全文获取类型
收费全文 | 2485篇 |
免费 | 158篇 |
国内免费 | 362篇 |
出版年
2024年 | 4篇 |
2023年 | 41篇 |
2022年 | 42篇 |
2021年 | 70篇 |
2020年 | 65篇 |
2019年 | 106篇 |
2018年 | 110篇 |
2017年 | 61篇 |
2016年 | 68篇 |
2015年 | 84篇 |
2014年 | 162篇 |
2013年 | 177篇 |
2012年 | 103篇 |
2011年 | 147篇 |
2010年 | 114篇 |
2009年 | 140篇 |
2008年 | 142篇 |
2007年 | 142篇 |
2006年 | 128篇 |
2005年 | 103篇 |
2004年 | 70篇 |
2003年 | 86篇 |
2002年 | 76篇 |
2001年 | 67篇 |
2000年 | 47篇 |
1999年 | 48篇 |
1998年 | 43篇 |
1997年 | 37篇 |
1996年 | 35篇 |
1995年 | 36篇 |
1994年 | 38篇 |
1993年 | 32篇 |
1992年 | 31篇 |
1991年 | 36篇 |
1990年 | 32篇 |
1989年 | 26篇 |
1988年 | 27篇 |
1987年 | 32篇 |
1986年 | 28篇 |
1985年 | 34篇 |
1984年 | 36篇 |
1983年 | 12篇 |
1982年 | 22篇 |
1981年 | 17篇 |
1980年 | 23篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1977年 | 6篇 |
1974年 | 2篇 |
1972年 | 3篇 |
排序方式: 共有3005条查询结果,搜索用时 31 毫秒
31.
Exponentially dividing culture forms of Trypanosoma brucei did not utilize glucose provided in the culture medium. The inclusion of 2-deoxyglucose in the medium had no effect on the growth of the trypanosomes. Glucose could be replaced by proline in the liquid phase of biphasic medium without affecting the doubling time of the organisms. Proline added to the culture medium in this way disappeared during the log phase of growth. Glucose in the culture medium was used by the trypanosomes only when the stationary growth phase had been reached. Lipid accumulated in stationary phase trypanosomes grown in glucose-containing medium, but there was no lipid accumulation in log phase organisms or in those which had been grown in proline-containing medium. Bloodstream trypanosomes transferred to liquid medium rapidly utilized glucose over the first 12 hr of culture, and this was accompanied by an accumulation of free pyruvate in the medium. The rate of glucose utilization fell off over the next 36 hr; this was accompanied by a lowering of free pyruvate in the medium and a rise in the proline oxidase activity of the trypanosomes. The possible biologic significance of proline to trypanosomes developing in the midgut of the tsetse vector is discussed. 相似文献
32.
Summary Ion: solute cotransporters frequency are incapable of achieving equilibrium between the solute accumulation and the transmembrane difference of the electrochemical potential of the ion. The presence of uncoupled flows of ion and solutes (leaks) is often advanced as an explanation. Here an alternative is discussed. The net accumulation of solute may be so slow that equilibrium can never be attained at finite times (e.g., several hours). Cotransporters may exhibit strong product inhibition, and the net influx of solute approaches zero far from equilibrium. The inherent slowness of net transport under these conditions is termed catalytic inefficiency. The likelihood that galactoside: H+ cotransport inEscherichia coli, hexose: H+ cotransport inChlorella vulgaris, andd-glucose: Na+ cotransport in brush-border membranes exhibit catalytic inefficiency is examined. The existence of strong product inhibition complicates the determination of the stoichiometry of cotransport and the characterization of chemically modified or mutant cotransporters. 相似文献
33.
34.
35.
36.
Ahmed S. Rahman Mieko Kimura Katsuhiko Yokoi Tanvir-E Naher Yoshinori Itokawa 《Biological trace element research》1996,53(1-3):57-64
Three groups of rats were fed two types of synthetic diets for 52 d. The—A group was allowed free access to a vitamin A-deficient
diet and showed classical signs of vitamin A deficiency. The brain was the only organ in our experiment where no significant
weight difference was present among the three groups. In the brain, calcium concentration was significantly higher in the—A
group when compared with the PF (Pair-fed; allowed restricted amount of control diet) and +A groups (allowed free access to
control diet). In the tibia, calcium and magnesium concentrations were significantly lower in the—A group when compared with
other two groups. Excessive accumulation of calcium in brain and apparently similar unbalance in bone, mineral concentration
were observed in central nervous system (CNS) degenerative diseases. Our results suggest that abnormal metabolism of calcium
and magnesium in some tissues and excessive accumulation of calcium in brain may be responsible for the development of neurological
disorders in vitamin A-deficient rats. 相似文献
37.
