全文获取类型
收费全文 | 7854篇 |
免费 | 466篇 |
国内免费 | 430篇 |
出版年
2024年 | 22篇 |
2023年 | 87篇 |
2022年 | 153篇 |
2021年 | 143篇 |
2020年 | 161篇 |
2019年 | 187篇 |
2018年 | 254篇 |
2017年 | 171篇 |
2016年 | 196篇 |
2015年 | 209篇 |
2014年 | 389篇 |
2013年 | 455篇 |
2012年 | 218篇 |
2011年 | 356篇 |
2010年 | 303篇 |
2009年 | 429篇 |
2008年 | 437篇 |
2007年 | 454篇 |
2006年 | 441篇 |
2005年 | 378篇 |
2004年 | 334篇 |
2003年 | 271篇 |
2002年 | 285篇 |
2001年 | 193篇 |
2000年 | 132篇 |
1999年 | 171篇 |
1998年 | 181篇 |
1997年 | 135篇 |
1996年 | 170篇 |
1995年 | 140篇 |
1994年 | 153篇 |
1993年 | 91篇 |
1992年 | 106篇 |
1991年 | 102篇 |
1990年 | 75篇 |
1989年 | 85篇 |
1988年 | 61篇 |
1987年 | 69篇 |
1986年 | 62篇 |
1985年 | 61篇 |
1984年 | 86篇 |
1983年 | 67篇 |
1982年 | 72篇 |
1981年 | 54篇 |
1980年 | 57篇 |
1979年 | 29篇 |
1978年 | 11篇 |
1976年 | 14篇 |
1975年 | 11篇 |
1973年 | 10篇 |
排序方式: 共有8750条查询结果,搜索用时 15 毫秒
141.
A. S. Feng W. -Y. Lin L. Sun 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1994,175(5):531-546
Physiological studies were carried out in the frog (Rana pipiens pipiens) eighth nerve to determine: (i) whether the modulation rate or the silent gap was the salient feature that set the upper limit of time-locking to pulsed amplitude-modulated (PAM) stimuli, (ii) the gap detection capacity of individual eighth nerve fibers. Time-locked responses of 79 eighth nerve fibers to PAM stimuli (at the fiber's characteristic frequency) showed that the synchronization coefficient was a low-pass function of the modulation rate. In response to PAM stimuli having different pulse durations, a fiber gave rise to non-overlapping modulation transfer functions. The upper cut-off frequency of time locking was higher when tonepulses in PAM stimuli had shorter duration. The fact that the cut-off frequency was different for the different PAM series suggested that the AM rate was neither the sole, nor the main, determinant for the decay in time-locking at high AM rates. Gap detection capacity was determined for 69 eighth nerve fibers by assessing fiber's spiking activities to paired tone-pulses during an OFF-window and an ON-window. It was found that the minimum detectable gap of eighth nerve fibers ranged from 0.5 to 10 ms with an average of 1.23–2.16 ms depending on the duration of paired tone pulses. For each fiber, the minimum detectable gap was longer when the duration of tone pulses comprising the twin-pulse stimuli was more than four times longer. When the synchronization coefficient was plotted against the silent gap between tones pulses in the PAM stimuli, the gap response functions of a fiber as derived from multiple PAM series were equivalent to gap response functions deriving from twin-pulse series suggesting that it was the silent gap which primarily determined the upper limit of time-locking to PAM stimuli.Abbreviations
MTF
modulation transfer function
-
PAM
pulse amplitude modulated
-
SAM
sinusoidally amplitude modulated
-
SC
synchronization coefficient
-
TW
time window 相似文献
142.
Ashok K. Raina Timothy G. Kingan Jadwiga M. Giebultowicz 《Archives of insect biochemistry and physiology》1994,25(4):317-327
Mating in most species of insects leads to a transient or permanent loss in sexual receptivity of the females. Among moths, this loss of receptivity is often accompanied with a loss of the sex pheromone in the absence of calling, which also could be temporary or permanent. Most of the earlier work on changes in reproductive behavior after mating was done with Diptera in which sperm and/or male accessory gland secretions were shown to be responsible for termination of receptivity. In the corn earworm moth, Helicoverpa zea, mated females become depleted of pheromone and become nonreceptive to further mating attempts, but only for the remainder of the night of mating. A pheromonostatic peptide isolated from the accessory glands of males may be responsible for the depletion of pheromone, while the termination of receptivity is independently controlled. In the gypsy moth, Lymantria dispar, the changes in behavior following mating are permanent. In this species, the switch from virgin to mated behavior involves three steps: a physical stimulation associated with mating, transfer of viable sperm to the spermatheca, and commencement of oviposition. Signals generated by these factors operate through neural pathways and, unlike in H. zea, accessory gland factors seem not to be involved. © 1994 Wiley-Liss, Inc. 1 This article is a US Government work and, as such, is in the public domain in the United States of America. 相似文献
143.
