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101.
Temperature, activating metal ions, and amino-acid substitutions are known to influence the CO2/O2 specificity of the chloroplast enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase. However, an understanding of the physical basis for enzyme specificity has been elusive. We have shown that the temperature dependence of CO2/O2 specificity can be attributed to a difference between the free energies of activation for the carboxylation and oxygenation partial reactions. The reaction between the 2,3-enediolate of ribulose 1,5-bisphosphate and O2 has a higher free energy of activation than the corresponding reaction of this substrate with CO2. Thus, oxygenation is more responsive to temperature than carboxylation. We have proposed possible transition-state structures for the carboxylation and oxygenation partial reactions based upon the chemical natures of these two reactions within the active site. Electrostatic forces that stabilize the transition state of the carboxylation reaction will also inevitably stabilize the transition state of the oxygenation reaction, indicating that oxygenase activity may be unavoidable. Furthermore, the reduction in CO2/O2 specificity that is observed when activator Mg2+ is replaced by Mn2+ may be due to Mg2+ being more effective in neutralizing the negative charge of the carboxylation transition state, whereas Mn2+ is a transition-metal ion that can overcome the triplet character of O2 to promote the oxygenation reaction.Abbreviations CABP
2-carboxyarabinitol 1,5-bisphosphate
- enol-RuBP
2,3-enediolate of ribulose 1,5-bisphosphate
- Kc
Kmfor CO2
- Ko
Kmfor O2
- Rubisco
ribulose-1,5-bisphosphate carboxylase/oxygenase
- RuBP
ribulose 1,5-bisphosphate
- Vc
V
max for carboxylation
- Vo
V
max for oxygenation 相似文献
102.
Graham N. Stone Andy Purvis 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1992,162(3):284-295
Summary This study examines the relationship between warm-up rate, body mass, metabolic rate, thermal conductance and normothermic body temperature in heterothermic mammals during arousal from torpor. Predictions based on the assumption that the energetic cost of arousal has been minimised are tested using data for 35 species. The observation that across-species warm-up rate correlates negatively with body mass is confirmed using a comparative technique which removes confounding effects due to the non-independence of species data due to shared common ancestry. Mean warm-up rate during arousal correlates negatively with basal metabolic rate and positively with the temperature difference through which the animal warms, having controlled for other factors. These results suggest that selection has operated to minimise the overall energetic, cost of warm-up. In contrast, peak warm-up rate during arousal correlates positively with peak metabolic rate during arousal, and negatively with thermal conductance, when body mass has been taken into account. These results suggest that peak warm-up rate is more sensitive to the fundamental processes of heat generation and loss. Although heterothermic marsupials have lower normothermic body temperatures and basal metabolic rates, marsupials and heterothermic eutherian mammals do not differ systematically in warm-up rate. Pre-flight warm-up rates in one group of endothermic insects, the bees, are significantly higher than predictions based on rates of arousal of a mammal of the same body mass.Abbreviations BMR
basal metabolic rate
- ICM
independent comparisons method
- MWR
mean warm-up rate
- PMR
peak metabolic rate
- PWR
peak·warm-up rate
-
Tbactivity
body temperature during activity
-
Tbtorpor
body temperature during torpor
- T
arousal
increase in body temperature during arousal 相似文献
103.
本文是26篇关于丝状真菌基因表达系统的研究论文的综述,包括两部份内容。前一部分叙述1979年开始建立并迅速发展起来的丝状真菌转化系统,着重介绍丝状真菌中转化系统的构建及转化的一般特点。后一部分叙述在转化系统发展基础上产生的丝状真菌基因工程,文中列出了截至1991年9月为止报道的一些成功的实例,说明它在丝状真菌工业育种和作为外源基因产物的生产和分泌系统中的应用。 相似文献
104.
105.
P.M. Matthews J.L. Bland D.G. Gadian G.K. Radda 《Biochimica et Biophysica Acta (BBA)/Molecular Cell Research》1982,721(3)
(1) 31P nuclear magnetic resonance was used to measure the creatine kinase-catalysed fluxes in Langendorff-perfused rat hearts consuming oxygen at different rates and using either of two exogenous substrates (11 mM glucose or 5 mM acetate). (2) Fluxes in the direction of ATP synthesis were between 3.5–12-times the steady-state rates of ATP utilization (estimated from rates of O2-consumption), demonstrating that the reaction is sufficiently rapid to maintain the cytosolic reactants near their equilibrium concentrations. (3) Under all conditions studied, the cytosolic free [ADP] was primarily responsible for regulating the creatine kinase fluxes. The enzyme displayed a Km for cytosolic ADP of 35 μM and an apparent Vmax of 5.5 mM/s in the intact tissue. (4) Although the reaction is maintained in an overall steady-state, the measured ratio of the forward flux (ATP synthesis) to the reverse flux (phosphocreatine synthesis) was significantly greater than unity under some conditions. It is proposed that this discrepancy may be a consequence of participation of ATP in reactions other than the PCr /ag ATP or ATP /ag ADP + Pi interconversions specifically considered in the analysis. (5) The results support the view that creatine kinase functions primarily to maintain low cytosolic concentrations of ADP during transient periods in which energy utilization exceeds production. 相似文献
106.
