全文获取类型
收费全文 | 2302篇 |
免费 | 192篇 |
国内免费 | 2篇 |
出版年
2023年 | 10篇 |
2022年 | 20篇 |
2021年 | 47篇 |
2020年 | 43篇 |
2019年 | 47篇 |
2018年 | 54篇 |
2017年 | 44篇 |
2016年 | 68篇 |
2015年 | 100篇 |
2014年 | 82篇 |
2013年 | 164篇 |
2012年 | 170篇 |
2011年 | 201篇 |
2010年 | 130篇 |
2009年 | 112篇 |
2008年 | 152篇 |
2007年 | 163篇 |
2006年 | 135篇 |
2005年 | 136篇 |
2004年 | 117篇 |
2003年 | 93篇 |
2002年 | 100篇 |
2001年 | 13篇 |
2000年 | 14篇 |
1999年 | 17篇 |
1998年 | 17篇 |
1997年 | 18篇 |
1996年 | 10篇 |
1995年 | 9篇 |
1994年 | 8篇 |
1993年 | 17篇 |
1992年 | 11篇 |
1991年 | 8篇 |
1990年 | 12篇 |
1989年 | 10篇 |
1988年 | 6篇 |
1987年 | 11篇 |
1986年 | 8篇 |
1985年 | 9篇 |
1984年 | 8篇 |
1983年 | 6篇 |
1982年 | 13篇 |
1981年 | 10篇 |
1979年 | 6篇 |
1978年 | 10篇 |
1976年 | 6篇 |
1975年 | 9篇 |
1974年 | 6篇 |
1973年 | 9篇 |
1972年 | 6篇 |
排序方式: 共有2496条查询结果,搜索用时 15 毫秒
1.
Halasz Norbert; Nowycky Martha C.; Shepherd Gordon M.; Hofelt Tomas 《Chemical senses》1985,10(2):203-218
aftographic exeperiments on the localization of radiolabelednoradrenaline, dopamine and dopa, as well as immunohistochemicalstudies on hydroxylase-like activity, are summarized and comparedin both rat and turtle olfactory bulbs. Evoked field potentialstudies on effects of dopamine are also discussed. The histochemicalstudies suggest that dopaminergic periglomerular neurons arethe most significant cellular component of the catecholaminergicsystem in the olfactory bulb of both species. Scattered fluorescentcell group was also present in the internal plexiform layerand superficial granule cell layer of the turtle olfactory bulb.Other fibres, not related to intrinsic bulbar neuronal cellbodies, were also labeled, mostly in the granule cell layerbut also in the external plexiform layer. These might belongto a centrifugal catecholaminergic system from brain stem neurons.In the in vitro turtle olfactory bulb, dopamine and apomorphinedepressed the amplitude of field potentials evoked by a singlevolley in the olfactory nerve or lateral olfactory tract, andreduced the depression and latency of reponses when paired volleywere delivered. It is suggested that catecholaminergic systemsplay a key role in modulating mitral cell activity through actionsin both superficial (glomerular) and deep (granule) layers.This may involve direct actions, or other, non-catecholaminergicinterneurons. 相似文献
2.
F. M. Swain J. Baysinger J. M. Bratt 《Origins of life and evolution of the biosphere》1976,7(3):239-257
Drill core samples of 42 Precambrian sedimentary, igneous, and metamorphic rocks were analyzed by heating under partial vacuum at 100°C and at 400°C to release hydrocarbons and other volatile products.The core samples yielded methane in amounts ranging from traces to 3 microliters per gram, but averaged much less. By way of comparison, samples of Middle Devonian Marcellus black shale, from Pennsylvania, yielded methane in amounts up to 7ul/g.Other straight chain hydrocarbons up to C11 were found in the volatile products, especially those obtained at 400°C, and benzene was a common product, also mainly in the 400°C experiments. Carbon dioxide and nitrogen appear to form a large part of the nonhydrocarbon volatiles in at least some of the samples.Spectral data indicate that the straight chain pyrolysis products of the Precambrian rocks are mainly alkenes, whereas those of the Devonian rocks, referred to above, are a mixture of alkanes and alkenes. Alkanes were however, obtained from several algae-bearing Middle Precambrian argillites. Available evidence indicates, although not conclusively, that the alkenes were contained in the rock rather than being produced from alkanes during pyrolysis.The writers believe that surface contamination in most of the drill cores was minimal owing to the low permeability of the rocks studied, and that contamination by drilling was also minimal.There is a reasonable possibility that the volatiles, if not formed from kerogen residues by the pyrolysis experiments, are in part juvenile igneous gases or are substances that were distilled out of the deeperlying rocks during intervals of folding and metamorphism, and subsequently accumulated at higher levels. 相似文献
3.
4.
The recovery of radioactivity in the urine of guineapigs following a bolus intravenous dose of chromatographically pure 14C-Ntau-methylhistidine was measured in order to test whether the excretion of Ntau-methylhistidine (Ntau-MH) is a valid index of myofibrillar protein breakdown in these animals. Four male and four female guineapigs were dosed and after 7 days, 91.65+/-2.82% and 3.58+/-0.91% of injected radioactivity was recovered in the excreta and tissues, respectively. The average total recovery of 95.2+/-3.0% was not significantly different from 100%. Male guineapigs excreted the radioactivity more slowly than females (70% of the dose excreted within 74 h vs 39 h, respectively) but cumulative excretion at 7 days was the same for each sex. Chromatographic analysis of the urine showed almost all of the radioactivity to be associated with a single peak corresponding to Ntau-MH, indicating a lack of significant metabolism. These data show that although the clearance of 14C-Ntau-MH is slower than in rats or humans the urinary excretion of Ntau-MH is a valid index for myofibrillar protein degradation in the guineapig. 相似文献
5.
