全文获取类型
收费全文 | 9426篇 |
免费 | 522篇 |
国内免费 | 359篇 |
出版年
2023年 | 82篇 |
2022年 | 138篇 |
2021年 | 195篇 |
2020年 | 235篇 |
2019年 | 214篇 |
2018年 | 239篇 |
2017年 | 180篇 |
2016年 | 160篇 |
2015年 | 271篇 |
2014年 | 576篇 |
2013年 | 539篇 |
2012年 | 496篇 |
2011年 | 629篇 |
2010年 | 434篇 |
2009年 | 368篇 |
2008年 | 454篇 |
2007年 | 372篇 |
2006年 | 316篇 |
2005年 | 318篇 |
2004年 | 272篇 |
2003年 | 264篇 |
2002年 | 195篇 |
2001年 | 115篇 |
2000年 | 132篇 |
1999年 | 135篇 |
1998年 | 119篇 |
1997年 | 134篇 |
1996年 | 128篇 |
1995年 | 126篇 |
1994年 | 148篇 |
1993年 | 110篇 |
1992年 | 99篇 |
1991年 | 117篇 |
1990年 | 96篇 |
1989年 | 90篇 |
1988年 | 85篇 |
1987年 | 61篇 |
1986年 | 77篇 |
1985年 | 86篇 |
1984年 | 166篇 |
1983年 | 123篇 |
1982年 | 139篇 |
1981年 | 126篇 |
1980年 | 123篇 |
1979年 | 120篇 |
1978年 | 98篇 |
1977年 | 90篇 |
1976年 | 117篇 |
1973年 | 148篇 |
1972年 | 96篇 |
排序方式: 共有10000条查询结果,搜索用时 687 毫秒
61.
S. Delventhal A. Brandis H. Ostertag R. Pabst 《Virchows Archiv. B, Cell pathology including molecular pathology》1992,62(1):271-274
The relevance of bronchus-associated lymphoid tissue (BALT) in man is still under discussion. Animal experiments indicate
that the development of BALT is dependent on microbial stimulation. Therefore, the incidence of BALT was investigated retrospectively
in specimens removed during surgical procedures on patients with chronic pulmonary inflammation. All these patients had severe
chronic bronchitis and bronchiectasis, but BALT was found in only 8%. In patients with BALT and a malignant tumor, occlusion
of a bronchus with poststenotic pneumonia was always present and BALT was observed exclusively in areas peripheral to the
occlusion. In man other compartments of the lung must be responsible for the immune function of BALT found in animals.
Partly presented at the Congress of the Eur. Respir. Soc, 21.- 26.9.1991 in Brussels. (Abstract: Eur Respir J. 4, Suppl. 14:217p) 相似文献
62.
Synopsis The effects of constant and diurnally fluctuating levels of dissolved oxygen on the growth of young-of-the-year winter flounder,Pseudopleuronectes americanus, were examined under controlled laboratory conditions. Fish were exposed for either 10 or 11 weeks to constant levels of 6.7 (high) and 2.2 (low) mg l–1, and a diurnal fluctuation, ranging from 2.5 to 6.4 mg 02l–1. Growth rates, calculated for both standard length and weight, for fish exposed to low and diurnally fluctuating levels were significantly reduced (p < 0.001) as compared to those for fish exposed to the high level. Growth rates of fish exposed to the high level were over twice those of fish held under low oxygen conditions. Under fluctuating conditions, fish grew at intermediate rates. Following these exposures, all fish were subsequently held at 7.2 mg Oz l–1 for five weeks. Growth rates increased over two and a half times for fish previously exposed to the low oxygen level and were significantly (p < 0.001) higher than for the other two groups. 相似文献
63.
Fabrizio Arigoni P. Alexandre Kaminski Hauke Hennecke Claudine Elmerich 《Molecular & general genetics : MGG》1991,225(3):514-520
Summary The nucleotide sequence of a 4.1 kb DNA fragment containing the fixABC region of Azorhizobium caulinodans was established. The three gene products were very similar to the corresponding polypeptides of Rhizobium meliloti. The C-terminal domains of both fixB products displayed a high degree of similarity with the -subunits of rat and human electron transfer flavoproteins, suggesting a role for the FixB protein in a redox reaction. Two open reading frames (ORF) were found downstream of fixC. The first ORF was identified as fixX on the basis of sequence homology with fixX from several Rhizobium and Bradyrhizobium strains. The second ORF potentially encoded a 69 amino acid product and was found to be homologous to a DNA region in the Rhodobacter capsulatus nif cluster I. Insertion mutagenesis of the A. caulinodans fixX gene conferred a Nif– phenotype to bacteria grown in the free-living state and a Fix– phenotype in symbiotic association with the host plant Sesbania rostrata. A crude extract from the fixX mutant had no nitrogenase activity. Furthermore, data presented in this paper also indicate that the previously identified nifO gene located upstream of fixA was probably a homologue of the nifW gene of Klebsiella pneumoniae and Azotobacter vinelandii. 相似文献
64.
