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951.
952.
953.
Rabbit neostriatal slices were prelabeled with [3H]dopamine (DA) and [14C]choline and then superfused. The electrical stimulation-evoked release of DA and of acetylcholine (ACh) was abolished by 0.33 microM tetrodotoxin and by low calcium concentrations (0.13 mM). Bromocriptine, a selective D2-DA receptor agonist, inhibited in a concentration-dependent manner the evoked overflow of DA and ACh, without affecting the basal efflux of both transmitters. The effects of bromocriptine were antagonized by sulpiride, a specific antagonist of D2-DA receptors. With stimulation at 0.3 Hz and 120 pulses, bromocriptine was eight times more potent in inhibiting the evoked overflow of DA (IC50: 11 nM) than that of ACh (IC50: 83 nM). Stimulations at 3 Hz and 360 pulses markedly reduced the potency of bromocriptine in inhibiting DA and ACh release, and diminished its selectivity for presynaptic receptors. These results indicate that DA receptors that modulate the release of DA and ACh are of the D2 subtype. The greater potency of bromocriptine at pre- than at postsynaptic sites suggests that these receptors may be different in quantity and/or quality [D2-alpha (presynaptic) versus D2-beta (postsynaptic)]. Finally, marked differences in the potency and efficacy of DA agonist actions on DA and ACh release modulatory receptors are obtained, depending on the parameters of stimulation used. 相似文献
954.
用比较样地法调查了内蒙古高原4类地带性针茅(Stipa)草原群落的β多样性特征。结果表明,Whittacker指数βWS与尺度有关,随取样面积的增加,βWS逐渐降低,取样面积相同时,4类群落的βWS比较接近,各群落 的Cody指数最初也随样方面积的扩大而增大,当样方面积扩大到一定尺度时,贝加尔针茅群落和大针茅群落为〉0.5m^2,克氏针茅群落和小针茅群落为〉2m^2,Cody指数趋于稳定。同一尺度 相似文献
955.
R. P. Singh X. Nie G. C. C. Tai 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》2000,100(3-4):401-408
The potato cultivar ’Shepody’ is susceptible to a number of potato viruses including potato virus Y (PVY, potyvirus) but was
found to possess extreme resistance to another potyvirus, potato virus A (PVA). ’Shepody’ plants were resistant to PVA infection
in manual and graft inoculations. PVA replication was not detected in any of the inoculated plants by ELISA, an infectivity
assay and RT-PCR. However, ’Shepody’ plants grafted with shoots containing PVA developed a novel symptomology which resembled
a virus infection in appearance and in rate of translocation to the entire plant. Efforts to transmit the symptom-inducing
agent manually failed. Graft-inoculation to potato virus indicator plants and PVA-susceptible potato plants showed that the
symptom inducer was PVA at an extremely low concentration, detected using RT-PCR followed by Southern blot assay. Tubers from
grafted but resistant ’Shepody’ plants had necrotic surfaces and internal spots. PVA was detected from necrotic areas but
not from the non-necrotic ones. However, plants resulting from necrotic tubers were free from aerial leaf symptoms observed
in grafted plants and produced non-necrotic normal tubers. A trace-back of the parental lineage of ’Shepody’ indicated that
the resistance had been introgressed from the cultivar ’Bake King’. Analysis of progeny of a cross of resistant ’Shepody’
to the susceptible ’Goldrus’ indicated that this resistance is controlled by two independent dominant complementary genes
in contrast to monogenic resistance reported for other potato viruses.
Received: 6 April 1999 / Accepted: 28 July 1999 相似文献
956.
Determining the growing season of land vegetation on the basis of plant phenology and satellite data in Northern China 总被引:23,自引:0,他引:23
The objectives of this study are to explore the relationships between plant phenology and satellite-sensor-derived measures
of greenness, and to advance a new procedure for determining the growing season of land vegetation at the regional scale.
