全文获取类型
收费全文 | 175803篇 |
免费 | 9449篇 |
国内免费 | 78篇 |
专业分类
185330篇 |
出版年
2018年 | 2153篇 |
2017年 | 2248篇 |
2016年 | 4349篇 |
2015年 | 7700篇 |
2014年 | 7627篇 |
2013年 | 8245篇 |
2012年 | 7733篇 |
2011年 | 5156篇 |
2010年 | 4429篇 |
2009年 | 4028篇 |
2008年 | 3490篇 |
2007年 | 3395篇 |
2006年 | 3268篇 |
2005年 | 9122篇 |
2004年 | 7821篇 |
2003年 | 5959篇 |
2002年 | 3368篇 |
2001年 | 5352篇 |
2000年 | 4430篇 |
1999年 | 4549篇 |
1998年 | 1342篇 |
1997年 | 1185篇 |
1996年 | 1004篇 |
1992年 | 4190篇 |
1991年 | 4278篇 |
1990年 | 4303篇 |
1989年 | 4016篇 |
1988年 | 3845篇 |
1987年 | 3655篇 |
1986年 | 3315篇 |
1985年 | 3322篇 |
1984年 | 2492篇 |
1983年 | 2056篇 |
1982年 | 1352篇 |
1981年 | 1262篇 |
1980年 | 1139篇 |
1979年 | 2451篇 |
1978年 | 1832篇 |
1977年 | 1585篇 |
1976年 | 1562篇 |
1975年 | 1980篇 |
1974年 | 2180篇 |
1973年 | 2272篇 |
1972年 | 2076篇 |
1971年 | 1923篇 |
1970年 | 1697篇 |
1969年 | 1736篇 |
1968年 | 1522篇 |
1967年 | 1524篇 |
1966年 | 1248篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
43.
Maria V. Sergeeva Vadim V. Mozhaev Joseph O. Rich Yuri L. Khmelnitsky 《Biotechnology letters》2000,22(17):1419-1422
A novel biocatalytic reaction of transamidation of non-activated amides with amines is reported. Among 45 different lipolytic and proteolytic enzymes tested, only the lipase from Candida antarcticawas able to catalyze this reaction. The reaction proceeded with up to ca. 80% conversion in anhydrous methyl tert-butyl ether and worked with both N-substituted and unsubstituted amides. The biocatalytic transamidation is an equilibrium process and, therefore, higher conversions to the desired amide were achieved by using increased concentrations of the amine nucleophile. 相似文献
44.
R H Ward B Modell M Petrou F Karag?zlu E Douratsos 《BMJ (Clinical research ed.)》1983,286(6377):1542-1544
Samples of chorionic villi were obtained in the first trimester by aspiration using a cannula passed transcervically under the guidance of real time ultrasound. In initial studies in 47 anaesthetised patients immediately before therapeutic abortion a method was developed giving a success rate of 89%. In 10 patients successful sampling was performed as an outpatient procedure without anaesthesia. In all, seven diagnostic procedures were undertaken and four of the five unaffected pregnancies continued. The technique of chorionic villous sampling using real time ultrasound is simple to learn and yields material for biochemical analysis and chromosomal study without the need for tissue culture. The exact obstetric risk, however, remains to be defined. 相似文献
45.
The site of hemolytic activity of a toxin isolated from Aspergillus fumigatus designated Asp-hemolysin was determined by photooxidation techniques. The hemolytic activity of this toxin was strongly inhibited by photooxidation with methylene blue, rose bengal, riboflavin, or eosin G as a sensitizer, whereas crystal violet, hematoxylin, naphthol yellow S, bromothymol blue, methyl orange, and cresol red had no effect. pH dependence of the inactivation with methylene blue was observed in the narrow range of pH values from 7.0 to 8.0, like that of the inactivation with rose bengal or riboflavin. The histidine, cysteine, methionine, tryptophan, and tyrosine content of methylene blue-photooxidized Asp-hemolysin was significantly decreased, while other amino acids were not affected. The hemolytic activity of the toxin was lost more slowly than the histidine residue, being maintained at about 50% even at the time when the histidine residue was completely lost after 30 min. Photooxidation of Asp-hemolysin in the presence of rose bengal also caused a decrease in histidine, methionine, and threonine content. These findings suggest that residues of cysteine, methionine, threonine, tryptophan, and/or tyrosine but not histidine may play an important role through stereostructure in the manifestation of the hemolytic activity of Asp-hemolysin. 相似文献
46.
