全文获取类型
收费全文 | 5271篇 |
免费 | 359篇 |
国内免费 | 1篇 |
出版年
2023年 | 23篇 |
2022年 | 39篇 |
2021年 | 115篇 |
2020年 | 71篇 |
2019年 | 95篇 |
2018年 | 130篇 |
2017年 | 108篇 |
2016年 | 174篇 |
2015年 | 232篇 |
2014年 | 265篇 |
2013年 | 307篇 |
2012年 | 469篇 |
2011年 | 412篇 |
2010年 | 280篇 |
2009年 | 214篇 |
2008年 | 320篇 |
2007年 | 319篇 |
2006年 | 318篇 |
2005年 | 246篇 |
2004年 | 263篇 |
2003年 | 268篇 |
2002年 | 238篇 |
2001年 | 51篇 |
2000年 | 41篇 |
1999年 | 55篇 |
1998年 | 66篇 |
1997年 | 56篇 |
1996年 | 36篇 |
1995年 | 46篇 |
1994年 | 27篇 |
1993年 | 22篇 |
1992年 | 28篇 |
1991年 | 27篇 |
1990年 | 31篇 |
1989年 | 21篇 |
1988年 | 22篇 |
1987年 | 19篇 |
1986年 | 13篇 |
1985年 | 16篇 |
1984年 | 13篇 |
1983年 | 18篇 |
1982年 | 8篇 |
1981年 | 19篇 |
1980年 | 11篇 |
1979年 | 15篇 |
1978年 | 6篇 |
1977年 | 10篇 |
1975年 | 16篇 |
1973年 | 9篇 |
1972年 | 4篇 |
排序方式: 共有5631条查询结果,搜索用时 46 毫秒
71.
The structural relationship of histocompatibility antigens W4 and W6 with other surface markers on cultured human lymphoid cells of the B type has been investigated, utilizing the lysostrip technique. W4 and W6 antigens were found to be structurally associated with antigens coded for by theB locus of theHLA region, but independent of receptors for breakdown products of the third complement component and xenogeneic red blood cells, and of antigenic moieties reacting with antibodies present in H-2 alloantisera. 相似文献
72.
Spectroscopic and binding studies of azide to type-2-copper-depleted ascorbate oxidase from zucchini
Luigi Casella Michele Gullotti Gianfranco Pallanza Alessandro Pintar Augusto Marchesini 《Biometals》1991,4(2):81-89
Summary Binding of azide to type-2-copper-depleted (T2D) zucchini ascorbate oxidase, containing reduced type-3 Cu centers, and met-T2D ascorbate oxidase, containing oxidized type-3 Cu centers, has been studied spectroscopically. In both cases titration with azide in 0.1 M phosphate pH 6.8 produces a broad near-ultraviolet band with maximum at 455 nm (e 2500 M–1 cm–1, with respect to the met-T2D enzyme) and shoulder at 390 nm (e 1700 M–1 cm–1), that are assigned to(azide)Cu(II) ligand-to-metal charge transfer (LMCT) transitions. This is accompanied by a reduction of absorbance at 330 nm in the met-T2D) enzyme adduct (e –1400 M–1 cm–1). A broad circular dichroic band of negative sign between 370–480 nm corresponds to the LMCT absorption band. Analysis of the titration data indicates that one azide ion binds independently to each of the binuclear T3 Cu couples with low affinity (K = 50 M–1). The ESR signal of the T1 Cu observed in frozen solutions of the T2D enzyme is also perturbed by the addition of azide. The analogies in the azide-binding characteristics between ascorbate oxidase and laccase are discussed. 相似文献
73.
Three patterns of mitochondrial DNA nucleotide divergence in the meadow vole,Microtus pennsylvanicus
Dorothy E. Pumo Carleton J. Phillips Michele Barcia Colleen Millan 《Journal of molecular evolution》1992,34(2):163-174
Summary The DNA sequence was determined for the cytochrome c oxidase II (COII), tRNALys, and ATPase 8 genes from the mitochondrial genome of the meadow vole, Microtus pennsylvanicus. When compared to other rodents, three different patterns of evolutionary divergence were found. Nucleotide variation in tRNALys is concentrated in the TC loop. Nucleotide variation in the COII gene in three genera of rodents (Microtus, Mus, Rattus) consists predominantly of transitions in the third base positions of codons. The predicted amino acid sequence in highly conserved (>92% similarity). Analysis of the ATPase 8 gene among four genera (Microtus, Cricetulus, Mus, Rattus) revealed more detectable transversions than transitions, many fixed first and second position mutations, and considerable amino acid divergence. The rate of nucleotide substitution at nonsynonymous sites in the ATPase 8 gene is 10 times the rate in the COII gene. In contrast, the estimated absolute mutation rate as determined by analysis of nucleotide substitutions at fourfold degenerate sites probably is the same for the two genes. The primary sequences of the ATPase 8 and COII peptides are constrained differently, but each peptide is conserved in terms of predicted secondary-level configuration. 相似文献
74.
