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91.
Gerhard Schenk Roy Layfield Judith M. Candy Ronald G. Duggleby Peter F. Nixon 《Journal of molecular evolution》1997,44(5):552-572
Members of the transketolase group of thiamine-diphosphate-dependent enzymes from 17 different organisms including mammals,
yeast, bacteria, and plants have been used for phylogenetic reconstruction. Alignment of the amino acid and DNA sequences
for 21 transketolase enzymes and one putative transketolase reveals a number of highly conserved regions and invariant residues
that are of predicted importance for enzyme activity, based on the crystal structure of yeast transketolase. One particular
sequence of 36 residues has some similarities to the nucleotide-binding motif and we designate it as the transketolase motif.
We report further evidence that the recP protein from Streptococcus pneumoniae might be a transketolase and we list a number of invariant residues which might be involved in substrate binding. Phylogenies
derived from the nucleotide and the amino acid sequences by various methods show a conventional clustering for mammalian,
plant, and gram-negative bacterial transketolases. The branching order of the gram-positive bacteria could not be inferred
reliably. The formaldehyde transketolase (sometimes known as dihydroxyacetone synthase) of the yeast Hansenula polymorpha appears to be orthologous to the mammalian enzymes but paralogous to the other yeast transketolases. The occurrence of more
than one transketolase gene in some organisms is consistent with several gene duplications. The high degree of similarity
in functionally important residues and the fact that the same kinetic mechanism is applicable to all characterized transketolase
enzymes is consistent with the proposition that they are all derived from one common ancestral gene. Transketolase appears
to be an ancient enzyme that has evolved slowly and might serve as a model for a molecular clock, at least within the mammalian
clade.
Received: 13 September 1995 / Accepted: 14 November 1996 相似文献
92.
Summary (S)-3-hydroxy-2-substituted propionaldehyde dimethyl or diethyl acetals 3, which are versatile synthons in dipeptide isostere synthesis, were synthesized in 54–95% enantiomeric excess by reduction of (S,R)-acetalized acyloxazolidinones 7 with LiAlH4. 相似文献
93.
A. H. Macpherson suggested that much of the current geographic diversity in Canadian Arctic mammals resulted from isolation in refugia during the Wisconsin glacial stage. This study evaluates the refugium hypothesis, insofar as it applies to Nearctic Dicrostonyx , by means of a statistical analysis of geographic variation in 13 skull characters. Overall, geographic variation among samples is not significant, although D. hudsonius and D. unalascensis are geographically and morphologically distinct. Some variation in skull shape is correlated with winter temperature. Partitioning tests on other measures of shape variation indicate some discontinuities consistent with the refugial hypothesis. Discrete samples reflect possible refugial populations in northern North America, Eastern Beringia and two southern periglacial refugia, one in eastern North America and at least one in western North America. 相似文献
94.
Electroencephalographic biofeedback of SMR and beta for treatment of attention deficit disorders in a clinical setting 总被引:1,自引:0,他引:1
Six children were provided with long-term biofeedback and academic treatment for attention deficit disorders. Their symptoms were primarily specific learning disabilities, and, in some cases, there were varying degrees of hyperkinesis. The training consisted of two sessions per week for 10 to 27 months, with a gradual phase-out. Feedback was provided for either increasing 12-to 15-Hz SMR or 16- to 20-Hz beta activity. Inhibit circuits were employed for blocking the SMR or beta when either gross movement, excessive EMG, or theta (4–8 Hz) activity was present. Treatment also consisted of combining the biofeedback with academic training, including reading, arithmetic, and spatial tasks to improve their attention. All children increased SMR or beta and decreased slow EEG and EMG activity. Changes could be seen in their power spectra after training in terms of increased beta and decreased slow activity. All six children demonstrated considerable improvement in their schoolwork in terms of grades or achievement test scores. None of the children are currently on any medications for hyperkinetic behavior. The results indicate that EEG biofeedback training, if applied comprehensively, can be highly effective in helping to remediate children who are experiencing attention deficit disorders.The authors would like to thank Mr. Kevin Bianchini for his assistance in this study. 相似文献
95.
Frank Marcus Ida Edelstein Judith Rittenhouse 《Biochemical and biophysical research communications》1984,119(3):1103-1108
Fructose 2,6-bisphosphate, a potent inhibitor of fructose-1,6-bisphosphatases, was found to be an inhibitor of the Escherichia coli enzyme. The substrate saturation curves in the presence of inhibitor were sigmoidal and the inhibition was much stronger at low than at high substrate concentrations. At a substrate concentration of 20 μM, 50% inhibition was observed at 4.8 μM fructose 2,6-bisphosphate. Escherichia coli fructose-1,6-bisphosphatase was inhibited by AMP (Kj = 16 μM) and phosphoenolpyruvate caused release of AMP inhibition. However, neither AMP inhibition nor its release by phosphoenolpyruvate was affected by the presence of fructose 2,6-bisphosphate. The results obtained, together with previous observations, provide further evidence for the fructose 2,6-bisphosphate-fructose-1,6-bisphosphatase active site interaction. 相似文献
96.
