全文获取类型
收费全文 | 18346篇 |
免费 | 1559篇 |
国内免费 | 4篇 |
专业分类
19909篇 |
出版年
2023年 | 65篇 |
2022年 | 151篇 |
2021年 | 317篇 |
2020年 | 175篇 |
2019年 | 230篇 |
2018年 | 322篇 |
2017年 | 282篇 |
2016年 | 402篇 |
2015年 | 791篇 |
2014年 | 844篇 |
2013年 | 1063篇 |
2012年 | 1484篇 |
2011年 | 1453篇 |
2010年 | 876篇 |
2009年 | 865篇 |
2008年 | 1196篇 |
2007年 | 1211篇 |
2006年 | 1103篇 |
2005年 | 1118篇 |
2004年 | 985篇 |
2003年 | 925篇 |
2002年 | 900篇 |
2001年 | 189篇 |
2000年 | 138篇 |
1999年 | 186篇 |
1998年 | 271篇 |
1997年 | 159篇 |
1996年 | 144篇 |
1995年 | 142篇 |
1994年 | 124篇 |
1993年 | 115篇 |
1992年 | 96篇 |
1991年 | 82篇 |
1990年 | 91篇 |
1989年 | 87篇 |
1988年 | 77篇 |
1987年 | 55篇 |
1986年 | 74篇 |
1985年 | 76篇 |
1984年 | 97篇 |
1983年 | 75篇 |
1982年 | 86篇 |
1981年 | 104篇 |
1980年 | 88篇 |
1979年 | 56篇 |
1978年 | 66篇 |
1977年 | 53篇 |
1976年 | 38篇 |
1974年 | 56篇 |
1973年 | 35篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
51.
The effect of the Abortion Act on gynaecological work in one provincial teaching hospital is that despite an increase in patient turnover of 45% the waiting list has increased by 200%. It is suggested that the Act has had little effect on the birth rate or illegitimacy and that there may have been an increase in criminal abortion. 相似文献
52.
A mass-spectrometric sequence study of the enzyme ribitol dehydrogenase from Klebsiella aerogenes 总被引:10,自引:6,他引:4
Howard R. Morris Dudley H. Williams Graeme G. Midwinter Brian S. Hartley 《The Biochemical journal》1974,141(3):701-713
The first detailed results of the application of a low-resolution mixture analysis approach to the sequence analysis of an enzyme, ribitol dehydrogenase, are given. Examples of the interpretation of the spectra of peptide mixtures derived from this protein are described. Evidence for new fragmentation patterns observed is reported, together with an explanation of the generation of ambiguous sequences by use of a low-specificity enzyme, thermolysin. The overall sequencing strategy evolved is assessed. 相似文献
53.
Brian Snoad 《Genetica》1966,37(1):247-254
By appropriate genetical studies using reciprocal translocation stocks it has been possible to confirmLamprecht's evidence that inPisum there are seven linkage groups.
Lamm's evidence for six linkage groups has been shown to be due to incomplete knowledge of the chromosomal structure of certain interchange lines.It is suggested thatLamprecht's system of chromosomal nomenclature should be universally adopted forPisum.Lamm &Miravelle's extended tester set has been accordingly renumbered and the intercrossing of the interchange lines completed in order to extend the confirmatory evidence as to their chromosomal constitution. 相似文献
54.
Brian J. S. Harley 《CMAJ》1960,82(22):1123-1126
55.
56.
Summary The present studies examined some of the properties of Cl– channels in renal outer medullary membrane vesicles incorporated into planar lipid bilayers. The predominant channel was anion selective having aP
Cl/P
K ratio of 10 and a unit conductance of 93 pS in symmetric 320mm KCl. In asymmetric KCl solutions, theI-V relations conformed to the Goldman-Hodgkin-Katz equation. Channel activity was voltage-dependent with a gating charge of unity. This voltage dependence of channel activity may account, at least in part, for the striking voltage dependence of the basolateral membrane Cl– conductance of isolated medullary thick ascending limb segments. The Cl– channels incorporated into the planar bilayers were asymmetrical: thetrans surface was sensitive to changes in ionized Ca2+ concentrations and insensitive to reducing KCl concentrations to 10mm, while thecis side was insensitive to changes in ionized Ca2+ concentrations, but was inactivated by reducing KCl concentrations to 50mm. 相似文献
57.
