全文获取类型
收费全文 | 6813篇 |
免费 | 587篇 |
国内免费 | 2篇 |
出版年
2023年 | 23篇 |
2021年 | 88篇 |
2020年 | 56篇 |
2019年 | 83篇 |
2018年 | 108篇 |
2017年 | 90篇 |
2016年 | 136篇 |
2015年 | 249篇 |
2014年 | 291篇 |
2013年 | 337篇 |
2012年 | 470篇 |
2011年 | 474篇 |
2010年 | 307篇 |
2009年 | 270篇 |
2008年 | 402篇 |
2007年 | 378篇 |
2006年 | 339篇 |
2005年 | 352篇 |
2004年 | 312篇 |
2003年 | 311篇 |
2002年 | 306篇 |
2001年 | 140篇 |
2000年 | 126篇 |
1999年 | 108篇 |
1998年 | 75篇 |
1997年 | 58篇 |
1996年 | 47篇 |
1995年 | 50篇 |
1994年 | 57篇 |
1993年 | 48篇 |
1992年 | 75篇 |
1991年 | 81篇 |
1990年 | 71篇 |
1989年 | 83篇 |
1988年 | 74篇 |
1987年 | 82篇 |
1986年 | 51篇 |
1985年 | 52篇 |
1984年 | 54篇 |
1983年 | 40篇 |
1982年 | 50篇 |
1981年 | 41篇 |
1980年 | 36篇 |
1979年 | 40篇 |
1978年 | 47篇 |
1977年 | 34篇 |
1976年 | 35篇 |
1975年 | 34篇 |
1974年 | 38篇 |
1973年 | 31篇 |
排序方式: 共有7402条查询结果,搜索用时 343 毫秒
51.
Michel A. Haring Steve Scofield Marianne J. Teeuwen-de Vroomen Gerjan S. Leuring H. John J. Nijkamp Jacques Hille 《Plant molecular biology》1991,17(5):995-1004
A Tam3 two-element system has been designed by combining an immobilized Tam3 element with a non-autonomous dTam3 element inserted into the HPT gene. The phenotypic assay employed, restored hygromycin resistance, indicated thattrans-activation of the non-autonomous dTam3 element occurred. Molecular analyses of the excision sites revealed that the ends of the dTam3 element remain in the empty donor sites. The predominant consequence of this type of excision appears to be that excised fragments fail to re-integrate into the tobacco genome. Only one case of dTam3 re-integration could be detected. The ends of this element had been degraded upon integration into the tobacco genome. Either the altered structure of the Tam3 derivatives or tobacco host factors are influencing thetrans-activation of a dTam3 element, resulting in aberrant excision. 相似文献
52.
Steve F. Perry Serge Thomas 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1991,161(5):489-497
Summary An extracorporeal circulation of rainbow trout (Oncorhynchus mykiss) was utilized to continuously monitor the rapid and progressive effects of endogenous or exogenous catecholamines on blood respiratory/acid-base status, and to provide in vivo evidence for adrenergic retention of carbon dioxide (CO2) in fish blood (cf. Wood and Perry 1985). Exposure of fish to severe aquatic hypoxia (final P
wO2=40–60 torr; reached within 10–20 min) elicited an initial respiratory alkalosis resulting from hypoxia-induced hyperventilation. However, at a critical arterial oxygen tension (P
aO2) between 15 and 25 torr, fish became agitated for approximately 5 s and a marked (0.2–0.4 pH unit) but transient arterial blood acidosis ensued. This response is characteristic of abrupt catecholamine mobilization into the circulation and subsequent adrenergic activation of red blood cell (RBC) Na+/H+ exchange (Fievet et al. 1987). Within approximately 1–2 min after the activation of RBC Na+/H+ exchange by endogenous catecholamines, there was a significant rise in arterial PCO2 (P
aCO2) whereas arterial PO2 was unaltered; the elevation of P
aCO2 could not be explained by changes in gill ventilation. Pre-treatment of fish with the -adrenoceptor antagonist phentolamine did not prevent the apparent catecholamine-mediated increase of P
aCO2. Conversely, pre-treatment with the -adrenoceptor antagonist sotalol abolished both the activation of the RBC Na+/H+ antiporter and the associated rise in P
aCO2, suggesting a causal relationship between the stimulation of RBC Na+/H+ exchange and the elevation of P
aCO2. To more clearly establish that elevation of plasma catecholamine levels during severe hypoxia was indeed responsible for causing the elevation of P
aCO2, fish were exposed to moderate hypoxia (final P
wO2=60–80 torr) and then injected intraarterially with a bolus of adrenaline to elicit an estimated circulating level of 400 nmol·l-1 immediately after the injection. This protocol activated RBC Na+/H+ exchange as indicated by abrupt changes in arterial pH (pHa). In all fish examined, P
aCO2 increased after injection of exogenous adrenaline. The effects on P
aO2 were inconsistent, although a reduction in this variable was the most frequent response. Gill ventilation frequency and amplitude were unaffected by exogenous adrenaline. Therefore, it is unlikely that ventilatory changes contributed to the consistently observed rise in P
aCO2. Pretreatment of fish with sotalol did not alter the ventilatory response to adrenaline injection but did prevent the stimulation of RBC Na+/H+ exchange and the accompanying increases and decreases in P
aCO2 and P
aO2, respectively. These results suggest that adrenergic elevation of P
aCO2, in addition to the frequently observed reduction of P
aO2 are linked to activation of RBC Na+/H+ exchange. The physiological significance and the potential mechanisms underlying the changes in blood respiratory status after addition of endogenous or exogenous catecholamines to the circulation of hypoxic rainbow trout are discussed.Abbreviations
P
aCO2
arterial carbon dioxide tension
-
P
aO2
arterial oxygen tension
-
P
da
dorsal aortic pressure
-
pHa
arterial pH
-
P
wO2
water oxygen tension
-
RBC
red blood cell
-
V
f
breathing frequency 相似文献
53.
