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排序方式: 共有24258条查询结果,搜索用时 125 毫秒
1.
Rapeseed oil methyl esters preparation using heterogeneous catalysts   总被引:55,自引:0,他引:55  
The classical method of fatty acids methyl esters (FAME) production is based on triglyceride transesterification to methyl esters. Sodium hydroxide dissolved in methanol is used as a catalyst. The purpose of this work was to examine a heterogeneous catalyst, in particular calcium compounds, to produce methyl esters of rapeseed oil. This research showed that the transesterification of rapeseed oil by methyl alcohol can be catalysed effectively by basic alkaline-earth metal compounds: calcium oxide, calcium methoxide and barium hydroxide. Calcium catalysts, due to their weak solubility in the reaction medium, are less active than sodium hydroxide. However, calcium catalysts are cheaper and lead to decreases in the number of technological stages and the amount of unwanted waste products. It was found that the transesterification reaction rate can be enhanced by ultrasound as well as by introducing an appropriate reagent into a reactor to promote methanol solubility in the rapeseed oil. Tetrahydrofuran was used as additive to accelerate the transesterification process.  相似文献
2.
A plant-derived edible vaccine against hepatitis B virus.   总被引:52,自引:0,他引:52  
The infectious hepatitis B virus represents 42 nm spherical double-shelled particles. However, analysis of blood from hepatitis B virus carriers revealed the presence of smaller 22 nm particles consisting of a viral envelope surface protein. These particles are highly immunogenic and have been used in the design of hepatitis B virus vaccine produced in yeast. Upon expression in yeast, these proteins form virus-like particles that are used for parenteral immunization. Therefore, the DNA fragment encoding hepatitis B virus surface antigen was introduced into Agrobacterium tumerifacience LBA4404 and used to obtain transgenic lupin (Lupinus luteus L.) and lettuce (Lactuca sativa L.) cv. Burpee Bibb expressing envelope surface protein. Mice that were fed the transgenic lupin tissue developed significant levels of hepatitis B virus-specific antibodies. Human volunteers, fed with transgenic lettuce plants expressing hepatitis B virus surface antigen, developed specific serum-IgG response to plant produced protein.  相似文献
3.
Based on previous in vivo genetic analysis of bacteriophage lambda growth, we have developed two in vitro lambda DNA replication systems composed entirely of purified proteins. One is termed 'grpE-independent' and consists of supercoiled lambda dv plasmid DNA, the lambda O and lambda P proteins, as well as the Escherichia coli dnaK, dnaJ, dnaB, dnaG, ssb, DNA gyrase and DNA polymerase III holoenzyme proteins. The second system includes the E.coli grpE protein and is termed 'grpE-dependent'. Both systems are specific for plasmid molecules carrying the ori lambda DNA initiation site. The major difference in the two systems is that the 'grpE-independent' system requires at least a 10-fold higher level of dnaK protein compared with the grpE-dependent one. The lambda DNA replication process may be divided into several discernible steps, some of which are defined by the isolation of stable intermediates. The first is the formation of a stable ori lambda-lambda O structure. The second is the assembly of a stable ori lambda-lambda O-lambda P-dnaB complex. The addition of dnaJ to this complex also results in an isolatable intermediate. The dnaK, dnaJ and grpE proteins destabilize the lambda P-dnaB interaction, thus liberating dnaB's helicase activity, resulting in unwinding of the DNA template. At this stage, a stable DNA replication intermediate can be isolated, provided that the grpE protein has acted and/or is present. Following this, the dnaG primase enzyme recognizes the single-stranded DNA-dnaB complex and synthesizes RNA primers. Subsequently, the RNA primers are extended into DNA by DNA polymerase III holoenzyme. The proposed model of the molecular series of events taking place at ori lambda is substantiated by the many demonstrable protein-protein interactions among the various participants.  相似文献
4.