The renal vasculature of the toad, Bufo marinus, was studied mainly by means of scanning electron microscopy of vascular corrosion casts. All arterial branches terminated in a glomerulus. Each glomerulus was supplied by only one afferent arteriole. No shunts between afferent and efferent arterioles were observed. The glomerular channels appeared to be permanent capillaries. No evidence supporting the theory of freely shifting glomerular blood channels was found. Efferent arterioles radiated out towards the dorsal surface of the kidney where they connected with peritubular vessels. The renal portal veins produced an anastomosing plexus on the dorsal surface of the kidney, giving rise to the peritubular vessels. Peritubular vessels ran radially toward the ventral surface of the kidney, where they formed the roots of the renal veins. Attention is drawn to the possibility of hairpin countercurrent exchange between the capillary-like efferent arterioles and the peritubular vessels in the dorsal kidney. 相似文献
97.
Peter H. Byers Karen A. Holbrook Judith G. Hall Paul Bornstein John W. Chandler 《Human genetics》1978,40(2):157-169
Summary Several individuals from one family are described with a unique form of spondyloepiphyseal dysplasia. Characteristic features include shorttrunked short stature, punctate corneal dystrophy and marked disorganization of dermal collagen fibrils when examined by transmission electron microscopy. Inheritance is compatible with either dominance and a variable expression or X-linkage. Although the basic defect has not been determined, the tissue distribution is consistent with a defect in a non-collagenous component that affects collagen fibril formation or stability. 相似文献
98.
Judith Huerta-Bahena Rafael Villalobos-Molina J.Adolfo García-Sáinz 《Life sciences》1982,30(20):1757-1762
Cycloheximide, a widely used inhibitor of protein synthesis, stimulates glycogenolysis, gluconeogenesis and ureogenesis in isolated rat hepatocytes. The effects of cycloheximide were compared to those of norepinephrine. Both agents, cycloheximide and norepinephrine, produced slight increases in the levels of cyclic AMP (30% increases) which were blocked by propranolol. Interestingly, it was found that the metabolic actions of norepinephrine and cycloheximide (stimulation of glycogenolysis, gluconeogenesis and ureogenesis) were only slightly diminished by the β adrenergic antagonist propranolol but abolished by the selective α1 adrenergic antagonist prazosin. The ability of cycloheximide to inhibit protein synthesis was not affected by either prazosin or propranolol. It is concluded that the stimulation of glycogenolysis, gluconeogenesis and ureogenesis by cycloheximide in rat hepatocytes, is an effect of the antibiotic independent of its ability to inhibit protein synthesis and that is mediated through activation of α1 adrenoceptors. The adrenergic activity of cycloheximide should be considered when this drug is used as an inhibitor of protein synthesis. 相似文献
99.
High molecular weight forms of adrenocorticotropin (ACTH) and endorphin were identified in extracts of guinea pig anterior and intermediate/posterior pituitary. Extracts of anterior pituitary contained ACTH immunoactive material with apparent molecular weights of 36,000, 24,000 and 4,500 daltons. The highest molecular weight form the ACTH co-migrated with a peak of endorphin immunoactive material. No material the size of glycosylated ACTH(1--39) was detected. Separated forms of high molecular weight ACTH prepared from mouse tumor cell culture medium stimulated the same maximal production of steroid as ACTH(1--39) in the guinea pig adrenal cell bioassay. Pro-ACTH/endorphin and ACTH biosynthetic intermediate were two orders of magnitude less potent than synthetic human ACTH(1--39); glycosylated ACTH(1--39) was equipotent to ACTH(1--39) although no similar material was detected in guinea pig pituitary extracts. Isolated guinea pig adrenal cortical cells were incubated with the various separated form of mouse tumor cell ACTH and products synthesized from (3H)pregnenolone were analyzed by two-dimensional thin-layer chromatography. The ratio of cortisol-related to corticosterone-related products was the same in response in glycosylated and nonglycosylated ACTH. 相似文献
100.
Summary The interaction of fluorescamine with living plant and animal cells was investigated to determine which subcellular structures and molecular species might react with the dye and to assess its effects on cell viability and function.Plasma and nuclear membranes ofXenopus erythrocytes, mitochondria of sea urchin sperm, growing apices of Timothy root hairs, and various organelles ofNitella andEuglena were labelled as judged by fluorescence microscopy. Cytoplasmic fluorescence was particulate inNitella and easily displaced by moderate centrifugal fields in sea urchin eggs. Chloroplasts and nuclei isolated from cells labelledin vivo exhibited fluorescamine dependent fluorescence.Reaction seemed to have little or no effect on cell viability (Euglena) photoautotrophic growth (Euglena), cell motility (sperm), fertilizability (sperm or egg), embryonic development (sea urchin), or cytoplasmic streaming (Nitella, Timothy).Quantitative fluorometric analysis of thein vivo reactants in sperm indicated a reaction preference for phospholipid over protein compared to control cells dissociated in SDS prior to labelling. The bulk of labelled lipid was phosphatidylethanolamine.These results suggest that fluorescamine is a true vital dye which can label the cell surface as well as penetrate deeply within cells to label a variety of organelles. The distribution of fluorescence and results of chemical analysis suggest thatin vivo the dye may preferentially react with membrane. 相似文献