全文获取类型
收费全文 | 7465篇 |
免费 | 770篇 |
国内免费 | 1篇 |
出版年
2021年 | 97篇 |
2019年 | 69篇 |
2018年 | 95篇 |
2017年 | 78篇 |
2016年 | 125篇 |
2015年 | 216篇 |
2014年 | 212篇 |
2013年 | 323篇 |
2012年 | 359篇 |
2011年 | 337篇 |
2010年 | 222篇 |
2009年 | 213篇 |
2008年 | 284篇 |
2007年 | 340篇 |
2006年 | 323篇 |
2005年 | 293篇 |
2004年 | 285篇 |
2003年 | 245篇 |
2002年 | 264篇 |
2001年 | 202篇 |
2000年 | 199篇 |
1999年 | 188篇 |
1998年 | 120篇 |
1997年 | 98篇 |
1996年 | 56篇 |
1995年 | 92篇 |
1994年 | 72篇 |
1993年 | 98篇 |
1992年 | 162篇 |
1991年 | 135篇 |
1990年 | 145篇 |
1989年 | 123篇 |
1988年 | 112篇 |
1987年 | 109篇 |
1986年 | 98篇 |
1985年 | 114篇 |
1984年 | 102篇 |
1983年 | 85篇 |
1982年 | 74篇 |
1981年 | 65篇 |
1980年 | 79篇 |
1979年 | 91篇 |
1978年 | 72篇 |
1977年 | 77篇 |
1976年 | 68篇 |
1975年 | 74篇 |
1974年 | 84篇 |
1973年 | 89篇 |
1972年 | 62篇 |
1970年 | 58篇 |
排序方式: 共有8236条查询结果,搜索用时 31 毫秒
181.
J R Ramsay G D Adams H C Morris R S Campbell C P Price P M Hammond 《Analytical biochemistry》1992,202(2):331-336
An enzyme-mediated assay has been developed for the measurement of salicylate using salicylate monooxygenase purified from Pseudomonas cepacia ATCC 29351. Two assay formulations were produced, based on either a multiple-reagent or a single-reagent formulation, to allow sufficient flexibility for automated use. The multiple-reagent formulation was especially suited to diagnostic laboratories performing infrequent manual salicylate estimation where stability of the reconstituted reagent is of paramount importance. This was achieved by preparing the enzyme and color reagents in separate vials, so keeping the enzyme at a stable pH. For more frequent assay use where a reconstituted reagent shelf life was less important, the single-reagent system offers advantages of convenience. However, the working reagent required a pH of 10.0 upon reconstitution. Although the enzyme was sufficiently active at this pH to give a reliable assay, its storage stability was poor at pH 10.0, preventing lyophilization of the reagent at a pH suitable for immediate use on reconstitution. This incompatibility was overcome by use of a layering technique. The enzyme was separated from the buffering solution in the same vial by freezing the buffering solution and then overlayering with the enzyme reagent prior to a second freezing cycle and subsequent freeze drying. 相似文献
182.
Establishment of polychlorinated biphenyl-degrading enrichment culture with predominantly meta dechlorination. 总被引:4,自引:4,他引:0 下载免费PDF全文
Enrichment of polychlorinated biphenyl (PCB)-dechlorinating microorganisms from PCB-contaminated sediments from the Upper Hudson River, N.Y., was attempted. The enrichment strategy was to use pyruvate as the electron donor and dechlorination of Aroclor 1242 as the electron acceptor. The enrichment medium also contained non-PCB-contaminated Hudson River sediments, which were required for the PCB-dechlorinating activity. An enrichment culture (that had stable PCBT-dechlorinating activity over nine serial transfers during 1 year) was established under these conditions; however, the rate of dechlorination did not increase after the second serial transfer. Dechlorination occurred primarily from the meta positions of the biphenyl molecule. Hydrogen could be substituted for pyruvate as the electron donor with equal activity, but when acetate was used as the electron donor a delay in dechlorination was observed. Sulfate and bromethane sulfonate inhibited dechlorination activity. The pyruvate-Aroclor 1242 enrichment also dechlorinated Aroclors 1248, 1254, and 1260; the extent of chlorine removed was the greatest for Aroclor 1254. For comparison, nonautoclaved non-PCB-contaminated Hudson River sediments used in the assay also dechlorinated Aroclors, but only after 12 to 16 weeks of incubation. This suggests that PCB-dechlorinating organisms were also present in these sediments but in numbers lower than those in the enrichment culture. 相似文献
183.
