全文获取类型
收费全文 | 3970篇 |
免费 | 317篇 |
出版年
2021年 | 32篇 |
2020年 | 25篇 |
2019年 | 30篇 |
2018年 | 40篇 |
2017年 | 34篇 |
2016年 | 67篇 |
2015年 | 116篇 |
2014年 | 127篇 |
2013年 | 164篇 |
2012年 | 197篇 |
2011年 | 210篇 |
2010年 | 138篇 |
2009年 | 134篇 |
2008年 | 196篇 |
2007年 | 221篇 |
2006年 | 161篇 |
2005年 | 169篇 |
2004年 | 175篇 |
2003年 | 189篇 |
2002年 | 185篇 |
2001年 | 67篇 |
2000年 | 46篇 |
1999年 | 66篇 |
1998年 | 61篇 |
1997年 | 26篇 |
1996年 | 44篇 |
1995年 | 43篇 |
1994年 | 35篇 |
1993年 | 41篇 |
1992年 | 36篇 |
1991年 | 48篇 |
1990年 | 41篇 |
1989年 | 49篇 |
1988年 | 41篇 |
1987年 | 38篇 |
1986年 | 29篇 |
1985年 | 39篇 |
1984年 | 43篇 |
1983年 | 45篇 |
1982年 | 50篇 |
1981年 | 46篇 |
1980年 | 37篇 |
1979年 | 36篇 |
1978年 | 49篇 |
1977年 | 31篇 |
1976年 | 25篇 |
1974年 | 37篇 |
1973年 | 56篇 |
1972年 | 30篇 |
1971年 | 33篇 |
排序方式: 共有4287条查询结果,搜索用时 31 毫秒
121.
OBJECTIVES: To determine the number of people who underwent treatment of urinary stones in Quebec before and after the introduction of extracorporeal shock-wave lithotripsy (ESWL) and to determine how the introduction of ESWL influenced resource utilization. DESIGN: Before-after study; data were obtained from administrative databases and hospital-based cost estimates. SETTING: The 68 acute care hospitals in Quebec in which treatment of urinary stones is undertaken. PATIENTS: Quebec residents admitted to hospital for treatment of urinary stones between the fiscal years 1984 and 1992. OUTCOME MEASURES: Number of people treated for urinary stones per year, total number of procedures per year (including open surgery, percutaneous procedures, retrograde procedures and ESWL), and annual resources (including number of hospital bed-days and direct costs) for treatment of urinary stones used overall and in hospitals with and without ESWL services. RESULTS: Over the study period the number of people treated for urinary stones increased by 59%. As well, the combined frequency of ESWL and surgery (the two main treatment methods) increased by 107%. These increases were largely due to rates of treatment that grew by 52% among women and by 34% among men. The total number of hospital bed-days decreased by 28%, which reflected shorter hospital stays for ESWL. However, despite this decrease, the total direct annual costs were 7% higher in 1992 than in 1984 because of the increased numbers of people treated and procedures performed. In the three hospitals that offered ESWL the number of hospital bed-days and the direct costs of treating urinary stones increased by 49% and $2.5 million respectively. In the 65 other hospitals these figures decreased by 41% and about $2.9 million respectively. CONCLUSIONS: Because of increased intervention rates the total cost of treating urinary stones has risen since the introduction of ESWL. The introduction of ESWL has also been associated with a shift in the use of resources for treating urinary stones to hospitals with a lithotriptor. The reasons for the increased intervention rates are unknown. However, given the possibility of negative health effects and the increased costs, studies to determine whether the increased rates improve health outcomes are warranted. 相似文献
122.
