全文获取类型
收费全文 | 6077篇 |
免费 | 712篇 |
国内免费 | 3篇 |
专业分类
6792篇 |
出版年
2021年 | 50篇 |
2018年 | 56篇 |
2016年 | 97篇 |
2015年 | 132篇 |
2014年 | 153篇 |
2013年 | 211篇 |
2012年 | 257篇 |
2011年 | 202篇 |
2010年 | 167篇 |
2009年 | 164篇 |
2008年 | 216篇 |
2007年 | 214篇 |
2006年 | 223篇 |
2005年 | 205篇 |
2004年 | 198篇 |
2003年 | 184篇 |
2002年 | 190篇 |
2001年 | 204篇 |
2000年 | 198篇 |
1999年 | 168篇 |
1998年 | 90篇 |
1997年 | 116篇 |
1996年 | 105篇 |
1995年 | 92篇 |
1994年 | 92篇 |
1993年 | 95篇 |
1992年 | 118篇 |
1991年 | 116篇 |
1990年 | 144篇 |
1989年 | 125篇 |
1988年 | 111篇 |
1987年 | 114篇 |
1986年 | 98篇 |
1985年 | 127篇 |
1984年 | 94篇 |
1983年 | 75篇 |
1982年 | 96篇 |
1981年 | 72篇 |
1980年 | 77篇 |
1979年 | 87篇 |
1978年 | 74篇 |
1977年 | 69篇 |
1976年 | 79篇 |
1975年 | 73篇 |
1974年 | 73篇 |
1973年 | 73篇 |
1972年 | 76篇 |
1971年 | 63篇 |
1969年 | 47篇 |
1968年 | 51篇 |
排序方式: 共有6792条查询结果,搜索用时 0 毫秒
41.
J N Evans R C Davies A S Boyd I Ichinose N E Mackenzie A I Scott R L Baxter 《Biochemistry》1986,25(4):896-904
High-field NMR spectroscopic methods have been applied to study the reactions catalyzed by porphobilinogen (PBG) deaminase and uroporphyrinogen III (uro'gen III) cosynthase, which are the enzymes responsible for the formation of the porphyrin macrocycle. The action of these enzymes in the conversion of PBG, [2,11-13C]PBG, and [3,5-13C]PBG to uro'gens I and III has been followed by 1H and 13C NMR, and assignments are presented. The principal intermediate that accumulated was the correspondingly labeled (hydroxymethyl)bilane (HMB), the assignments for which are also presented. 相似文献
42.
Summary Leaves of birch (Betula pendula Roth) and sycamore (Acer pseudoplatanus L.) were initiated and grown either in a simulated shadelight (80 mol m-2 s-1, R/FR ratio 0.28)/dark photoenvironment or a white light (250 mol m-2 s-1, R/FR>1)/dark photoenvironment. Until the leaves were more than 50% expanded, growth rates (measured every 24 h) were the same for both species in both environments. After this time, growth rate slowed and this correlated well with a decrease in wall extensibility (WEX). Birch leaves in shadelight showed reduced surface acidification and were the first to show reduced growth. WEX under these conditions was particularly low.Daily patterns of leaf growth of the two species were very different. Sycamore leaves showed a slightly higher growth rate in the dark than in shadelight, while birch leaves grew more rapidly in shadelight than in the dark. Limitation of growth of sycamore leaves in light may be explained by a very high yield threshold turgor for growth (Y). The daily pattern of leaf growth shown by birch is more difficult to explain but the importance of a possible limitation of growth by solute availability and a diurnal variation in Y are discussed. 相似文献
43.
T. H. Noel Ellis Claire Domoney Judy Castleton Wendy Cleary D. Roy Davies 《Molecular & general genetics : MGG》1986,205(1):164-169
Summary We describe four genomic clones of pea 7S storage protein gene, one of which corresponds to convicilin, and the others to
vicilin. Hybridization studies exploiting these clones, and previously identified cDNA clones, have enabled us to define six
different loci. Three of these loci have been mapped to positions on chromosome 7. 相似文献
44.
Wounding Inhibits Protein Synthesis yet Stimulates Polysome Formation in Aged, Excised Pea Epicotyls 总被引:2,自引:0,他引:2
Wounding of aged, previously-excised pea epicotyl segments byremoval of the basal 12 mm resulted in a rapid (beginningwithin 15 min) recruitment of monosomes on to polysomes andan even more rapid (maximal between 612 min) inhibitionof protein synthesis in the remaining tissue. This inhibitionof protein synthesis in vivo did not appear to be an artefactcaused by the removal of highly active tissue (e.g., callus,contaminating bacteria), since wounds inflicted at a site distantfrom the region analyzed still elicited the response, and proteinsynthesis in the 12 mm slices (normally discarded) wasinhibited even more strongly than it was in the remaining tissue.The proportion of radioactive methionine in nascent chains (boundto polysomes) increased, while the production of completed polypeptidesdecreased, after wounding. Cycloheximide, a known inhibitorof the ribosome translocation/release process mimicked someof the effects of wounding. We interpret the results to indicatethat the initial effect of wounding is to inhibit translationby inhibiting the ribosome translocation/release process, whereasthe subsequent recovery in protein synthesis is brought aboutpartly by a recovery in ribosome translocation/release and partlyby enhanced initiation.
1 Present address: Harvard-MIT Division of Health Science andTechnology, MIT, Cambridge, Massachusetts 02139, U.S.A.
2 Present address: Institute of Agricultural Environment Control,College of Agriculture, Ehime University, 3-5-7 Tarumi, Matsuyama790, Japan. (Received May 26, 1986; Accepted August 4, 1986) 相似文献
45.