Summary
Chamaedorea bartlingiana is a dioecious palm that grows in the cloud forest understories of the Venezuelan Andes. Age and sexual differences in phenology and reproductive patterns were studied in labelled individuals of all age categories. This species has long-lived leaves and low leaf production, both characteristic of understory plants. Growth rates are lower in juveniles than in adults and in females than in males, as in other palms. Male and female individuals show different reproductive patterns. Male inflorescences are always produced at the same rate and the probability of surviving until anthesis is constant. Females produce reproductive buds at the same rate as males, but these buds have a 35% probability of becoming a ripe infrutescence if the plant has infrutescences already growing, and 70% if it does not. This pattern and the slow growth of inflorescences (1 year for males from bud to flowers, 2 years for females from bud to ripe fruits) cause a pluriannual reproductive pattern at the population level. Field germination does not follow this pattern, but shows one annual peak probably related to environmental conditions. 相似文献
75.
The effect of phosphate deficiency on mitochondrial activity and adenylate levels in bean roots 总被引:6,自引:0,他引:6
Anna M. Rychter Michele Chauveau Jean-Loup Bomsel Claude Lance 《Physiologia plantarum》1992,84(1):80-86
Bean plants ( Phaseolus vulgaris ) were grown for 16–20 days with or without phosphate in Knop nutrient medium. It was found in previous experiments that for roots grown on a Pi -deficient medium respiration is mainly carried out by the cyanide-insensitive pathway. Mitochondria isolated from—Pi , roots had poor respiratory control and their respiration exhibited 62% inhibition by cyanide and was inhibited (30%) by salicylhydroxamic acid (SHAM). In contrast, mitochondria obtained with control (+Pi ) roots had respiratory control and ADP/O ratios typical for succinate as the substrate; their respiration was inhibited to 95% by cyanide and insensitive to SHAM. The integrity of mitochondrial membranes was similar in both types of mitochondria. Cytochrome oxidase activity, however, was about 20% lower in -Pi mitochondria, but the cytochrome composition was the same in both types of mitochondria. The cytochrorae pathway was not operating at full capacity in mitochondria isolated from—Pi roots but the alternative oxidation pathway participated in a great part in mitochondrial respiration, similar to in vivo whole roots. The participation of the non-phosphorylating., alternative pathway decreased the respiratory control ratio in mitochondria and had an effect on the total adenine nucleotide pool and energy charge values which were lower (16 and 13% respectively) in -Pi roots. About 50% lower ADP and 20% lower ATP levels were observed whereas AMP levels were several times higher. 相似文献
76.
Michael J. Renner Allyson J. Bennett Michele L. Ford Peter J. Pierre 《Primates; journal of primatology》1992,33(3):315-327
Investigatory behavior with novel, inanimate objects by two groups of four juvenile greater bushbabies (Otolemur garnettii) was examined in the laboratory. Substantial investigatory behavior was shown by all subjects. In the first study, subjects
showed interest in a wide variety of nonfood stimulus objects. In the second, subjects displayed sustained interest in and
investigation of non-food stimulus objects over three sessions. Bushbabies showed preferences for larger, more manipulable
objects and variations in total contact over days. Individual differences were observed in the duration and types of contact
with objects. These observations contradict earlier reports that prosimians show little interest in inanimate, non-food objects. 相似文献
77.
78.
Darja Barlič Maganja Borut Štrukelj Jože Pungerčar Franc Gubenšek Vito Turk Igor Kregar 《Plant molecular biology》1992,20(2):311-313
A genomic DNA clone encoding an aspartic proteinase inhibitor of potato was isolated from a lambda EMBL3 phage library using the aspartic proteinase inhibitor cDNA as a hybridization probe. The gene has all characteristic sequences normally found in eucaryotic genes. Typical CAAT and TATA box sequences were found in the 5-upstream region. In this part are also two putative regulatory AGGA box sequences located. In the genomic sequence there are no intron sequences interrupting the coding region. An open reading frame of the gene encodes a precursor protein of 217 amino acids which shows high percent identity with the aspartic proteinase inhibitor cDNA. 相似文献
79.
The mosquito fern, Azolla filiculoides Lam., was grown in a growth chamber on a nitrogen-free culture solution at 24 C under the following photoperiod: 16 hr light/8 hr darkness. Shoot tips were fixed every 2 hr for 24 hr to determine the mitotic index for the apical cell, immediate derivatives, and remaining cells to the level of the first leaf or lateral shoot primordium. Mitotic indices were 6.9%, 6.5% and 6.3%, respectively. The colchicine method was employed to determine the cell-cycle durations and duration of mitosis for the same populations of cells. The cell-cycle duration and duration of mitosis of the apical cell were 28.2 hr and 2.8 hr, respectively; for the immediate derivatives, 26.7 hr and 2.5 hr; for the remaining cells, 23.6 hr and 2.1 hr. Conclusions: the apical cell is as mitotically active as its immediate derivatives, and there is no evidence of a quiescent apical cell. 相似文献
80.
A microtechnique has been developed for the measurement of alkaline phosphatase in minute amounts of renal tissue. This microtechnique utilizes the known fluorescent property of 4-methylumbelliferyl phosphate following enzymatic hydrolysis. The reaction is sensitive and reproducible and is inhibited by l-bromotetramisole, a specific alkaline phosphatase inhibitor. The microdetermination of alkaline phosphatase activity in the various segments of the mouse nephron allowed the localization of the enzyme in the glomeruli, and in the proximal convoluted tubule where the activity progressively decreases from the capsule of Bowman to the more distal segments. The enzyme was absent from the pars recta or S3 and from the rest of the nephron. This technique is applicable to very small amounts (0.1 μg of protein) of any tissue containing alkaline phosphatase. 相似文献