全文获取类型
收费全文 | 7344篇 |
免费 | 535篇 |
国内免费 | 17篇 |
专业分类
7896篇 |
出版年
2023年 | 35篇 |
2022年 | 81篇 |
2021年 | 154篇 |
2020年 | 84篇 |
2019年 | 96篇 |
2018年 | 127篇 |
2017年 | 99篇 |
2016年 | 201篇 |
2015年 | 319篇 |
2014年 | 339篇 |
2013年 | 419篇 |
2012年 | 525篇 |
2011年 | 564篇 |
2010年 | 361篇 |
2009年 | 287篇 |
2008年 | 409篇 |
2007年 | 436篇 |
2006年 | 409篇 |
2005年 | 354篇 |
2004年 | 319篇 |
2003年 | 324篇 |
2002年 | 314篇 |
2001年 | 63篇 |
2000年 | 46篇 |
1999年 | 70篇 |
1998年 | 77篇 |
1997年 | 64篇 |
1996年 | 53篇 |
1995年 | 62篇 |
1994年 | 46篇 |
1993年 | 54篇 |
1992年 | 51篇 |
1991年 | 22篇 |
1990年 | 27篇 |
1989年 | 23篇 |
1988年 | 34篇 |
1987年 | 24篇 |
1986年 | 27篇 |
1985年 | 36篇 |
1984年 | 51篇 |
1983年 | 35篇 |
1982年 | 51篇 |
1981年 | 45篇 |
1980年 | 49篇 |
1979年 | 23篇 |
1978年 | 34篇 |
1977年 | 30篇 |
1976年 | 25篇 |
1975年 | 21篇 |
1973年 | 21篇 |
排序方式: 共有7896条查询结果,搜索用时 6 毫秒
11.
Manickam Sugumaran Steven J. Saul Victor Semensi 《Archives of insect biochemistry and physiology》1988,9(4):269-281
The mechanism of formation of quinone methide from the sclerotizing precursor N-acetyldopamine (NADA) was studied using three different cuticular enzyme systems viz. Sarcophaga bullata larval cuticle, Manduca sexta pharate pupae, and Periplaneta americana presclerotized adult cuticle. All three cuticular samples readily oxidized NADA. During the enzyme-catalyzed oxidation, the majority of NADA oxidized became bound covalently to the cuticle through the side chain with the retention of o-diphenolic function, while a minor amount was recovered as N-acetylnorepinephrine (NANE). Cuticle treated with NADA readily released 2-hydroxy-3′,4′-dihydroxyacetophenone on mild acid hydrolysis confirming the operation of quinone methide sclerotization. Attempts to demonstrate the direct formation of NADA-quinone methide by trapping experiments with N-acetylcysteine surprisingly yielded NADA-quinone-N-acetylcysteine adduct rather than the expected NADA-quinone methide-N-acetylcysteine adduct. These results are indicative of NADA oxidation to NADA-quinone and its subsequent isomerization to NADA-quinone methide. Accordingly, all three cuticular samples exhibited the presence of an isomerase, which catalyzed the conversion of NADA-quinone to NADA-quinone methide as evidenced by the formation of NANE—the water adduct of quinone methide. Thus, in association with phenoloxidase, newly discovered quinone methide isomerase seems to generate quinone methides and provide them for quinone methide sclerotization. 相似文献
12.
In this work, we have calculated the optimal surface area per amphiphile molecule using a two-dimensional gas of electric dipoles interacting with hard The optimal surface area can be evaluated according to a formula (in the two-body approximation) or as a root of an equation (in the three-body approximation). If the equation has more than one positive root, which is greater than the hard core area, then a polydisperse dimensional distribution of supramolecular aggregates is possible. 相似文献
13.
