全文获取类型
收费全文 | 298篇 |
免费 | 43篇 |
国内免费 | 2篇 |
出版年
2021年 | 3篇 |
2017年 | 5篇 |
2016年 | 8篇 |
2015年 | 6篇 |
2014年 | 7篇 |
2013年 | 15篇 |
2012年 | 13篇 |
2011年 | 16篇 |
2010年 | 17篇 |
2009年 | 15篇 |
2008年 | 20篇 |
2007年 | 9篇 |
2006年 | 11篇 |
2005年 | 6篇 |
2004年 | 10篇 |
2003年 | 10篇 |
2002年 | 11篇 |
2001年 | 16篇 |
2000年 | 13篇 |
1999年 | 12篇 |
1998年 | 11篇 |
1997年 | 13篇 |
1996年 | 10篇 |
1995年 | 5篇 |
1994年 | 6篇 |
1993年 | 10篇 |
1992年 | 3篇 |
1991年 | 8篇 |
1990年 | 10篇 |
1989年 | 3篇 |
1983年 | 2篇 |
1981年 | 2篇 |
1979年 | 2篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1959年 | 1篇 |
1955年 | 1篇 |
1951年 | 2篇 |
1948年 | 1篇 |
1947年 | 2篇 |
1946年 | 1篇 |
1945年 | 1篇 |
1944年 | 2篇 |
1942年 | 2篇 |
1941年 | 2篇 |
1940年 | 1篇 |
1939年 | 1篇 |
1938年 | 1篇 |
1936年 | 1篇 |
1931年 | 1篇 |
排序方式: 共有343条查询结果,搜索用时 31 毫秒
1.
2.
Physical chemical studies of short-chain lecithin homologues. II. Micellar weights of dihexanoyl- and diheptanoyllecithin 总被引:2,自引:0,他引:2
The micellar weights of dihexanoyl- and diheptanoyllecithin in aqueous solutions are calculated from light scattering and ultracentrifugation data. A monomer-micelle assocation model is used and corrections for the thermodynamic nonideality, on the basis of rigid noninteracting particles, are applied. A few experiments on the influence of high NaCI concentrations (up to 3 M) are described. Dihexanoyllecithin forms micelles with micellar weight of 15 000 to 20 000 and with rather narrow weight distributions. Diheptanoyllecithin micelles however, have broad size distributions with micellar weights of 20 000 up to about 100 000 in the concentration range studied. Micelles are assumed to be spherical or to have sphero-cylindrical shapes depending on the molecular weights. Two models are used: (1) a compact structure, where no attention is paid to the hydrocarbon-water contact (2) micelles with as little hydrocarbon-water contact as possible. 相似文献
3.
Synergistic effect of 1-aminocyclopropane-1-carboxylic acid and ethylene during senescence of isolated carnation petals 总被引:2,自引:0,他引:2
The effects of ethylene (C2 H4 ), (2-chloroethyl)phosphonic acid (ethefon) and 1-aminocyclopropane-1-carboxylic acid (ACC) on senescence of isolated intact petals and of upper petal parts of carnation flowers ( Dianthus caryophyllus L. cv. White Sim) were investigated.
Isolated upper petal parts did not respond to treatment with ethefon or ACC. These tissues did, however, show severe wilting in intact petals that were treated with ethefon or ACC. When isolated upper petal parts were simultaneously treated with ACC and ethefon or ACC and ethylene, a marked synergistic effect on senescence was found. Treatment of isolated petals with radiolabeled ACC led to the accumulation of radiolabeled ACC and N-malonyl-ACC (MACC) in the upper parts. The formation of ethylene and the malonylation of ACC were inhibited by pretreatment of the flower with the inhibitor of ethylene action, silver thiosulphate (STS), which indicates that both were induced by endogenously produced ethylene. Treatment of isolated upper parts with ACC slightly increased their ethylene production. However, when these petal parts were simultaneously treated with ethylene and ACC, the conversion of ACC to ethylene was markedly stimulated.
The results indicate that, in intact petals, ethylene may be translocated from the basal to the upper part where it stimulates the activity of the ethylene-forming enzyme (EFE), thereby making the tissue receptive to ACC.
In addition, it was found that upon incubation of petal portions in radiolabeled ACC, both the petal tissue and the incubation solutions produced radiolabeled carbon dioxide. This was shown to be due to microorganisms that were able to metabolize the carbon atoms in the 2 and 3 position of ACC into carbon dioxide. 相似文献
Isolated upper petal parts did not respond to treatment with ethefon or ACC. These tissues did, however, show severe wilting in intact petals that were treated with ethefon or ACC. When isolated upper petal parts were simultaneously treated with ACC and ethefon or ACC and ethylene, a marked synergistic effect on senescence was found. Treatment of isolated petals with radiolabeled ACC led to the accumulation of radiolabeled ACC and N-malonyl-ACC (MACC) in the upper parts. The formation of ethylene and the malonylation of ACC were inhibited by pretreatment of the flower with the inhibitor of ethylene action, silver thiosulphate (STS), which indicates that both were induced by endogenously produced ethylene. Treatment of isolated upper parts with ACC slightly increased their ethylene production. However, when these petal parts were simultaneously treated with ethylene and ACC, the conversion of ACC to ethylene was markedly stimulated.
