全文获取类型
收费全文 | 109556篇 |
免费 | 1445篇 |
国内免费 | 861篇 |
专业分类
111862篇 |
出版年
2023年 | 160篇 |
2022年 | 320篇 |
2021年 | 589篇 |
2020年 | 420篇 |
2019年 | 541篇 |
2018年 | 12296篇 |
2017年 | 11049篇 |
2016年 | 8099篇 |
2015年 | 1563篇 |
2014年 | 1369篇 |
2013年 | 1513篇 |
2012年 | 5522篇 |
2011年 | 13901篇 |
2010年 | 12637篇 |
2009年 | 8809篇 |
2008年 | 10512篇 |
2007年 | 12087篇 |
2006年 | 953篇 |
2005年 | 1130篇 |
2004年 | 1548篇 |
2003年 | 1543篇 |
2002年 | 1230篇 |
2001年 | 541篇 |
2000年 | 409篇 |
1999年 | 230篇 |
1998年 | 117篇 |
1997年 | 110篇 |
1996年 | 87篇 |
1995年 | 84篇 |
1994年 | 79篇 |
1993年 | 95篇 |
1992年 | 151篇 |
1991年 | 136篇 |
1990年 | 80篇 |
1989年 | 98篇 |
1988年 | 101篇 |
1987年 | 109篇 |
1986年 | 80篇 |
1985年 | 69篇 |
1984年 | 76篇 |
1983年 | 62篇 |
1982年 | 38篇 |
1981年 | 45篇 |
1980年 | 38篇 |
1979年 | 51篇 |
1978年 | 37篇 |
1975年 | 40篇 |
1973年 | 56篇 |
1972年 | 272篇 |
1971年 | 296篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
981.
Cintia Camila Silva Angelieri Christine Adams-Hosking Katia Maria Paschoaletto Micchi de Barros Ferraz Marcelo Pereira de Souza Clive Alexander McAlpine 《PloS one》2016,11(1)
A mosaic of intact native and human-modified vegetation use can provide important habitat for top predators such as the puma (Puma concolor), avoiding negative effects on other species and ecological processes due to cascade trophic interactions. This study investigates the effects of restoration scenarios on the puma’s habitat suitability in the most developed Brazilian region (São Paulo State). Species Distribution Models incorporating restoration scenarios were developed using the species’ occurrence information to (1) map habitat suitability of pumas in São Paulo State, Southeast, Brazil; (2) test the relative contribution of environmental variables ecologically relevant to the species habitat suitability and (3) project the predicted habitat suitability to future native vegetation restoration scenarios. The Maximum Entropy algorithm was used (Test AUC of 0.84 ± 0.0228) based on seven environmental non-correlated variables and non-autocorrelated presence-only records (n = 342). The percentage of native vegetation (positive influence), elevation (positive influence) and density of roads (negative influence) were considered the most important environmental variables to the model. Model projections to restoration scenarios reflected the high positive relationship between pumas and native vegetation. These projections identified new high suitability areas for pumas (probability of presence >0.5) in highly deforested regions. High suitability areas were increased from 5.3% to 8.5% of the total State extension when the landscapes were restored for ≥ the minimum native vegetation cover rule (20%) established by the Brazilian Forest Code in private lands. This study highlights the importance of a landscape planning approach to improve the conservation outlook for pumas and other species, including not only the establishment and management of protected areas, but also the habitat restoration on private lands. Importantly, the results may inform environmental policies and land use planning in São Paulo State, Brazil. 相似文献
982.
The chitosan with three-dimensional porous structure greatly increased the effective electrode surface for loading of platinum
nanoparticles and promoted efficient electron transfer. The resulting biosensor had a response time (within 5 s) and a linear
response from 6 μM to 4.2 mM glucose with a detection limit of 2 μM (S/N = 3). Moreover, the methodology can be applied for
the immobilization of other enzymes. 相似文献
983.
Cell migration and growth are essential components of the development of multicellular organisms. The role of various cues
in directing cell migration is widespread, in particular, the role of signals in the environment in the control of cell motility
and directional guidance. In many cases, especially in developmental biology, growth of the domain also plays a large role
in the distribution of cells and, in some cases, cell or signal distribution may actually drive domain growth. There is an
almost ubiquitous use of partial differential equations (PDEs) for modelling the time evolution of cellular density and environmental
cues. In the last 20 years, a lot of attention has been devoted to connecting macroscopic PDEs with more detailed microscopic
models of cellular motility, including models of directional sensing and signal transduction pathways. However, domain growth
is largely omitted in the literature. In this paper, individual-based models describing cell movement and domain growth are
studied, and correspondence with a macroscopic-level PDE describing the evolution of cell density is demonstrated. The individual-based
models are formulated in terms of random walkers on a lattice. Domain growth provides an extra mathematical challenge by making
the lattice size variable over time. A reaction–diffusion master equation formalism is generalised to the case of growing
lattices and used in the derivation of the macroscopic PDEs. 相似文献
984.
Andrew J. Ghio 《Biometals》2016,29(5):751-779
Over several decades, asthma has evolved from being recognized as a single disease to include a diverse group of phenotypes with dissimilar natural histories, pathophysiologies, responses to treatment, and distinctive molecular pathways. With the application of Occam’s razor to asthma, it is proposed that there is one cause underlying the numerous phenotypes of this disease and that the responsible molecular pathway is a deficiency of iron in the lung tissues. This deficiency can be either absolute (e.g. asthma in the neonate and during both pregnancy and menstruation) or functional (e.g. asthma associated with infections, smoking, and obesity). Comparable associations between asthma co-morbidity (e.g. eczema, urticaria, restless leg syndrome, and pulmonary hypertension) with iron deficiency support such a shared mechanistic pathway. Therapies directed at asthma demonstrate a capacity to impact iron homeostasis, further strengthening the relationship. Finally, pathophysiologic events producing asthma, including inflammation, increases in Th2 cells, and muscle contraction, can correlate with iron availability. Recognition of a potential association between asthma and an absolute and/or functional iron deficiency suggests specific therapeutic interventions including inhaled iron. 相似文献
985.