P700 is rapidly, but only transiently photooxidized upon illuminating dark-adapted leaves. Initial oxidation is followed by a reductive phase even under far-red illumination which excites predominantly photosystem (PS) I. In this phase, oxidized P700 is reduced by electrons coming from PSII. Charge separation in the reaction center of PSI is prevented by the unavailability of electron acceptors on the reducing side of PSI. It is subsequently made possible by the opening of an electron gate which is situated between PSI and the electron acceptor phosphoglycerate. Electron acceptors immediately available for reduction while the gate is closed corresponded to 10 nmol · (mg chlorophyll)–1 electrons in geranium leaves, 16 nmol · (mg chlorophyll)–1 in sunflower and 22 nmol · (mg chlorophyll)–1 in oleander. Reduction of NADP during the initial phase of P700 oxidation showed that the electron gate was not represented by ferredoxin-NADP reductase. Availability of ATP indicated that electron flow was not hindered by deactivation of the thylakoid ATP synthetase. It is concluded that NADP-dependent glyceraldehydephosphate dehydrogenase is completely deactivated in the dark and activated in the light. The rate of activation depends on the length of the preceding dark period. As chloroplasts contain both NAD- and NADP-dependent glyceraldehydephosphate dehydrogenases, deactivation of the NADP-dependent enzyme disconnects chloroplast NAD and NADP systems and prevents phosphoglycerate reduction in the dark at the expense of NADPH and ATP which are generated by glucose-6-phosphate oxidation and glycolytic starch breakdown, respectively.Abbreviations Chl
chlorophyll
- P700
electron donor pigment in the reaction center of photosystem I
Cooperation of the Institute of Botany of the University of Würzburg with the Institute of Astrophysics and Atmospheric Physics of the Estonian Academy of Sciences in Tartu was supported by the Deutsche Forschungsgemeinschaft and the Estonian Academy of Sciences. This work was performed within the Sonderforschungsbereich 251 of the University of Würzburg. 相似文献
65.
Oscillations in photosynthesis are initiated and supported by imbalances in the supply of ATP and NADPH to the Calvin cycle 总被引:4,自引:0,他引:4
Agu Laisk Katharina Siebke Ulvi Gerst Hillar Eichelmann Vello Oja Ulrich Heber 《Planta》1991,185(4):554-562
Oscillations in the rate of photosynthesis of sunflower (Helianthus annuus L.) leaves were induced by subjecting leaves, whose photosynthetic apparatus had been activated, to a sudden transition from darkness or low light to high-intensity illumination, or by transfering them in the light from air to an atmosphere containing saturating CO2. It was found that at the first maximum, light-and CO2-saturated photosynthesis can be much faster than steady-state photosynthesis. Both QA in the reaction center of PS II and P700 in the reaction center of PS I of the chloroplast electron-transport chain were more oxidized during the maxima of photosynthesis than during the minima. Maxima of P700 oxidation slightly preceded maxima in photosynthesis. During a transition from low to high irradiance, the assimilatory force FA, which was calculated from ratios of dihydroxyacetone phosphate to phosphoglycerate under the assumption that the reactions catalyzed by NADP-dependent glyceraldehydephosphate dehydrogenase, phosphoglycerate kinase and triosephosphate isomerase are close to equilibrium, oscillated in parallel with photosynthesis. However, only one of its components, the calculated phosphorylation potential (ATP)/(ADP)(Pi), paralleled photosynthesis, whereas calculated NADPH/NADP ratios exhibited antiparallel behaviour. When photosynthetic oscillations were initiated by a transition from low to high CO2, the assimilatory force FA declined, was very low at the first minimum of photosynthesis and increased as photosynthesis rose to its second maximum. The observations indicate that the minima in photosynthesis are caused by lack of ATP. This leads to overreduction of the electron-transport chain which is indicated by the reduction of P700. During photosynthetic oscillations the chloroplast thylakoid system is unable to adjust the supply of ATP and NADPH rapidly to demand at the stoichiometric relationship required by the carbonreduction cycle.Abbreviations PGA
3-phosphoglycerate
- DHAP
dihydroxyacetone phosphate
- P700
electron-donor pigment in the reaction enter of PS I
- QA
quinone acceptor in the reaction center of PS II
This work received support from the Estonian Academy of Sciences, the Bavarian Ministry of Science and Art and the Sonderforschungsbereich 251 of the University of Würzburg. We are grateful for criticism by D.A. Walker, Robert Hill Institute, University of Sheffield, U.K. and by Mark Stitt, Institute of Botany, University of Heidelberg, FRG. 相似文献
66.