Sulphate uptake by the unicellular marine red algaRhodella maculata conforms to Michaelis-Menten kinetics. Two uptake systems have been found: a low affinity system with an apparentK
m
of 22 mM, and a high affinity system with an apparentK
m
of 63.4 M. Transition from the low to the high affinity system can occur within 2.5 min, in response to a decrease in the ambient sulphate concentration to below 10 mM. Assimilation rates in the dark are about 20% those in the light, although enhancement by light is independent of the quanlity of light supplied above 27 mol m-2 s-1. Use of metabolic inhibitors indicates that photophosphorylation provides the main source of energy for sulphate assimilation, through both cyclic and non-cyclic electron flow.Abbreviations used APS-kinase
ATP:adenylyl-sulphate 3-phosphotransferase (E.C. 2.7.1.25)
- ATP-sulphurylase
ATP:sulphate adenylyltransferase (E.C.2.7.74)
- DCMU
[3-(3,4-dichlorophenyl)]-1,1 dimethylurea
- 2,4 DNP
2,4-dinitrophenol
- DBMIB
Dibromothymoquinone (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone) 相似文献
107.
R. Vunsh D. Aviv E. Galun 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1982,64(1):51-58
Summary Protoplasts were derived from haploid and diploid Nicotiana sylvestris and N. tabacum. Exposure of the protoplasts to mutagenic doses of ultraviolet (U.V.) radiation prior to two selection rounds in the presence of 4 mM (or 5 mM) and 8 mM of valine, respectively, was required to obtain cell lines with persistent valine resistance. Such lines were obtained from haploid and diploid N. sylvestris protoplasts as well as from haploid protoplasts of N. tabacum but not from (1.8 × 107) diploid N. tabacum protoplasts. The ratio between number of verified valine-resistant cell lines and the initial number of U.V. exposed protoplasts enabled the estimation of the following order of mutation frequency: haploid N. sylvestris > haploid N. tabacum > diploid N. sylvestris. Plants which retained the valine resistance and transmitted it to their sexual progeny were derived from the resistant cell lines. 相似文献
108.
Solomon Goldstein Melvin M. Belsky Suzanne Sesnowitz-Horn 《Archives of microbiology》1980,126(2):161-165
The relationship between the respiration and the presence and utilization of endogenous and exogenous substrates was studied in the non-filamentous obligately marine fungus Thraustochytrium aureum. Using isotopic and manometric methods, it was shown that almost all exogenous glucose is assimilated, whilst almost all the oxygen consumption in the presence of exogenous glucose was due to oxidation of endogenous reserves. In contrast, exogenous glutamate, which cannot serve as the sole carbon source for growth, inhibits respiration of endogenous materials, and is itself rapidly oxidized. The uncoupler 2,4-dinitrophenol stimulates the oxidation of endogenous reserves without affecting the uptake and use of exogenous glucose. These data strongly support the idea of physiologic compartmentation in this organism. 相似文献
109.
Corrado Barghigiani Giuliano Colombetti Francesco Lenci Rosalba Banchetti Maria Pia Bizzaro 《Archives of microbiology》1979,120(3):239-245
We have investigated the effect of some metabolic drugs, 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), 2,4-dinitrophenol (DNP), sodium azide (NaN3), on the photobehavior of single cells of Euglena gracilis, in order to clarify the relevance of different metabolic pathways in the process of photoperception and sensory transduction in this alga. The results obtained show that the photophobic response of Euglena is not affected by the action of these drugs. This suggests that neither the photosynthetic process nor oxidative phosphorylation play a significant role in the phenomenon of photosensory transduction in Euglena.List of Abbreviations DNP
2,4-dinitrophenol
- DCMU
3-(3,4-dichlorophenyl)-1,1-dimethylurea
- PSI
Photosystem I
- PSII
Photosystem II 相似文献
110.
Harold B. White III 《Journal of molecular evolution》1976,7(2):101-104
Summary A metabolic system composed of nucleic acid enzymes is proposed to have existed prior to the evolution of ribosomal protein synthesis. Vestiges of these nucleic acid enzymes persist in contemporary coenzymes. This proposal rationalizes the fact that many coenzymes are nucleotides or heterocyclic bases which could be derived from nucleotides. 相似文献