全文获取类型
收费全文 | 2296篇 |
免费 | 233篇 |
出版年
2023年 | 7篇 |
2022年 | 21篇 |
2021年 | 49篇 |
2020年 | 25篇 |
2019年 | 58篇 |
2018年 | 60篇 |
2017年 | 54篇 |
2016年 | 72篇 |
2015年 | 100篇 |
2014年 | 97篇 |
2013年 | 140篇 |
2012年 | 170篇 |
2011年 | 160篇 |
2010年 | 87篇 |
2009年 | 99篇 |
2008年 | 116篇 |
2007年 | 120篇 |
2006年 | 110篇 |
2005年 | 96篇 |
2004年 | 89篇 |
2003年 | 69篇 |
2002年 | 82篇 |
2001年 | 67篇 |
2000年 | 63篇 |
1999年 | 60篇 |
1998年 | 30篇 |
1997年 | 27篇 |
1996年 | 30篇 |
1995年 | 21篇 |
1994年 | 14篇 |
1993年 | 15篇 |
1992年 | 32篇 |
1991年 | 41篇 |
1990年 | 36篇 |
1989年 | 22篇 |
1988年 | 29篇 |
1987年 | 23篇 |
1986年 | 22篇 |
1985年 | 21篇 |
1984年 | 8篇 |
1983年 | 5篇 |
1981年 | 5篇 |
1980年 | 6篇 |
1978年 | 9篇 |
1977年 | 7篇 |
1975年 | 4篇 |
1974年 | 11篇 |
1973年 | 6篇 |
1972年 | 5篇 |
1969年 | 4篇 |
排序方式: 共有2529条查询结果,搜索用时 767 毫秒
31.
Javier G. Fernández Velasco Norma L. Pucheu Oscar Romero Norma L. Kerber Raúl A. Fernández Augusto F. García 《Archives of microbiology》1985,143(2):192-195
The conditions under which an oscillatory behaviour is observed during net hydrolysis or synthesis of ATP in chromatophores of Rhodospirillum rubrum FR1 are described. In the case of ATPase the oscillations are observed at low temperature (ca. 11°C) in the dark after an initial transient behaviour. These oscillations are attenuated or disappear by the addition of an uncoupler.Oscillations are also observed during ATP synthesis. At 3°C the oscillations appear spontaneously if photophosphorylation is measured during a sufficiently long time. At 30°C the mere intercalation of a dark period also at 30°C is sufficient to trigger the oscillations in the following light period.Abbreviations Bchl
Bacteriochlorophyll
- FCCP
carbonyl cyanide p-trifluoromethoxyphenyl hydrazone
- PMS
phenazine methosulfate
- TMPD, N,N,N,N
tetramethyl-1,4-phenylenediamine
Dedicated to Prof. Dr. Gerhart Drews as a homage for his permanent example as hard worker and careful scientist and also for his remarkable human quality 相似文献
32.
Francisco Romero Francisco Javier Caballero Francisco Castillo José Manuel Roldán 《Archives of microbiology》1985,143(2):111-116
Anti-glutamine synthetase serum was raised in rabbits by injecting purified glutamine synthetase (GS) of the phototrophic bacterium Rhodopseudomonas capsulata E1F1. The antibodies were purified to monospecificity by immunoaffinity chromatography in GS-sepharose gel. These anti-GS antibodies were used to measure the antigen levels in crude extracts from bacteria, grown phototrophically with dinitrogen, nitrate, nitrite, ammonia, glutamate, glutamine or alanine as nitrogen sources. The amount of GS detected by rocket immunoelectrophoresis was proportional to Mn2+-dependent transferase activity measured in the crude extracts. Addition of GS inhibitor l-methionine-d,l-sulfoximine (MSX) to the actively growing cells promoted increased antigen levels, that were not found in the presence of glutamine or chloramphenicol. The ammonia-induced decrease in GS relative levels was reverted by MSX. GS levels remained constant when phototrophically growing cells were kept in the dark.Abbreviations GS
glutamine synthetase
- MOPS
2-(N-morpholine) propane sulfonate
- MSX
l-methionine-d,l-sulfoximine 相似文献
33.