K Razdan R L Heinrikson H Zurcher-Neely P W Morris L E Anderson 《Archives of biochemistry and biophysics》1992,298(1):192-197
Two cDNAs which correspond to two very similar Class I aldolases have been isolated from a pea (Pisum sativum L.) cDNA library. With the exception of one codon they match the experimentally determined N-terminal sequence of a pea chloroplast aldolase. The deduced C-terminal sequence of one of these clones is unique among Class I aldolases. The deduced C-terminus of the other is more like the C-terminus of other eucaryotic Class I aldolases. Comparisons of sequence homology suggest that the pea chloroplast isozymes are only marginally more closely related to the anaerobically induced plant aldolases than to aldolases from animals. 相似文献
184.
Association of a polymorphism of the angiotensin I-converting enzyme gene with essential hypertension. 总被引:14,自引:0,他引:14
R Y Zee Y K Lou L R Griffiths B J Morris 《Biochemical and biophysical research communications》1992,184(1):9-15
Angiotensin I-converting enzyme (ACE) is responsible for production of angiotensin II and breakdown of kinins, leading to increased blood pressure (BP). Furthermore, ACE inhibitors are effective antihypertensive agents. A 287 bp insertion/deletion polymorphism in intron 16 of the ACE gene (ACE) was examined by PCR in a cross-sectional study of 80 hypertensive (HT) and 93 normotensive (NT) subjects whose parents had a similar BP status at age greater than or equal to 50. The frequency of the insertion allele was 0.56 in HTs and 0.41 in NTs, and the difference between observed alleles in all subjects in each group was significant (chi 2 = 7.6, P less than 0.01). The data thus provide evidence in favour of an association of HT with a polymorphism at the ACE locus (17q23), so implicating this locus, and possibly a genetic variant of ACE itself, in human essential hypertension. 相似文献
185.
186.
S E Thomas S J Morris Z Xu D M Byers F B Palmer M W Spence H W Cook 《Biochimica et biophysica acta》1992,1126(2):125-134
Plasmalogens (1-O-alk-1'-enyl-2-acyl-sn-glycero-3-phosphoethanolamine) are major phospholipids in many tissues and cells, particularly of neural origin. Using cultured C6 glioma cells and subcellular fractions isolated on Percoll gradients we investigated selectivity for esterification of several polyunsaturated fatty acids (PUFA) in the sn-2 position of plasmalogens compared to [1-14C]hexadecanol, representative of de novo synthesis of the ether-linked sn-1 position. In whole cells at a final concentration of 105 microM PUFA, 2-4 nmol plasmalogen/mg protein was labeled in 4 h and 10-14 nmol in 24 h, representing 8-15% and 35-50%, respectively, of initial plasmalogen mass. Incorporation of label from hexadecanol was lower than PUFA incorporation (20:5(n-3) greater than 20:4(n-6) greater than 18:3(n-3) much greater than 18:2(n-6)) suggesting deacylation-reacylation at the sn-2 position. Plasmalogens accounted for 50% of total cell ethanolamine phospholipids and 75% in plasma membrane. Using a novel, improved method for extraction of subcellular fractions containing Percoll, plasma membrane also was enriched in plasmalogen relative to microsomes (107.4 +/- 5.2 vs. 40.0 +/- 2.9 nmol/mg protein). Selectivity for esterification at the sn-2 position of plasmalogens with respect to chain length and unsaturation of the fatty acyl chain was similar in both subcellular fractions and reflected that of whole cells. Labeling of plasma membrane with PUFA and fatty alcohol lagged behind that of microsomes. Chase experiments in cells prelabeled with [1-14C]18:3(n-3) for 2 h showed no significant reduction of label in plasmalogen of any subcellular fraction although accumulation of label in the microsomal fraction was slowed initially. Reduction of plasmalogen label (40-50%) did occur in microsomes and plasma membrane when cells prelabeled for 24 h were switched to chase medium with or without chase fatty acid. Our data suggest that esterification of PUFA to plasmalogen may occur at the endoplasmic reticulum with subsequent translocation to plasma membrane resulting in accumulation of relatively stable pools of plasmalogen that are not readily accessible for deacylation-reacylation exchange with newly appearing PUFA. Alternatively, deacylation-reacylation may occur in a more stable phospholipid pool within the plasma membrane but would involve a slower process than at the endoplasmic reticulum. 相似文献
187.
