全文获取类型
收费全文 | 4047篇 |
免费 | 464篇 |
国内免费 | 5篇 |
出版年
2021年 | 48篇 |
2020年 | 31篇 |
2019年 | 45篇 |
2018年 | 49篇 |
2017年 | 60篇 |
2016年 | 61篇 |
2015年 | 133篇 |
2014年 | 161篇 |
2013年 | 162篇 |
2012年 | 205篇 |
2011年 | 220篇 |
2010年 | 129篇 |
2009年 | 113篇 |
2008年 | 176篇 |
2007年 | 162篇 |
2006年 | 178篇 |
2005年 | 143篇 |
2004年 | 157篇 |
2003年 | 137篇 |
2002年 | 147篇 |
2001年 | 112篇 |
2000年 | 128篇 |
1999年 | 98篇 |
1998年 | 64篇 |
1997年 | 42篇 |
1996年 | 45篇 |
1995年 | 44篇 |
1994年 | 42篇 |
1992年 | 83篇 |
1991年 | 78篇 |
1990年 | 75篇 |
1989年 | 54篇 |
1988年 | 64篇 |
1987年 | 63篇 |
1986年 | 53篇 |
1985年 | 68篇 |
1984年 | 61篇 |
1983年 | 48篇 |
1982年 | 40篇 |
1981年 | 33篇 |
1980年 | 43篇 |
1979年 | 62篇 |
1978年 | 30篇 |
1977年 | 43篇 |
1976年 | 31篇 |
1975年 | 37篇 |
1974年 | 47篇 |
1973年 | 44篇 |
1971年 | 29篇 |
1967年 | 34篇 |
排序方式: 共有4516条查询结果,搜索用时 171 毫秒
991.
Velocity of the creatine kinase reaction in the neonatal rabbit heart: role of mitochondrial creatine kinase 总被引:2,自引:0,他引:2
To examine the role of changes in the distribution of the creatine kinase (CK) isoenzymes [BB, MB, MM, and mitochondrial CK (mito-CK)] on the creatine kinase reaction velocity in the intact heart, we measured the creatine kinase reaction velocity and substrate concentrations in hearts from neonatal rabbits at different stages of development. Between 3 and 18 days postpartum, total creatine kinase activity did not change, but the isoenzyme distribution and total creatine content changed. Hearts containing 0, 4, or 9% mito-CK activity were studied at three levels of cardiac performance: KCl arrest and Langendorff and isovolumic beating. The creatine kinase reaction velocity in the direction of MgATP production was measured with 31P magnetization transfer under steady-state conditions. Substrate concentrations were measured with 31P NMR (ATP and creatine phosphate) and conventional biochemical analysis (creatine) or estimated (ADP) by assuming creatine kinase equilibrium. The rate of ATP synthesis by oxidative phosphorylation was estimated with oxygen consumption measurements. These results define three relationships. First, the creatine kinase reaction velocity increased as mito-CK activity increased, suggesting that isoenzyme localization can alter reaction velocity. Second, the reaction velocity increased as the rate of ATP synthesis increased. Third, as predicted by the rate equation, reaction velocity increased with the 3-fold increase in creatine and creatine phosphate contents that occurred during development. 相似文献
992.
The effects of alterations in extracellular calcium concentration on prostaglandin (PGE) and thromboxane (TXB2) syntheses were studied in isolated epithelial cells from the urinary bladder of the toad, Bufo marinus. In epithelial cells prepared using collagenase, basal iPGE synthesis was greater than iTXB2 synthesis. Increasing extracellular calcium from zero to 1 mm increased iPGE synthesis and decreased iTXB2 synthesis equivalently such that total conversion of endogenous arachidonate to these two metabolites was unaltered. Vasopressin stimulated iPGE and iTXB2 syntheses when the incubation buffer contained 1 mm calcium but had no effect in the presence of 0.4 μm calcium. In contrast, using an EDTA isolation method, basal iPGE and iTXB2 syntheses were equal in the presence of zero calcium. Increasing extracellular calcium concentration to 1 mm caused a greater enhancement in iTXB2 synthesis compared to iPGE. Increasing extracellular calcium to 2 mm was associated with a decline in iPGE and iTXB2 syntheses back to the levels observed with no calcium added to the medium. The effect of increasing the calcium concentration was greater in phosphate than in bicarbonate buffer. In a Tris buffer the effect of altered calcium was almost completely abrogated. These studies demonstrate that the choice of buffer and alterations in extracellular calcium concentration differentially alter basal arachidonic acid metabolism to prostaglandins and thromboxane in isolated toad urinary bladder cells. The results suggest that there may exist several endogenous pools of arachidonic acid which are differentially influenced by calcium. Furthermore, the pool sensitive to vasopressin has an absolute requirement for calcium. 相似文献
993.