Exercise,agonistic behaviour and food acquisition in Arctic charr,Salvelinus alpinus 总被引:1,自引:0,他引:1
Colin E. Adams Felicity A. Huntingford Jaroslav Krpal Malcolm Jobling Scott J. Burnett 《Environmental Biology of Fishes》1995,43(2):213-218
Synopsis Although swimming is energetically costly, a number of studies on salmonid species have demonstrated increased growth rates in fishes forced to swim for prolonged periods at moderate speeds (typically 1–2 body lengths per sec). This suggests that additional energetic costs of swimming are more than met by alternative compensatory gains. The mechanisms underlying such effects are not fully understood. In this paper, we describe an experiment designed to examine one possible mechanism, namely a swimming-induced inhibition of aggression, with consequent beneficial effects on growth. The study used Arctic charr,Salvelinus alpinus, a species for which a positive relationship between exercise and growth has been clearly established. Using direct behavioural observations on small groups, we demonstrate that individuals displaying high levels of aggressive behaviour are able to monopolise access to food and that enforced swimming at a moderate speed (1 body length per sec) reduces the incidence of aggression although not the degree of monopolisation of food shown by aggressive individuals. These results suggest that the enhanced growth rates accompanying enforced swimming may reflect lower energetic costs of reduced aggressive activity rather than improved access to food by subordinates. 相似文献
123.
Kenneth J. Snibson David Woodcock Jacqueline M. Orian Malcolm R. Brandon Timothy E. Adams 《Transgenic research》1995,4(2):114-122
We have examined transgene methylation in the DNA from the livers of a pedigree of mice carrying three copies of an integrated MToGH1 transgene. Utilizing the methylation-sensitive isoschizomersMsp I andHpa II, Southern blot analysis revealed that all second generation animals derived from a transgenic female had hypermethylated DNA, whereas first generation animals sired by a transgenic male displayed a range of methylation phenotypes ranging from no methylation to hypermethylation of the transgene sequences. Of the mice that exhibited hypermethylation of the transgene in CpG dinucleotides (CmCGG), a minority of these animals also exhibited apparent CpC methylation (i.e. inhibition ofMsp I cutting, presumably blocked by methylation of the outer C of CCGG). Methylation was also examined in the inner C of CC(A/T)GG sequences in the MToGH1 transgene using the isoschizomer pairBstN I andEcoR II. A minority of MToGH1 animals in the F1 generation showed clear evidence of methylation in these sites as well as in the inner and outer Cs of CCGG sites. An examination of MToGH1 expression in terms of oGH levels in serum revealed that there was a high degree of variation in the levels of circulating oGH between animals of this pedigree. There was a weak inverse relationship between the serum level of oGH and the extent of methylation of the transgene. In particular, mice exhibiting CpC together with CpG methylation were found to have very low levels of circulating oGH. Our results highlight the nature and complexity of epigenetic factors associated with transgene sequences which may ultimately influence expression of introduced genes in the mammalian genome. 相似文献
124.
Malcolm N. Jones Philip Manley Angela Holt 《International journal of biological macromolecules》1984,6(2):65-68
The binding of sodium n-dodecyl sulphate to lysozyme has been measured by equilibrium dialysis at 25°C and pH 3.2 over a range of ionic strengts from 0.0119 to 0.2119. Binding isotherms in the region corresponding to ionic binding between the surfactant anions and cationic amino acid residues on the protein have been interpreted in terms of the Hill equation and exhibit positive cooperativity with Hill coefficients in the region of 7–11. The Gibbs energies of binding have been calculated from the Hill binding constants and from the Wyman binding potentials. The stability of the surfactant-protein complexes is discussed in relation to the stability of surfactant micelles. Ionic binding of the surfactant is weakened and hydrophobic binding strengthened by increasing ionic strength. 相似文献
125.
126.
R Deslauriers G C Levy W H McGregor D Sarantakis I C Smith 《European journal of biochemistry》1977,75(2):343-346
13C nuclear magnetic resonance spectroscopy in used to gain information on the flexibility of the backbone in peptide hormones and peptide hormone analogs. 13C spin-lattice relaxation times (T1) were measured on luliberin, the luteinizing-hormone-releasing hormone and des(Gly-NH2)10-luliberin-N-ethylamide in aqueous solution at 25.2 and 67.9 MHz at temperatures of 32 degrees, 40 degrees and 55 degrees C. The 13C spin-lattice relaxation times indicate increased flexibility of the peptide backbone in the immediate environment of glycyl residues in luliberin (less than Glu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2) and the hormone analog des(Gly-NH2)10-luliberin-N-ethylamide (less than Glu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-NH-CH2CH3) in aqueous solutions. 13 C NMR spectroscopy is shown to be a sensitive technique for monitoring the time-averaged conformational flexibility of peptides in solution. Activation energies (Ea) of about 25 kJ/mol were obtained for rotational reorientation of non-terminal alpha-carbons in the peptide backbone. Rotation of methyl groups was characterized by an Ea of 9.6 kJ/mol whereas reorientation of the N-terminal pyroglutamyl residue showed an Ea value of 14.6 kJ/mol. The Ea values of individual carbons in the side-chains of prolyl, arginyl and leucyl residues in the peptides were similar to those obtained for the alpha-carbon of the same amino acid residue in the peptide backbone of the hormones. 相似文献
127.
