全文获取类型
收费全文 | 16635篇 |
免费 | 1309篇 |
国内免费 | 1218篇 |
出版年
2023年 | 244篇 |
2022年 | 322篇 |
2021年 | 531篇 |
2020年 | 625篇 |
2019年 | 888篇 |
2018年 | 579篇 |
2017年 | 454篇 |
2016年 | 446篇 |
2015年 | 489篇 |
2014年 | 918篇 |
2013年 | 1114篇 |
2012年 | 807篇 |
2011年 | 1016篇 |
2010年 | 744篇 |
2009年 | 765篇 |
2008年 | 828篇 |
2007年 | 860篇 |
2006年 | 763篇 |
2005年 | 727篇 |
2004年 | 648篇 |
2003年 | 579篇 |
2002年 | 542篇 |
2001年 | 356篇 |
2000年 | 324篇 |
1999年 | 350篇 |
1998年 | 261篇 |
1997年 | 244篇 |
1996年 | 202篇 |
1995年 | 163篇 |
1994年 | 175篇 |
1993年 | 189篇 |
1992年 | 138篇 |
1991年 | 114篇 |
1990年 | 94篇 |
1989年 | 81篇 |
1988年 | 95篇 |
1987年 | 64篇 |
1986年 | 78篇 |
1985年 | 131篇 |
1984年 | 207篇 |
1983年 | 131篇 |
1982年 | 154篇 |
1981年 | 111篇 |
1980年 | 124篇 |
1979年 | 125篇 |
1978年 | 67篇 |
1977年 | 60篇 |
1976年 | 72篇 |
1975年 | 51篇 |
1974年 | 37篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Bhuvana Gopalakrishnan Burachai Sonthayanon Rownak Rahmatullah Subbaratnam Muthukrishnan 《Plant molecular biology》1991,16(3):463-467
Protoplasts were prepared from barley aleurone layers using Onozuka cellulase digestion and purification through a Percoll gradient. Protoplasts prepared by this procedure had a viability ranging from 60% to 80% during the first two days of culture. They were responsive to gibberellic acid (GA) as measured by the stimulation of -amylase synthesis. The GA stimulation was counteracted by abscisic acid (ABA). In the presence of polyethylene glycol (PEG), the protoplasts took up exogenously added plasmid DNA containing the reporter gene coding for chloramphenicol acetyl transferase (CAT) linked to a 35S promoter from cauliflower mosaic virus (CaMV) or to barley -amylase gene promoters and expressed CAT activity. Therefore, barley aleurone layer protoplasts are suitable for analysis of hormoneresponsive elements in hydrolase genes. 相似文献
82.
A detailed study of the regulation and evolution of the two classes of patatin genes in Solanum tuberosum L. 总被引:4,自引:0,他引:4
Xiang-Yun Liu Mario Rocha-Sosa Sabine Hummel Lothar Willmitzer Wolf B. Frommer 《Plant molecular biology》1991,17(6):1139-1154
The class-specific expression of patatin genes was investigated by analysing four new patatin genes. A class I patatin gene from cv. Berolina as well as a class I and two class II patatin genes from the monohaploid cultivar AM 80/5793 were isolated and partially sequenced. Sequence comparison indicates rearrangements as the major source for the generation of diversity between the different members of the classes. The expression of single genes was studied in potato plants transformed with chimaeric genes where the putative patatin promoters were fused to the GUS reporter gene. A detailed histochemical analysis reveals that both class I genes are expressed as the previously described class I patatin gene B33 from cv. Berolina [1], i.e. in the starch-containing cells of potato tubers and in sucrose-induced leaves. The class II gene pgT12 shows the same pattern as the previously described class II gene pgT2 [2], i.e. expression in root tips and in the vascular tissue of tubers, whereas no activity was detectable for pgT4. Thus the expression pattern of both classes of genes seems to be stable at least within or even between different cultivars. 相似文献
83.
