全文获取类型
收费全文 | 2867篇 |
免费 | 246篇 |
出版年
2021年 | 27篇 |
2019年 | 25篇 |
2018年 | 46篇 |
2017年 | 27篇 |
2016年 | 66篇 |
2015年 | 97篇 |
2014年 | 91篇 |
2013年 | 167篇 |
2012年 | 161篇 |
2011年 | 148篇 |
2010年 | 102篇 |
2009年 | 69篇 |
2008年 | 122篇 |
2007年 | 146篇 |
2006年 | 102篇 |
2005年 | 114篇 |
2004年 | 92篇 |
2003年 | 91篇 |
2002年 | 105篇 |
2001年 | 74篇 |
2000年 | 78篇 |
1999年 | 65篇 |
1998年 | 38篇 |
1997年 | 23篇 |
1996年 | 27篇 |
1995年 | 39篇 |
1994年 | 23篇 |
1992年 | 43篇 |
1991年 | 58篇 |
1990年 | 63篇 |
1989年 | 58篇 |
1988年 | 45篇 |
1987年 | 27篇 |
1986年 | 32篇 |
1985年 | 31篇 |
1984年 | 29篇 |
1983年 | 28篇 |
1982年 | 35篇 |
1981年 | 29篇 |
1979年 | 31篇 |
1978年 | 31篇 |
1977年 | 19篇 |
1974年 | 19篇 |
1973年 | 25篇 |
1972年 | 23篇 |
1971年 | 21篇 |
1970年 | 26篇 |
1968年 | 19篇 |
1967年 | 21篇 |
1966年 | 19篇 |
排序方式: 共有3113条查询结果,搜索用时 15 毫秒
81.
An electron microscopical investigation of the cell walls in young antheridia ofPolypodium crassifolium andPlatycerium alcicorne confirms the classical developmental model as postulated byStrasburger and byKny. The structure of the basal cell walls, both of the funnel cell and of the operculum, and especially the evidence of plasmodesmata in those walls, disprove the widely accepted interpretation presented 1951 byDavie.
相似文献
82.
An enzyme that catalyzes the formation of 6-(D-erythro-1',2',3'-trihydroxypropyl)-7,8-dihydropterin triphosphate (D-erythrodihydroneopterin triphosphate) and formic acid from GTP has been purified about 3700-fold from homogenates of chicken liver. The molecular weight of the enzyme, D-erythrodihydroneopterin triphosphate synthetase (GTP cyclohydrolase), has been estimated to be 125,000 by gel filtration on Ultrogel AcA-34. The enzyme functions optimally between pH 8.0 and 9.2 and is considerably heat-stable. No cofactors or metal ions have been demonstrated to be required for activity; however, the reaction is strongly inhibited by Cu2+ and Hg2+. GTP is the most efficient substrate, with GDP being 1/17 as active and guanosine, GMP, and ATP being inactive. The Km for GTP has been found to be 14 micrometer. Although the overall reaction catalyzed by D-erythrodihydroneopterin triphosphate synthetase from chicken liver is identical with that from Escherichia coli GTP cyclohydrolase, immunological studies show no apparent homology between the two enzymes. 相似文献
83.
