全文获取类型
收费全文 | 4629篇 |
免费 | 354篇 |
国内免费 | 1篇 |
出版年
2022年 | 40篇 |
2021年 | 96篇 |
2020年 | 49篇 |
2019年 | 66篇 |
2018年 | 72篇 |
2017年 | 70篇 |
2016年 | 114篇 |
2015年 | 147篇 |
2014年 | 190篇 |
2013年 | 256篇 |
2012年 | 295篇 |
2011年 | 296篇 |
2010年 | 192篇 |
2009年 | 161篇 |
2008年 | 216篇 |
2007年 | 226篇 |
2006年 | 237篇 |
2005年 | 185篇 |
2004年 | 178篇 |
2003年 | 206篇 |
2002年 | 182篇 |
2001年 | 111篇 |
2000年 | 93篇 |
1999年 | 92篇 |
1998年 | 62篇 |
1997年 | 35篇 |
1996年 | 45篇 |
1995年 | 43篇 |
1994年 | 27篇 |
1993年 | 38篇 |
1992年 | 57篇 |
1991年 | 69篇 |
1990年 | 43篇 |
1989年 | 34篇 |
1988年 | 60篇 |
1987年 | 47篇 |
1986年 | 43篇 |
1985年 | 41篇 |
1984年 | 39篇 |
1983年 | 36篇 |
1982年 | 33篇 |
1981年 | 33篇 |
1979年 | 24篇 |
1978年 | 23篇 |
1977年 | 26篇 |
1976年 | 27篇 |
1975年 | 26篇 |
1974年 | 37篇 |
1973年 | 23篇 |
1972年 | 30篇 |
排序方式: 共有4984条查询结果,搜索用时 15 毫秒
71.
Cloning of Pf3, a filamentous bacteriophage of Pseudomonas aeruginosa, into the pBD214 vector of Bacillus subtilis. 总被引:2,自引:1,他引:1
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The genome of Pf3, a filamentous single-stranded DNA bacteriophage of Pseudomonas aeruginosa (a gram-negative organism) was cloned into pBD214, a plasmid cloning vector of Bacillus subtilis (a gram-positive organism). Cloning in the gram-positive organism was done to avoid anticipated lethal effects. The entire Pf3 genome was inserted in each orientation at a unique Bc/I site within a thymidylate synthetase gene (from B. subtilis phage beta 22) on the plasmid. Additional clones were made by inserting EcoRI fragments of Pf3 DNA into a unique EcoRI site within this gene. 相似文献
72.
The distribution of immunoreactive-dynorphin (ir-Dyn) in isolated subpopulations of bovine adrenal chromaffin cells was examined and compared with that of adrenaline (A), noradrenaline (NA) and ir-Leucine-Enkephalin (ir-Leu-Enk). Using a stepwise bovine serum albumin (BSA) gradient, various populations of catecholamine-storing cells were separated and designated as cell layers I, II and III. Cell layer I contained more NA than A; cell layer II contained slightly more A than NA whereas cell layer III was highly enriched in A. The original cell preparation contained 2.9 times more ir-Leu-Enk than ir-Dyn (4.7 and 1.6 pmoles per 10(6) cells, respectively). After separation of the cells on BSA gradient, ir-Dyn was mainly detected in cell layer I (4.0 pmoles/10(6) cells) whereas ir-Leu-Enk was concentrated in cell layer III (8.3 pmoles/10(6) cells). Both peptides were secreted in response to acetylcholine (5 x 10(-5) M), but the amount secreted was in accordance with the cell content in each peptide. After subcellular fractionation of the adrenal medulla, the neuropeptides were found in close association with catecholamines in the secretory granules. These results indicate that bovine adrenal chromaffin cells can be isolated according to their specific content in A, NA and opioid peptides and are consistent with the hypothesis of distinct biosynthetic pathways for Dyn and the Enk. 相似文献
73.
