全文获取类型
收费全文 | 8740篇 |
免费 | 1058篇 |
国内免费 | 99篇 |
专业分类
9897篇 |
出版年
2024年 | 15篇 |
2023年 | 192篇 |
2022年 | 180篇 |
2021年 | 292篇 |
2020年 | 363篇 |
2019年 | 426篇 |
2018年 | 389篇 |
2017年 | 346篇 |
2016年 | 305篇 |
2015年 | 369篇 |
2014年 | 438篇 |
2013年 | 555篇 |
2012年 | 280篇 |
2011年 | 363篇 |
2010年 | 283篇 |
2009年 | 431篇 |
2008年 | 477篇 |
2007年 | 387篇 |
2006年 | 453篇 |
2005年 | 392篇 |
2004年 | 357篇 |
2003年 | 335篇 |
2002年 | 263篇 |
2001年 | 163篇 |
2000年 | 168篇 |
1999年 | 183篇 |
1998年 | 165篇 |
1997年 | 158篇 |
1996年 | 143篇 |
1995年 | 113篇 |
1994年 | 102篇 |
1993年 | 89篇 |
1992年 | 97篇 |
1991年 | 83篇 |
1990年 | 64篇 |
1989年 | 74篇 |
1988年 | 61篇 |
1987年 | 55篇 |
1986年 | 41篇 |
1985年 | 57篇 |
1984年 | 47篇 |
1983年 | 20篇 |
1982年 | 36篇 |
1981年 | 23篇 |
1980年 | 18篇 |
1979年 | 20篇 |
1978年 | 4篇 |
1977年 | 7篇 |
1976年 | 6篇 |
1973年 | 4篇 |
排序方式: 共有9897条查询结果,搜索用时 15 毫秒
21.
A model is proposed for the effect of gramicidin A on the order and structure of phospholipid dispersions. According to this model, the addition of gramicidin A influences the surrounding lipids via two independent mechanisms. The first arises from a drop in surface pressure for those lipids substantially bounded by gramicidin A. The second mechanism arises from the increase in the phospholipid headgroup spacing due to the small polar region of the polypeptide. The model provides an explanation for the currently available NMR, X-ray diffraction and Langmuir monolayer results. The model also suggests mechanisms for the ability of gramicidin A to trigger a transition of the lipid from the lamellar to hexagonal II phase, the dependence of this transition on the lipid chain length and the formation of a lamellar phase with lysophosphatidylcholine.Abbreviations NMR
nuclear magnetic resonance
- DMPC
dimyristoylphosphatidylcholine
- S
molecular order parameter
- CSA
chemical shift anisotropy
- DPPC
dipalmitoylphosphati-dylcholine
- LPC
lysophosphatidylcholine 相似文献
22.
Kallar grass is a highly salt-tolerant grass grown as a pioneer plant on alkaline, salt-affected soils in Pakistan. Nitrogen-fixing bacteria and kallar grass were found to be in close association, which was even root-zone specific: rhizoplane and endorhizosphere were colonized by two different populations. Among theAzospirillum isolates originating from the root surface, some were of a new species, now namedA. halopraeferens. To study plant-bacterium interactions, this natural kallar grass association was chosen. The possible role of bacterial chemotaxis and oxygen tolerance are discussed. 相似文献
23.
Neurotransmitter receptors and ion channels play a critical role in the transduction of signals at chemical synapses. The modulation of neurotransmitter receptor and ion channel function by protein phosphorylation is one of the major regulatory mechanisms in the control of synaptic transmission. The nicotinic acetylcholine receptor (nAcChR) has provided an excellent model system in which to study the modulation of neurotransmitter receptors and ion channels by protein phosphorylation since the structure and function of this receptor have been so extensively characterized. In this article, the structure of the nAcChR from the electric organ of electric fish, skeletal muscle, and the central and peripheral nervous system will be briefly reviewed. Emphasis will be placed on the regulation of the phosphorylation of nAcChR by second messengers and by neurotransmitters and hormones. In addition, recent studies on the functional modulation of nicotinic receptors by protein phosphorylation will be reviewed. 相似文献
24.
