全文获取类型
收费全文 | 11435篇 |
免费 | 957篇 |
国内免费 | 16篇 |
出版年
2023年 | 25篇 |
2022年 | 80篇 |
2021年 | 254篇 |
2020年 | 150篇 |
2019年 | 188篇 |
2018年 | 267篇 |
2017年 | 220篇 |
2016年 | 389篇 |
2015年 | 645篇 |
2014年 | 677篇 |
2013年 | 767篇 |
2012年 | 1018篇 |
2011年 | 893篇 |
2010年 | 536篇 |
2009年 | 461篇 |
2008年 | 725篇 |
2007年 | 656篇 |
2006年 | 565篇 |
2005年 | 545篇 |
2004年 | 527篇 |
2003年 | 389篇 |
2002年 | 391篇 |
2001年 | 307篇 |
2000年 | 229篇 |
1999年 | 199篇 |
1998年 | 96篇 |
1997年 | 68篇 |
1996年 | 59篇 |
1995年 | 48篇 |
1994年 | 52篇 |
1993年 | 38篇 |
1992年 | 86篇 |
1991年 | 69篇 |
1990年 | 61篇 |
1989年 | 65篇 |
1988年 | 58篇 |
1987年 | 61篇 |
1986年 | 48篇 |
1985年 | 41篇 |
1984年 | 34篇 |
1983年 | 29篇 |
1982年 | 34篇 |
1981年 | 34篇 |
1980年 | 25篇 |
1979年 | 37篇 |
1978年 | 26篇 |
1977年 | 31篇 |
1976年 | 26篇 |
1975年 | 28篇 |
1974年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
71.
Using the sealed jar technique (also called residual oxygen bioassay), rainbow trout fry infected with Cryptobia salmositica were more susceptible than non-infected fish to environmental hypoxia. The Winkler technique (azide modification) was used to determine the residual dissolved oxygen in the water. Susceptibility of infected fish increased with 1) time after infection and was most evident in 3-7 wk infections, 2) the severity of anemia, and 3) increasing parasitemia. In prolonged infections, susceptibility was reduced when there were decreases in anemia and parasitemia; however, these infected fish were still more susceptible than non-infected fish. The increase in susceptibility of infected fish to hypoxia may be an important contributing factor to mortality of fish in hatcheries where there is inadequate water flow and overcrowding. The sealed jar technique is recommended in future studies on the pathogenesis of parasitic fish diseases, especially if the metabolic and/or respiratory systems are affected by the infection. 相似文献
72.
Simultaneous Measurements of Steady State Chlorophyll a Fluorescence and CO(2) Assimilation in Leaves: The Relationship between Fluorescence and Photosynthesis in C(3) and C(4) Plants 总被引:3,自引:3,他引:0
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Rates of CO2 assimilation and steady state chlorophyll a fluorescence were measured simultaneously at different intercellular partial pressures of CO2 in attached cotton (Gossypium hirsutum L. cv Deltapine 16) leaves at 25°C. Electron transport activity for CO2 assimilation plus photorespiration was calculated for these experiments. Under light saturating (1750 microeinsteins per square meter per second) and light limiting (700 microeinsteins per square meter per second) conditions there was a good correlation between fluorescence and the calculated electron transport activity at 19 and 200 millibars O2, and between fluorescence and rates of CO2 assimilation at 19 millibars but not 200 millibars O2. The values of fluorescence measured at about 220 microbars intercellular CO2 were not greatly affected by increasing O2 from 19 to 800 millibars. Fluorescence increased with light intensity at any one intercellular CO2 partial pressure. But the values obtained for fluorescence, expressed as a ratio of the maximum fluorescence obtained in DCMU-treated tissue, over the same range of CO2 partial pressure at 500 microeinsteins per square meter per second were similar to those obtained at 1000 and 2000 microeinsteins per square meter per second. There were two phases in the observed correlation between fluorescence and calculated electron transport activity: an initial inverse relationship at low CO2 partial pressures which reversed to a positive correlation at higher values of CO2 partial pressures. Similar results were observed in the C3 species Helianthus annuus L., Phaseolus vulgaris L., and Brassica chinensis. In all C4 species (Zea mays L., Sorghum bicolor L., Panicum maximum Jacq., Amaranthus edulis Speg., and Echinochloa frumentacea [Roxb.] Link) examined changes in fluorescence were directly correlated with changes in CO2 assimilation rates. The nature and the extent to which Q (primary quencher) and high-energy state (qE) quenching function in determining the steady state fluorescence obtained during photosynthesis in leaves is discussed. 相似文献
73.
Cloning, expression, and characterization of the Anabaena thioredoxin gene in Escherichia coli. 总被引:8,自引:5,他引:3
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The gene encoding thioredoxin in Anabaena sp. strain PCC 7119 was cloned in Escherichia coli based on the strategy that similarity between the two thioredoxins would be reflected both in the gene sequence and in functional cross-reactivity. DNA restriction fragments containing the Anabaena thioredoxin gene were identified by heterologous hybridization to the E. coli thioredoxin gene following Southern transfer, ligated with pUC13, and used to transform an E. coli strain lacking functional thioredoxin. Transformants that complemented the trxA mutation in E. coli were identified by increased colony size and confirmed by enzyme assay. Expression of the cloned Anabaena thioredoxin gene in E. coli was substantiated by subsequent purification and characterization of the algal protein from E. coli. The amino acid sequence derived from the DNA sequence of the Anabaena gene was identical to the known amino acid sequence of Anabaena thioredoxin. The E. coli strains which expressed Anabaena thioredoxin complemented the TrxA- phenotype in every respect except that they did not support bacteriophage T7 growth and had somewhat decreased ability to support bacteriophages M13 and f1. 相似文献
74.
