全文获取类型
收费全文 | 15495篇 |
免费 | 1521篇 |
国内免费 | 11篇 |
专业分类
17027篇 |
出版年
2023年 | 119篇 |
2022年 | 273篇 |
2021年 | 603篇 |
2020年 | 282篇 |
2019年 | 349篇 |
2018年 | 418篇 |
2017年 | 347篇 |
2016年 | 617篇 |
2015年 | 973篇 |
2014年 | 980篇 |
2013年 | 1114篇 |
2012年 | 1423篇 |
2011年 | 1386篇 |
2010年 | 828篇 |
2009年 | 658篇 |
2008年 | 907篇 |
2007年 | 845篇 |
2006年 | 852篇 |
2005年 | 681篇 |
2004年 | 623篇 |
2003年 | 548篇 |
2002年 | 498篇 |
2001年 | 136篇 |
2000年 | 105篇 |
1999年 | 124篇 |
1998年 | 126篇 |
1997年 | 66篇 |
1996年 | 80篇 |
1995年 | 43篇 |
1994年 | 54篇 |
1993年 | 61篇 |
1992年 | 70篇 |
1991年 | 54篇 |
1990年 | 56篇 |
1989年 | 48篇 |
1988年 | 23篇 |
1987年 | 41篇 |
1986年 | 35篇 |
1985年 | 50篇 |
1984年 | 46篇 |
1983年 | 37篇 |
1982年 | 31篇 |
1981年 | 23篇 |
1980年 | 23篇 |
1979年 | 23篇 |
1978年 | 25篇 |
1977年 | 23篇 |
1975年 | 21篇 |
1973年 | 28篇 |
1972年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
122.
The hemoprotein content of 17 strains of Bradyrhizobium japonicum bacteroids from field-grown plants and the corresponding strains of cultured cells was determined spectrally. The major terminal oxidases, cytochromes (cyt) aa3 and o, were present in all strains of cultured cells. cyt aa3 was present in significant amounts in bacteroids only in strains of DNA homology group II. cyt o appeared to be present in bacteroids of all strains, and the average level was the same as in cultured cells. cyt b and c in the membrane fractions were higher in bacteroids of all strains compared with cultured cells. cyt P-450 was present in both the membrane and soluble fractions of bacteroids of most strains. The total P-450 content varied sixfold among strains. A CO-reactive hemoprotein, P-422, was present in the soluble fraction of all strains of cultured cells. P-422 may be a hemoglobinlike protein, and it was present in significant amounts in bacteroids only in DNA homology group I strains. 相似文献
123.
124.
Sarah Ann Woodin 《Journal of experimental marine biology and ecology》1982,60(1):35-45
Data on the morphology and behavior of five species of spionid polychaete from muddy sand habitats in both the United Kingdom and the northwest coast of North America are presented. The data are consistent with the hypothesis that browsing predation by visual predators is important now and has been important in the evolutionary past. As predicted by such a hypothesis, regenerating individuals are common in all the populations examined. Individuals belonging to species that expose their anterior ends while feeding and/or defecating have cryptic anteriors (Pygospio elegans Claparède, Rhynchospio glutaeus (Ehlers), Malacoceros fuliginosus (Claparède), and Spiophanes bombyx (Claparède)). Individuals belonging to species that do not expose their anteriors while feeding and/or defecating do not have cryptic anteriors (Pseudopolydora kempi (Southern)). Given these data it is still unclear as to how many of the geographic patterns of coloration that have been reported for infauna are associated with the presence of visual predation. Further investigations are suggested. 相似文献
125.
J. M. Field P. R. Shewry B. J. Miflin J. F. March 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1982,62(4):329-336
Summary Homologous high molecular weight storage prolamins were purified from grain of wheat, rye and barley using combinations of gel filtration, ion-exchange chromatography and preparative isoelectric focusing. Sodium dodecylsulphate polyacrylamide gel electrophoresis showed that the components were single bands with apparent mol.wts. of above 100,000. Molecular weights determined by sedimentation equilibrium ultracentrifugation were considerably lower; 54,700, 67,600 and 69,600 for the components from barley, rye and wheat respectively. Amino acid analysis showed the presence of 13.6 to 16.5 mol% glycine, 29.6 to 34.0 mol% glutamate + glutamine, 11.4 to 13.7 mol% proline and a total of 4.0 to 5.7 mol% basic amino acids. Automated N-terminal amino acid sequencing of the component from wheat showed the presence of cysteine residues at positions 5 and 10, and this is discussed in relation to the possible role of these proteins in the visco-elastic gluten network. 相似文献
126.