Dr. Marie-Joelle Virolle Victor J. Morris Mervyn J. Bibb 《Journal of industrial microbiology & biotechnology》1990,5(5):295-301
Summary A simple, reliable and sensitive assay for alpha-amylase activity is reported, together with its theoretical derivation, that overcomes many of the problems encountered with other assays, especially when attempting to assay alpha-amylase activity in crude cell extracts or culture supernatants. The method relies on the reduction in turbidity that occurs upon digestion of a starch suspension with alpha-amylase. The initial rate of decrease in turbidity is shown to be proportional to a wide range of enzyme concentrations, permitting a rapid spectrophotometric and kinetic determination of alpha-amylase activity. 相似文献
188.
189.
Felix Gisela; Altmann Thomas; Uwer Ursula; Jessop Alison; Willmitzer Lothar; Morris Peter-Christian 《Journal of experimental botany》1996,47(8):1007-1017
A novel Arabidopsis thaliana (L.) Heynh. developmental mutant,waldmeister (wam), is described. This mutant was found in theprogeny arising from an Ac-Ds tagging experiment, but does notappear to be tagged by an introduced transposon. This recessivenuclear mutation maps between GAPB and ap1 on chromosome 1 andshows extreme morphological and physiological changes in bothfloral and vegetative tissues. Changes to the vegetative phenotypeinclude altered leaf morphology, multiple rosettes, stem fasciation,retarded senescence and disturbed geotropic growth. Changesto the floral phenotype include delayed flowering, increasednumber of inflorescences, determinate inflorescences, alterednumber and morphology of floral organs, chimeric floral organs,and ectopic ovules . wam was crossed to a number of previouslydescribed floral mutants: apetela 2, apetela 3, pistillata,agamous, and leafy. The phenotype of the double mutant was ineach case additive. In the case of agamous, however, the indeterminaterepetitive floral structure of agamous was lacking, emphasizingthe determinate inflorescence growth of wam. The extreme phenotypeof the wam mutant is suggestive of a disturbance to a gene ofglobal importance in the regulation of plant growth and development. Key words: Arabidopsis thaliana, waldmeister, developmental mutant, flower mutant 相似文献
190.
González Mariela Montoya Rolando Candia Arturo Gómez Patricia Cisternas Manuel 《Hydrobiologia》1996,326(1):229-234
The extreme phenotypic variability recognized among the species of Gracilaria has highlighted the need for the application of refined methods to help solve taxa identifications. In Chile, there still exists uncertainty about the exact number of Gracilaria species. Our investigations are centered on DNA analyses of morphotypes collected from different geographical locations, namely Lenga and Isla Santa María, Region VIII (36°00 S to 38°00 S), and Maullín, Region X (39°30 S to 43°40 S). These two regions of Chile are considered as areas of confluence of G. chilensis, G. verrucosa, and a species of Gracilariopsis. In this study four morphotypes, from a natural bed located in Maullín, were analyzed for RFLP of plastid DNA and the results compared with data of four morphotypes from a bed in Lenga. The DNA banding patterns from each enzyme digest were identical irrespective of morphotypes and/or locations. In an attempt to unravel the nature of the morphological differences found among Lenga and Maullín morphotypes, RAPD analyses of nuclear DNA were also performed; however, no polymorphism has been found yet. Therefore, the data of this study, as well as concurrent data from preliminary interfertility tests, suggest that all morphotypes belong to a single taxon, Gracilaria chilensis.Departamento de Botánica, Facultad de Ciencias Naturales y Oceanográficas 相似文献