Expression and sequence analysis of a cDNA encoding the orotidine-5'-monophosphate decarboxylase domain from Ehrlich ascites uridylate synthase 总被引:4,自引:0,他引:4
C A Ohmstede S D Langdon C B Chae M E Jones 《The Journal of biological chemistry》1986,261(9):4276-4282
Orotidine-5'-monophosphate decarboxylase (OD-Case) catalyzes the conversion of orotidine 5'-monophosphate to UMP. In mammals, ODCase is present as part of a bifunctional protein which also contains orotate phosphoribosyltransferase; the preceding enzyme in the de novo UMP biosynthetic pathway. We have isolated a plasmid (pMEJ) which contains a cDNA for the ODCase domain of UMP synthase. Insertion of this sequence into an Escherichia coli expression vector (pUC12) has allowed for the expression of ODCase and not orotate phosphoribosyltransferase in E. coli. The molecular weight of the expressed protein is 26,000-27,300 from immunoblot analysis which corresponds closely to the molecular weight of the ODCase domain (28,500) isolated by tryptic digestion of UMP synthase. We have sequenced the cDNA insert of pMEJ and deduced the amino acid sequence. The molecular weight of the ODCase domain calculated from the amino acid sequence in 28,654. Comparison of the deduced amino acid sequence from pMEJ with that for yeast ODCase (a monofunctional protein) demonstrated that 52% of the amino acids were identical when the two sequences are compared. Furthermore, several stretches of the amino acid sequence have 80% or greater absolute homology. 相似文献
75.
Sidney K. Woo A.Peter Roszkowski L.David Waterbury Gabriel L. Garay 《Prostaglandins & other lipid mediators》1986,32(2)
The potent antiulcer prostaglandin enprostil binds with high affinity to porcine gastic mucosal tissues. This binding is saturable, dissociable and displaceable by compounds with similar structures. Various characteristics of binding such as pH optimum and displacement potencies suggest that enprostil binds to mucosal PGE2 sites. Structure-activity and gastric mucosal binding relationships were also examined. 相似文献
76.
The structure of the capsular polysaccharide from Klebsiella K79 was determined by the techniques of methylation, periodate oxidation, beta-elimination, chromic acid oxidation, and partial hydrolysis. N.m.r. spectroscopy (1H and 13C) was used extensively to establish the nature of the anomeric linkages of the polysaccharide and of oligosaccharides derived through degradative procedures. The polysaccharide was found to have the heptasaccharide, "5 + 2" repeating unit: (Formula: see text). 相似文献
77.
W T Schroeder M F Miller S L Woo G F Saunders 《American journal of human genetics》1985,37(5):868-872
In situ hybridization of a recombinant cDNA probe containing the human alpha 1-antitrypsin gene to metaphase chromosomes demonstrated significant hybridization to chromosomal segment 14q31-32. A high percentage of cells analyzed (31%) displayed labeling on chromosome 14. Of all labeled sites on chromosome 14, 60% were found on segment 14q31-32. These results refine the previous assignment of the human alpha 1-antitrypsin gene to segment 14q24.1-32.1. 相似文献
78.
Z Damuni H Y Lim Tung L J Reed 《Biochemical and biophysical research communications》1985,133(3):878-883
A potent, heat-stable protein inhibitor of branched-chain alpha-keto acid dehydrogenase (BCKDH) phosphatase has been identified and purified to near homogeneity from bovine kidney mitochondria (Damuni, Z., Humphreys, J. S., and Reed, L. J., Proc. Natl. Acad. Sci. U.S.A., in press). This protein is a noncompetitive inhibitor of BCKDH phosphatase, with a Ki about 0.13 nM. By contrast, this protein inhibitor did not affect the activity of the cytosolic protein phosphatase-1 and phosphatase-2A or the mitochondrial pyruvate dehydrogenase (PDH) phosphatase at concentrations up to 10 nM. The cytosolic protein phosphatase inhibitor-1 and inhibitor-2 had no effect on the activity of BCKDH phosphatase or PDH phosphatase at concentrations up to 50 and 300 nM respectively. These results, together with previous evidence, demonstrate that BCKDH phosphatase and its inhibitor protein are distinct from the cytosolic protein phosphatase-1 and phosphatase-2A and from protein phosphatase inhibitor-1 and inhibitor-2, respectively. 相似文献
79.
Nitrogen Metabolism in Senescent Flag Leaves of Wheat (Triticum aestivum L.) in the Light 总被引:5,自引:2,他引:3
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Nitrogen metabolism was examined in senescent flag leaves of 90- to 93-day-old wheat (Triticum aestivum L. cv Yecora 70) plants. CO2 assimilation and the levels of protein, chlorophyll, and nitrogen in the leaves decreased with age. Glutamine synthetase activity decreased to one-eighth of the level in young flag leaves. Detached leaves were incubated (with the cut base) in 15N-labeled NH3, glutamate, or glycine in the light (1.8 millieinstein per square meter per second) at 25°C in an open gas exchange system under normal atmospheric conditions for up to 135 minutes. The 15N-enrichment of various amino acids derived from these 15N-substrates were examined. The amido-N of glutamine was the first 15N-labeled product in leaves incubated with 15NH4Cl whereas serine, closely followed by the amido- and amino-N of glutamine, were the most highly 15N-labeled products during incubation with [15N]glycine. In contrast, aspartate and alanine were the first 15N-labeled products when [15N] glutamate was used. These results indicate that NH3 was assimilated via glutamine synthetase and glutamate synthase activities and the photorespiratory nitrogen cycle remained functional in these senescent wheat flag leaves. In contrast, an involvement of glutamate dehydrogenase in the assimilation of ammonia could not be detected in these tissues. 相似文献
80.