U. Skiba K. J. Hargreaves I. J. Beverland D. H. ONeill D. Fowler J. B. Moncrieff 《Plant and Soil》1996,181(1):139-144
Measurements of N2O emission fluxes from a 3 ha field of winter wheat were measured using eddy covariance and relaxed eddy accumulation continuously over 10 days during April 1994. The measurements averaged fluxes over approximately 105 m2 of the field, which was fertilised with NH4NO3 at a rate of 43 kg N ha-1 at the beginning of the measurements. The emission fluxes became detectable after the first heavy rainfall, which occured 4 days after fertiliser application. Emissions of N2O increased rapidly during the day following the rain to a maximum of 280 ng N m-2s-1 and declined over the following week. During the period of significant emission fluxes, a clear diurnal cycle in N2O emission was observed, with the daytime maximum coinciding with the soil temperature maximum at 12 cm depth. The temperature dependence of the N2O emission was equivalent to an activation energy for N2O production of 108 kJ mol-1. The N2O fluxes measured using relaxed eddy accumulation, averaged over 30 to 270 min, were in agreement with those of the eddy covariance system within 60%. The total emission of N2O over the period of continuous measurement (10 days) was equivalent to about 10 kg N2O-N, or 0.77% of the N fertiliser applied. 相似文献
38.
P. J. den Besten C. A. Schmidt M. Ohm M. M. Ruys J. W. van Berghem C. van de Guchte 《Journal of Aquatic Ecosystem Stress and Recovery (Formerly Journal of Aquatic Ecosystem Health)》1995,4(4):257-270
The quality of sediment was assessed in 46 sites in the delta of the rivers Rhine and Meuse (The Netherlands) by means of physical-chemical analysis, field observations on the macrobenthic community structure, accumulation studies and bioassays using Chironomus riparius, Daphnia magna and Photobacterium phosphoreum. The results of chemical analyses were classified using national criteria for sediment quality. Results of field studies and bioassays were classified using criteria derived from research in reference areas or based on data from literature. Risk assessment was carried out according to the sediment quality triad and by means of multi criteria analysis (MCA). The Triad approach was used to demonstrate causal relations between effects on the macrozoobenthos community structure, effects demonstrated in bioassays and sediment pollution. This was done by making a comparison of sediment contamination levels with toxicity data from literature for the test organisms of the bioassays. Using the MCA method, for each site a numerical value was derived for total environmental risk in the present situation, based on observed effects. In this way, a relative risk ranking of all sites was realized. The MCA values for the present situation were also compared with MCA scores based on estimated risks after remediation in 1995, in order to set priorities for sites where remediation is expected to cause a significant reduction in environmental risk. In most of the 46 sites studied so far, the macrofauna community was poorly developed, judged by a low number of benthic species, low abundances and a high dominance of species regarded as relatively tolerant to chemical pollution. In bioassays high sediment toxicity was demonstrated for 24 sites. Using the sediment quality triad approach, 25 sites were identified as areas where pollution can be held responsible for the effects observed in the field. From a comparison of contaminant concentrations in different types of food with maximum tolerable risk levels, and the application of a bioaccumulation model it was concluded that the sediment pollution also implies high risks for plant-, benthos- or fish-eating birds (secondary poisoning of top predators). In the Nieuwe Merwede highest MCA risk scores were found for shallow parts where highly polluted sediments are found. It is concluded that the sediment quality triad and the MCA provide additional information which can be used to establish priorities for remedial action. Based on an ecotoxicological evaluation of the improved quality of new sediments that will be deposited after removal of the polluted sediments in the Nieuwe Merwede, it is concluded that in this upstream part of the Rhine delta remedial action will be effective. 相似文献
39.