A system was devised for the in vitro culture of soybean fruits. The culture system consisted of a single fruit attached to a short piece of stem through which the nutrients were supplied. The fruit explants were taken when pods were fully expanded and the seeds at initial stages of growth. During a 7-day culture period, the seeds accumulated dry matter and protein in quantities comparable to those in situ. Omission of the C source (sucrose) from the medium resulted in no dry matter accumulation in the seeds, but omission of the N source (glutamine) still led to some protein accumulation, indicating mobilization of N from other parts of the fruit explant. Optimum protein accumulation occurred when glutamine was supplied at 1.2 mg N ml-1. Protein accumulation in the seeds was highly dependent on the nature of the N source. Glutamine, asparagine and the ureide, allantoin, were equally the most efficient sources, whereas several other amino acids tested showed lower degrees of efficiency. The data indicate a high metabolic capacity of the fruit tissues for principal N transport compounds of soybean, namely allantoin, asparagine and glutamine. The culture system described should prove useful for developmental and metabolic studies where the complex influence of the rest of the plant is to be avoided.Abbreviations ALN
allantoin
- ALC
allantoic acid
Preliminary report presented at the IV World Soybean Research Conference, Buenos Aires, Arggentina, March 1989. 相似文献
84.
P. N. Nehete N. K. Shah V. Ramamurthy R. M. Kothari 《World journal of microbiology & biotechnology》1992,8(4):446-450
An economical protocol, which is simple, rapid and reproducible for the production of maltose by enzymatic hydrolysis of tapioca starch, has been optimized. The protocol involves liquefaction of 35% (w/w) tapioca starch by bacterial -amylase at 78±2°C to 3 to 5% (w/w) reducing sugars, followed by maximal (85±3% w/w maltose equivalent) saccharification with barley -amylase and pullulanase at 50°C for 24 to 30 h. The post-saccharification recovery protocol comprised decolourization by charcoal, de-dextrinization by denatured spirit precipitation, de-ionization by passage through cation and anion exchangers and dehydration by vacuum drying. A white crystalline maltose powder was obtained with specifications comparable to commercial high purity maltose. The protocol yields at least 60% (w/w) recovery of maltose and is suitable for use by the pharmaceutical industry. The protocol is unique in that it utilizes cheap and easily hydrolysed tapioca starch, leaves no mother liquor, enabling higher recovery of maltose, and allows almost quantitative recovery of limit maltodextrins, a value-added marketable by-product. 相似文献
85.
D. -L. Wei S. -C. Chang Y. -H. Wei Y. -W. Lin C. -L. Chuang S. -C. Jong 《World journal of microbiology & biotechnology》1992,8(2):141-146
Eighteen strains of xylariaceous fungi have been screened for higher activities of cellulolytic enzymes,Trichoderma reesei QM 9414 was also examined for comparison. Strains ofXylaria anisopleura andX. regalis had higher endocellulase (CMCase) and exocellulase (Avicelase) activities after 2 weeks' incubation.Hypoxylon stygium produced the highest activity of -glucosidase 3 days after inoculation. The optimum pH for these cellulolytic enzymes was approx. 5.0 and the optimum temperatures ranged from 37 to 50°C. A mixed culture process usingT. reesei QM 9414 andH. stygium was developed to obtain enhanced synthesis of cellulase. -Glucosidase activities in the mixed culture increased within 48h whenH. stygium was introduced after 24h. 相似文献
86.
Stephanie J. Mihalik William J. Rhead 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1991,160(6):671-675
Summary The oxidation ofl-pipecolic acid to -aminoadipic acid was studied in eight species of mammals using an assay system more sensitive than those previously employed. After percoll-gradient fractionation, activity was localized to the mitochondrial-enriched fractions in tissues from rabbit, guinea pig, pig, dog, and sheep, with guinea pig kidney cortex showing greatest specific activity. These results contrast with the peroxisomal oxidation ofl-pipecolic acid observed in macaques and man (Mihalik and Rhead 1989; Mihalik et al. 1989). Rats and mice had undetectable levels of both peroxisomal and mitochondriall-pipecolic acid oxidation. In the rat, peroxisomal oxidation activity was not induced by feeding with either clofibrate or clofibrate andl-pipecolic acid. Thus, among mammals, both the ability to oxidizel-pipecolic acid and the organellar location of this oxidation is species dependent. 相似文献
87.
88.