Six 'core' subunits of pea photosystem I have been isolated and their N-terminal amino acid sequences determined by gas-phase or solid-phase sequencing. On average more than thirty residues were determined from the N-terminus of each polypeptide. This sequence analysis has revealed three polypeptides with charged N-terminal regions (21, 17 and 11 kDa subunits), one polypeptide with a predominantly hydrophobic N-terminal region (9 kDa subunit), one polypeptide which is cysteine-rich (8 kDa subunit) and one which is alanine-rich (13 kDa subunit). 相似文献
54.
The beta-glucosidase, linamarase, which specifically hydrolyzes cyanogenic substrates, linamarin and lotaustralin, in white clover, is synthesized in the early stages of leaf and seedling development in genetically competent plants. Plants, from natural populations, possessing at least one Li allele synthesize linamarase but plants with only li alleles do not, nor do they produce inactive but antigenically related linamarase. Linamarase is known to be a mannosyl glycoprotein, which in its active form is a dimer, with a subunit size of 62,000 Mr. We demonstrate that the antibiotic tunicamycin, which prevents N-acetyl-asparagine linked glycosylation, reduces in vivo synthesis of linarmarase. In vitro translation of mRNA from a Li Li plant yields a 59,000 Mr immunoprecipitated linamarase polypeptide which is modified to a 62,000 Mr product by the addition of dog pancreas microsomes. No anti-linamarase immunoprecipitable product is obtained from the in vitro translation products of mRNA from a li li plant. 相似文献
55.
The kinetics of anaerobic degradation of a molasses wastewater were measured under constant pH conditions in a laboratory scale packed bed reactor. In continuous and batch experiments the formation and degradation rates of the organic acids (butyric, propionic and acetic) have been followed. The influence of hydrogen gas on the acid degradation rates has been measured and, contrary to the literature and the thermo-dynamic calculations, no inhibition was detected, biofilm diffusional effects may be the reason. Two dynamic simulation models were tested, a heterogeneous model, which considered the biofilm diffusion-reaction phenomena and a quasihomogeneous model with the same kinetics. Except for hydrogen, the diffusion effects were found to be negligible. Otherwise both models gave essentially the same results and the time profiles of acids, hydrogen, carbon dioxide and methane agreed relatively well with dynamic startup experiments. Batch experiments showed the acid concentrations to be highly sensitive to the initial molasses concentration. This aspect was not included in the model but is being investigated further. 相似文献
56.
Contact sensitization to components of the urushiol oils of poison oak and poison ivy appears to require covalent bond formation between the o-quinones derived from urushiol catechols and nucleophilic groups on proteins. Previous studies using a murine delayed hypersensitivity model demonstrated that 5-methyl-3-pentadecylcatechol (5-Me-PDC) is an epicutaneous tolerogen to the parent compound and a weak sensitizer to itself. To investigate further the structural requirements for sensitization vs suppression, 5,6-dimethyl-3-pentadecylcatechol (5,6-di-Me-PDC) and 4,5,6-trimethylpentadecylcatechol (4,5,6-tri-Me-PDC) were synthesized. The former compound is blocked at both preferred sites for covalent bond formation and the latter is completely blocked towards conjugate addition reactions. These compounds were tested for sensitizing and suppressive ability. Epicutaneous application of both analogs suppressed subsequent induction of sensitization to 3-pentadecylcatechol (PDC) and 3-heptadecylcatechol (HDC). Lymph node cells from animals treated with 5,6-di-Me-PDC could transfer suppression. The dimethyl analog, 5,6-di-Me-PDC, but not the trimethyl analog also exhibited weak sensitizing capacity. The urushiol analogs 5-pentadecylresorcinol (PDR) and 3-heptadecylveratrole (HDV) which cannot form o-quinones were found to be ineffective sensitizers as well. HDV in addition produced no blastogenesis in draining lymph nodes whereas lymph node cell proliferation induced by 4,5,6-tri-Me-PDC followed the same kinetics as previously observed for HDC. PDR elicited weak proliferation with a different time course. These and previous studies indicate that blocking the C5-position on the catechol ring favors the induction of suppression, although some sensitizing capacity may be retained. Covalent bond formation may not be necessary for the induction of active suppressor cell populations. 相似文献
57.