Electron spin resonance evidence for the formation of free radicals in plants exposed to ozone 总被引:9,自引:0,他引:9
Electron spin resonance (ESR) spectroscopy was used to demonstrate that free radicals are formed in O3 -fumigated plant leaves prior to the formation of visible leaf injury. ESR signals with a g-value of 2.0037 to 2.0043, were observed in pea ( Pisum sativum L. cv. Feltham first) and bean ( Phaseolus vulgaris L. cv. Pinto) plants that had been fumigated for 4 h with 70–300 nl l−1 of ozone after they had been treated with the spin-trap N- t -butyl-α-phenylnitrone (PBN). The size of the ESR signals increased with the concentration of ozone used but the nature of the trapped radicals could not be identified. However, further experiments using an inhibitor of ethylene biosynthesis, arninoethoxyvinyl glycine (AVG), showed that the reaction between ozone and ethylene is the cause for ozone toxicity. 相似文献
58.
David G. Griffiths Michael D. Partis Perry Churchill Stephen C. Brenner Sidney Fleischer Roger J. Moore R. Brian Beechey 《Journal of bioenergetics and biomembranes》1990,22(5):691-707
A series of amphiphilic polymethylenecarboxymaleimides has been synthesized for use as sulfhydryl reagents applicable to membrane proteins. Physical properties of the compounds which are relevant to their proposed mode of action have been determined. By comparing rates of reaction in aqueous and aprotic solvents, the compounds have been shown to react exclusively with the thiolate ion. The effects of the reagents on three membrane-associated proteins are reported, and in two cases a comparative study has been made of the effects on the proteins in the absence of membranes. A mechanism is proposed whereby the reagents are anchored at the lipid/water interface by the negatively charged carboxyl group, thus siting the reactive maleimide in a plane whose depth is defined by the length of the reagent. Supporting evidence for this model is provided by the inability of the reagents to traverse membranes, and variation of their inhibitory potency with chain length when the proteins are embedded in the membrane, but not when extracted into solution. As examples of general use of the reagents to probe sulfhydryl groups in membrane proteins, the reagents have been used to (a) determine the depths in the membrane at which two populations of sulfhydryl groups occur in the mitochondrial phosphate transporter; (b) locate a single sulfhydryl associated with the active site ofD--hydroxybutyrate dehydrogenase in the inner mitochondrial membrane; (c) examine sulfhydryl groups in theD-3-glyceraldehyde phosphate dehydrogenase associated with the human red blood cell membrane. 相似文献
59.
Ian T. Forrester Anthony C. Grabski Chittra Mishra Brian D. Kelley W. Nick Strickland Gary F. Leatham Richard R. Burgess 《Applied microbiology and biotechnology》1990,33(3):359-365
Summary Extracellular culture filtrates from ligninolytic cultures of the lignin-degrading basidiomycete Lentinula (syn. Lentinus) edodes (Berk.) Pegler contained one major peroxidase when grown on a commercial oak-wood substrate. The peroxidase was purified by polyethylenimine clarification, anion-exchange chromatography, and hydrophobic-interaction HPLC. The enzyme (MnP1) was a heme-iron protein with an apparent molecular weight of 44 600 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels and an isoelectric point of pH 3.2. The native enzyme had an absorption maximum at 407 nm, which shifted to 420 nm upon H2O2 addition. The pyridine-hemochrome-absorption spectrum indicated that one heme group was present per enzyme as protoporphyrin IX. N-terminal amino acid sequencing showed that MnP1 had higher sequence homology with manganese peroxidases than with lignin peroxidases reported from Phanerochaete chrysosporium. L. edodes MnP1 was capable of oxidizing lignin and lignin-model compounds in the presence of manganese and H2O2.On leave from the Department of Biochemistry, University of Otago, P. O. Box 56, Dunedin, New Zealand.Research carried out while a visiting scientist at the USDA Forest Products Laboratory from the National Chemistry Laboratory, Pune, India 41 1008
Offprint requests to: I. T. Forrester 相似文献
60.
Patricia A. McLenachan Peter J. Lockhart H. Rick Faber Brian C. Mansfield 《Journal of molecular evolution》1996,42(2):273-280
The pregnancy-specific 1-glycoproteins (PSG) form a large family of closely related proteins. Using newly developed methods of sequence analysis, in combination with protein modeling, we provide a framework for investigating the evolution and biological function of genes like the PSG. Evolutionary trees, based on C-terminal sequence, group PSG genes in a manner consistent with their genomic organization. Trees constructed using the N-terminal domain sequences are unreliable as an indicator of phylogeny because of non-neutral processes of sequence change. During duplication of the PSG genes, evolutionary pressures have resulted in a gradient of constrained change across each gene. The N-terminal domains show a nonrandom pattern of amino acid substitutions clustered in the immunoglobulin complementarity-determining region (CDR)-like regions, which appear to be important in the function of the protein. 相似文献