Increased concentrations of the endogenous tryptophan metabolite 3-hydroxykynurenine (3-HK) were measured in the brains of vitamin B6 deficient neonatal rats. Mean concentrations of 3-HK in B6 deficient cerebellum, corpus striatum, frontal cortex, and pons/medulla ranged from 9.7 to 18.6 and 102 to 142 nmol/g of wet tissue at 14 and 18 days of age, respectively. 3-HK was not significantly increased in control neonatal or adult rat brain, vitamin B6 deficient rat brain at 7 days of age, or in brains from adult rats deprived of vitamin B6 for 58 days. The administration of daily intraperitoneal injections of vitamin B6 from the 14th to the 18th day of age decreased the concentration of 3-HK to control levels. 3-HK has been shown by other investigators to produce seizures when injected into the cerebral ventricles of adult rodents. Thus, our studies show the accumulation in brain of a putative endogenous convulsant as the result of a nutritional deficiency. 相似文献
6.
Tomas Jacobsen Bo Jensen Jørgen Olsen Knud Allermann 《Applied microbiology and biotechnology》1989,32(3):256-261
Summary The imperfect fungus Geotrichum candidum produced extracellular lipase in a basic peptone-salt medium. By adding olive oil or Tween 80 to the basic medium the lipase yields could be enhanced and the maximal yields were found with Tween 80, which resulted in a sixfold increase in extracellular lipase activity as compared with basic medium. During the early phase of growth in medium with olive oil the proportion of cell-bound activity was higher than that of extracellular activity, and a delay in the secretion of extracellular lipase was found. The proportion of cell-bound activity from growth in basic medium and in basic medium with Tween 80 was lower than that of extracellular activity during the entire growth phase. Analyses by polyacrylamide gel electrophoresis showed that the lipase activity from growth in all three media could be ascribed to equivalent protein bands at 57 000 and 61 000 daltons. Immunodiffusion showed that the cell-bound preparation contained lipase that was immunologically identical with purified extracellular lipase from G. candidum. 相似文献
7.
8.
Per-Ola Arvidsson CharlotteEva Bratt Lars-Erik Andréasson Hans-Erik ?kerlund 《Plant Growth Regulation》1991,10(4):381-382
Book Review
Photosynthesis: Molecular biology and bioenergeticsG.S. Singhal, J. Barber, R.A. Dilley, Govindjee, R. Haselkorn and P. Mohanty (Eds), Proceedings of the International Workshop on Application of Molecular Biology and Bioenergetics of Photosynthesis. Berlin, Heidelberg, New York, London, Paris, Tokyo, Hong Kong and Narosa: Springer-Verlag, 1989. xiv+441 pages. DM 148,00. ISBN 3-540-50451-6 相似文献9.
The carbon-13 nuclear magnetic resonance spectra of aquocobalamin, adenosylcobalamin, methylcobalamin, and (carboxymethyl)cobalamin have been interpreted. The assignments were made by a comparison of the spectra with that of cyanocobalamin, by a study of the pH dependence of the chemical shifts, by an analysis of the effect of the axial ligands on the carbon atoms of the corrin ring, and by a study of the specific line broadening effect of the paramagnetic ions Mn2+ and Gd3+. The chemical shift changes that accompany the "base-on"----"base-off" conversion of the organocobalamins demonstrate that the conformation of the "western" half of the corrin ring and the conformations of the a, b, c, d, f, and g side chains are relatively constant. In contrast, the conformations of the "eastern" half of the corrin ring and the e propionamide side chain are highly variable. 相似文献
10.
Farkas Vánky Eva Klein Jan Willems Kim Böök Torbjörn Ivert Arpád Péterffy Ulf Nilsonne Andris Kreicbergs Tomas Aparisi 《Cancer immunology, immunotherapy : CII》1986,21(1):69-76
Summary Lymphocyte-mediated lysis of autologous tumor cells (autologous lymphocyte cytotoxocity ALC) was tested at the time of surgery in 108 patients (46 squamous cell carcinomas of the lung, 25 adenocarcinomas of the lung, 19 soft tissue sarcomas and 18 osteosarcomas). The clinical course of these patients in relation to the test results has been published previously. The group was evaluated again after an extended observation time, now with a mean of 80.2 months (range 36–108). The test was rarely positive in patients with metastasis (2 out of 28 experiments).There was a correlation between the ALC results and the postsurgical clinical course for patients without detectable metastasis in that (1) a negative test was invariably a bad prognostic sign, i.e., all 32 patients with negative ALC died within 3 years (mean survival time 16.1 months). (2) The remission and survival times were longer for the ALC positive patients (p<0.001). (3) All 37 individuals who are alive at present without recurrence belong to the reactive group.The ALC results correlated with the clinical course in 88% of patients. The correlation was highest for the groups of soft tissue sarcoma and adenocarcinoma of the lung. There was no correlation between killing of K562 cells and ALC, or between lymphoproliferative response to PHA and ALC reactivity. 相似文献