全文获取类型
收费全文 | 6348篇 |
免费 | 728篇 |
国内免费 | 591篇 |
出版年
2024年 | 33篇 |
2023年 | 212篇 |
2022年 | 278篇 |
2021年 | 453篇 |
2020年 | 498篇 |
2019年 | 643篇 |
2018年 | 402篇 |
2017年 | 239篇 |
2016年 | 284篇 |
2015年 | 275篇 |
2014年 | 398篇 |
2013年 | 474篇 |
2012年 | 273篇 |
2011年 | 332篇 |
2010年 | 222篇 |
2009年 | 267篇 |
2008年 | 268篇 |
2007年 | 267篇 |
2006年 | 223篇 |
2005年 | 234篇 |
2004年 | 184篇 |
2003年 | 171篇 |
2002年 | 147篇 |
2001年 | 75篇 |
2000年 | 68篇 |
1999年 | 75篇 |
1998年 | 66篇 |
1997年 | 57篇 |
1996年 | 53篇 |
1995年 | 46篇 |
1994年 | 41篇 |
1993年 | 47篇 |
1992年 | 32篇 |
1991年 | 27篇 |
1990年 | 31篇 |
1989年 | 22篇 |
1988年 | 25篇 |
1987年 | 23篇 |
1986年 | 26篇 |
1985年 | 25篇 |
1984年 | 29篇 |
1983年 | 18篇 |
1982年 | 23篇 |
1981年 | 13篇 |
1980年 | 19篇 |
1979年 | 6篇 |
1978年 | 10篇 |
1977年 | 6篇 |
1976年 | 7篇 |
1974年 | 6篇 |
排序方式: 共有7667条查询结果,搜索用时 15 毫秒
131.
A comparison was made of energy metabolism of nodulated N2 fixing plants and non-nodulated NO3-fed plants of Lupinus albus L. Growth, N-increment, root respiration (O2 uptake and CO2 production) and the contribution of a SHAM-sensitive oxidative pathway (the alternative pathway) in root respiration were measured. Both growth rate and the rate of N-increment were the same in both series of plants. The rate of root respiration, both O2 uptake and CO2 production, and the activity of the SHAM-sensitive pathway were higher in NO3-fed plants than in N2 fixing plants. The rate of ATP production in oxidative phosphorylation was computed also to be higher in NO3-fed plants. It is concluded that both carbohydrate costings and ATP costings for synthesis + maintenance of root material were lower in N2 fixing than in NO3-fed plants. The respiratory quotient of root respiration was 1.6 in N2-fixing plants and 1.4 in NO3-fed plants. These values were slightly higher than the values calculated on the basis of CO2 output due to N-assimilation and the experimental values of O2 uptake, but showed the same trend: highest in N2 fixing plants. Root respiration of NO3-fed plants showed a diurnal pattern (both O2 uptake, CO2 production and the activity of the SHAM-sensitive pathway), whilst no diurnal variation in root respiration was found in N2 fixing plants. However, C2H2 reduction did show a diurnal rhythm, which is suggested to be related to the diurnal variation in transpiration. Addition of NO3 to N2 fixing plants increased the rate of root respiration and the activity of the alternative pathway. This treatment did not decrease C2H2 reduction and H2 evolution within 4 days. Withdrawal of NO3-supply from NO3-fed plants decreased the rate of root respiration but had no effect on the relative activity of the alternative pathway. It is suggested that the higher rate of root respiration and the higher activity of the SHAM-sensitive pathway in NO3-fed plants is due to a larger supply of carbohydrates to the roots, partly due to a better photosynthetic performance of the shoots and partly due to a higher capacity of the roots to attract carbohydrates. 相似文献
132.