Karl Hård Albert Mekking Johannis P. Kamerling Georges A. A. Dacremont Johannes F. G. Vliegenthart 《Glycoconjugate journal》1991,8(1):17-28
Five brain-derived and 17 urinary oligomannose-type oligosaccharides were isolated by ion-exchange chromatography on Mono Q or Dowex, followed by HPLC on Lichrosorb-NH2 from a Persian cat suffering from -mannosidosis. The structures ofthe carbohydrate chains were determined by 500- or 600-MHz1H-NMR spectroscopy. Different oligosaccharide patterns were found in brain and urine. 99% of the urinary oligosaccharides possess an (1-6)-linked mannose residue attached to -mannose, whereas only 5% of the brain-derived oligosaccharides contain such a residue. Furthermore, of the urinary carbohydrate chains 71% end with Man1-4GlcNAc1-4GlcNAc and 29% end with Man1-4GlcNAc, whereas the corresponding amounts are 23% and 77%, respectively, for the brain-derived oligosaccharides.Abbreviations MLEV-17
composite pulse devised by M. Levitt
- HOHAHA
homonuclear Hartman-Hahn spectroscopy
- TPPI
time-proportional phase incrementation
- 2D
two dimensional
- GlcNAc
N-acetylglucosamine
- Man
mannose
- Fuc
fucose 相似文献
89.
S. Frillingos M. Frangou-Lazaridis K. Seferiadis J. D. Hulmes Y. -C. E. Pan O. Tsolas 《Molecular and cellular biochemistry》1991,108(1):85-94
Goat prothymosin , a highly acidic polypeptide of pl 3.5, 109 amino acid residues, has been isolated from lymphoid and non-lymphoid tissues of young female goats. Unlike rat, murine and porcine prothymosins , goat prothymosin appears at a higher concentration in the spleen compared with the thymus. The sequence of segments of the polypeptide involving known mutations has been determined, by automatic sequencing of its tryptic peptide fragments. The acidic amino acid-rich segment in the middle of the molecule, including residues 49–83, has not been sequenced. Goat prothymosin closely resembles bovine prothymosin , with only one substitution, proline for alanine at position 85. It also resembles human prothymosin , with only three substitutions. It differs more significantly from rat and murine prothymosins , by two deletions and three substitutions. The results show the highly conserved nature of the molecule, with substitutions at given positions only.Abbreviations ProT
Prothymosin
- T1
Thymosin 1
- MLR
Mixed Lymphocyte Response
- HPLC
High Performance Liquid Chromatography
- RIA
Radioimmunoassay
- B
Aspartic acid or Asparagine
- Z
Glutamic acid or Glutamine 相似文献
90.
Rolf Altenburger Sibylle Abarzua Rainer Callies L. Horst Grimme Adalbert Mayer Dieter Leibfritz 《Archives of microbiology》1991,156(6):471-476
Cultures of the cyanobacterium Microcystis firma show rhythmic uptake and release of ammonia under conditions of carbon limitation. The massive removal of ammonia from the medium during the first light phase has little impact on the intracellular pH: a pH shift of less than 0.2 U towards the alkaline can be measured by in vivo 31P NMR. Furthermore, the energy status of the cells remains regulated. In vivo 15N NMR of M. firma, cultivated either with labelled nitrate or ammonia as the sole nitrogen source, reveals only gradual differences in the pool of free amino acids. Additionally both cultivation types show -aminobutyric acid, acid amides and yet unassigned secondary metabolites as nitrogen storing compounds. Investigating the incorporation of nitrogen under carbon limitation, however, only the amide nitrogen of glutamine is found permanently labelled in situ. While transamination reactions are blocked, nitrate reduction to ammonia can still proceed. Cation exchange processes in the cell wall are considered regarding the ammonia disappearance in the first phase, and the control of ammonia uptake is discussed with respect to the avoidance of intracellular toxification.Abbreviations GABA
-aminobutyric acid
- GOGAT
glutamate synthase
- GS
glutamine synthetase
- MDP
methylene diphosphonate
- MOPSO
3-(N-morpholino)-2-hydroxy-propanesulfonic acid
- NDPS
nucieoside diphosphosugars
- NOE
nuclear Overhauser effect
- NMR
nuclear magnetic resonance
For convenience, the term ammonia is used throughout to denote ammonia or ammonium ion when there is no good evidence as to which chemical species is involved 相似文献