Rotation of cytochrome P-450. II. Specific interactions of cytochrome P-450 with NADPH-cytochrome P-450 reductase in phospholipid vesicles 总被引:2,自引:0,他引:2
J Gut C Richter R J Cherry K H Winterhalter S Kawato 《The Journal of biological chemistry》1982,257(12):7030-7036
Purified rat liver microsomal cytochrome P-450 and NADPH-cytochrome P-450 reductase were co-reconstituted in phosphatidylcholine-phosphatidylethanolamine-phosphatidylserine vesicles using a cholate dialysis technique. The co-reconstitution of the enzymes was demonstrated in proteoliposomes fractionated by centrifugation in a glycerol gradient. The proteoliposomes catalyzed the N-demethylation of a variety of substrates. Rotational diffusion of cytochrome P-450 was measured by detecting the decay of absorption anisotropy r(t), after photolysis of the heme.CO complex by a vertically polarized laser flash. The rotational mobility of cytochrome P-450, when reconstituted alone, was found to be dependent on the lipid to protein ratio by weight (L/P450) (Kawato, S., Gut, J., Cherry, R. J., Winterhalter, K. H., and Richter, C. (1982) J. Biol. Chem. 257, 7023-7029). About 35% of cytochrome P-450 was immobilized and the rest was rotating with a mean rotational relaxation time phi 1 of about 95 mus in L/P450 = 1 vesicle. In L/P450 = 10 vesicles, about 10% of P-450 was immobile and the rest was rotating with phi 1 congruent to 55 mus. Co-reconstitution of equimolar amounts of NADPH-cytochrome P-450 reductase into the above vesicles results in completely mobile cytochrome P-450 with a phi 1 congruent to 40 mus. Only a small decrease in the immobile fraction of cytochrome P-450 is observed when the molar ratio of cytochrome P-450 to the reductase is 5. The results suggest the formation of a monomolecular 1:1 complex between cytochrome P-450 and NADPH-cytochrome P-450 reductase in the liposomes. 相似文献
84.
D. W. Johnson G. S. Henderson D. D. Huff S. E. Lindberg D. D. Richter D. S. Shriner D. E. Todd J. Turner 《Oecologia》1982,54(2):141-148
Summary Sulfur (S) cycling in a chestnut oak forest on Walker Branch Watershed, Tennessee, was dominated by geochemical processes involving sulfate. Even though available SO
4
2-
was present far in excess of forest nutritional requirements, the ecosystem as a whole accumulated 60% of incoming SO4–S. Most (90%) of this accumulation occurred by SO
4
2-
adsorption in sesquioxide-rich subsurface soils, with a relatively minor amount accumulating and cycling as SO
4
2-
within vegetative components. Organic sulfates are thought to constitute a large proportion of total S in surface soils, also, and to provide a pool of readily mineralized available S within the ecosystem.Research sponsored by Division of Biomedical and Environmental Research, U.S. Department of Energy, under contract W-7405-eng-26 with Union Carbide Corporation. Soil ester sulfate work sponsored by contract RP-1813-1 with the Electric Power Research Institute. Publication No. 1990, Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 相似文献
85.
Oscillations and efficiency in glycolysis 总被引:6,自引:0,他引:6
We suggest that temporal oscillations of concentrations of intermediates in biochemical reaction systems may enhance the efficiency of free energy conversion (reduce dissipation) in those reactions. Experiments on glycolysis are used to estimate the Gibbs free energy changes along the glycolysis mechanism, and to postulate a construct for the glycolysis "machine" which involves: the PFK reaction as the primary oscillophor; the GAPDH reaction as a phase-shifting device; and the PK reaction with the property of intrinsic oscillatory response at resonance with the driving frequency. Analysis of a simple reaction mechanism with these postulated properties shows that the conversion of free energy from reactants to products is more efficient in an oscillatory than a steady state operation. The efficiency of free energy conversion in glycolysis from glucose + ADP to products + ATP is estimated to be increased by 5--10% due to oscillations. This may have been relevant for the evolutionary development of oscillations such as in glycolysis, especially in anaerobic cells. 相似文献
86.
Questioning of reported evidence for guanosine tetraphosphate synthesis in a ribosome system from mouse embryos. 总被引:2,自引:0,他引:2
In 1974, Irr, Kaulenas and Unsworth reported that ppGpp is synthesized by cytosolic ribosomes from mouse embryos and proposed a role for ppGpp in the process of differentiation. This proposal is being challenged because ribosomes of mouse embryos from various stages of development and of mouse embryoid bodies were completely inactive in ppGpp formation. 相似文献
87.
Summary On a soil rich in CaCO3 in a semiaride climate in Algeria a hard soil layer, impermeable for plant roots, was formed in a depth of 20–40 cm after
farmyard manure application and irrigation.