Comparative study of selective media for enumeration of Pseudomonas aeruginosa from water by membrane filtration 总被引:2,自引:0,他引:2
A de Vicente J J Borrego F Arrabal P Romero 《Applied and environmental microbiology》1986,51(4):832-840
In the present study, mPA-D and mPA-E agar, modifications of mPA-C agar that reduce background fecal streptococci that interfere with the differentiation and enumeration of the Pseudomonas aeruginosa colonies grown in other mPA media, are proposed for use in analyzing natural water samples. In addition, the efficiencies of several culture media for the recovery of P. aeruginosa in water after membrane filtration and multiple-tube techniques are compared. The degree of selectivity, precision, efficiency, and sensitivity achieved with the proposed media exceeded that achieved by current methods. Furthermore, they yielded equal rates of accuracy and specificity. Incubation at 36 degrees C resulted in an improved recovery of stressed P. aeruginosa. In conclusion, we propose the use of mPA-D and mPA-E agar, both incubated at 36 degrees C for 24 to 48 h, for analyzing river water and seawater, respectively. 相似文献
34.
The rate of photosynthetic nitrate utilization in Anacystis nidulans is strongly influenced by the availability of carbon dioxide. This dependence can be relieved by inhibiting the metabolism of the ammonium derived from nitrate reduction. Nitrate uptake seems to be modulated through a sensitive regulatory system integrating the photosynthetic metabolism of carbon and nitrogen, with CO2 fixation products antagonizing the inhibitory effect of ammonium derivatives. 相似文献
35.
We compared the histamine responsiveness of canine airways and parenchymal tissues in six anesthetized paralyzed open-chest mongrel dogs, partitioning total lung resistance (RL) into airway resistance (Raw) and tissue viscance (Vti). Pressure was measured during tidal breathing (frequency was 0.3 Hz) at the trachea and in three alveolar regions by use of alveolar capsules. Measurements were taken before and after the delivery of increasing concentrations of aerosolized histamine (0.1-30 mg/ml). We found that Vti accounted for 78 +/- 8% of RL under base-line conditions; this proportion remained relatively constant throughout the histamine concentration-response curve. There was a significant correlation between percent change in Vti and percent change in Raw at all levels of histamine-induced constriction (P less than 0.001). Moreover, the sensitivity of the tissues and airways (defined as the concentration of histamine required to double resistance) was remarkably similar. We conclude that, at this frequency of ventilation, Vti accounts for the major portion of RL both under base-line conditions and after histamine-induced constriction. Although increases in RL cannot be attributed solely to events occurring in the airways, the close correlation between changes in Raw and Vti and the similar sensitivities of the two support the use of indexes reflecting changes in airway caliber as an indicator of overall lung histamine responsiveness. 相似文献
36.
Bacterial endotoxin and tumor necrosis factor stimulate prostaglandin production by human decidua 总被引:3,自引:0,他引:3
R Romero M Mazor Y K Wu C Avila E Oyarzun M D Mitchell 《Prostaglandins, leukotrienes, and essential fatty acids》1989,37(3):183-186
The purpose of these studies was to determine the effect of bacterial endotoxin and tumor necrosis factor (TNF) on prostaglandin (PG) secretion by human decidua. Decidual explants were established from women undergoing elective cesarean sections before the onset of labor. Escherichia Coli endotoxin and purified human recombinant TNF (rh TNF) were incubated with decidual explants. PGF2 alpha and PGE2 biosynthesis was measured by radioimmunoassay. A significant increase in the release of all PGs into the media occurred in response to LPS and TNF. In the setting of an extraamniotic infection, bacterial and host secretory products (TNF) could trigger the onset of labor, activating the decidua to produce PGs. 相似文献
37.