Kelem Kassahun Pei Hu Mark P. Grillo Margaret R. Davis Lixia Jin Thomas A. Baillie 《Chemico-biological interactions》1994,90(3):253-275
The ability of 2-n-propyl-4-pentenoic acid (Δ4-VPA) and 2-n-propyl-2(E)-pentenoic acid ([E]-Δ2-VPA), two unsaturated metabolites of valproic acid (VPA), to form reactive intermediates, deplete hepatic glutathione (GSH) and cause accumulation of liver triglycerides was investigated in the rat. With the aid of ionspray liquid chromatography-tandem mass spectrometry (LC-MS/MS), three GSH adducts were detected in the bile of Δ4-VPA-treated animals and were identified as 4-hydroxy-5-glutathion-S-yl-VPA-γ-lactone, 5-glutathion-S-yl-(E)-Δ3-VPA and 3-oxo-5-glutathion-S-yl-VPA. A fourth conjugate was identified tentatively as 4-glutathion-S-yl-5-hydroxy-VPA. Quantitative analysis of the corresponding N-acetylcysteine (NAC) conjugates in urine indicated that metabolism of Δ4-VPA via the GSH-dependent pathways accounted for approximately 20% of an acute dose (100 mg kg−1 i.p.). In contrast, when rats were given an equivalent dose of (E)-Δ2-VPA, only one GSH adduct (5-glutathion-S-yl-(E)-Δ3-VPA) was detected at low concentrations in bile. In vitro experiments with rat liver mitochondria demonstrated that Δ4-VPA undergoes coenzyme A- and ATP-dependent metabolic activation in this organelle via the β-oxidation pathway to intermediates which bind covalently to proteins. When liver homogenates and hepatic mitochondria from rats injected with Δ4-VPA, (E)-Δ2-VPA or VPA were analyzed for GSH content, it was found that only Δ4-VPA depleted GSH pools significantly. Treatment of rats with Δ4-VPA and (to a lesser extent) VPA led to an accumulation of liver triglycerides, whereas (E)-Δ2-VPA had no measurable effect. It is concluded that Δ4-VPA undergoes metabolic activation by both microsomal cytochrome P-450-dependent and mitochondrial coenzyme A-dependent processes, and that the resulting electrophilic intermediates, which are trapped in part by GSH, may mediate the hepatotoxic effects of this compound. In contrast, (E)-Δ2-VPA is not transformed to any appreciable extent to reactive metabolites, which thus accounts for the apparent lack of hepatotoxicity of this positional isomer in the rat. 相似文献
40.
Physiological Elevations of Glucocorticoids Potentiate Glutamate Accumulation in the Hippocampus 总被引:7,自引:0,他引:7
Becky A. Stein-Behrens Walter J. Lin Robert M. Sapolsky 《Journal of neurochemistry》1994,63(2):596-602
Abstract: Glucocorticoids (GCs) are secreted during stress and can damage the hippocampus over the course of aging and impair the capacity of hippocampal neurons to survive excitotoxic insults. Using microdialysis, we have previously observed that GCs augment the extracellular accumulation of glutamate and aspartate in the hippocampus following kainic acid-induced seizures. In that study, adrenalectomized rats maintained on minimal GC concentrations were compared with those exposed to GCs elevated to near-pharmacological levels. We wished to gain insight into the physiological relevance of these observations. Thus, we have examined the effects of GCs over the normal physiological range on glutamate and aspartate profiles; this was done by implanting adrenalectomized rats with GC-secreting pellets, which produce stable and controllable circulating GC concentrations. We observe that incremental increases in GC concentrations cause incremental increases in glutamate accumulation before the kainic acid insult, as well as in the magnitude of the glutamate response to kainic acid. Elevating GC concentrations from the circadian trough to peak doubled cumulative glutamate accumulation, whereas a rise into the stress range caused a fourfold increase in accumulation. Similar, although smaller, effects also occurred with aspartate accumulation (as well as with taurine but not glutamine accumulation). These data show that the highly elevated GC concentrations that accompany neurological insults such as seizure or hypoxia-ischemia will greatly exacerbate the glutamate accumulation at that time. Furthermore, stress levels of GCs augmented glutamate accumulation even in the absence of an excitotoxic insult, perhaps explaining how sustained stress itself damages the hippocampus. Finally, even the moderately ?levated basal GC concentrations that typically occur in aged rats augmented glutamate accumulation, perhaps explaining how GCs damage the hippocampus over the course of normal aging. 相似文献