全文获取类型
收费全文 | 5185篇 |
免费 | 625篇 |
国内免费 | 2篇 |
专业分类
5812篇 |
出版年
2021年 | 51篇 |
2018年 | 62篇 |
2017年 | 51篇 |
2016年 | 92篇 |
2015年 | 156篇 |
2014年 | 157篇 |
2013年 | 202篇 |
2012年 | 223篇 |
2011年 | 237篇 |
2010年 | 149篇 |
2009年 | 129篇 |
2008年 | 185篇 |
2007年 | 163篇 |
2006年 | 183篇 |
2005年 | 155篇 |
2004年 | 151篇 |
2003年 | 124篇 |
2002年 | 184篇 |
2001年 | 170篇 |
2000年 | 159篇 |
1999年 | 148篇 |
1998年 | 68篇 |
1997年 | 45篇 |
1996年 | 62篇 |
1995年 | 82篇 |
1994年 | 59篇 |
1993年 | 72篇 |
1992年 | 134篇 |
1991年 | 117篇 |
1990年 | 137篇 |
1989年 | 98篇 |
1988年 | 119篇 |
1987年 | 109篇 |
1986年 | 91篇 |
1985年 | 100篇 |
1984年 | 81篇 |
1983年 | 74篇 |
1982年 | 76篇 |
1981年 | 73篇 |
1980年 | 67篇 |
1979年 | 95篇 |
1978年 | 91篇 |
1977年 | 67篇 |
1976年 | 64篇 |
1975年 | 56篇 |
1973年 | 49篇 |
1972年 | 58篇 |
1971年 | 50篇 |
1968年 | 40篇 |
1967年 | 43篇 |
排序方式: 共有5812条查询结果,搜索用时 0 毫秒
31.
32.
Spectrophotometric Measurements of Phytochrome in vivo and Their Correlation with Photomorphogenic Responses of Phaseolus 总被引:7,自引:7,他引:0 下载免费PDF全文
Direct in vivo measurements of phytochrome have been made in Phaseolus vulgaris by 2-filter difference spectrophotometry (Ratiospect). All measurements were made at 730 versus 800 nm and it is assumed that the Δ (ΔOD) is directly proportional to the PFR concentration of phytochrome present. Dose response curves were determined for both physiological and spectrophotometric responses for red induction and far-red photoinactivation. For induction, saturation occurs at 100 mj/cm2 and for inactivation at 30 mj/cm2. The rate of hook opening and the physiological response measured 20 hours after induction are both shown to be directly proportional to the initial amount of PFR present spectrophotometrically. The sensitivity of the tissue correlates well with the absolute amount of phytochrome present, the inner portion of the hook having the maximum concentration of 0.042 Δ (ΔOD)/g fresh weight. If the total reversible phytochrome concentration is reduced by exposure to red light and allowing PFR to decay out of the system the remaining sensitivity of the tissue is shown to be directly correlated with the amount of PR remaining in the tissue. PFR disappears rapidly in the dark at 25°, and is not detectable after 6 hours. There is no indication that PFR reverts in the system to PR. At 4°, PFR does not disappear measurably up to 1 hour and is nearly totally reversible to PR. 相似文献
33.
Resistance to H-2-restricted but not to allo-H2-specific graft and cytotoxic T lymphocyte responses in lymphoma mutant 总被引:2,自引:0,他引:2
C Ohlén J Bastin H G Ljunggren L Foster E Wolpert G Klein A R Townsend K K?rre 《Journal of immunology (Baltimore, Md. : 1950)》1990,145(1):52-58
The lymphoma mutant RMA-S escaped graft rejection after transplantation over a minor histocompatibility barrier, whereas it was rejected in H-2 allogeneic mice. The parental control line was rejected in both situations. The mutant, which had been selected against MHC class I molecules retained 5 to 10% of the wild-type H-2Db, Kb, and beta 2-microglobulin expression on the cell surface. It remained sensitive to allo-H-2b CTL in vitro, but was completely resistant to minor histocompatibility antigen-specific, H-2b-restricted CTL. It was equally resistant to other H-2b-restricted responses against internally derived Ag, such as tumor-specific CTL or a CTL clone specific for the influenza virus nucleoprotein. The results indicate a target cell defect that selectively abolishes the sensitivity to H-2-restricted CTL directed against internally processed Ag. This appears sufficient to shift the transplantation response over a minor histocompatibility Ag barrier from rejection to acceptance. There are two possible explanations for the results: 1) a block in the MHC class I-directed pathway for internal Ag processing, and 2) subthreshold H-2/Ag ligand density in relation to triggering requirements of restricted CTL. Regardless of the type of defect, the results demonstrate a difference between allo-H-2-specific and H-2-restricted CTL recognition at the level of the target cell. 相似文献
34.
