全文获取类型
收费全文 | 8695篇 |
免费 | 680篇 |
专业分类
9375篇 |
出版年
2023年 | 89篇 |
2022年 | 128篇 |
2021年 | 253篇 |
2020年 | 135篇 |
2019年 | 179篇 |
2018年 | 215篇 |
2017年 | 199篇 |
2016年 | 305篇 |
2015年 | 471篇 |
2014年 | 517篇 |
2013年 | 662篇 |
2012年 | 742篇 |
2011年 | 691篇 |
2010年 | 408篇 |
2009年 | 441篇 |
2008年 | 526篇 |
2007年 | 447篇 |
2006年 | 422篇 |
2005年 | 408篇 |
2004年 | 374篇 |
2003年 | 330篇 |
2002年 | 316篇 |
2001年 | 100篇 |
2000年 | 63篇 |
1999年 | 85篇 |
1998年 | 94篇 |
1997年 | 80篇 |
1996年 | 63篇 |
1995年 | 50篇 |
1994年 | 44篇 |
1993年 | 49篇 |
1992年 | 36篇 |
1991年 | 28篇 |
1990年 | 29篇 |
1989年 | 27篇 |
1988年 | 22篇 |
1987年 | 29篇 |
1986年 | 13篇 |
1985年 | 28篇 |
1984年 | 31篇 |
1983年 | 24篇 |
1982年 | 24篇 |
1981年 | 22篇 |
1980年 | 14篇 |
1978年 | 15篇 |
1977年 | 23篇 |
1975年 | 11篇 |
1974年 | 17篇 |
1973年 | 12篇 |
1972年 | 11篇 |
排序方式: 共有9375条查询结果,搜索用时 15 毫秒
61.
A preparation of ATPase from the membranes of Micrococcus lysodeikticus, solubilized and more than 95 %. pure, showed two main bands in analytical polyacrylamide gel electrophoresis. They did not correspond to isoenzymes because one band could be converted into the other by exposure to a mildly alkaline pH value. The conversion was paralleled by changes in molecular weight, circular dichroism and catalytic properties. Denaturation by pH at 25 °C was followed by means of circular dichroism, ultracentrifugation and polyacrylamide gel electrophoresis. A large conformational transition took place in the acid range with midpoints at about pH = 3.6 (I = 10?4 M), 4.3 (I = 0.03 M) and 5.3 (I = 0.1 M). The transition was irreversible. Strong aggregation of the protein occurred in this range of pH. The final product was largely random coil, but even at pH 1.5 dissociation into individual subunits was not complete. However, partial dissociation took place at pH 5 (I = 0.028 M). At this pH value the enzyme was inactive, but 20–30 % of the activity could be recovered when the pH was returned to 7.5.In the alkaline region the midpoint of the transition occurred near pH = 11 (I = 0.028 M). The pK of most of the tyrosine residues of the protein was about 10.9. The unfolding was irreversible and the protein was soon converted into peptide species with molecular weights lower than those determined for the subunits by gel clectrophoresis in the presence of sodium dodecyl sulphate. Conventional proteolysis did not account for the transformation. 相似文献
62.
63.
Exogenous polyamines improve rooting of hazel microshoots 总被引:7,自引:0,他引:7
Manuel Rey Carmen Díaz-Sala Roberto Rodríguez 《Plant Cell, Tissue and Organ Culture》1994,36(3):303-308
A strong positive effect of polyamines on rooting of microshoots of adult hazel (Corylus avellana L., cv. Gironell) is described. The effect of polyamines, both in the root induction solution and in the actual rooting medium, was assessed in order to study the effect on the successive rooting phases. Polyamines improved rooting of indole-3-butyric acid-treated microshoots in a synergistic fashion, perhaps by favouring a better induction of roots, with an acceleration of the response (only half the time required for rooting compared to the control). When applied without indole-3-butyric acid, polyamines had only a limited positive effect on rooting, although longer exposure times and/or higher concentrations could increase their effect. Possible rapid uptake and translocation of polyamines in the xylem in our system is discussed. The results offer a new approach to enhance rooting ability of species that are normally difficult to root.Abbreviations BM
basal medium
- IAA
indole-3-acetic acid
- IBA
indole-3-butyric acid
- NAA
1-naphthaleneacetic acid
- Put
putrescine
- Spd
spermidine
- Spm
spermine 相似文献
64.