In the Hupc mutants of Bradyrhizobium japonicum SR, regulation of expression of hydrogenase is altered; the mutants synthesize hydrogenase constitutively in the presence of atmospheric levels of oxygen. The DNA gyrase inhibitors nalidixic acid, novobiocin, and coumermycin were used to inhibit growth of wild-type and mutant cells. For each inhibitor tested, growth of mutant and wild-type strains was equally sensitive. However, in contrast to the wild type, the Hupc mutants synthesized hydrogenase in the presence of high levels of any inhibitor. Cells were incubated with the drugs and simultaneously labeled with 14C-labeled amino acids, and hydrogenase was immunoprecipitated with antibody to the large subunit of the enzyme. Fluorograms of antibody blots then were scanned to determine the relative amount of hydrogenase (large subunit) synthesized in the presence or absence of the gyrase inhibitors. The amount of hydrogenase synthesized by the Hupc mutants in the presence of 300 micrograms of nalidixic acid per ml was near the level of enzyme synthesized in the absence of the inhibitor. No hydrogenase was detected in antibody blots of wild-type cultures which were derepressed for hydrogenase in the presence of 100 micrograms of coumermycin or novobiocin per ml. In contrast, hydrogenase was synthesized by the Hupc mutants in the presence of 100 micrograms of either drug per ml. The amount synthesized ranged from 5 to 32% and 20 to 49%, respectively, of that in the absence of those inhibitors, but nevertheless, hydrogenase synthesis was detected in all of the mutants examined.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
994.
Hydrogenase synthesis in Bradyrhizobium japonicum Hupc mutants is altered in sensitivity to DNA gyrase inhibitors. 总被引:2,自引:1,他引:1
下载免费PDF全文
![点击此处可从《Applied microbiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
In the Hupc mutants of Bradyrhizobium japonicum SR, regulation of expression of hydrogenase is altered; the mutants synthesize hydrogenase constitutively in the presence of atmospheric levels of oxygen. The DNA gyrase inhibitors nalidixic acid, novobiocin, and coumermycin were used to inhibit growth of wild-type and mutant cells. For each inhibitor tested, growth of mutant and wild-type strains was equally sensitive. However, in contrast to the wild type, the Hupc mutants synthesized hydrogenase in the presence of high levels of any inhibitor. Cells were incubated with the drugs and simultaneously labeled with 14C-labeled amino acids, and hydrogenase was immunoprecipitated with antibody to the large subunit of the enzyme. Fluorograms of antibody blots then were scanned to determine the relative amount of hydrogenase (large subunit) synthesized in the presence or absence of the gyrase inhibitors. The amount of hydrogenase synthesized by the Hupc mutants in the presence of 300 micrograms of nalidixic acid per ml was near the level of enzyme synthesized in the absence of the inhibitor. No hydrogenase was detected in antibody blots of wild-type cultures which were derepressed for hydrogenase in the presence of 100 micrograms of coumermycin or novobiocin per ml. In contrast, hydrogenase was synthesized by the Hupc mutants in the presence of 100 micrograms of either drug per ml. The amount synthesized ranged from 5 to 32% and 20 to 49%, respectively, of that in the absence of those inhibitors, but nevertheless, hydrogenase synthesis was detected in all of the mutants examined.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
995.
996.
Vladimir J. A. Novak 《Origins of life and evolution of the biosphere》1984,14(1-4):513-522
In agreement with the views of Oparin, Fox, Dose etc., the theory assumes that coacervation of protein-like polyaminoacids began with their accumulation along the coasts of the Archaic water basins. Unlike the above authors, however, the present author views the original coacervates as a suitable culture medium from which the first polynucleotides orginated and their partial replication started. Their base sequence was not fortuitous, but determined by the proteinoids on the basis of their mutual affinity. The polyfunctional enzymic activity of the proteinoids catalyzed their replication as well as other activities. Around the replicating DNA molecules secondary coacervates (coacervates in coacervates) accumulated which developed gradually to the first prokaryotic cells. Their most probable evolution to the first eukaryotic organisms is discussed on the basis of the modified Studitsky's synbacteriogenesis theory. 相似文献
997.
Summary From acetylene reduction assays over a 10-month period starting in April 1979, nodule activities averaged 18.78 (se 4.67) moles C2H4 g nodule dw–1 h–1 forAlnus rubra and 59.95 (se 12.14) moles C2H4 g nodule dw–1 h–1 forCytisus scorparius. Plant rates were 1.91 (se. 47) moles C2H4 plant–1 h–1 forA. rubra and 0.55 (se. 17) moles C2H4 plant–1 h–1 forC. Scoparius. Plant activity and total leaf N were strongly correlated with the dw of other plant parts, but nodule activity and percent leaf N were not. Plant and nodule activities were not associated with temperature, moisture stress, precipitation events or percent light for either species over the growing season nor for 54A. rubra sampled in mid-season 1979 on one replication. After 5 to 6 growing seasons, 14A. rubra on the same site ranged from 30 to 332 cm in height and showed strong correlation between nodule dw, leaf dw, plant size and total leaf N. Results from this study and others indicate logistic equations may be modified to predict the effect of adding a N2 fixing plant to a population of non N2 fixing trees. 相似文献
998.