Cytokinin-induced switch in development in excised cotyledons of radiata pine cultured in vitro 总被引:2,自引:0,他引:2
Victor M. Villalobos Melvin J. Oliver Edward C. Yeung Trevor A. Thorpe 《Physiologia plantarum》1984,61(3):483-489
Cotyledons of Pinus radiata D. Don were cultured under shoot-forming (plus cytokinin) and elongating (minus cytokinin) conditions. Using. autoradiographic and precursor incorporation techniques, the sites and rate of macromolecular synthesis were examined during the first five days in culture. Active incorporation of 3 H-thymidine, 3 H-uridine and 3 H-leucine occurred. In shoot-forming cotyledons the incorporation became preferentially located in the epidermal and sub-epidermal cell layers in contact with the medium. In elongating cotyledons, in contrast, incorporation was randomly distributed, and the amount of incorporation declined with time. Biochemically, differences in DNA, RNA and total protein synthetic patterns were observed. In elongating cotyledons the rates of RNA and protein synthesis were higher during the first 48 h than in shoot-forming tissues, after which the synthetic rates were similar. Two peaks of newly formed DNA were observed in both tissues. These findings indicate that the cytokinin-induced changes in developmental pathways began within 24 h in culture. 相似文献
14.
Abstract: Extracellular protein fractions were obtained (1) by mild, isotonic irrigation of freshly perfused brain tissue; (2) by collection of proteins released into super-fusing medium by physiologically viable slices of rat hippocampus; and (3) by sampling the CSF of anesthetized rats. Analysis of the S-100 protein content of these fractions gave values of 2.8, 4.2, and 1.8 μg S-100/mg protein, respectively. These values were three- to sixfold higher than the S-100 content of the soluble cytoplasmic protein fractions from the same tissue. This several-fold higher S-100 content of the extracellular protein fractions relative to the intracellular cytoplasmic protein fractions indicates that S-100 is selectively released into the extracellular spaces of the brain. We suggest that the biological function of this CNS protein may involve intercellular transfer. 相似文献
15.
Abstract: Dopamine (DA) uptake into synaptosomes from rat corpus striatum was studied in the presence of a monoamine oxidase (MAO) inhibitor and dithiothreitol, by means of a filtration technique. Under these conditions a steady state develops rapidly in which the synaptosomal DA content remains constant while the continuing DA uptake is counterbalanced by DA efflux from the synaptosome. Exchange of synaptosomal [3H]DA and [14C]DA was measured under these conditions. In timecourse experiments it was found that exchange could be described significantly better by a three-compartment model than by a two-compartment model. However, if synaptosomes from reserpine-pretreated animals were used, analysis according to a three-compartment model did not result in a significantly better fit compared with a two-compartment model. Subsequently, kinetic transfer parameters describing DA fluxes between compartments at different DA concentrations were calculated from the fitted exchange curves. A Michaelis-Menten kinetic analysis indicated that only the in-series three-compartment configuration, in which DA is taken up from the medium into one synaptosomal compartment, from which it can subsequently be transferred to a second compartment without direct access to the medium, gave kinetically acceptable results. Transfer parameters in synaptosomes from reserpine-treated rats were comparable to those parameters describing DA transport between the medium and the first intrasynaptosomal compartment as measured under control conditions. Morover, it was found that potassium depolarization of synaptosomes resulted in a release of DA in a quantity similar to that found in the second intrasynaptosomal compartment. It is suggested that the two intrasynaptosomal compartments found correspond to a cytoplasmatic and vesicular DA pool, respectively. The functional significance of these findings is discussed in terms of the regulation of DA levels within the nerve terminal. 相似文献
16.
Victor G. Bruce 《Genetics》1974,77(2):221-229
Mutants affecting the period length of the biological clock in Chlamydomonas reinhardi have been isolated and a start has been made on analyzing the genetics of this system. In four mutants, the long period characteristic seems to be controlled by single genes at separate loci. Crosses between single mutants, as well as crosses involving three or four mutant genes, yielded progeny with periods characteristic of the parents as well as recombinant types, including normal period (wild type) and extra-long periods (double, triple and quadruple mutants). It was found that the period lengthening effect is additive; that is, the period of double mutants is lengthened by the sum of the period lengthening of the single mutants. 相似文献
17.
18.