The results indicate that, in intact petals, ethylene may be translocated from the basal to the upper part where it stimulates the activity of the ethylene-forming enzyme (EFE), thereby making the tissue receptive to ACC.
In addition, it was found that upon incubation of petal portions in radiolabeled ACC, both the petal tissue and the incubation solutions produced radiolabeled carbon dioxide. This was shown to be due to microorganisms that were able to metabolize the carbon atoms in the 2 and 3 position of ACC into carbon dioxide. 相似文献
4.
L. S. van Overbeek J. D. van Elsas J. T. Trevors M. E. Starodub 《Microbial ecology》1990,19(3):239-249
One year after introduction into agricultural drainage waterPseudomonas fluorescens R2f (RP4),Pseudomonas putida CYM318 (pRK2501), andKlebsiella aerogenes NCTC418 (pBR322) could be recovered on agar media. Survival of the introduced strains depended on competition with the indigenous microflora, the presence of nutrients, and the availability of air.In contrast toK. aerogenes NCTC418 (pBR322), bothPseudomonas species lost their plasmids, as indicated by the consistently lower colony counts on selective medium compared with the counts on nonselective medium. The plasmid loss did not depend on nutrient status and oxygen supply.
P. fluorescens R2f cells could be detected with the immunofluorescence (IF) technique. Total cell counts determined by IF were consistently higher than corresponding colony counts. Even in samples where no colonies were recovered, R2f cells could be detected by IF. This indicated the occurrence of nonculturable R2f cells in drainage water. Homology with32P-labelled plasmid RP4 DNA was found in several drainage water samples that originally receivedP. fluorescens R2f (RP4), by using the cell suspension filter hybridization technique.
P. putida CYM318 andK. aerogenes NCTC418 cells could also be detected in sterile drainage water samples, after nonspecific staining with fluorescein isothiocyanate. Cell counts of both strains were consistently higher than corresponding plate counts. 相似文献
5.
The metabolic pathway collection from EMP: the enzymes and metabolic pathways database. 总被引:4,自引:1,他引:3 下载免费PDF全文
E Selkov S Basmanova T Gaasterland I Goryanin Y Gretchkin N Maltsev V Nenashev R Overbeek E Panyushkina L Pronevitch E Selkov Jr I Yunus 《Nucleic acids research》1996,24(1):26-28
The Enzymes and Metabolic Pathways database (EMP) is an encoding of the contents of over 10 000 original publications on the topics of enzymology and metabolism. This large body of information has been transformed into a queryable database. An extraction of over 1800 pictorial representations of metabolic pathways from this collection is freely available on the World Wide Web. We believe that this collection will play an important role in the interpretation of genetic sequence data, as well as offering a meaningful framework for the integration of many other forms of biological data. 相似文献
6.
B L Maidak G J Olsen N Larsen R Overbeek M J McCaughey C R Woese 《Nucleic acids research》1996,24(1):82-85
The Ribosomal Database Project (RDP) is a curated database that offers ribosome-related data, analysis services and associated computer programs. The offerings include phylogenetically ordered alignments of ribosomal RNA (rRNA) sequences, derived phylogenetic trees, rRNA secondary structure diagrams and various software for handling, analyzing and displaying alignments and trees. The data are available via anonymous ftp (rdp.life.uiuc.edu), electronic mail (server@rdp.life.uiuc.edu), gopher (rdpgopher.life.uiuc.edu) and World Wide Web (WWW)(http://rdpwww.life.uiuc.edu/). The electronic mail and WWW servers provide ribosomal probe checking, screening for possible chimeric rRNA sequences, automated alignment and approximate phylogenetic placement of user-submitted sequences on an existing phylogenetic tree. 相似文献
7.
We have broadly defined the DNA regions regulating esterase6 activity in
several life stages and tissue types of D. melanogaster using P-
element-mediated transformation of constructs that contain the esterase6
coding region and deletions or substitutions in 5' or 3' flanking DNA.
Hemolymph is a conserved ancestral site of EST6 activity in Drosophila and
the primary sequences regulating its activity lie between -171 and -25 bp
relative to the translation initiation site: deletion of these sequences
decrease activity approximately 20-fold. Hemolymph activity is also
modulated by four other DNA regions, three of which lie 5' and one of which
lies 3' of the coding region. Of these, two have positive and two have
negative effects, each of approximately twofold. Esterase6 activity is
present also in two male reproductive tract tissues; the ejaculatory bulb,
which is another ancestral activity site, and the ejaculatory duct, which
is a recently acquired site within the melanogaster species subgroup.