W Y Mok R C Luiz?o M do S Barreto da Silva 《Applied and environmental microbiology》1982,44(3):570-575
A total of 2,886 bats captured in the Amazon Basin of Brazil were processed for the isolation of fungi. From the livers, spleens, and lungs of 155 bats (5.4%), 186 fungal isolates of the genera Candida (123 isolates), Trichosporon (26 isolates), Torulopsis (25 isolates), Kluyveromyces (11 isolates), and Geotrichum (1 isolate) were recovered. Seven known pathogenic species were present: Candida parapsilosis, C. guilliermondii, C. albicans, C. stellatoidea, C. pseudotropicalis, Trichosporon beigelii, and Torulopsis glabrata. Twenty-three culture-positive bats showed identical fungal colonization in multiple organs or mixed colonization in a single organ. The fungal isolation rates for individual bat species varied from 1 fungus per 87 bats to 3 fungi per 13 bats, and the mycoflora diversity for members of an individual fungus-bearing bat species varied from 16 fungi per 40 bats to 7 fungi per 6 bats. Of the 38 fungal species isolated, 36 had not been previously described as in vivo bat isolates. Of the 27 culture-positive bat species, 21 had not been previously described as mammalian hosts for medically or nonmedically important fungi. 相似文献
47.
Biochemical and phenotypical correction of an envelope mutant of Salmonella typhimurium 总被引:1,自引:0,他引:1
R A Pizarro G O Boselli L V Orce 《Archives internationales de physiologie et de biochimie》1984,92(5):333-337
Salmonella typhimurium DA 361 bears an env D1 mutation with the following abnormal phenotypical and biochemical characteristics: a) it autolyses at stationary phase in nutrient broth; b) it grows in chains of short rods; c) it is a poor maltose fermenter and d) it has a diphosphatidylglycerol (DPG) content twice as high than its isogenic non-lytic pair DA 362 (env D+) and LT2, of which both are derivatives. Growth of DA 361 in the presence of 400 mM ethanol leads on a 50% decrease of DPG level, thereby equalling its PG/DPG ratio with those of the control strain. Consequently, a correction on the other phenotypical and biochemical anomalies are induced since the DA 361 strain decreases its autolytic activity, ferments normally maltose and appear as rods undifferentiated from DA 362. 相似文献
48.
Postural sway was compared for humans touching an external object while standing on an immobile or slowly moving posturographic platform. When the platform moves, the central nervous system may interpret the movement of the point of the contact with the external object as the movement of the body relative to the support or as the movement of the support itself. Thus, the information concerning the body position that is provided by the touch becomes ambiguous. It was demonstrated that contact with an external object during standing on an unstable support leads to a decrease in support sway. When a subject stands on a moving platform, this decrease is smaller than in the case of an immobile platform. Contact with an external object causes a decrease in postural responses to shank muscle vibrations on an immobile platform. On a moving platform, this decrease is nonsignificant. The change in postural sway depending on the unambiguity of afferent information is discussed in terms of the interaction between afferent signals of different modalities on the basis of the body scheme in subjects maintaining balance.Translated from Fiziologiya Cheloveka, Vol. 31, No. 1, 2005, pp. 59–65.Original Russian Text Copyright © 2005 by Kazennikov, Shlykov, Levik. 相似文献
49.
We have studied the effect of parathyroid hormone (PTH) on adenylate cyclase of microvessels isolated from rat cerebral cortex. Native bovine (b) PTH-(1–84), the synthetic amino-terminal fragment bPTH-(1–34) and the synthetic analog [Nle8, Nle18, Tyr34]-bPTH- (1–34) amide stimulated adenylate cyclase in a dose-dependent manner with apparent ED50 values of 16 nM, 6.3 nM and 15 nM respectively. The stimulation by bPTH was greatly enhanced by guanosine triphosphate. The PTH antagonist, [Nle8, Nle18, Tyr34]-bPTH-(3–34) amide inhibited the action of bPTH-(1–84) and bPTH-(1–34). In summary, PTH stimulated adenylate cyclase in rat cerebral microvessels in a very similar manner to its stimulation in the renal cortex. 相似文献
50.
M Sj?lund J Nilsson L Palmberg J Thyberg 《Differentiation; research in biological diversity》1984,27(2):158-162
The effects of prostaglandin E1 (PGE1) on the phenotypic state of enzymatically isolated arterial smooth-muscle cells in primary culture were studied by transmission electron microscopy, thymidine autoradiography, and cell counting. Early in culture (day 0-2), PGE1 stimulated conversion of the cells from contractile (less euchromatic nucleus and cytoplasm dominated by myofilament bundles) to synthetic state (more euchromatic nucleus and cytoplasm dominated by cisternae of rough endoplasmic reticulum and a large Golgi complex). The rate of entrance of the cells into DNA synthesis and mitosis was also increased at this time. Later on (day 3-6), when the majority of the cells had entered synthetic state, PGE1 inhibited DNA synthesis and cellular proliferation. These observations indicate that the effect of prostaglandins on arterial smooth muscle is dual in nature and dependent on the state of differentiation of the cells. 相似文献