Primary productivity and related parameters in three different types of inland waters in Sri Lanka 总被引:1,自引:1,他引:0
Gross and net primary production together with chlorophyll-a biomass were investigated with respect to depth and diurnal changes in three categories of inland waters (reservoirs, temporary ponds, brackish water lagoons) in Sri Lanka. Ten field sites, in both the dry and wet zones of the island, were investigated. Bimodal productivity profiles were recorded in two of the three reservoirs studied. The diel pattern of net photosynthetic rate varied between sites although peak photosynthetic efficiency occurred at solar noon. Surface photoinhibition was characteristic of the reservoirs and brackish water lagoons but not of the temporary ponds. Mean gross primary production was 3.02 g C m–2 d–1 but was higher in the temporary ponds than in the reservoirs. The gross primary production in the brackish water Koggala Lagoon at 0.08 g C m–2 d–1 is a record low for tropical lagoons and was 2.5 times less than the two other lagoons investigated. Variability in net primary production between sites was similar to the variation in gross production with a relatively low mean value for tropical inland waters of 0.495 C m–2 d–1. Mean maximum photosynthetic rate was 0.30 mg C m–3 h–1 but was lower in the reservoirs than in the temporary ponds and lagoons. 相似文献
46.
Retinoic acid rapidly induces the accumulation of a specific enzyme, tissue transglutaminase (EC 2.3.2.13), in mouse macrophages. We have used the induction of tissue transglutaminase to study the regulation of gene expression by retinoic acid. In this study we report that pertussis toxin can inhibit retinoic acid-induced expression of tissue transglutaminase in mouse resident peritoneal macrophages. This inhibition is paralleled by the ADP-ribosylation of 41,000-dalton macrophage membrane protein. 相似文献
47.
Enhanced neuronal K+ conductance: a possible common mechanism for sedative-hypnotic drug action 总被引:2,自引:0,他引:2
P L Carlen N Gurevich M F Davies T J Blaxter M O'Beirne 《Canadian journal of physiology and pharmacology》1985,63(7):831-837
It is commonly thought that central nervous system depressant drugs exert their actions through enhancement of gamma-aminobutyrate (GABA)-mediated mechanisms. Recently, the cellular electrophysiological evidence from this laboratory and others suggests that both sedative hypnotics and general anaesthetics inhibit central neurons by increasing potassium conductance (GK). We have utilized the mammalian in vitro hippocampal and cerebellar slice preparations at 34-36 degrees C. Intracellular recordings from CA1, CA3, and cerebellar Purkinje cells were obtained. Low dose (sedative) concentrations of ethanol (less than or equal to 20 mM), two different benzodiazepines (midazolam and clonazepam in low nanomolar concentrations), and pentobarbital (10(-6) to 10(-4) M) were applied by pressure ejection or were bath perfused. All drugs caused a hyperpolarization with decreased spontaneous activity, and enhanced post spike afterhyperpolarizations (AHPs). These long-lasting AHPs are presumably due to enhanced calcium-mediated GK. Increased responsiveness to focally applied GABA was only seen at higher doses (ethanol, 100 mM; midazolam, 10(-7) M; pentobarbital, 10(-4) M). These data suggest that the above neurodepressant drugs, when applied at sedative doses to hippocampal pyramidal cells, enhance GK and not the actions of GABA. 相似文献
48.
The subcutaneous injection of diethylstilbestrol (2.5 micrograms/day) into pregnant guinea pigs from the 28th or 40th day to term resulted in an accelerated tempo of differentiation of the genital tract of the female offspring at birth, as well as intense estrogenic stimulation. Estradiol (50 micrograms per day) injected over the same period caused insignificant estrogenic stimulation in the newborn. The normal embryogenesis of the genital tract is described and a pattern of caudocranial differentiation identified. The normal genital tract is described as a basis for the analysis of results in the newborn. Comparative studies in other animals are discussed. 相似文献
49.
Structure of an antifreeze polypeptide and its precursor from the ocean pout, Macrozoarces americanus 总被引:5,自引:0,他引:5
X M Li K Y Trinh C L Hew B Buettner J Baenziger P L Davies 《The Journal of biological chemistry》1985,260(24):12904-12909
Serum antifreeze polypeptides (AFP) from Newfoundland ocean pout have been resolved by ion exchange chromatography and reverse phase high performance liquid chromatography into at least 12 components. The protein sequences of three of the AFP were determined using a combination of protein Edman degradation and cDNA sequencing. The AFP precursor protein encodes for a preprotein of 87 amino acids with no obvious prosequences. Two of the AFP (SP1-A and SP1-C) were separate gene products with minor amino acid sequence differences. The protein structure of SP1-C precursor is MKSVILTGLLFVLLCVDHMTASQSVVAT QLIPINTALTPAMMEGKVTNPIGIPFAEMSQIVGKQVNTPVAKGQTLMPNMVKTYVAGK. The third AFP (SP1-B) is a post-translation modification product of SP1-C. These experiments indicate that the ocean pout AFP are a multigene family with protein structure different from any other known polypeptide antifreezes. 相似文献
50.
D. Francis Nanette D. Davies J. A. Bryant S. G. Hughes D. R. Sibson P. N. Fitchett 《Experimental cell research》1985,158(2)
We have examined by fibre autoradiography the spacing of replicons in pea root meristems during synchronized entry into S phase from arrest at the G1/S boundary. Pretreatment with the DNA cross-linking agent, psoralen, produces a marked shortening of replicon spacing, suggesting that premature arrest of the replication fork results in the recruitment of additional initiation points within a given replicon family. This is discussed in relation to models for the control of DNA replication. 相似文献