Three phenological stations were selected as sample sites to represent different climatic zones and vegetation types in northern
China. The mixed data set consists of occurrence dates of all observed phenophases for 50–70 kinds of trees and shrubs from
1983 to 1988. Using these data, we calculated the cumulative frequency of phenophases in every 5-day period (pentad) throughout
each year, and also drew the cumulative frequency distribution curve for all station-years, in order to reveal the typical
seasonal characteristics of these plant communities. The growing season was set as the time interval between 5% and 95% of
the phenological cumulative frequency. Average lengths of the growing season varied between 188 days in the northern, to 259
days in the southern part of the research region. The beginning and end dates of the surface growing season were then applied
each year as time thresholds, to determine the corresponding 10-day peak greenness values from normalized difference vegetation
index curves for 8-km2 pixels overlying the phenological stations. Our results show that, at the beginning of the growing season, the largest average
greenness value occurs in the southern part, then in the northern, and finally the middle part of the research region. In
contrast, at the end of the growing season, the largest average greenness value is measured in the northern part, next in
the middle and lastly the southern part of the research region. In future studies, these derived NDVI thresholds can be applied
to determine the growing season of similar plant communities at other sites, which lack surface phenological data.
Received: 29 November 1999 / Revised: 14 March 2000 / Accepted: 15 March 2000 相似文献
957.
958.
959.
A theoretical study of anthracene and phenanthrene derivatives acting as A-T specific intercalators. 下载免费PDF全文
Theoretical computations are performed on the comparative A-T versus G-C binding selectivities of two DNA intercalating molecules recently synthesized by Wilson et al. These are derivatives of phenanthrene and anthracene with side chains containing an hydroxy group bound to its C alpha carbon and a cationic amino group bound to its C beta carbon. We have optimized the binding energies of these phenanthrene and anthracene derivatives (1 and 2, respectively) to the double-stranded tetramers d(ATAT)2 and d(GCGC)2, the intercalation occurring in the central pyrimidine (3'-5') purine sequence. The sum of the intercalator-oligonucleotide intermolecular interaction energy plus the conformational energy variation of the intercalator upon binding were computed by the SIBFA procedures, which use empirical formulas based on ab initio SCF computations. Both compounds are found to bind more favourably to the AT sequence than to the GC one. Moreover, the affinity of 1 for the AT oligomer is computed to be larger than that of 2, whereas conversely that of 2 is larger than that of 1 for the GC oligomer. The AT versus GC binding selectivity of 1 is significantly larger than that of 2. These results are in excellent agreement with the experimental findings of Wilson et al. However, contrary to the suggestion of these authors the alpha-hydroxy group of the side chain of the intercalators does not seem to play a decisive role in determining the A-T specificity. 相似文献
960.
Enzymatic conversion of beta-carotene into beta-apo-carotenals and retinoids by human, monkey, ferret, and rat tissues 总被引:2,自引:0,他引:2
X D Wang G W Tang J G Fox N I Krinsky R M Russell 《Archives of biochemistry and biophysics》1991,285(1):8-16
Whether the conversion of beta-carotene into retinoids involves an enzymatic excentric cleavage mechanism was examined in vitro with homogenates prepared from human, monkey, ferret, and rat tissue. Using high-performance liquid chromatography, significant amounts of beta-apo-12'-, -10'-, and -8'-carotenals, retinal, and retinoic acid were found after incubation of intestinal homogenates of the four different species with beta-carotene in the presence of NAD+ and dithiothreitol. No beta-apo-carotenals or retinoids were detected in control incubations done without tissue homogenates. The production of beta-apo-carotenals was linear for 30 min and up to tissue protein concentrations of 1.5 mg/ml. The rate of formation of beta-apo-carotenals from 2 microM beta-carotene was about 7- to 14-fold higher than the rate of retinoid formation in intestinal homogenates, and the rate of beta-apo-carotenal production was fivefold greater in primate intestine vs rat or ferret intestine (P less than 0.05). The amounts of beta-apo-carotenals and retinoids formed were markedly reduced when NAD+ was replaced by NADH, or when dithiothreitol and cofactors were deleted from the incubation mixture. Both beta-apo-carotenal and retinoid production from beta-carotene were inhibited completely by adding disulfiram, an inhibitor of sulfhydryl-containing enzymes. Incubation of beta-carotene with liver, kidney, lung, and fat homogenates from each species also resulted in the appearance of beta-apo-carotenals and retinoids. The identification of three unknown compounds which might be excentric cleavage products is ongoing. These data support the existence of an excentric cleavage mechanism for beta-carotene conversion. 相似文献