Nocardia sp. DSM 1069 was grown on mineral salt media with coniferyl alcohol, 4-methoxybenzoic acid, 3-methoxybenzoic acid or veratric acid as sole sources of carbon and energy. During incubation on coniferyl alcohol, the formation of coniferyl aldehyde, ferulic acid and quantitative accumulation of vanillic acid and proteocatechuic acid could be achieved with mutants. Washed cell suspensions of N. sp. grown on 4-methoxybenzoic acid, oxidized 4-hydroxybenzoic acid and protocatechuic acid. Cells grown on veratric acid, oxidized vanillic acid, isovanillic acid, and protocatechuic acid. Cell extracts were shown to cleave protocatechuic acid by ortho-fission.A mutant without protocatechuate 3,4-dioxygenase activity was not influenced in its growth on 3 methoxybenzoic acid. Cell free extracts of cells grown on 3-methoxybenzoic acid were shown to catalyze the oxidation of gentisic acid (2,5-dihydroxybenzoic acid). The resulting ring cleavage product was further metabolized by a glutathione dependent reaction.The specificity of the demethylation reactions has been investigated with a mutant unable to grow on vanillic acid. This mutant was not impaired in the degradation of isovanillic acid, 4-methoxy-, or 3-methoxybenzoic acid, whereas growth of this mutant on veratric acid (3,4-dimethoxybenzoic acid) was only half as much as that of the wild type. Concomitantly with growth on veratric acid this mutant accumulated vanillic acid with a yield of about 50%.A pathway for the catabolism of coniferyl alcohol, involving oxidation and shortening of the side chain, and of 4-methoxybenzoic acid and veratric acid with protocatechuic acid as intermediate is being proposed. A second one is proposed for the degradation of 3-methoxybenzoic acid with gentisic acid as intermediate. 相似文献
133.
134.
Thomas L.J. Boehm Willi Kreis Dusan Drahovsky 《Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression》1982,696(1):52-56
The incorporation of [14C]deoxycytidine, [3H]deoxyuridine, and [3H]thymidine, respectively into pyrimidine bases of DNA has been measured in rapidly proliferating P815 mouse mastocytoma cells in the presence of hydroxyurea. The incorporation of [14C]deoxycytidine-derived radioactivity into DNA cytosines is increased when compared to the incorporation into DNA thymines. The [3H]deoxyuridine-derived radioactivity is incorporated solely into DNA thymines and this incorporation is inhibited by hydroxyurea in a dose-dependent manner. This suggests an inhibitory effect of hydroxyurea on the thymidylate synthase which was proved in experiments in which the conversion of deoxyuridine monophosphate into deoxythymidine monophosphate catalysed by a crude enzyme preparation from P815 cells was inhibited in the presence of hydroxyurea. Enzymatic DNA methylation as measured by the conversion of incorporated [14C]deoxycytidine into 5-methylcytosines was not affected by hydroxyurea. 相似文献
135.
Trypanosomatid protozoa: survey of acetylornithinase and ornithine acetyltransferase 总被引:1,自引:0,他引:1
Trypanosomatid protozoa (Crithidia deanei, C. deanei aposymbiotic, C. oncopelti, C. fasciculata, C. acanthocephali, Leptomonas seymouri, L. collosoma, L. samueli, Herpetomonas samuelpessoai, H. sp., H. megaseliae, H. muscarum muscarum, Leishmania donovani, L. braziliensis, Trypanosoma cruzi, T. conorhini and T. mega) were examined for the presence of acetylornithinase (EC 3.5.1.16) and ornithine acetyltransferase (EC 2.3.1.35). As a rule, species of the genus Crithidia presented one of the two enzymes for the conversion of acetylornithine into ornithine. Crithidia fasciculata and C. acanthocephali presented acetylornithinase, while C. deanei and C. oncopelti, species harboring symbionts, presented ornithine acetyltransferase. The enzyme was absent in the aposymbiotic strain of C. deanei, which suggests that the enzyme belongs to the symbiont. Among the other trypanosomatids examined only Herpetomonas samuelpessoai presented acetylomithinase. The participation of acetylornithinase and ornithine acetyltransferase in the metabolism of trypanosomatids is discussed in the light of their nutritional requirements and possession of enzymes of the arginineornithine metabolism. 相似文献
136.
