全文获取类型
收费全文 | 263724篇 |
免费 | 155282篇 |
国内免费 | 40471篇 |
出版年
2022年 | 5241篇 |
2021年 | 9589篇 |
2020年 | 6987篇 |
2019年 | 10166篇 |
2018年 | 9690篇 |
2017年 | 7551篇 |
2016年 | 10174篇 |
2015年 | 13939篇 |
2014年 | 16276篇 |
2013年 | 17140篇 |
2012年 | 20580篇 |
2011年 | 19434篇 |
2010年 | 14452篇 |
2009年 | 17785篇 |
2008年 | 14093篇 |
2007年 | 13346篇 |
2006年 | 11188篇 |
2005年 | 9672篇 |
2004年 | 8693篇 |
2003年 | 7625篇 |
2002年 | 7640篇 |
2001年 | 8016篇 |
2000年 | 5570篇 |
1999年 | 9929篇 |
1998年 | 10520篇 |
1997年 | 10679篇 |
1996年 | 9843篇 |
1995年 | 9994篇 |
1994年 | 9334篇 |
1993年 | 8694篇 |
1992年 | 9222篇 |
1991年 | 8959篇 |
1990年 | 9464篇 |
1989年 | 8649篇 |
1988年 | 7800篇 |
1987年 | 6833篇 |
1986年 | 6258篇 |
1985年 | 5704篇 |
1984年 | 4294篇 |
1983年 | 3527篇 |
1982年 | 3730篇 |
1981年 | 3366篇 |
1980年 | 3272篇 |
1979年 | 3408篇 |
1978年 | 3079篇 |
1977年 | 3001篇 |
1976年 | 2808篇 |
1974年 | 2575篇 |
1972年 | 2921篇 |
1971年 | 2651篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
841.
C-Glycosylflavonoids. The chemistry of aspalathin 总被引:1,自引:0,他引:1
1. The isolation of aspalathin, the principal phenolic constituent in the leaves of Aspalathus linearis, is described and its properties are discussed. 2. The compound has been identified as 3'-C-beta-d-glucopyranosyl-2',3,4,4',6'-pentahydroxydihydrochalcone by the preparation and analysis of various derivatives, by photochemical oxidation to 2,3-dihydroiso-orientin and by nuclear-magnetic-resonance studies. 相似文献
842.
843.
844.
W S Peart A M Lloyd G N Thatcher N Payne N Stone A F Lever 《The Biochemical journal》1965,96(3):31contd-31con3c
845.
Rates of parasitism ofHelicoverpa (=Heliothis) zea (Boddie) (Lepidoptera: Noctuidae) by two tachinid parasitoid species differing in larviposition habits were measured in field
cages on tomato plant lines with or without methyl-ketone (2-tridecanone and 2-undecanone)-mediated insect resistance. Hosts
were placed on resistant or susceptible plants, exposed to parasitoids for 24–48 h, then held on artificial diet for parasitoid
emergence. Rates of parasitism byArchytas marmoratus (Townsend) (Diptera: Tachinidae), which larviposits on its host's food plant, were significantly reduced on the resistant
plants, relative to those on the susceptible plants. Parastism by another Tachinid,Eucelatoria bryani Sabrosky, which larviposits directly into its host and does not directly contact the foliage of its hosts' food plant as
a small larva, was not affected by the methyl-ketone mediated resistance. 相似文献
846.
A retrovirus carrying the polyomavirus middle T gene induces acute thrombocythemic myeloproliferative disease in mice. 总被引:2,自引:0,他引:2 下载免费PDF全文
A Fusco G Portella M Grieco G Tajana G Di Minno N Polli A Pinto 《Journal of virology》1988,62(1):361-365
Mice inoculated with an artificially constructed retrovirus carrying the middle T gene of polyomavirus develop acute myeloproliferative disease with severe thrombotic and hemorrhagic disorder and impaired platelet function. The megakaryocytic lineage appears to be a target for polyoma-murine leukemia virus infection and middle T gene expression. This newly described disease represents a unique model system for studying disorders of the megakaryocytic lineage. 相似文献
847.
Interaction between the Effects of Inside and Outside Na and K on Bullfrog Skin Potential 总被引:2,自引:1,他引:1
Daniel E. Leb Charles Edwards Barry D. Lindley T. Hoshiko with the technical assistance of James A. Dugan 《The Journal of general physiology》1965,49(2):309-320
The composition of the solution bathing one border of the isolated frog skin affects the response of the potential across the skin to changes in the composition of the solution bathing the opposite border. Increasing the K concentration of the inside (corium) bathing solution decreased the sensitivity of the potential to a change in outside Na concentration. Decreasing the outside Na concentration decreased the sensitivity of the potential to a change in inside K concentration. Increasing the total ionic strength of the outside bathing solution or of both bathing solutions decreased the sensitivity of the potential to a change in outside Na concentration. 相似文献
848.
Kimberly A. Aeling Kirsty A. Salmon José M. Laplaza Ling Li Jennifer R. Headman Alex H. Hutagalung Stephen Picataggio 《Journal of industrial microbiology & biotechnology》2012,39(11):1597-1604
We have integrated and coordinately expressed in Saccharomyces cerevisiae a xylose isomerase and cellobiose phosphorylase from Ruminococcus flavefaciens that enables fermentation of glucose, xylose, and cellobiose under completely anaerobic conditions. The native xylose isomerase was active in cell-free extracts from yeast transformants containing a single integrated copy of the gene. We improved the activity of the enzyme and its affinity for xylose by modifications to the 5′-end of the gene, site-directed mutagenesis, and codon optimization. The improved enzyme, designated RfCO*, demonstrated a 4.8-fold increase in activity compared to the native xylose isomerase, with a Km for xylose of 66.7?mM and a specific activity of 1.41?μmol/min/mg. In comparison, the native xylose isomerase was found to have a Km for xylose of 117.1?mM and a specific activity of 0.29?μmol/min/mg. The coordinate over-expression of RfCO* along with cellobiose phosphorylase, cellobiose transporters, the endogenous genes GAL2 and XKS1, and disruption of the native PHO13 and GRE3 genes allowed the fermentation of glucose, xylose, and cellobiose under completely anaerobic conditions. Interestingly, this strain was unable to utilize xylose or cellobiose as a sole carbon source for growth under anaerobic conditions, thus minimizing yield loss to biomass formation and maximizing ethanol yield during their fermentation. 相似文献
849.
850.