A. Nelson A. De Soyza S.J. Bourke J.D. Perry S.P. Cummings 《Letters in applied microbiology》2010,51(3):272-277
Aim: The aim of this study was to quantitatively and qualitatively assess the effect of sample storage on the metabolically active microbial community found in sputum samples from patients with cystic fibrosis (CF). Methods: Sputum samples were collected and split in two equal aliquots one of which was immersed in RNAlater and refrigerated immediately, the second stored at room temperature for 24 h and RNAlater was subsequently added. mRNA was extracted, and RT‐PCR‐DGGE and qPCR analysis of the bacterial and fungal communities was carried out. Results: Significant differences in the bacterial communities between the two protocols were observed but there were no significant difference seen in the fungal community analyses. Analysis by qPCR demonstrated that room temperature storage gave statistically significant increases in eubacteria and Pseudomonas spp. and a statistically significant decrease in those of Haemophilus influenzae. Conclusions: The analysis of metabolically active microbial communities from CF sputum using molecular techniques indicated that samples should be stored at 4°C upon addition of RNAlater to obtain an accurate depiction of the CF lung microbiota. Also, storing respiratory samples at room temperature may cause an over representation of Pseudomonas aeruginosa and mask the presence of other clinically significant organisms. 相似文献
999.
Kristopher Novak Marcus Schaub Jürg Fuhrer John M. Skelly Beat Frey Norbert Kräuchi 《Environmental and Experimental Botany》2008,62(3):212-220
Seedlings of Fagus sylvatica (beech) and Viburnum lantana (Viburnum) grown in monoculture and mixture were exposed to ambient and sub-ambient (charcoal-filtered) ozone concentrations in open-top chambers over the course of the 2003 and 2004 growing seasons at the WSL Lattecaldo open-top chamber facility in southern Switzerland. The aim of the study was to determine how the sensitivity to ozone in ambient air of these two species would differ between monocultures and mixtures in terms of growth and visible foliar injury development. Ambient ozone concentrations were consistently higher from the end of April to the middle of October in 2003 than in 2004 with seasonal peaks and means reaching 147 and 50 parts-per-billion (ppb) in 2003 compared to 124 and 40 ppb in 2004. Ambient AOT40 (ozone concentration accumulated over a threshold of 40 ppb during daylight hours with global radiation >50 W m?2) values from the end of April to the middle of October reached 48.3 and 26.8 parts-per-billon hours (ppm h) in 2003 and 2004, respectively. In general, Viburnum was a stronger competitor than beech over the course of this 2-year study. Seedlings of Viburnum benefited from interspecific competition in terms of both height growth and above-ground biomass accumulation at the expense of beech seedlings, which showed significantly reduced growth in the mixture as compared to the monoculture. However, as this was only the case for Viburnum growing in the charcoal-filtered treatment, ozone seemed to counteract the beneficial effect of interspecific competition on above-ground biomass accumulation in Viburnum, while at the same time decreasing relative biomass allocation to roots. Foliar sensitivity of the two species was also altered under interspecific competition suggesting that results based on seedlings of single species grown in monocultures may significantly over- or under-estimate foliar sensitivity to ozone. These results demonstrate that competition is an important factor affecting plant responses to ozone stress, but the direction and severity of these effects depend on the interacting species. 相似文献
1000.
Frank B. Jensen Claudio Agnisola Ivana Novak 《Comparative biochemistry and physiology. Part A, Molecular & integrative physiology》2009,152(3):351-356
The present study tested the hypothesis that rainbow trout erythrocytes release ATP upon deoxygenation, a mechanism that enables mammalian erythrocytes to produce local vasodilation. We also investigated ATP release and ectonucleotidase activity in the coronary circulation of the isolated trout heart. Erythrocytes suspended in an albumin-containing saline and equilibrated at physiological Pco2 showed negligible hemolysis (< 0.1%), and notably they released small amounts of ATP. The elevation of extracellular [ATP] was higher in the presence of the ectonucleotidase inhibitor ARL 67156 than in its absence, revealing the presence of ectonucleotidase activity. The induction of either a slow (minutes) or a fast (seconds) decrease in hemoglobin O2 saturation did not lead to additional ATP release. An elevation of Pco2 was also without influence on erythrocyte ATP release. In the saline-perfused coronary circulation, [ATP] increased as the perfusate moved through the vessels in the presence of ARL 67156. When ATP was added to the inflowing saline, most ATP disappeared during passage of the coronary bed when ARL 67156 was absent but not when it was present. We conclude that rainbow trout erythrocytes and vasculature possess the key elements for ATP signaling, i.e. cellular ATP release and balanced ATP degradation by ectonucleotidases, but that erythrocyte ATP release is not influenced by oxygenation degree. The latter is suggested to be related to the lack of a deoxygenation-dependent interaction of trout hemoglobin with the cytoplasmic domain of band 3. 相似文献