A cross-platform public domain PC image-analysis program for the comet assay   总被引:47,自引:0,他引:47  
The single-cell gel electrophoresis, also known as the comet assay, has gained wide-spread popularity as a simple and reliable method to measure genotoxic and cytotoxic effects of physical and chemical agents as well as kinetics of DNA repair. Cells are generally stained with fluorescent dyes. The analysis of comets--damaged cells which form a typical comet-shaped pattern--is greatly facilitated by the use of a computer image-analysis program. Although several image-analysis programs are available commercially, they are expensive and their source codes are not provided. For Macintosh computers a cost-free public domain macro is available on the Internet. No ready for use, cost-free program exists for the PC platform. We have, therefore, developed such a public domain program under the GNU license for PC computers. The program is called CASP and can be run on a variety of hardware and software platforms. Its practical merit was tested on human lymphocytes exposed to gamma-rays and found to yield reproducible results. The binaries for Windows 95 and Linux, together with the source code can be obtained from: http://www.casp.of.pl.  相似文献
5.
D Skowyra  C Georgopoulos  M Zylicz 《Cell》1990,62(5):939-944
Pelham previously proposed that the hsp70 family of heat shock proteins could prevent the formation and/or allow the dissolution of protein aggregates created during stress conditions. We confirmed this hypothesis by showing that the E. coli hsp70 homolog, the dnaK gene product, protects the host RNA polymerase enzyme from heat inactivation in an ATP-independent reaction. In addition, we show that heat-inactivated and aggregated RNA polymerase is both disaggregated and reactivated following simultaneous incubation with DnaK protein and hydrolyzable ATP. The DnaK756 mutant protein has lost the ability to disaggregate the inactivated RNA polymerase enzyme. Our results demonstrate that the DnaK protein contributes to E. coli's growth not only by protecting some enzymes from denaturation but also by reactivating some once they are misfolded or aggregated.  相似文献
6.
 High concentrations of heavy metals in soil have an adverse effect on micro-organisms and microbial processes. Among soil microorganisms, mycorrhizal fungi are the only ones providing a direct link between soil and roots, and can therefore be of great importance in heavy metal availability and toxicity to plants. This review discusses various aspects of the interactions between heavy metals and mycorrhizal fungi, including the effects of heavy metals on the occurrence of mycorrhizal fungi, heavy metal tolerance in these micro-organisms, and their effect on metal uptake and transfer to plants. Mechanisms involved in metal tolerance, uptake and accumulation by mycorrhizal hyphae and by endo- or ectomycorrhizae are covered. The possible use of mycorrhizal fungi as bioremediation agents in polluted soils or as bioindicators of pollution is also discussed. Accepted: 23 June 1997  相似文献
7.
Alloxan and streptozotocin are widely used to induce experimental diabetes in animals. The mechanism of their action in B cells of the pancreas has been intensively investigated and now is quite well understood. The cytotoxic action of both these diabetogenic agents is mediated by reactive oxygen species, however, the source of their generation is different in the case of alloxan and streptozotocin. Alloxan and the product of its reduction, dialuric acid, establish a redox cycle with the formation of superoxide radicals. These radicals undergo dismutation to hydrogen peroxide. Thereafter highly reactive hydroxyl radicals are formed by the Fenton reaction. The action of reactive oxygen species with a simultaneous massive increase in cytosolic calcium concentration causes rapid destruction of B cells. Streptozotocin enters the B cell via a glucose transporter (GLUT2) and causes alkylation of DNA. DNA damage induces activation of poly ADP-ribosylation, a process that is more important for the diabetogenicity of streptozotocin than DNA damage itself. Poly ADP-ribosylation leads to depletion of cellular NAD+ and ATP. Enhanced ATP dephosphorylation after streptozotocin treatment supplies a substrate for xanthine oxidase resulting in the formation of superoxide radicals. Consequently, hydrogen peroxide and hydroxyl radicals are also generated. Furthermore, streptozotocin liberates toxic amounts of nitric oxide that inhibits aconitase activity and participates in DNA damage. As a result of the streptozotocin action, B cells undergo the destruction by necrosis.  相似文献
8.