BRIGITTE FIALA ULRICH MASCHWITZ 《Biological journal of the Linnean Society. Linnean Society of London》1991,44(4):287-305
Some species of the paleotropical tree genus Macaranga (Euphorbiaceae) live in close association with ants. The genus comprises the full range of species from those not regularly inhabited by ants to obligate myrmecophytes. In Malaysia (Peninsular and Borneo) 23 of the 52 species are known to be ant-associated (44%). The simplest structural adaptation of plants to attract ants are extrafloral nectaries. We studied the distribution of extrafloral nectaries in the genus Macaranga to assess the significance of this character as a possible predisposition for the evolution of obligate myrmecophytism. All species have marginal glands on the leaves. However, only the glands of non- myrmecophytic species function as nectaries, whereas liquids secreted by these glands in myrmecophytic species did not contain sugar. Some non-myrmecophytic Macaranga and transitional Macaranga species in addition have extrafloral nectaries on the leaf blade near the petiole insertion. All obligatorily myrmecophytic Macaranga species, however, lack additional glands on the lamina. The non-myrmecophytic species are visited by a variety of different ant species, whereas myrmecophytic Macaranga are associated only with one specific ant-partner. Since these ants keep scale insects in the hollow stems, reduction of nectary production in ant-inhabited Macaranga seems to be biologically significant. We interpret this as a means of (a) saving the assimilates and (b) stabilization of maintenance of the association's specificity. Competition with other ant species for food rewards is avoided and thereby danger of weakening the protective function of the obligate ant- partner for the plant is reduced. A comparison with other euphorb species living in the same habitats as Macaranga showed that in genera in which extrafloral nectaries are widespread, no myrmecophytes have evolved. Possession of extrafloral nectaries does not appear to be essential for the development of symbiotic ant-plant interactions. Other predispositions such as nesting space might have played a more important role. 相似文献
25.
Summary We investigated the effects of thorns and spines on the feeding of 5 herbivore species in arid Australia. The herbivores were the rabbit (Oryctolagus cuniculus), euro kangaroo (Macropus robustus), red kangaroo (Macropus rufus), sheep (Ovis aries), and cattle (Bos taurus). Five woody plants without spines or thorns and 6 woody plants with thorns were included in the study. The spines and thorns were not found to affect the herbivores' rates of feeding (items ingested/min), but they did reduce the herbivores' rates of biomass ingestion (g-dry/item). The reduction in biomass ingested occurred in two ways: at a given diameter, twigs with spines and thorns had less mass than undefended plants, and the herbivores consumed twigs with smaller diameters on plants with spines and thorns. The relative importance of the two ways that twigs with spines and thorns provided less biomass varied with herbivore body mass. Reduced twig mass was more important for small herbivores, while large herbivores selected smaller diameters. The effectiveness of spines and thorns as anti-herbivore defenses did not vary with the evolutionary history of the herbivores (i.e. native vs. introduced). Spines and thorns mainly affected the herbivores' selection of maximum twig sizes (reducing diameter and mass), but the minimum twig sizes selected were also reduced. 相似文献
26.
Summary A factorial experiment examined the effects of varying concentrations of the allelochemical rutin in caterpillars and the length of time the caterpillars had fed on the behavioral interactions of predatory stinkbugs (Podisus maculiventris) and their prey (Manduca sexta). Diet had no significant effect on defensive behavior of the caterpillars. The length of time that the caterpillars had fed (1 vs. 24 h) only influenced the frequency of caterpillars knocking the attacking stinkbugs away, with caterpillars knocking the stinkbugs away more often after 24 h of feeding. A second experiment tested the effects of diet (prey fed various concentrations of rutin), temperature (18° C and 28° C) and gender on consumption and growth parameters of fifth instar stinkbugs. At the cooler temperature, the bugs ate more, gained more weight but took twice as long to complete the stadium and consequently had reduced relative consumption and relative growth rates. Diet had no significant effect on biomass gained or stadium duration, but rutin-fed caterpillars did depress the stinkbugs' relative consumption rates. The effect of food quality on relative growth rate (RGR) was temperature dependent; rutin had no significant effect at the cooler temperature, but a high dose of rutin reduced RGR at the warmer temperature. Rutin had a greater negative impact on the females than the males. The effect of rutin on these predators was different than the effect on their prey (this study compared to Stamp (1990, 1992)): the negative effects of rutin seem to impact on the stinkbug's growth rather than on molting. 相似文献
27.
