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31.
Fluxes of h and k in corn roots : characterization and stoichiometries using ion-selective microelectrodes 总被引:14,自引:6,他引:8
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We report here on an experimental system that utilizes ion-selective microelectrodes to measure the electrochemical potential gradients for H+ and K+ ions within the unstirred layer near the root surface of both intact 4-day-old corn seedlings and corn root segments. Analysis of the steady state H+ and K+ electrochemical potential gradients provided a simultaneous measure of the fluxes crossing a localized region of the root surface. Net K+ influx values obtained by this method were compared with unidirectional K+ (86Rb+) influx kinetic data; at any particular K+ concentration, similar values were obtained by either technique. The ionspecific microelectrode system was then used to investigate the association between net H+ efflux and net K+ influx. Although the computed H+:K+ stoichiometry is dependent upon the choice of diffusion coefficients, the values obtained were extremely variable, and net K+ influx rarely appeared to be charge-balanced by H+ efflux. In contrast to earlier studies, we found the cortical membrane potential to be highly K+ sensitive within the micromolar K+ concentration range. Simultaneous measurements of membrane potential and K+ influx, as a function of K+ concentration, revealed similar Km values for the depolarization of the potential (Km 6-9 micromolar K+) and net K+ influx (Km 4-7 micromolar K+). These data suggest that K+ may enter corn roots via a K+-H+ cotransport system rather than a K+/H+ antiporter. 相似文献
32.
Glucose stimulates the biosynthesis of rat I and II insulin to an equal extent in isolated pancreatic islets 总被引:2,自引:0,他引:2
The effects of glucose on insulin biosynthesis were studied by measuring the incorporation of radiolabelled amino acids into proinsulin/insulin in isolated rat islets. The islets were pulse labelled for 15 min with [3H]leucine (present in rat insulin I and II) or [35S]methionine (unique to rat insulin II) and then incubated for a 165 min post-label (chase) period during which the majority of labelled proinsulin was converted to insulin but under conditions whereby greater than 95% of radiolabelled proinsulin or insulin was retained in the islets. The newly synthesized, labelled, insulin was analyzed by high performance liquid chromatography. Rat I and II insulin biosynthesis was stimulated by 16.7 mM glucose to the same extent. 相似文献
33.
Abstract The genomes of DNA phage ΦX174 and of RNA phage MS2 each encode a single lysis protein, E protein and L protein, respectively. Based on the known DNA and protein sequences, and with the aid of structural predictions of the proteins, a chimeric gene was constructed. The resulting protein was composed of the N-terminal sequence of E protein and the C-terminal sequence of L protein. The truncated E and L polypeptides used in this construction were functionally inactive. However, the chimeric gene product had very high lysis-inducing activity. This construction is an example which could be extended to the engineering of other lysis proteins designed with specific biotechnological processes in mind. 相似文献
34.
35.
Aminoglycoside suppression at UAG, UAA and UGA codons in Escherichia coli and human tissue culture cells 总被引:6,自引:0,他引:6
Robin Martin Anne E. Mogg Louise A. Heywood Lars Nitschke Julian F. Burke 《Molecular & general genetics : MGG》1989,217(2-3):411-418
Summary We have compared the suppression of nonsense mutations by aminoglycoside antibiotics inEscherichia coli and in human 293 cells. Six nonsense alleles of the chloramphenicol acetyl transferase (cat) gene, in the vector pRSVcat, were suppressed by growth in G418 and paromomycin. Readthrough at UAG, UAA and UGA codons was monitored with enzyme assays
for chloramphenicol acetyl transferase (CAT), in stably transformed bacteria and during transient expression from the same
plasmid in human 293 tissue culture cells. We have found significant differences in the degree of suppression amongst three
UAG codons and two UAA codons in different mRNA contexts. However, the pattern of these effects are not the same in the two
organisms. Our data suggest that context effects of nonsense suppression may operate under different rules inE. coli and human cells. 相似文献
36.
Anin situ method, derived from anin vivo method, was used to determine nitrate reductase activity (NRA) in:i) excised barley and corn shoots and excised soybean leaves
during a N-depletion experiment and; ii) roots and shoots of N-depleted barley and corn seedlings during induction of nitrate,
reductase (NR). Nitrate reduction, calculated from thesein situ RNA measurements, was compared with estimates of each organ's nitrate reduction in light aerobic conditions from NO
3
−
consumption and a15N model (Gojonet al., 1986b).
Thein situ RNA of roots strongly underestimated their15NO
3
−
reduction. In contrast, in barley and corn shoots and in the first trifoliolate leaves from 26-day-old, soybean, thein situ NRA assay gave a fair approximation of the true NO
3
−
reduction rate (relative differences ranging from −14 to +32%). In young soybean leaves (from 20-day-old plants), however,
thein situ NRA strongly underestimated the actual NO
3
−
reduction. The physiological significance of thein situ NRA assay in shoots and roots, and its value for field studies are discussed from these results. 相似文献
37.