Metallothionein (MT) is a ubiquitous mammalian protein comprising 61 or 62 nonaromatic amino acids of which 20 are cysteine residues. The high sulfhydryl content imparts to this protein a unique and remarkable ability to bind multiple metal ions in structurally significant metal–thiolate clusters. MT can bind seven divalent metal ions per protein molecule in two domains with exclusive tetrahedral metal coordination. The domain stoichiometries for the M7S20 structure are M4(Scys)11 (α domain) and M3(Scys)9 (β domain). Up to 12 Cu(I) ions can displace the 7 Zn2+ ions bound per molecule in Zn7–MT. The incoming Cu(I) ions adopt a trigonal planar geometry with domain stoichiometries for the Cu12S20 structure of Cu6(Scys)11 and Cu6(Scys)9 for the α and β domains, respectively. The circular dichroism (CD) spectra recorded as Cu+ is added to Zn7–MT to form Cu12–MT directly report structural changes that take place in the metal binding region. The spectrum arises under charge transfer transitions between the cysteine S and the Cu(I); because the Cu(I)–thiolate cluster units are located within the chiral binding site, intensities in the CD spectrum are directly related to changes in the binding site. The CD technique clearly indicates stoichiometries of several Cu(I)–MT species. Model Cu(I)–thiolate complexes, using the tripeptide glutathione as the sulfhydryl source, were examined by CD spectroscopy to obtain transition energies and the Cu(I)–thiolate coordination geometries which correspond to these bands. Possible structures for the Cu(I)–thiolate clusters in the α and β domains of Cu12–MT are proposed. © 1994 Wiley-Liss, Inc. 相似文献
144.
A stationary bubble-swarm has been used to aerate a mammalian cell culture bioreactor with an extremely low gas flow rate. Prolonging the residence time of the gas bubbles within the medium improved the efficiency of the gas transfer into the liquid phase and suppressed foam formation. An appropriate field of speed gradients prevented the bubbles from rising to the surface. This aeration method achieves an almost 90% transfer of oxygen supplied by the bubbles. Consequently, it is able to supply cells with oxygen even at high cell densities, while sparging with a gas flow of only 0.22·10–3–1.45·10–3 vvm (30–200 ml/h).The reactor design, the oxygen transfer rates and the high efficiency of the system are presented. Two repeated batch cultures of a rat-mouse hybridoma cell line are compared with a surface-aerated spinner culture. The used cell culture medium was serum-free, either with or without BSA and did not contain surfactants or other cell protecting agents. One batch is discussed in detail for oxygen supply, amino acid consumption and specific antibody production. 相似文献
145.
Sites polluted with organic compounds frequently contain inorganic pollutants such as heavy metals. The latter might inhibit the biodegradation of the organics and impair bioremediation. Chromosomally located polychlorinated biphenyl (PCB) catabolic genes ofAlcaligenes eutrophus A5,Achromobacter sp. LBS1C1 andAlcaligenes denitrificans JB1 were introduced into the heavy metal resistantAlcaligenes eutrophus strain CH34 and related strains by means of natural conjugation. Mobile elements containing the PCB catabolic genes were transferred fromA. eutrophus A5 andAchromobacter sp. LB51C1 intoA. eutrophus CH34 after transposition onto their endogenous IncP plasmids pSS50 and pSS60, respectively. The PCB catabolic genes ofA. denitrificans JB1 were transferred intoA. eutrophus CH34 by means of RP4::Mu3A mediated prime plasmid formation. TheA. eutrophus CH34 transconjugant strains expressed both catabolic and metal resistance markers. Such constructs may be useful for the decontamination of sites polluted by both organics and heavy metals. 相似文献
146.