Exogenous NAD Effects on Plant Mitochondria: A Reinvestigation of the Transhydrogenase Hypothesis 总被引:4,自引:4,他引:0
Addition of NAD+ to purified potato (Solanum tuberosum L.) mitochondria respiring α-ketoglutarate and malate in the presence of the electron transport inhibitor rotenone, stimulated O2 uptake. This stimulation was prevented by incubating mitochondria with N-4-azido-2-nitrophenyl-aminobutyryl-NAD+ (NAP4-NAD+), an inhibitor of NAD+ uptake, but not by 1 mm EGTA, an inhibitor of external NADH oxidation. NAD+-stimulated malate-cytochrome c reductase activity, and reduction of added NAD+ by intact mitochondria, could be duplicated by rupturing the mitochondria and adding a small quantity to the cuvette. The extent of external NAD+ reduction was correlated with the amount of extra mitochondrial malate dehydrogenase present. Malate oxidation by potato mitochondria depleted of endogenous NAD+ by storing on ice for 72 hours, was completely dependent on added NAD+, and the effect of NAD+ on these mitochondria was prevented by incubating them with NAP4-NAD+. External NAD+ reduction by these mitochondria was not affected by NAP4-NAD+. We conclude that all effects of exogenous NAD+ on plant mitochondrial respiration can be attributed to net uptake of the NAD+ into the matrix space. 相似文献
74.
Effects of flooding on leaf litter decomposition in microcosms 总被引:3,自引:0,他引:3
Frank P. Day Jr. 《Oecologia》1983,56(2-3):180-184
Summary The effects of hydroperiod on decomposition rates of senescent Acer rubrum leaves were tested in microcosms in a controlled laboratory environment. Microcosm treatments included continuously flooded, continuously unflooded, and fluctuating hydroperiods. All flooding treatments promoted decomposition but variations in hydroperiod had no significant effects. A leaching experiment indicated the higher decay rates under flooded conditions were primarily due to high leaching losses from soaking. Unlike nutrient dynamics in the field, where net accumulation occurs, nitrogen and phosphorus in the litter in the microcosms exhibited net losses. The major external inputs which provide a source of nitrogen and phosphorus for immobilization in the field were lacking in the microcosms. Calcium, magnesium, and potassium exhibited net losses except for calcium in the unflooded microcosms. The microcosm results demonstrated the importance of external inputs to litter nutrient relations. 相似文献
75.
The regulation of glycolysis and electron transport in roots 总被引:3,自引:0,他引:3
The respiration of roots and isolated root mitochondria was investigated in Phaseolus vulgaris L., Spinacea oleracea L.; Triticum aestivum L., and Zea mays L. Although the respiration of both intact roots and isolated mitochondria displayed resistance to cyanide and sensitivity to SHAM, the percentage resistance and inhibition in roots was not the same as that in the mitochondria, with the exception of wheat. Adding FCCP to roots stimulated oxygen uptake and equalized the effects of SHAM and cyanide on roots and mitochondria. In spinach and maize roots, FCCP stimulated both the cytochrome and alternative pathways, while in bean roots, only the alternative pathway was stimulated. FCCP had little effect on wheat root respiration rates. Potential in vivo rates of oxygen uptake were estimated by expressing rates obtained with isolated mitochondria on a fumarase activity basis, and fumarase activity on a root weight basis. In wheat roots the potential rate was approximately equal to the measured in vivo rate; in the other species the potential rates were substantially greater than measured rates, but approximately equal to uncoupled in vivo rates. Key glycolytic intermediates in roots were measured, and it was found that the phosphofructokinase and pyruvate kinase reactions were displaced far from equilibrium, the degree of displacement being approximately equal in roots with little, and roots with substantial, alternative path engagement. Thus, although glycolysis is controlled, the regulation of this pathway appears to be quite flexible. The results are discussed in terms of possible regulatory mechanisms. 相似文献
76.
Cyanide-resistant respiration in roots and leaves. Measurements with intact tissues and isolated mitochondria 总被引:3,自引:0,他引:3
A comparison was made between the oxygen uptake of roots and leaves and of mitochondria isolated from the same tissues. Ten species were included in this study: three legumes, one C3-monocotyledon, one C4-monocotyledon, the rest non-leguminous C3-dicotyledons. Root and leaf respiration in all species examined displayed substantial resistance to KCN (0.1–1.0 mM) and the cyanide-resistant respiration was completely inhibited by salicylhydroxamic acid (SHAM; 10–20 mM). SHAM alone inhibited oxygen uptake to varying degrees, depending on the species. Mitochondria were isolated from roots and leaves of many of the species examined and also displayed cyanide-resistant oxygen uptake, which was sensitive to both SHAM and tetraethylthiuram disulfide (disulfiram). Concentrations of SHAM greater than 2 mM caused inhibition of the cytochrome path as well as of the alternative path in isolated mitochondria. Respiration rates of intact roots and leaves in the presence of varying concentrations of SHAM alone were plotted against those obtained in the presence of both SHAM and KCN. This plot showed that in vivo the cytochrome pathway was not affected by 10 or 20 mM SHAM in the external solution. We conclude that the activity of the alternative pathway in intact roots and leaves can be reliably estimated by comparing SHAM-sensitivity and cyanide-resistance of respiration. 相似文献
77.