Komandoor E. Achyuthan Ann Mary Robin Bhaermani Charles S. Greenberg 《Molecular and cellular biochemistry》1989,85(1):57-65
Calcium ions are crucial for expression of transglutaminase activity. Although lanthanides have been reported to substitute for calcium in a variety of protein functions, they did not replace the calcium requirement during transglutaminase activity measurements. Furthermore, lanthanides strongly inhibited purified liver transglutaminase activity using either casein or fibrinogen as substrates. Terbium (III) inhibition of transglutaminase-catalyzed putrescine incorporation into casein was not reversed by the presence of 10–200 fold molar excess of calcium ions (Ki for Tb(III)=60 µM). Conformational changes in purified liver transglutaminase upon Tb(III) binding were evident from a biphasic effect of Tb(III) on transglutaminase binding to fibrin. Low concentrations of Tb(III) (1 µM to 10 µM inhibited the binding of transglutaminase to fibrin, whereas higher concentrations (20 µM to 100 µM promoted binding. Conformational changes in purified liver transglutaminase consequent to Tb(III) binding were also demonstrated by fluorescence spectroscopy due to Forster energy transfer. Fluorescence emission was stable to the presence of 200 mM NaCl and 100 mM CaCl2 only partially quenched emission. Purified liver transglutaminase strongly bound to Tb(III)-Chelating Sepharose beads and binding could not be disrupted by 100 mM CaCl2 solution. Our data suggest that Tb(III)-induced conformational changes in transglutaminase are responsible for the observed effects on enzyme structure and function. The potential applications of Tb(III)-transglutaminase interactions in elucidating the structure-function relationships of liver transglutaminase are discussed. 相似文献
38.
Pedro J. N. Silva Richard K. Koehn Walter J. Diehl III Robin P. Ertl Elaine B. Winshell Mauro Santos 《Biochemical genetics》1989,27(7-8):451-467
Four samples of the musselMytilus edulis were taken between 1984 and 1987 from Stony Brook, New York, and used to study the glucose-6-phosphate isomerase (GPI) polymorphism
in this species.In vitro specific activity andin vivo flux measured in the same animals were found to be significantly correlated. A significant effect of GPI genotype on flux
was observed in one of the samples; overall, significant evidence of effect of genotype on enzyme activity was also obtained.
GPI activities of common genotypes tend to deviate less from the population mean than those of rare (frequency less than 5%)
genotypes. This suggests the possibility that rare GPI genotypes are rare as a consequence of having biochemical properties
that deviate from an optimum level and, therefore, having a lower fitness. In support of this hypothesis, we found in one
of our samples that shell length is a concave function of GPI activity with an intermediate optimum activity level.
The financial support provided to P.J.N.S. by the Luso-American Educational Commission (Fulbright Program), the Instituto
Nacional de Investigacao Científica (Portugal), and the Faculdade de Ciências da Universidade de Lisboa during several stages
of this research is gratefully acknowledged. Financial support from the Ministerio de Educatión y Ciencia (Spain) in the form
of a postdoctoral Fulbright/MEC fellowship to M.S. is also gratefully acknowledged. Research was supported by National Science
Foundation Grant BSR-8415060 to R.K.K. This is contribution No. 736 from the Program in Ecology and Evolution, State University
of New York at Stony Brook.
On leave from Departamento de Biologia Vegetal, Faculdade de Ciências, Universidade de Lisboa, Campo Grande C2, Lisboa, Portugal. 相似文献
39.
A water-soluble fraction from adult bone stimulates the differentiation of cartilage in explants of embryonic muscle 总被引:2,自引:0,他引:2
Paul A. Lucas Glenn T. Syftestad Arnold I. Caplan 《Differentiation; research in biological diversity》1988,37(1):47-52
A water-soluble fraction of a 4 M guanidine HCl extract of demineralized adult bovine bone stimulated the differentiation of cartilage in explants of minced skeletal muscle from embryonic chick legs; cartilage was also induced by a semipurified protein preparation. Cartilage could be identified in treated cultures at 1 week with muscle from day-9 embryos, not before 2 weeks with muscle from day-12 embryos, and not before 3 weeks with muscle from day-19 embryos. The ability to respond to this water-soluble fraction by exhibiting cartilage differentiation was dose-dependent, but not confined to any particular muscle region of the day-12 embryonic leg. These observations indicate that bone-derived soluble chondroinductive agents act on cells in minced embryonic muscle preparations. The induction of cartilage is dependent upon the accessibility of the responding cells to the agents, on the concentration of inductive agents, and on the developmental age of the responsive tissue. 相似文献
40.
Menstrual-cycle phase and sexual behavior in semi-free-ranging stumptail macaques (Macaca arctoides)
The sexual behavior and female reproductive cycles of a group of island-dwelling stumptail macaques (Macaca arctoides)were monitored over a 6-month period, yielding 530 observation hr and 268 copulations. Compared to nondominant males, the
dominant male copulated at a relatively high rate throughout the cycle, but largely with one high-ranking female. The non-dominant
males copulated most frequently at midcycle. Female presenting was highest at midcycle, but only to the dominant male. Cross-study
discrepancies may be due to different observation methods and restricted environmental conditions that mask female-initiated
sexual behavior. The more naturalistic setting of this study allowed for a fuller expression of proceptivity. Contrary to
some previous conclusions, present findings suggest that both hormonal and socioenvironmental factors influence the patterns
of sexual behavior found in stumptail macaque colonies. 相似文献