J. F. Romero Collazos M. D. Marrodán E. Gutierrez Redomero 《International Journal of Anthropology》1996,11(1):11-19
Anthropometric characteristics of proportionality, body composition and somatotype have been determined in a group of 72 Spanish
judoists. The sample includes the junior male and female National Team, and seniors competitors in the last Olimpic Games
held in 1992, and participants in the National Championship of 1993. The methodology has been used according to Weiner and
Lourie(1981)and MOGAP procedures described by Borms et al. (1979). The obtained results show a similar proportionality profile
and mesoendomorphic mean somatotype in both male and female series. However, were found significant differences between sexes
as well as depending of weight categories. 相似文献
38.
C. Benito M. P. Romero N. Henriques-Gil F. Llorente A. M. Figueiras 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1996,93(5-6):926-931
The variation in recombination frequency (rf) is important to plant breeders since their major objective is to obtain favorable recombinants of linked genes. One source of variation in rf is sex. Sex differences for recombination frequencies were studied in four of the seven chromosomes of Secale cereale L. cv Ailés using isozyme and storage protein loci and were determined on the basis of reciprocal crosses between heterozygous plants of cv. Ailés and homozygous plants of the inbred line Riodeva. The differences were found to be strongly segmentspecific. In some cases the level of crossing-over in male and female meiosis was about the same (between Pgm1 and Ndh1 loci on chromosome arm 4RS). However, for most of the chromosome segments in 1R, 3RL and 6RL the male rf was significantly higher than the female rf. Different hypotheses about the mechanisms of plant sex differences for recombination are discussed. 相似文献
39.
Rafael Navarro-González Mitsuhiko Akaboshi Alfredo Romero Cyril Ponnamperuma 《Journal of biological physics》1996,22(2):87-100
Akaboshi et al. (1990) has found an unexpected protection of the achiral amino acid, glycine, towards ionizing radiation at the expense of the selective destruction of the chiral amino acids, alanine and aspartic acid. The present work examines the mechanism of this protection for the case of alanine. We have developed a computer model for the radiolysis of glycine, alanine and glycine-alanine mixtures in aqueous solution. It is established that this protection is due in part to the reaction of the α-radical of glycine with alanine to regenerate a more stable α-radical, according to the following reaction, $$ \cdot CH(NH_3^ + )CO_2^ - + CH_3 CH(NH_3^ + )CO_2^ - \to CH_2 (NH_3^ + )CO_2^ - + CH_3 \dot C(NH_3^ + )CO_2^ -$$ The rate constant of this reaction was estimated to be ≤104M-1s-1. The implications for this selective protection of glycine are considered for a hypothetical case in which there would be an enrichment of about 10% ofL-alanine in the primitive ocean and taking the glycine/alanine ratios obtained in CH4-and CO2- dominated atmospheres using electric discharge experiments. It is predicted that alanine would be rapidly destroyed and radioracemized in spite of the fact that the concentration of alanine is equal or significantly lower than that of glycine. Assuming that chiral amino acids were a prerequisite for the origin of life, it can be deduced that life could have appeared in a relatively short period of time unless there was a constant supply of optical amino acids from extraterrestrial sources. 相似文献
40.
Characterization of homologous DNA recombination activity in normal and immortal mammalian cells. 总被引:1,自引:0,他引:1
下载免费PDF全文
![点击此处可从《Nucleic acids research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
B Thyagarajan M McCormick-Graham D P Romero C Campbell 《Nucleic acids research》1996,24(20):4084-4091
Homologous DNA recombination levels were measured in normal and spontaneously immortalized murine and human fibroblasts, and in a number of primate and murine established fibroblast cell lines. Immortal cell lines and tumor-derived clones homologously recombined extrachromosomal plasmid substrates at frequencies approximately 100-fold higher than did normal cells. To further explore the mechanism responsible for this phenotype, homologous recombination frequency was measured using nuclear extracts derived from normal and immortalized murine and human fibroblasts. Extracts prepared from immortal cells catalyzed high levels of homologous recombination, whereas very little recombination activity was detected in extracts prepared from normal fibroblasts. Similarly, only extracts derived from immortal cells contained strand-transferase activity as measured by the recently described pairing-on-membrane assay. Mixing experiments indicated that a recombination enhancing factor or factors present in immortal cells, rather than a recombination inhibitor in normal cells, was responsible for the enhanced homologous recombination activity observed using extracts derived from the former. 相似文献