The effect of exogenous adenine nucleotides on CO2 fixation and oxygen evolution was studied with mesophyll protoplast extracts of the C4 plant Digitaria sanguinalis. Exogenous ATP was found to stimulate the rate of pyruvate and pyruvate + oxalacetate induced CO2 fixation, as well as reverse the inhibition of CO2 fixation by carbonyl cyanide m-chlorophenyl hydrazone and several electron transport inhibitors. The ATP-dependent stimulation of CO2 fixation varied from 40 to 70 μmol CO2 fixed/mg chlorophyll per h, suggesting that ATP was crossing the chloroplast membranes at rates of 80–140 μmol/mg chlorophyll per h, since 2 ATP are required for each CO2 fixed. Fixation of CO2 could also be induced in the dark by exogenous ATP, in which case ADP accumulated outside the chloroplasts. This suggests that external ATP is exchanging for internal ADP. In contrast, ADP and AMP were found not to traverse chloroplast membranes, on the basis that neither nucleotide inhibited CO2 fixation or stimulated oxygen evolution that was limited by available ADP for phosphorylation. Further evidence that ATP can enter the chloroplasts was obtained by direct measurements of the increase in ATP in the chloroplasts due to addition of exogenous ATP in the dark. These studies yielded minimal rates of ATP uptake on the order of 30–40 μmol/mg chlorophyll per h. It is suggested that a membrane translocator exists that specifically transports ATP into the chloroplasts in exchange for ADP. The significance of these findings are considered with respect to the C4 pathway of photosynthesis. 相似文献
137.
Iain M. Campbell 《Phytochemistry》1975,14(3):683-693
Mathematical expressions governing incorporation and dilution values in a simple, three component biosynthetic system are established and used to track the influence of pathway structure and sampling time on the numerical value of these parameters. A more productive manner of using the incorporation concept is suggested. 相似文献
138.
The metabolism of d-gluconate-[1-14C] and -[6-14C] by segments from etiolated hypocotyls of Phaseolus mungo has been studied. The release of 14CO2 from gluconate-[1-14C] was greater than that from gluconate-[6-14C] in all parts of hypocotyls examined. Incorporation of the radioactivity from gluconate-[6-14C] into RNA, lignin and aromatic amino acid fractions was greater in the upper (younger) part of the hypocotyls. Incorporation into sugars was greater in the lower (more mature) parts. 相似文献
139.
The effect of exogenous proline on the activity of the glycolate pathway in Nicotiana tabacum cv. Xanthi n.c. An exogenous proline supply in the light provokes an increase in free glycine concentration in apical tissues or in leaf disks of vegetative Nicotiana tabacum L. cv. Xanthi n.c. This does not occur in the equivalent tissues of tobacco plants after floral induction, these being naturally rich in proline. In vegetative tobacco, we have tried to determine this specific action of exogenous proline. With 14C glycine, 14CO2 experiments (Pulse-chase) and glycine decarboxylase activity determinations, we observed that glycine-serine transformation was inhibited by proline supply. Presently it is important to determine if endogenous proline acts on the same reaction. 相似文献
140.
R.P. Herd L. Ko S.E. Weisbrode D.D. Heath 《International journal for parasitology》1984,14(2):141-149
Herd R. P., Ko L., Weisbrode S. E. and Heath D. D. 1984. Sequential morphologic changes in adult Echinococcus granulosus during complement-mediated lysis in vitro. International Journal for Parasltology14:141–149. Sequential changes (5,10, 20, 30,40, 50 min, 1, 2, 3, 4, 5, 6, 7, 8, h) were observed by light, scanning and transmission electron microscopy after 38-day-old adult Echinococcus granulosus were exposed to 50% guinea pig serum in vitro. Early changes within 3 h included contraction of worms, fusion of microtriehes, vacuolization and vesiculization of the distal cytoplasm, followed by rupture of vesicles leading to erosion and loss of the distal cytoplasm. This was most marked in the terminal proglottid but ultimately there was complete erosion of the distal cytoplasm of all proglottids and the scolex. After 3 h there was loss of definition of organelles, apparent edema of the perinuclear cytoplasm and, in some instances, rupture of the circular muscle layer with extrusion of parenchyma. Adult tape-worms exposed to heat-inactivated complement showed none of these changes. Lysis and death of the parasite was attributed to osmotic changes subsequent to the formation of trans-membrane channels induced by complement-mediated attack of the tegument after activation of the alternate pathway by factors present in the cestode tegument. 相似文献