Chemical analysis of Melampodium americanum yielded, besides the known melampodinin A, four new melampolides, melampodinin B, melampodinin C, 9-des 相似文献
128.
129.
Populations of Methane-Producing Bacteria and In Vitro Methanogenesis in Salt Marsh and Estuarine Sediments 总被引:5,自引:5,他引:0
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)
Most probable numbers (MPNs) of methanogens in various salt marsh and estuarine sediments were determined with an anaerobic, habitat-simulating culture medium with 80% H2 plus 20% CO2 as substrate. Average MPNs for the short Spartina (SS) marsh sediments of Sapelo Island, Ga., were maximal at the 5- to 7-cm depth (1.2 × 107/g of dry sediment). Populations decreased to approximately 880/g of dry sediment at the 34- to 36-cm depth. There was no significant difference between summer and winter populations. In tall Spartina (TS) marsh sediments, average populations were maximal (1.2 × 106/g of dry sediment) in the upper 0- to 2-cm zone; populations from the 5- to 36-cm zones were similar (average of 9 × 104/g of dry sediment). Methanogenic populations for TS sediments of James Island Creek marsh, Charleston, S.C., were similar (average of 3 × 106/g of dry sediment) for all depths tested (0 to 22 cm), which was comparable to the trend observed for TS sediments at Sapelo Island, Ga. Sediment grab samples collected along a transect of James Island Creek and its adjacent Spartina marsh had MPNs that were approximately 20 times greater for the region of Spartina growth (average of 106/g of dry sediment) compared with the channel (approximately 5 × 104 methanogens per g of dry sediment). A similar trend was found at Pawley's Island marsh, S.C., but populations were approximately one order of magnitude lower. In vitro rates of methanogenesis with SS sediments incubated under 80% H2-20% CO2 showed that the 5- to 7-cm region exhibited maximal activity (51 nmol of CH4 g−1 h−1), which was greater than rates for sediments above and below this depth. SS sediment samples (5 to 7 cm) incubated under 100% N2 and supplemented with formate exhibited rates of methanogenesis similar to those generated by samples under 80% H2-20% CO2. Replacing the N2 atmosphere with H2 resulted in an eightfold decrease in the rate of methanogenesis. In vitro methanogenic activity by TS salt marsh sediments, incubated under 80% H2-20% CO2, was similar for all depths tested (0 to 22 cm). TS sediment samples (0 to 7 cm) supplemented with formate and incubated under 100% N2 had greater rates of methanogenesis compared with unsupplemented samples. 相似文献
130.
John L. McGregor Kenneth J. Clemetson Elizabeth James Ernst F. Luscher Marc Dechavanne 《生物化学与生物物理学报:生物膜》1980,599(2):473-483
Intact human blood platelets were radioactively labelled at the surface by techniques specific for proteins or glycoproteins. Labelled platelet samples were analyzed by a high-resolution two-dimensional separation system involving isoelectric focusing in the first dimension and discontinuous sodium dodecyl sulphate-polyacrylamide gel electrophoresis in the second. The major platelet membrane glycoprotein (GP) bands (Ib, IIb, IIIa and IIIb) were found to be highly heterogeneous even after removal of terminal sialic acid residues. Lactoperoxidase-catalyzed iodination of platelets showed that the major labelled proteins (Ib, IIb, IIIa and IIIb) had altered isoelectric points () and molecular weights after neuraminidase treatment. A number of membrane glycoproteins previously undetected by one-dimensional gel electrophoresis were demonstrated and good evidence provided that the major platelet surface proteins are glycosylated. 相似文献