Activities in these tissues are at least in part independently regulated:
activity in the ejaculatory bulb is conferred by sequences between -273 and
-172 bp (threefold decrease when deleted), while activity in the
ejaculatory duct is conferred by more distal sequences between -844 and
-614 bp (fourfold decrease when deleted). The reproductive tract activity
is further modulated by two additional DNA regions, one in 5' DNA (-613 to
-284 bp; threefold decrease when deleted) and the other in 3' DNA (+1860 to
+2731 bp; threefold decrease when deleted) that probably overlaps the
adjacent esteraseP gene. Collating these data with previous studies
suggests that expression of EST6 in the ancestral sites is mainly regulated
by conserved proximal sequences while more variable distal sequences
regulate expression in the acquired ejaculatory duct site.
相似文献
8.
9.
Chlorophyll loss in leaves of cut flowers of alstroemeria (Alstroemeria pelegrina L. cv. Westland) was rapid in darkness and counteracted by irradiation and treatment of the flowers with gibberellic acid (GA3). The mechanism of the effect of GA3 under dark conditions was investigated. The content of various carbohydrates in the leaves under dark conditions rapidly decreased; this was not influenced by treatment with GA3. indicating that the loss of carbohydrates in the leaves did not induce the loss of chlorophyll. Placing the cut flowers in various solutions of organic and inorganic nutrients exhibited no significant effect on the retention of chlorophyll in leaves of dark-senescing flowers. The total nitrogen content in leaves of dark-senescing cut flowers decreased with time. Leaves of GA3-treated flowers retained more nitrogen. In contrast, the buds of GA3-treated flowers retained less nitrogen during senescence in the dark than control buds. To investigate whether GA3 affects export of assimilates from the leaf to various parts of control and GA3-treated flowers, we labelled one leaf with radioactive carbon dioxide. 14C-assimilates accumulated preferentially in the flowers, in which the relative specific activity of the youngest floral buds was highest. No significant differences were observed in the distribution of 14C-labelled compounds between the buds of control and GA3-treated flowers. To establish the importance of source-sink relations for the loss of leaf chlorophyll we removed the flower buds (i. e. the strongest sink) from the cut flowers. This removal only slightly delayed chlorophyll loss as compared to the large delay caused by GA3-treatment. In addition, detached leaf tips exhibited chlorophyll loss in the dark, which was delayed by GA3-treatment in a fashion comparable with that in flowers. Together these data demonstrate that interactions of the leaves with other plant organs are not essential for chlorophyll loss during senescence in the dark. Additionally, we have found no evidence that GA3 delays the loss of chlorophyll by affecting the transport of nutrients within the cut flowers. 相似文献
10.
Litman GW; Rast JP; Shamblott MJ; Haire RN; Hulst M; Roess W; Litman RT; Hinds- Frey KR; Zilch A; Amemiya CT 《Molecular biology and evolution》1993,10(1):60-72
Immunoglobulins are encoded by a large multigene system that undergoes
somatic rearrangement and additional genetic change during the development
of immunoglobulin-producing cells. Inducible antibody and antibody-like
responses are found in all vertebrates. However, immunoglobulin possessing
disulfide-bonded heavy and light chains and domain-type organization has
been described only in representatives of the jawed vertebrates. High
degrees of nucleotide and predicted amino acid sequence identity are
evident when the segmental elements that constitute the immunoglobulin gene
loci in phylogenetically divergent vertebrates are compared. However, the
organization of gene loci and the manner in which the independent elements
recombine (and diversify) vary markedly among different taxa. One striking
pattern of gene organization is the "cluster type" that appears to be
restricted to the chondrichthyes (cartilaginous fishes) and limits
segmental rearrangement to closely linked elements. This type of gene
organization is associated with both heavy- and light-chain gene loci. In
some cases, the clusters are "joined" or "partially joined" in the germ
line, in effect predetermining or partially predetermining, respectively,
the encoded specificities (the assumption being that these are expressed)
of the individual loci. By relating the sequences of transcribed gene
products to their respective germ-line genes, it is evident that, in some
cases, joined-type genes are expressed. This raises a question about the
existence and/or nature of allelic exclusion in these species. The
extensive variation in gene organization found throughout the vertebrate
species may relate directly to the role of intersegmental
(V<==>D<==>J) distances in the commitment of the individual
antibody-producing cell to a particular genetic specificity. Thus, the
evolution of this locus, perhaps more so than that of others, may reflect
the interrelationships between genetic organization and function.
相似文献