterations in lignin monomer composition. These observations open up the possibility of genetically manipulating plants with reduced lignin for improved processing and biomass digestibility. 相似文献
9.
Joana Pais De Faria Vitor H. Paiva Sara N. Veríssimo Catarina S. Lopes Rita Soares João Oliveira Ivo dos Santos Ana C. Norte Jaime A. Ramos 《Ibis》2023,165(1):312-321
Gulls, as largely flexible opportunistic individuals, have been increasingly breeding in many cities around the world, but it is still unclear whether urban habitats are of equal or higher quality than traditional natural habitats or represent an ecological trap with immediate reproductive benefits but longer-term detrimental consequences to health. Here we present a study of breeding parameters (nest density, egg dimensions, clutch size, hatching success and adult body condition) and physiological parameters (erythrocyte sedimentation rate, heterophil/lymphocyte ratio, haemoglobin concentration and measurements of oxidative stress) as indicators of the general health condition of Yellow-legged Gull Larus michahellis adults and chicks from natural and urban colonies. Yellow-legged Gulls in the largest urban area (Porto) laid smaller eggs and clutches, showed a significantly lower occurrence of inflammatory processes in chicks, and showed a slower early chick growth than in the natural colony of Deserta. This suggests that urban gulls might be facing important trade-offs between the advantages of breeding in lower density urban colonies, with fewer intraspecific interactions and a lower disease transmission probability, and the disadvantages of having an anthropogenic diet usually lower in nutritional value. 相似文献
10.
Madan K. Bhattacharyya Nancy L. Paiva Richard A. Dixon Kenneth L. Korth Bruce A. Stermer 《Plant molecular biology》1995,27(1):1-15
3-Hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) catalyzes a key step in isoprenoid metabolism leading to a range of compounds that are important for the growth, development and health of the plant. We have isolated 7 classes of genomic clones encoding HMGR from a potato genomic library. Comparison of nucleic acid sequences reveals a high degree of identity between all seven classes of clones and the potato hmg 1 gene described by Choi et al. (Plant Cell 4: 1333, 1992), indicating that all are members of the same subfamily in potato. A representative member (hmg 1.2) of the most abundant class of genomic clones was selected for further characterization. Transgenic tobacco and potato containing the -glucuronidase (GUS) reporter gene under the control of the hmg 1.2 promoter expressed GUS activity constitutively at a low level in many plant tissues. High levels of GUS activity were observed only in the pollen. GUS assays of isolated pollen, correlations of GUS activity with the HMGR activity of anthers, hmg 1.2 promoter deletion studies, and segregation analysis of the expression of hmg 1.2::GUS among the R2 pollen of R1 progeny plants demonstrated that the hmg 1.2 promoter controls pollen expression. 相似文献