BACKGROUND: The exposure of gastric mucosa to damaging factors, such as ethanol, water restraint stress, or ischemia followed by reperfusion, produces pathological changes: inflammatory process, hemorrhagic erosions, even acute ulcers. The base of these changes is a disturbance of protective mechanisms and disrupture of gastric mucosal barrier. Previous studies pointed out the role of disturbances of gastric blood flow, mucus secretion and involvement of prostaglandins and nitric oxide formation in the pathomechanism of gastric mucosa lesions. The role of reactive oxygen species (ROS) in these processes has been little studied. Aim: The purpose of our present investigations is to explain the participation of ROS in acute gastric mucosal damage by various irritants. MATERIAL AND METHODS: Experiments were carrying out on 80 male Wistar rats. To assess gastric blood flow (GBF) laser Doppler flowmeter was used. The area of gastric lesions was established by planimetry. The levels of proinflammatory cytokines were measured by ELISA technique. The colorimetric assays were used to determine of malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE) as well as superoxide dismutase (SOD) activity. RESULTS: We demonstrated that 3.5 h of water immersion and restraint stress (WRS), 30 min of gastric ischemia followed by 60 min of reperfusion or intragastric administration of 100% ethanol, all resulted in appearance of acute gastric mucosal lesions accompanied by a significant decrease of gastric blood flow. These lesions are also accompanied by the significant increase of proinflammatory cytokines including interleukin-1 beta (IL-1beta) and tumor necrosis factor alpha (TNFalpha) plasma level. Biological effects of ROS were estimated by measuring tissue level of MDA and 4-HNE, the products of lipid peroxydation by ROS, as well as the activity of SOD, the scavanger of ROS. It was established that 3.5 h of WRS, ischemia-reperfusion and 100% ethanol lead to significant increase of MDA and 4-HNE mucosal level, accompanied by a decrease of SOD activity (significant in WRS and ethanol application). CONCLUSIONS: The pathogenesis of experimental mucosal damage in rat stomach includes the generation of ROS that seem to play an important role, namely due to generation of lipid peroxides, accompanied by impairment of antioxidative enzyme activity of cells.  相似文献
9.
10.
Cadmium (Cd) accumulation has been found to vary between cultivars of durum wheat (Triticum turgidum var. durum), and it is hypothesized that low-molecular-weight organic acids (LMWOAs) produced at the soil-root interface (rhizosphere) may play an important role in the availability and uptake of Cd by these plants. The objective of this study, therefore, was to (1) investigate the nature and quantity of LMWOAs present in the rhizosphere of durum wheat cultivars Arcola (low Cd accumulator) and Kyle (high Cd accumulator) grown in three different soils: Yorkton, Sutherland and Waitville, and (2) determine the relationship between Cd accumulation in these plants and LMWOAs present in the rhizosphere. Plants were grown for two weeks in pot-cultures under growth chamber conditions. Oxalic, fumaric, succinic, L-malic, tartaric, citric, acetic, propionic and butyric acids were found and quantified in the water extracts of rhizosphere soil, with acetic and succinic acids being predominant. No water extractable LMWOAs were identified in the bulk soil. Total amount of LMWOAs in the rhizosphere soil of the high Cd accumulator (Kyle) was significantly higher than that for the low Cd accumulator (Arcola) in all three soils. Furthermore, large differences in amounts of LMWOAs were found in the rhizosphere soil for the same cultivars grown in different soils and followed the pattern: Sutherland > Waitville > Yorkton. Extractable soil Cd (M NH4Cl) and Cd accumulation in the plants also followed the same soil sequence as LMWOA production. Cadmium accumulation by the high and low Cd accumulating cultivars was proportional to the levels of LMWOAs found in the rhizosphere soil of each cultivar. These results suggest that the differing levels of LMWOAs present in the rhizosphere soil played an important role in the solubilization of particulate-bound Cd into soil solution and its subsequent phytoaccumulation by the high and low Cd accumulating cultivars.  相似文献
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