To find the reason soil samples of this field were taken and leaching experiments were carried out in the laboratory, with
the result that much more Caions and HCO3-ions were leached out of the soils with farmyard manure application than from the soils without manure.
Probably the high amount of CO2, being liberated by the organic matter, and the irrigation water dissolved the CaCO3 in the soil, and the formed Ca- and HCO3-ions followed the movement of water in the soil. Where the Ca(HCO3)2 reached soil layers with a less amount of CO2, CaCO3 precipitated and formed the hard soil layer.
In order to avoid the formation of such calcareous crusts on irrigated, limy soils in a dry climate it is recommended to fertilize
rather often small quantities instead of rarely big quantities of farmyard manure. 相似文献
88.
The term monophagy is applied to animal species which are linked to a single plant-reprospecies at least during part of their ontogeny. Aspects of subsequent evolution of monophagous animal species and their plant host species are discussed. 相似文献
89.
R S Phillips I Richter P Gollnick P Brzovic M F Dunn 《The Journal of biological chemistry》1991,266(28):18642-18648
Lysine 269 in Escherichia coli tryptophan indole-lyase (tryptophanase) has been changed to arginine by site-directed mutagenesis. The resultant K269R mutant enzyme exhibits kcat values about 10% those of the wild-type enzyme with S-(o-nitrophenyl)-L-cysteine, L-tryptophan, and S-benzyl-L-cysteine, while kcat/Km values are reduced to 2% or less. The pH profile of kcat/Km for S-benzyl-L-cysteine for the mutant enzyme exhibits two pK alpha values which are too close to separate, with an average value of 7.6, while the wild-type enzyme exhibits pK alpha values of 6.0 and 7.8. The pK alpha for the interconversion of the 335 and 412 nm forms of the K269R enzyme is 8.3, while the wild-type enzyme exhibits a pK alpha of 7.4. Steady-state kinetic isotope effects on the reaction of [alpha-2H]S-benzyl-L-cysteine with the K269R mutant enzyme (Dkcat = 2.0; D(kcat/Km) = 3.9) are larger than those of the wild-type enzyme (Dkcat = 1.4; D(kcat/Km) = 2.9). Rapid scanning stopped-flow kinetic studies demonstrate that the K269R mutant enzyme does not accumulate quinonoid intermediates with L-alanine, L-tryptophan, or S-methyl-L-cysteine, but does form quinonoid absorption peaks in complexes with S-benzyl-L-cysteine and oxidolyl-L-alanine, whereas wild-type enzyme forms prominent quinonoid bands with all these amino acids. Single wavelength stopped-flow kinetic studies demonstrate that the alpha-deprotonation of S-benzyl-L-cysteine is 6-fold slower in the K269R mutant enzyme, while the intrinsic deuterium kinetic isotope effect is less for the K269R enzyme (Dk = 4.2) than for the wild-type (Dk = 7.9). The decay of the K269R quinonoid intermediate in the presence of benzimidazole is 7.1-fold slower than that of the wild-type enzyme. These results demonstrate that Lys-269 plays a significant role in the conformational changes or electrostatic effects obligatory to the formation and decomposition of the quinonoid intermediate, although it is not an essential basic residue. 相似文献
90.
Atrial natriuretic peptide (ANP) is a high-affinity substrate for rat insulin-degrading enzyme. 总被引:2,自引:0,他引:2
A cytosolic protein specifically binding to and degrading atrial natriuretic peptide (ANP) was purified from rat brain homogenate. Based on partial amino acid sequences and enzymatic properties, this protein with an apparent molecular mass of 112 kDa has been identified as the rat insulin-degrading enzyme (IDE). In addition to the known substrates, insulin and transforming-growth-factor alpha IDE binds also with high affinity (apparent Kd 60 nM) to ANP. Competition studies with structural variants of ANP demonstrate that both the C terminus and the disulfide loop of the molecule are essential for high-affinity binding. The data suggest that IDE might be involved in the cellular processing and/or metabolic clearance of ANP. 相似文献