Rifampin inhibited 20 strains of Mycobacterium tuberculosis in concentrations of 0.005 to 0.02 mug/ml in 7H-9 broth with Tween 80 and killed all or nearly all of the inoculum in four to eight times greater concentrations. In the same medium without Tween 80, as well as on 7H-10 agar, about 16 to 64 times these amounts were required to produce the same effect. Rifampin was also active against M. kansasii and some of the nonchromogenic mycobacteria. The incidence of mycobacterial cells resistant to rifampin within the cultures studied was in the range of one to four per 10(8) to 10(9) colony-forming units with concentrations of 4 to 125 mug of rifampin per ml. Only one of the Battey cultures and that of M. fortuitum yielded cells resistant to rifampin at 125 mug/ml but not at 500 mug/ml. The same strains yielded more than double that number of organisms resistant to streptomycin and up to 100 times more organisms resistant to isoniazid. All three drugs stopped the growth or reduced the mycobacterial population in growing cultures after contact for 24 to 48 hr. Complete inhibition of growth was produced by rifampin at 1.0 mug/ml in an average of 6 days and by streptomycin at 5.0 mug/ml in 3 days. After an average contact of 10.7 days with rifampin, five of seven strains resumed growth and all strains began regrowth after exposure to streptomycin for 9.4 days. The marked susceptibility of M. tuberculosis and of atypical mycobacteria to rifampin in vitro and the relatively low incidence of resistant mutants suggests that this agent may have clinical usefulness in the treatment of tuberculosis and some other mycobacterioses. 相似文献
19.
Victor Lorian 《Applied microbiology》1966,14(4):603-607
A study of growth and colony morphology of mycobacteria was performed on 7H10 medium and compared with the same medium to which WR 1339 was added. Collection strains of human, bovine, and atypical mycobacteria, in addition to 1,199 sputa, were planted on both media. The results showed that WR 1339 modifies the growth and colony morphology of human, bovine, atypical group I, and, to a lesser degree, atypical groups II, III, and IV mycobacteria. The great majority of human and bovine strains exhibit definite cords when examined at a magnification of 100 times. The colonies are larger when WR 1339 is added, especially if there is a small number of colonies. The addition of WR 1339 spreads the growth on the surface of the agar, producing a thin but larger colony as compared with 7H10 medium alone on which the colonies grow more vertically, thus producing thick but smaller colonies. WR 1339 spreads the growth, producing thin and transparent colonies where the cords are oriented in uniplane and easily visible directly on the isolation media. Half of the positive sputum cultures were easily identified at 12 days. The presence of typical cords permits a quick screening diagnosis of species directly on the isolation media and, in most instances, elimination of the possibility of atypical or contaminating colonies. 相似文献
20.
Kailash Prasad Victor A. Laxdal Ming Yu Barbara L. Raney 《Molecular and cellular biochemistry》1996,165(1):55-63
While antibiotics are broadly used in dental and medical therapy, little attention has been directed towards the potential toxic side effects of antibiotics on tissue regeneration. Here we examined the effect of a quinolone antibiotic, pefloxacin (Rhone Poulenc) on rat parotid gland responses to chronic isoproterenol treatment. Groups of rats received injections of isoproterenol to induce glandular growth, saline (controls), pefloxacin, or isoproterenol and pefloxacin in combination. Parotid gland weight decreased significantly after pefloxacin treatment for 7 days as well as inhibiting glandular enlargement provoked by isoproterenol. The same trend was observed for the rates of DNA synthesis, with the incorporation of [3H]-thymidine in isoproterenol/pefloxacin-treated rats reduced to 49% of isoproterenol treatment alone levels. Saline-treated animals were 42% of the rate of [3H]-thymidine incorporation into DNA observed in isoproterenol treated rats. While isoproterenol treatment increased steady-state mRNA levels for fos, jun, myc, src, c-erbB-2, ras and topo II, inclusion of pefloxacin with the isoproterenol regimen blocked these increases. Pefloxacin treatment by itself did not alter proto-oncogene mRNA levels in the parotid gland. Glandular amylase activity was decreased in the pefloxacin treated group, while the combination of isoproterenol with pefloxacin did not decrease glandular amylase levels to the extent of that observed with -agonist treatment alone. In acute experiments, pefloxacin significantly decreased the volume of saliva secreted by the parotid gland. These results suggest that quinolone-based antibiotics disturb the secretory function of the parotid gland and can inhibit cell proliferation and regeneration. (Mol Cell Biochem 165: 55–63, 1996) 相似文献