Dodier Y Banderali U Klein H Topalak O Dafi O Simoes M Bernatchez G Sauvé R Parent L 《The Journal of biological chemistry》2004,279(8):6853-6862
The substituted cysteine accessibility method (SCAM) was used to map the external vestibule and the pore region of the ECaC-TRPV5 calcium-selective channel. Cysteine residues were introduced at 44 positions from the end of S5 (Glu515) to the beginning of S6 (Ala560). Covalent modification by positively charged MTSET applied from the external medium significantly inhibited whole cell currents at 15/44 positions. Strongest inhibition was observed in the S5-linker to pore region (L520C, G521C, and E522C) with either MTSET or MTSES suggesting that these residues were accessible from the external medium. In contrast, the pattern of covalent modification by MTSET for residues between Pro527 and Ile541 was compatible with the presence of a alpha-helix. The absence of modification by the negatively charged MTSES in that region suggests that the pore region has been optimized to favor the entrance of positively charged ions. Cysteine mutants at positions -1, 0, +1, +2 around Asp542 (high Ca2+ affinity site) were non-functional. Whole cell currents of cysteine mutants at +4 and +5 positions were however covalently inhibited by external MTSET and MTSES. Altogether, the pattern of covalent modification by MTS reagents globally supports a KcsA homology-based three-dimensional model whereby the external vestibule in ECaC-TRPV5 encompasses three structural domains consisting of a coiled structure (Glu515 to Tyr526) connected to a small helical segment of 15 amino acids (527PTALFSTFELFLT539) followed by two distinct coiled structures Ile540-Pro544 (selectivity filter) and Ala545-Ile557 before the beginning of S6. 相似文献
65.
Lizbeth Castro-Concha Victor M. Loyola-Vargas José L. Chan Manuel L. Robert 《Plant Cell, Tissue and Organ Culture》1990,22(2):147-151
Agave tequilana stem explants were used to produce adventitious shoots under a set of different water potentials induced by different concentrations of gelrite in the medium. At high water potentials all shoots were vitrified; as the medium water potential became more negative the degree of vitrification decreased but the number of shoots per explant also diminished. The enzymes NADH and NAD-GDH (EC. 1.4.1.2) were measured along the water potential gradient. GDH activity was high in the non-vitrified tissues and decreased significantly in the vitrified ones.Abbreviations GDH
glutamate dehydrogenase
- MS
Murashige and Skoog medium
- MSO
methionine sulfoximine
- PVP
polyvinylpolypyrrolidone
- GS
glutamine synthetase
- GOGAT
glutamine: oxoglutarate amino transferase 相似文献
66.
Cremer C Kaufmann R Gunkel M Pres S Weiland Y Müller P Ruckelshausen T Lemmer P Geiger F Degenhard S Wege C Lemmermann NA Holtappels R Strickfaden H Hausmann M 《Biotechnology journal》2011,6(9):1037-1051
For the improved understanding of biological systems on the nanoscale, it is necessary to enhance the resolution of light microscopy in the visible wavelength range beyond the limits of conventional epifluorescence microscopy (optical resolution of about 200 nm laterally, 600 nm axially). Recently, various far-field methods have been developed allowing a substantial increase of resolution ("superresolution microscopy", or "lightoptical nanoscopy"). This opens an avenue to 'nano-image' intact and even living cells, as well as other biostructures like viruses, down to the molecular detail. Thus, it is possible to combine light optical spatial nanoscale information with ultrastructure analyses and the molecular interaction information provided by molecular cell biology. In this review, we describe the principles of spectrally assigned localization microscopy (SALM) of biological nanostructures, focusing on a special SALM approach, spectral precision distance/position determination microscopy (SPDM) with physically modified fluorochromes (SPDM(Phymod) . Generally, this SPDM method is based on high-precision localization of fluorescent molecules, which can be discriminated using reversibly bleached states of the fluorophores for their optical isolation. A variety of application examples is presented, ranging from superresolution microscopy of membrane and cytoplasmic protein distribution to dual-color SPDM of nuclear proteins. At present, we can achieve an optical resolution of cellular structures down to the 20-nm range, with best values around 5 nm (~1/100 of the exciting wavelength). 相似文献
67.
Molecular Genetics of Cystinuria: Identification of Four New Mutations and Seven Polymorphisms, and Evidence for Genetic Heterogeneity 总被引:1,自引:2,他引:1 下载免费PDF全文
Paolo Gasparini Maria Julia Calonge Luigi Bisceglia Jesus Purroy Irma Dianzani Angelo Notarangelo Ferran Rousaud Michele Gallucci Xavier Testar Alberto Ponzone Xavier Estivill Antonio Zorzano Manuel Palacin Virginia Nunes Leopoldo Zelante 《American journal of human genetics》1995,57(4):781-788
A cystinuria disease gene (rBAT) has been recently identified, and some mutations causing the disease have been described. The frequency of these mutations has been investigated in a large sample of 51 Italian and Spanish cystinuric patients. In addition, to identify new mutated alleles, genomic DNA has been analyzed by an accurate and sensitive method able to detect nucleotide changes. Because of the lack of information available on the genomic structure of rBAT gene, the study was carried out using the sequence data so far obtained by us. More than 70% of the entire coding sequence and 8 intron-exon boundaries have been analyzed. Four new mutations and seven intragenic polymorphisms have been detected. All mutations so far identified in rBAT belong only to cystinuria type I alleles, accounting for ~44% of all type I cystinuric chromosomes. Mutation M467T is the most common mutated allele in the Italian and Spanish populations. After analysis of 70% of the rBAT coding region, we have detected normal sequences in cystinuria type II and type III chromosomes. The presence of rBAT mutated alleles only in type I chromosomes of homozygous (type I/I) and heterozygous (type I/III) patients provides evidence for genetic heterogeneity where rBAT would be responsible only for type I cystinuria and suggests a complementation mechanism to explain the intermediate type I/type III phenotype. 相似文献
68.