67.
Motility and orientation has been studied in the unicellular photosynthetic flagellate, Euglena gracilis, using real time image analysis capable of tracking up to 200 cells simultaneously in the slow rotating centrifuge microscope (NIZEMI) which allows one to observe the cells' swimming behavior during centrifugation accelerations between 1 g and 5 g. At 1 g the cells show a weak negative gravitaxis, which increases significantly at higher accelerations up to about 3 g. Though most cells were capable of swimming even against an acceleration of 4.5 g, the degree of gravitaxis decreased and some of the cells were passively moved downward by the acceleration force; this is true for most cells at 5 g. The velocity of cells swimming against 1 g is about 10% lower than that of cells swimming in other directions. The velocity decreases even more drastically in cells swimming against higher acceleration forces than those at 1 g. The degree of gravitactic orientation drastically decreases after short exposure to artificial UV radiation which indicates that gravitaxis may be due to an active physiological perception rather than a physical effect such as an asymmetry of the center of gravity within the cell.
Offprint requests to: D.-P. Häder 相似文献
68.
R. Casadio 《European biophysics journal : EBJ》1991,19(4):189-201
In chromatophores from photosynthetic bacteria the interaction of the fluorescent monoamine, 9-amino, 6-chloro, 2-methoxyacridine (ACMA), with the membrane is evaluated and described by an S-shaped adsorption isotherm. This phenomenon is hysteretic, as indicated by the difference between the adsorption and desorption branches of the binding isotherm. Maximal saturation of adsorption is reached at one ACMA per one to four lipid molecules, indicating that the probe binds in its neutral form. Adsorption of the probe on the membrane causes a large quenching of its fluorescence, which is explaind as being due to hypochromic effects following stacking and aggregation in a medium of low dielectric constant. A further quenching of fluorescence is brought about by imposing artificially induced transmembrane pH's. This latter phenomenon titrates in at increasing pH values and approaches saturation when pH is 2. The dependence of pH on the observed quenching of fluorescence is predicted by considering a model based on the equilibrium distribution of the amine between two phases at different pH's, in which adsorption of the probe on the membrane is used to evaluate its free concentration in the inner and outer compartments of the chromatophore vesicle. It is proposed that the equation thus obtained should be used to measure pH from the quenching of ACMA fluorescence.Abbreviations pH
transmembrane pH difference between the inner and outer compartments
- Q
quenching of fluorescence
- BChl
Bacteriochlorophyll
- ACMA
9-amino-6-chloro-2-methoxyacridine
- 9AA
9-aminoacridine
- Tricine
N-tris-(hydroxymethyl)methylglycine
- MES
2-morpholinoethanesulfonic acid
- FCCP
carbonylcyanide-p-trifluoro-methoxy-phenylhydrazone
- CCCP
carbonylcyanide-m-chloro-phenylhydrazone 相似文献
69.
Quantitative receptor autoradiography was used to examine the binding of [125I]-human CGRP in the dorsal horn of the L4 spinal segment of rats with a chronic constriction injury (CCI) of the sciatic nerve or unilateral dorsal rhizotomies of spinal segments L1–L6. At the times selected for study, we found no change in the amount of CGRP binding in any areas examined following CCI. In contrast, our results showed a temporally related increase in the amount of CGRP binding in areas within laminae I–II and in lateral lamina V of the dorsal horn ipsilateral to the rhizotomies. These results indicate that CGRP binding sites are regulated, most likely, by changes in the release of CGRP. Further, our results suggest that the release of CGRP from primary afferent neurons is unchanged in animals with a CCI. 相似文献
70.
Summary A unique cytoplasmic connection between erythroblasts was studied by electron microscopy in mouse hemopoietic tissues (fetal liver, fetal and neonatal spleen and adult bone marrow). Many pairs of interphase erythroblasts were connected by a cytoplasmic bridge that was very thin and sometimes long in comparison with telophase bridges. The stage of maturation of the cells in a pair was similar. Small numbers of microtubules ran along the cytoplasmic bridge; a mid-body was not seen. The plasma membrane at approximately the middle of the bridge bulged to form a ring-shaped ridge filled with dense amorphous substances; this was called a bulging ring. Thus, the cytoplasmic bridge between erythroblasts did not morphologically correspond to the telophase bridge in the usual cytokinesis. Cytoplasmic bridges were observed in various differentiating stages of erythroblasts, whereas other cell types of the hemopoietic lineage did not have such a bridge. The cytoplasmic bridge is unique to erythroblasts and provides an evidence for the atypical cytokinesis of the erythroblastic lineage. 相似文献