Philip L. Smith Stephanie A. Orellana Michael Field M.D. 《The Journal of membrane biology》1981,63(3):199-206
Summary Unidirectional fluxes of35SO4 across and into rabbit ileal epithelium were measured under short-circuit conditions, mostly at a medium SO4 concentration of 2.4mm. Unidirectional mucosa (m)-to-serosa (s) ands-to-m fluxes (J
ms,J
sm) were 0.456 and 0.067 moles hr–1 cm–2, respectively.J
ms was 2.7 times higher in distal ileum than in mid-jejunum. Ouabain abolished net SO4 transport (J
net) by reducingJ
ms. Epinephrine, a stimulus of Cl absorption, had no effect on SO4 fluxes. Theophylline, a stimulus of Cl secretion, reducedJ
ms without affectingJ
sm, causing a 33% reduction inJ
net. Other secretory stimuli (8-Br-cAMP, heat-stable enterotoxin, Ca-ionophore A23187) had similar effects. Replacement of all Cl with gluconate markedly reducedJ
net through both a decrease inJ
ms and an increase inJ
sm. The anion-exchange inhibitor, 4-acetoamido-4-isothiocyano-2,2-sulfonic acid stilbene (SITS), when added to the serosal side, reducedJ
ms by 94%, nearly abolishingJ
net. SITS also decreasedJ
sm by 75%. Mucosal SITS (50 m) was ineffective. 4,4-diisothiocyano-2,2-sulfonic acid stilbene (DIDS) had effects similar to SITS but was less potent. Measurements of initial rates of epithelial uptake from the luminal side (J
me) revealed the following: (1)J
me is a saturable function of medium concentration with aV
max of 0.94 moles hr–1 cm–2 and aK
1/2 of 1.3mm; (2) replacing all Na with choline abolishedJ
me; (3) replacing all Cl with gluconate increasedJ
me by 40%; (4) serosal SITS had no effect onJ
me; and (5) stimuli of Cl secretion had no effect onJ
me or increased it slightly. Determination of cell SO4 with35SO4 indicated that, at steady-state, the average mucosal concentration is 1.1 mmoles per liter cell water, less than half the medium concentration. Cell SO4 was increased to 3.0mm by adding SITS to the serosal side. Despite net transport rates greater than 1.4 Eq hr–1 cm–2, neither addition of SO4 to the SO4-free medium nor addition of SITS to SO4-containing medium altered short-circuit current. The results suggest that (1) ileal SO4 absorption consists of Na-coupled influx (symport) across the brush border and Cl-coupled efflux (antiport) across the basolateral membrane; (2) the overall process is electrically neutral; (3) the medium-to-cell Cl concentration difference may provide part of the driving force for net SO4 absorption; and (4) since agents affecting Cl fluxes (both absorptive and secretory) have little effect on SO4 fluxes, the mechanisms for their transcellular transports are under separate regulation. 相似文献
127.
Localized secretion of acid phosphatase reflects the pattern of cell surface growth in saccharomyces cerevisiae 总被引:39,自引:20,他引:19 下载免费PDF全文
Secretion of cell wall-bound acid phosphatase by Saccharomyces cerevisiae occurs along a restricted portion of the cell surface. Acid phosphatase activity produced during derepressed synthesis on a phosphate-limited growth medium is detected with an enzyme-specific stain and is localized initially to the bud portion of a dividing cell. After two to three generations of phosphate-limited growth, most of the cells can be stained; if further phosphatase synthesis is repressed by growth in excess phosphate, dividing cells are produced in which the parent but not the bud can be stained. Budding growth is interrupted in α-mating-type cells by a pheromone (α-factor) secreted by the opposite mating type; cell surface growth continues in the presence of α-factor and produces a characteristic cell tip. When acid phosphatase synthesis is initiated during α-factor treatment, only the cell tip can br stained; when phosphate synthesis is repressed during α-factor treatment, the cell body but not the tip can be stained. A mixture of derepressed α cells and phosphatase-negative α cells form zygotes in which mainly one parent cell surface can be stained. The cell cycle mutant, cdc 24 (Hartwell, L.H. 1971. Exp. Cell Res. 69:265-276), fails to bud and, instead, expands symmetrically as a sphere at a nonpermissive temperature (37 degrees C). This mutant does not form a cell tip during α-factor treatment at 37 degrees C, and although acid phosphatade secretion occurs at this temperature, it is not localized. These results suggest that secretion reflects a polar mode of yeast cell- surface growth, and that this organization requires the cdc 24 gene product. 相似文献
128.
John I. Gallagher Sarah E. Robinson William J. Pfeiffer Denise M. Seliskar 《Hydrobiologia》1979,67(1):3-9
The biological oxygen demand (BOD) of filtered water from Lake Wingra, Wisconsin is significantly higher in the littoral zone than in the pelagial zone. Laboratory experiments indicate that BOD is not influenced by water temperature at the time of sampling or by enrichment with nitrate or ammonia. Rather, enrichment with macrophyte leachate sharply increases BOD, and enrichment with phosphate produces a small but significant increase in BOD. We conclude that high BOD in littoral waters of the lake is an indication of production of labile organic matter in the water by dense beds of the macrophyte Myriophyllum spicatum. 相似文献
129.
J W Hand H Walker S Hornsey S B Field 《International journal of radiation biology and related studies in physics, chemistry, and medicine》1979,35(6):521-528
The effects of both hyperthermia alone and X-rays combined with hyperthermia on mouse testis have been investigated. Testis weight on heating time was observed for temperatures in the range 39.5 to 43.75 degrees C. The relationship between the reaction rate and the reciprocal of absolute temperature indicated that, over the temperature range considered, the activation energy associated with such thermal damage was (646 +/- 45) x 10(3) J mol-1. No evidence was obtained to indicate a change in slope of the Arrhenius plot over this temperature range. Finally, despite the high sensitivity of the testis to heat and X-rays, no thermal enhancement of the weight loss after irradiation was observed when thermal treatments which, if given alone would produce some observable damage, were administered immediately after irradiation. 相似文献
130.