Víctor Rodríguez-Prieto Deborah Kukielka Beatriz Martínez-López Ana Isabel de las Heras José Ángel Barasona Christian Gortázar José Manuel Sánchez-Vizcaíno Joaquín Vicente 《European Journal of Wildlife Research》2013,59(6):859-867
Porcine reproductive and respiratory syndrome (PRRS) is a swine infectious disease causing major economic problems on the intensive pig industry. This virus has been reported worldwide in domestic pigs and there is evidence of PRRS virus (PRRSV) infection in wild boar (Sus scrofa). Nonetheless, the epidemiological role of wild boar and extensively kept domestic pigs remains unclear. The aim of this study was to determine the occurrence of PRRS in wild boar and Iberian pigs in the dehesa ecosystem of the Castile-La Mancha region of Spain, which boasts one of the most important free-roaming porcine livestock and hunting industries in the country. Using geo-spatial analysis of literature data, we first explored the relationship between domestic pig density and PRRS occurrence in wild boar in Europe. Results revealed that PRRS occurrence in wild boar may be influenced, albeit not significantly, by domestic pig density. Next, we analyzed sera from 294 wild boar and 80 Iberian pigs by indirect enzyme-linked immunosorbent assay for PRRSV antibodies. The sera and 27 wild boar tissue samples were analyzed by two real-time RT-PCR assays, targeting the most conserved genes of the PRRSV genome, ORF1 and ORF7. Seven wild boar (2.4 %) and one Iberian pig (1.3 %) were seropositive, while none of the animals tested positive for PRRSV by RT-PCR. Our results confirm the limited spread of PRRSV in free-roaming Iberian pigs and wild boar living in mutual contact. Further studies would be necessary to address whether this low seroprevalence found in these animals reflects transmission from intensively kept pigs or the independent circulation of specific strains in free-roaming pigs. 相似文献
69.
Sebastian Schneider Arno Schintlmeister Manuel Becana Michael Wagner Dagmar Woebken Stefanie Wienkoop 《Plant, cell & environment》2019,42(4):1180-1189
Legume–rhizobia symbioses play a major role in food production for an ever growing human population. In this symbiosis, dinitrogen is reduced (“fixed”) to ammonia by the rhizobial nitrogenase enzyme complex and is secreted to the plant host cells, whereas dicarboxylic acids derived from photosynthetically produced sucrose are transported into the symbiosomes and serve as respiratory substrates for the bacteroids. The symbiosome membrane contains high levels of SST1 protein, a sulfate transporter. Sulfate is an essential nutrient for all living organisms, but its importance for symbiotic nitrogen fixation and nodule metabolism has long been underestimated. Using chemical imaging, we demonstrate that the bacteroids take up 20‐fold more sulfate than the nodule host cells. Furthermore, we show that nitrogenase biosynthesis relies on high levels of imported sulfate, making sulfur as essential as carbon for the regulation and functioning of symbiotic nitrogen fixation. Our findings thus establish the importance of sulfate and its active transport for the plant–microbe interaction that is most relevant for agriculture and soil fertility. 相似文献
70.
Covarrubias SA de-Bashan LE Moreno M Bashan Y 《Applied microbiology and biotechnology》2012,93(6):2669-2680
When the freshwater microalga Chlorella sorokiniana and the plant growth-promoting bacterium Azospirillum brasilense were deployed as free suspensions in unsterile, municipal wastewater for tertiary wastewater treatment, their population
was significantly lower compared with their populations in sterile wastewater. At the same time, the numbers of natural microfauna
and wastewater bacteria increased. Immobilization of C. sorokiniana and A. brasilense in small (2–4 mm in diameter), polymer Ca-alginate beads significantly enhanced their populations when these beads were suspended
in normal wastewater. All microbial populations within and on the surface of the beads were evaluated by quantitative fluorescence
in situ hybridization combined with scanning electron microscopy and direct measurements. Submerging immobilizing beads in
wastewater created the following sequence of events: (a) a biofilm composed of wastewater bacteria and A. brasilense was created on the surface of the beads, (b) the bead inhibited penetration of outside organisms into the beads, (c) the
bead inhibited liberation of the immobilized microorganisms into the wastewater, and (d) permitted an uninterrupted reduction
of ammonium and phosphorus from the wastewater. This study demonstrated that wastewater microbial populations are responsible
for decreasing populations of biological agents used for wastewater treatment and immobilization in alginate beads provided
a protective environment for these agents to carry out uninterrupted tertiary wastewater treatment. 相似文献