S. Hentrich M. Hebeler L. H. Grimme D. Leibfritz A. Mayer 《European biophysics journal : EBJ》1993,22(1):31-39
ATP synthesis and consumption in respiring cells of the green alga Chlamydomonas reinhardtii were measured with 31P in vivo NMR saturation transfer experiments to determine the intracellular compartmentation of inorganic phosphate. Most of the observed flux towards ATP synthesis was catalyzed by the coupled enzymes glyceraldehyde-3-phosphate dehydrogenase/phosphoglycerate kinase (GAPDH/PGK). The attribution of the measured flux to these enzymes is supported by the observation, that (i) the magnetization transfer was strongly reduced by iodoacetate, an irreversible inhibitor of GAPDH and that (ii) the unidirectional flux was much greater than the net flux through the mitochondrial F0F1-ATPase as determined by oxygen consumption measurements. In Chlamydomonas, glycolysis is divided into a chloroplastidic and a cytosolic part with the enzymes GAPDH/PGK being located in the chloroplast stroma (Klein 1986). The 31P-NMR signal of inorganic phosphate must, therefore, originate from the chloroplast. The life time of the magnetic label transferred to Pi by these enzymes is too short for it to be transported to the cytosol via the phosphate translocator of the chloroplast envelope. When the intracellular compartmentation of Pi was taken into consideration the calculated unidirectional ATP synthesis rate was equal to the consumption rate, indicating operation of GAPDH/PGK near equilibrium. The assignment of most of the intracellular Pi to the chloroplast is in contradiction to earlier reports, which attributed the Pi signal to the cytosol. This is of special interest for the use of the chemical shift of the Pi signal as an intracellular pH-marker in plant cells.Abbreviations 3-PGA
3-phosphoglycerate
- CW
continuous wave
- dG6P
2-deoxyglucose-6-phosphate
- GAPDH
glyceraldehyde-3-phosphate dehydrogenase
- MO
equilibrium z-magnetization
- M0
instantaneous z-magnetization after selective saturation for time t
- MDP
methylene-diphosphonic acid
- PDE
phosphodiester
- PGK
phosphoglycerate kinase
- Pi
inorganic orthophosphate
- polyP
polyphosphate
- T1
longitudinal relaxation time
- 1
longitudinal relaxation time with chemical exchange
- TCA cycle
tricarboxylic acid cycle
Correspondence to: A. Mayer 相似文献
147.
We use the electron-conformational interaction approach to develop a physical model which self-consistently describes the photomobilized electron transfer kinetics and structure conformational transitions in reaction centers (RCs) of purple bacteria. We consider the kinetics of electron transition from pigment onto primary acceptor and the subsequent charge recombination accounting for the change of distance between the above-mentioned cofactors. It is shown that, given natural values of RC parameters, the kinetic constant's dependence on the acting light intensity is monotone. As opposed to the previous case, similar dependencies for the chain of electron transfer between primary and secondary quinone acceptors revealed anS-like relationship. This can lead to bistability of the RC optical transmission coefficient and a fundamental dependence of charge recombination kinetics upon the prehistory of the RC's interaction with exciting radiation. 相似文献
148.
Bruce C. Hill 《Journal of bioenergetics and biomembranes》1993,25(2):115-120
Kinetic studies of the electron transfer processes performed by cytochrome oxidase have assigned rates of electron transfer between the metal centers involved in the oxidation of ferrocytochromec by molecular oxygen. Transient-state studies of the reaction with oxygen have led to the proposal of a sequence of carriers from cytochromec, to CuA, to cytochromea, and then to the binuclear (i.e., cytochromea
3-CuB) center. Electron exchange rates between these centers agree with relative center-to-center distances as follows; cytochromec to CuA 5–7 Å, cytochromec to cytochromea 20–25 Å, CuA to cytochromea 14–16 Å and cytochromea to cytochrome a3-CuB 8–10 Å. It is proposed that the step from cytochromea to the binuclear center is the key control point in the reaction and that this step is one of the major points of energy transduction in the reaction cycle. 相似文献
149.
The structure of the predicted amino acid sequence in the FX domain of Photosystem 1 was studied by molecular modeling and a working hypothesis was developed for the functional interaction of PsaC with the core heterodimer. We propose that the intervening sequences between homologous cysteines in the FX cluster form two flexible loops and participate in the binding of PsaC, and that the arginine residues in the two surface-exposed loops may promote the interaction between the P700–FX core and the subunit. The model was tested experimentally; chemical modification of arginine residues in the P700–FX core using phenylglyoxal prevented reconstitution of the core with PsaC and PsaD after insertion of FeS clusters in vitro. Treatment of the P700–FX core with trypsin also prevented reconstitution of terminal electron transfer to FAFB, although neither treatments affected the electron transfer to FX as judged by flash kinetic spectrophotometry. Electron transfer in the P700–FAFB complex was not impaired by either phenylglyoxal or trypsin treatment indicating that the small subunit(s) protect the arginine residues that become chemically modified or cleaved. The data are consistent with the working model and point to additional experiments designed to identify the specific residues involved in the interaction between the P700–FX core and PsaC.Abbreviations PG-
phenylglyoxal
- PS 1-
Photosystem 1 相似文献
150.
接合转移诱动系统在遗传分析和体内基因操作中的应用赵巍,张成刚,蔺继尚(中国科学院沈阳应用生态研究所,110015)细菌间DNA的转移主要有转化、转导、接合和原生质体融合等几种形式。接合是通过供体菌和受体菌完整细胞间的直接接触,而传递大段DNA的方法,... 相似文献