Column centrifugation generates an intersubunit disulfide bridge in Escherichia coli F1-ATPase 总被引:1,自引:0,他引:1
Passage of F1-ATPase through a centrifuge column [Penefsky, H. S. (1979) Methods Enzymol. 56, 527-530] caused formation of a product with a relative molecular mass of 72,000 as determined by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. The product was identified as cross-linked alpha and delta subunits by using Western blots and subunit-specific monoclonal antibodies. The cross-link was reversed by 50 mM dithiothreitol implying that it was a disulfide bridge. Formation of the cross-link was inhibited by 2 mM EDTA and was stimulated in some buffers by the addition of 10 microM CuCl2. Time course experiments indicated that the majority of the cross-link formed while the enzyme was passing through the column. Thus the cross-link induced by column centrifugation arose from the rapid, heavy-metal-ion-catalysed oxidation of two sulfhydryl groups, one on the alpha subunit and one on the delta subunit, to a disulfide. These results demonstrate that care must be exercised when running proteins through centrifuge columns as potentially deleterious disulfide formation can result. An anti-beta monoclonal antibody was capable of immunoprecipitating the entire enzyme including the cross-linked subunits, implying that the cross-linked alpha and delta subunits were still a part of F1. The formation of the cross-link affected neither the hydrolytic activity of the enzyme nor its susceptibility to inhibition by epsilon subunit. The cross-linked enzyme was unable to bind to F1-depleted membranes in experiments in which soluble F1 and membranes were separated by centrifugation. Column centrifugation did not generate the cross-link on membrane-bound enzyme. These results indicate that the alpha-delta cross-link results in a loss of binding affinity between F1 and F0. 相似文献
58.
Factors affecting the reliability of the McMaster technique 总被引:3,自引:0,他引:3
Factors detracting from the reliability of faecal egg counts based on the McMaster technique include variation in flotation time (interval between loading chamber and counting eggs) and sample dilution (ratio of faecal material to salt solution). We recommend standardization of both these variables as normal laboratory procedure, and propose optima of a 30 minute flotation time and a sample dilution of 15 ml salt solution/g faeces for use of the McMaster technique in the estimation of the fecundity of Heligmosomoides polygyrus (Nematoda). 相似文献
59.
Anti-VPg antibody precipitation of product RNA synthesized in vitro by the poliovirus polymerase and host factor is mediated by VPg on the poliovirion RNA template. 总被引:11,自引:10,他引:1 下载免费PDF全文
Antibody to the poliovirus genome-linked protein, VPg, specifically immunoprecipitated the product RNA synthesized in vitro by the poliovirus RNA polymerase and HeLa cell host factor when VPg-linked poliovirion RNA was used as a template. The largest product RNA that was immunoprecipitated was twice the size of the template RNA. The complete denaturation of the product RNA with CH3HgOH had no effect on the immunoprecipitation reaction. In contrast, CH3HgOH denaturation prevented the immunoprecipitation of the oligo(U)-primed product RNA. Immunoprecipitation of the product RNA synthesized in the host-factor-dependent reaction was prevented if VPg was removed from the template RNA by pretreatment with proteinase K or if an RNA template without VPg was used in the reaction. The results support our previous evidence that a covalent linkage exists between the labeled negative-strand product RNA and the VPg-linked template RNA and suggest that the purified polymerase and host factor initiated RNA synthesis in vitro in the absence of VPg or a VPg-precursor protein. 相似文献
60.
Production of pea lectin in Escherichia coli 总被引:2,自引:0,他引:2
In order to explore the molecular basis for the glycopeptide specificity of legume lectins, we have developed an experimental system in which specific amino acid alterations can be introduced into the carbohydrate binding site of pea lectin. This system is based on the production of pea lectin in Escherichia coli. The plasmid coding for the lectin was constructed from two lectin cDNA sequences isolated from Pisum sativum seeds (Higgins, T. J. V., Chandler, P. M., Zurawski, G., Button, S. C., and Spencer, D. (1983) J. Biol. Chem. 258, 9544-9549) and an expression vector based on the gene for the outer membrane lipoprotein of E. coli (Nakamura, K., and Inouye, M. (1982) EMBO J. 1, 771-775). The lectin is produced as a single polypeptide chain and forms insoluble aggregates in E. coli cells (2-5 mg/liter). Functional lectin is recovered by solubilization of the aggregates in guanidinium hydrochloride, renaturation in the presence of MnCl2 and CaCl2, and affinity purification on Sephadex. This procedure yields a homogeneous 28,000-dalton protein. Comparison of the recombinant lectin with natural pea lectin in an inhibition of hemagglutination assay demonstrated that there is no detectable difference in the carbohydrate binding properties of the two lectins. 相似文献