78.
Acquisition and maintenance of enterotoxin plasmids in wild-type strains of Escherichia coli 总被引:3,自引:0,他引:3
Enterotoxigenic strains of Escherichia coli (ETEC) may produce a heat-labile enterotoxin (LT), a heat-stable enterotoxin (ST) or both enterotoxins. Certain serogroups are represented more frequently than others in ETEC isolated from humans. The transfer of three plasmids encoding enterotoxin production (Ent) to 22 non-toxigenic E. coli strains of many different O:H serotypes was studied. The Ent plasmids encoded ST (TP276), or LT (TP277), or ST + LT (TP214), and all carried antibiotic-resistance determinants. Twenty-one recipient strains acquired TP214, 18 acquired TP277 and 14 acquired TP276. Strains of those serotypes to which ETEC in diarrhoeal studies commonly belong neither acquired nor maintained Ent plasmids with a higher frequency than strains of those serotypes to which ETEC rarely belong. The recipient strains, with one exception, all expressed ST, or LT, or ST and LT, when they had acquired the appropriate plasmid; a non-motile strain belonging to O serogroup 88 expressed LT but failed to express ST when it acquired TP214 or TP277. 相似文献
79.
Silver and mercury probing of deoxyribonucleic acid structures in the filamentous viruses fd, If1, IKe, Xf, Pf1, and Pf3 总被引:3,自引:0,他引:3
Ag+ binding and Hg2+ binding to both double-stranded DNA (dsDNA) and single-stranded DNA (ssDNA) have been examined in some detail, and the results have been applied to study the structures of circular ssDNA in several filamentous viruses. It has been known for some time that Ag+ and Hg2+ bind to the bases of DNA producing characteristic large changes in absorbance and circular dichroism (CD) spectra, as well as changes in sedimentation rates. In the case of Ag+, it is known that there are three modes of binding to isolated dsDNA, referred to as types I, II, and III. Type III binding, by definition, occurs when Ag+ binds to Ag-dsDNA complexes having sites for binding types I and II extensively occupied, if not saturated. It produces CD spectra, assigned in this study, and absorbance spectra that are isosbestic with those of the Ag-dsDNA complexes present prior to its onset. In phosphate buffers binding is restricted to types I and II, whereas in borate buffers weaker type III binding can occur. Characteristics of types I, II, and III were observed for the DNAs in fd, If1, IKe, and Xf, but not for those in Pf1 and Pf3. Similarly, many of the spectral changes seen when Hg2+ binds to isolated double-stranded DNA are mimicked by Hg2+ binding to the DNAs within fd, IKe, If1, and Xf, but not for those in Pf1 and Pf3. The Ag+ and Hg2+ results indicate the presence of right-handed DNA helices in fd, If1, IKe, and Xf, with the two antiparallel strands of the covalently closed single-stranded DNAs having the bases directed toward the virion axes. For Pf1 and Pf3, Ag+ and Hg2+ binding cause large absorbance changes but only small CD changes. The very different results for Pf1 and Pf3 are consistent with the presence of inverted DNA structures (I-DNA) with the bases directed away from the structure axes, but the two structures differ from one another. Sedimentation velocity changes with Ag+ and Hg2+ binding strongly suggest structural linkages between the DNA and the surrounding protein sheath in each of the viruses. 相似文献
80.
There is some apprehension about the possible effect of the introduced American mink Mustela vison upon the indigenous British fauna, either as a predator exerting new and excessive pressure upon native prey species, or as a carnivore competing for prey resources against native carnivores. Concern is also felt by preservers of game, and by keepers of small domestic animals (poultry and rabbits) about the mink's potential as a pest. To investigate the food habits of the mink in England and Wales the alimentary canals of 1165 trapped animals were examined, yielding 204 samples for analysis. The resulting information provides a general picture of the mink's diet, including seasonal variations. Comparisons were possible with similar work carried out in other countries, and with the diets of other British carnivores. Tentative conclusions were reached regarding the broad pattern of food relationships between the mink and other British animals, wild and domestic. 相似文献