Summary Production of lipase by free and alginate-entrapped protoplasts was studied in batch culture. Cell-wall-degrading enzymes Novozym 234 and cellulase CP improved lipase secretion of normal mycelium by 25%–100%. The protoplast-regenerated mycelium exhibited several-fold higher lipase activity in batch replacements in TRIS buffer over normal spore-derived mycelium. The specific lipase activity of immobilized protoplasts was about four times higher than normal mycelial beads. Protoplasts beads were stable and retained high enzyme activity even after three buffer replacements lasting 120 h; TRIS buffer was better than acetate or normal glucose medium. A minimum of 8 h regeneration period was necessary for lipase synthesis. Triolein, olive oil, tributyrin and oleic acid butylester were able to induce lipase in immobilized protoplasts. Tween 80 enhanced lipase activity of the immobilized protoplasts. Partially degraded immobilized mycelium was nearly as effective as normal immobilized protoplasts for lipase secretion. Both free and immobilized protoplasts could be reused for up to 200 h with some loss in enzyme activity. 相似文献
35.
Marc Klein 《Invertebrate neuroscience : IN》1995,1(1):15-24
Up and down-regulation of calcium and potassium conductances are associated with several forms of short-term synaptic modulation.
Detailed investigation of synaptic plasticity in the marine gastropodAplysia, and in other mollusks, indicates that synaptic transmission can be influenced in a number of ways by modulatory neurotransmitters
acting through several second-messenger cascades. Modulation at the synapse itself occurs by means of the regulation of calcium
current as well as through effects on processes directly involved in transmitter mobilization and exocytosis. Modulation of
potassium current plays a major role in controlling neuronal excitability and may contribute to a lesser extent to the regulation
of transmitter release through actions on the resting potential and on action potential configuration. 相似文献
36.
Uri S. Ladror Gary T. Wang William L. Klein Thomas F. Holzman Grant A. Krafft 《Journal of Protein Chemistry》1994,13(4):357-366
Fluorogenic peptide substrates designed to encompass the reported-secretory and amyloidogenic cleavage sites of the amyloid- precursor protein (PP) were used to analyze proteinase activities in brain extracts from control patients and those with Alzheimer's disease (AD). Activity against the secretory substrate atpH 7.5 in control and AD brains produced a major endopeptidase cleavage at the Lys687-Leu688 bond (PP770 numbering), consistent with thePP secretase cleavage. Activity in control brains against the amyloidogenic substrate atpH 7.5 produced one cleavage at the Ala673-Glu674 bond, two residues C-terminal to the amyloidogenic Met-Asp site. However, in three of four AD brains, the major cleavage was at the Asp-Ala bond, one residue from the amyloidogenic site. Both endopeptidase and carboxypeptidase activities in AD brains were lower than in control brains. Proteinase activities against the secretory substrate had a major optimum atpH 3.0–4.0 and another atpH 6.0–7.5. Proteinase activities against the amyloidogenic substrate had a major optimum at or belowpH 3.0 and another atpH 6.0. Using both substrates, activities at lowpH were higher in AD brains than in controls, while atpH above 6.5, activities in control brains were higher than in AD. These results indicate that the levels of proteolytic enzymes in AD brains are altered relative to controls.Abbreviations A
Amyloid-
- ACN
acetonitrile
- AD
Alzheimer's disease
- PP
amyloid- precursor protein
- DABCYL
4-(4-dimethylaminophenylazo)-benzoic acid
- EDANS
5-{(2-aminoethyl)amino}napthalene-1-sulfonic acid
- MES
morpholinoethane sulfonic acid
- MOPS
morpholino-propane sulfonic acid
- RP-HPLC
reverse-phase high-performance liquid chromatography
- SDS-PAGE
sodium do-decyl sulfate-polyacrylamide gel electrophoresis
- TFA
tri-fluoroacetic acid
- Tris
tris(hydroxyethyl)aminomethane 相似文献
37.
38.
39.
S. C. Klein J. G. Golverdingen A. G. M. Bouwens M. G. J. Tilanus R. A. de Weger 《Immunogenetics》1995,41(1):57-57
The nucleotide sequence data presented in this paper have been submitted to the EMBL nucleotide sequence database and have been assigned the accession number X81851 相似文献
40.