Functional mapping of the surface of Escherichia coli ribose-binding protein: mutations that affect chemotaxis and transport. 总被引:3,自引:3,他引:0 下载免费PDF全文
R. A. Binnie H. Zhang S. Mowbray M. A. Hermodson 《Protein science : a publication of the Protein Society》1992,1(12):1642-1651
Ribose-binding protein is a bifunctional soluble receptor found in the periplasm of Escherichia coli. Interaction of liganded binding protein with the ribose high affinity transport complex results in the transfer of ribose across the cytoplasmic membrane. Alternatively, interaction of liganded binding protein with a chemotactic signal transducer, Trg, initiates taxis toward ribose. We have generated a functional map of the surface of ribose-binding protein by creating and analyzing directed mutations of exposed residues. Residues in an area on the cleft side of the molecule including both domains have effects on transport. A portion of the area involved in transport is also essential to chemotactic function. On the opposite face of the protein, mutations in residues near the hinge are shown to affect chemotaxis specifically. 相似文献
28.
A. Benayad D. Benamar N. Van Mau G. Page F. Heitz 《European biophysics journal : EBJ》1991,20(4):209-213
Three different gramicidin A analogues bearing acyl chains of various length on the ethanolamine moiety have been studied by investigating their single channel behaviour and their monolayer properties. It is shown that the single channel conductance does not depend on the substitution of the ethanolamine OH group and that the channel lifetime is roughly proportional to the length of the alkyl chain. The monolayer study indicates that acylation of gramicidin A produces compounds which have medium-dependent conformations. These acylated compounds are miscible with lipids, while GA is not, and the surface potential is not modified by the esterification of the alcohol group.
Offprint requests to: F. Heitz 相似文献
29.
Cindy Pfeiffer-Linn Raymon M. Glantz 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1991,168(3):373-381
1. | The actions of GABA on three classes of visual interneurons in crayfish, Procambarus clarkii, medulla externa are examined. The effect of GABA on the visual response is compared to GABA's action on agonist-elicited responses purported to mediate the visual response. |
2. | GABA produces a shunting type of inhibition in medullary amacrine cells which is associated with a small depolarization (Figs. 2, 3), a large increase in input conductance (Gn) and a reversal potential close to rest (Fig. 4). GABA is a potent antagonist to the depolarizing action of acetylcholine (ACh) (Fig. 5). |
3. | GABA depolarizes dimming fibers (Fig. 2), and the response is mediated by an increase in Gn (Fig. 6). GABA antagonizes the light-elicited IPSP and the hyperpolarizing action of ACh (Fig. 7). |
4. | Sustaining fibers (SF) do not appear to have GABA receptors but GABA inhibits the excitatory visual input pathway to the SFs (Fig. 8). Conversely, the GABA antagonist, bicuculline, potentiates the SF light response (Fig. 9). |
5. | GABA has at least three different modes of antagonist action in the medulla: i) Increased conductance and depolarization in dimming fibers and medullary amacrine neurons; ii) Decreased chloride conductance in tangential cells; and iii) An inhibitory action on the visual pathway which drives SFs. |
30.
Summary A model has been developed for 5-nitroxide stearate, I(12,3), distribution in human erythrocyte ghosts which accurately predicts ESR spectral alterations observed with increased probe/total lipid (P/L) at 37°C. This spin probe occupies a class of high-affinity, noninteracting sites at low loading. Saturation occurs with increasing probe concentration, and, at higher loading, the probe inserts itself at initially dilute sites to form membranebound clusters of variable size. No low probe remains at high P/L where all I(12,3) clusters in a concentrated phase. This model allows determination of the dilute/clustered probe ratio, and shows that I(12,3) segregates in erythrocytes at what might otherwise be considered low P/L (e.g., 1/359). These findings validate the earlier use of empirical parameters to estimate probe sequestration in biological membranes. 相似文献