Aleksandra Topic Marina Milenkovic Snezana Uskokovic-Markovic Dragana Vucicevic 《Biological trace element research》2010,134(3):296-306
Investigations of effective, orally active, and safe antidiabetic metallopharmaceuticals have been carried out during the
last two decades. It has been reported that tungsten compounds mimic the action of insulin in intact cell systems. As insulin
mimetics, the most investigated tungsten compound was sodium tungstate (ST), rarely investigated was tungstophosphoric acid
(WPA), but never alanine complex of tungstophosphoric acid (WPA-A). In this study, the insulin mimetic activity of three different
tungsten compounds, ST, WPA, and WPA-A, was evaluated by means of in vitro measurements of the glucose uptake and inhibition
of free fatty acids release from epinephrine-treated isolated rat white adipocytes. We investigated the influence of concentration
(lower and higher, 0.1 and 1.0 mM, respectively) and solvent: isotonic salt solution—saline (0.9% w/v of NaCl) and dimethyl sulfoxide (DMSO; 2% v/v), on the biological effect of tested compounds. Our experimental data showed that all of the three investigated tungsten
compounds possess insulin mimetic activity in vitro on the isolated adipocytes. Influence of concentration and solvents on
insulin mimetic effect for the certain tungsten compounds were: WPA was shown effect independently of concentration and solvents;
higher concentration and DMSO were significant decreasing insulin mimetic effect of ST; lower concentration and saline led
to decreasing effect of WPA-A. Generally, there were no differences in insulin mimetic effect of three tungsten compounds
in lower concentration and dissolved in DMSO. When saline was used as solvent, it was needed higher concentration of investigated
compounds to accomplish the same effect. In conclusion, our results suggest that low concentration (0.1 mM) of ST, WPA, and
WPA-A dissolved in 2% DMSO could be the good candidates for in vivo investigation of their antidiabetic properties. 相似文献
986.
M H Silva B D Hammock 《Comparative biochemistry and physiology. B, Comparative biochemistry》1987,87(1):95-102
An affinity purification system based on elution of cytosolic epoxide hydrolase from a methoxycitronellyl thiol ligand with 4-azidochalcone oxide was applied to a variety of samples including liver from human, monkey, baboon, rabbit, rat and mouse as well as mammary gland from mouse. Hepatic tissues yielded a major 58 kDa band on SDS-PAGE, but the system had to be modified slightly to remove a 33 kDa band for rat. All of the affinity purified hydrolases showed similar properties with regard to substrate selectivity, pH dependence and mobilities on SDS-PAGE. 相似文献
987.
Jolanta J. Adamczyk 《Ecological Research》2011,26(3):547-554
Macrofungal communities were investigated in four associations of xerothermic swards: Festucetum pallentis, Origano-Brachypodietum, Adonido-Brachypodietum pinnati and Diantho-Armerietum elongatae in a Jurassic area of the Częstochowa Upland (southern Poland). A total of 47 species were recorded. The self-organising
map (SOM)—an unsupervised algorithm for artificial neural networks—was used to recognise patterns in the macrofungal communities
of diverse xerothermic swards. Only two associations were mycologically similar: Origano-Brachypodietum and Adonido-Brachypodietum pinnati. Species with high and significant IndVal (the species indicator value) for each investigated phytocoenoses are presented.
The presence of macrofungal species and the participation of indicator species were connected with habitat factors of plant
associations, as documented by the IndVal application. In the least fertile phytocoenoses, macrofungal communities were poor
with few indicator species. The more fertile phytocoenoses had richer and more varied communities of macrofungi with higher
numbers of indicator species. The ordering methods applied in this study were very effective for analyzing the macrofungal
communities existing in plant associations. 相似文献
988.
The most accepted hypothesis of magnetoreception for social insects is the ferromagnetic hypothesis which assumes the presence of magnetic material as a sensor coupled to sensitive structures that transmit the geomagnetic field information to the nervous system. As magnetite is the most common magnetic material observed in living beings, it has been suggested as basic constituent of the magnetoreception system. Antennae and head have been pointed as possible magnetosensor organs in social insects as ants, bees and termites. Samples of three antenna joints: head-scape, scape-pedicel and pedicel-third segment joints were embedded in epoxi resin, ultrathin sectioned and analyzed by transmission electron microscopy. Selected area electron diffraction patterns and X-ray energy dispersive spectroscopy were obtained to identify the nanoparticle compound. Besides iron oxides, for the first time, nanoparticles containing titanium have been identified surrounded by tissue in the antennae of ants. Given their dimension and related magnetic characteristics, these nanoparticles are discussed as being part of the magnetosensor system. 相似文献
989.
Huiru Lu Jun Chen Hui Huang Mengxue Zhou Qing Zhu Shao Q. Yao Zhifang Chai Yi Hu 《Biometals》2017,30(4):599-607
Both monoamine oxidase B (MAO-B) and iron accumulation are associated with neurologic diseases including Parkinson’s disease. However, the association of iron with MAO-B activity was poorly understood. Here we took advantage of highly sensitive and specific fluorescence probes to examine the change in MAO-B activity in human dopaminergic neuroblastoma (SH-SY5Y) cells upon iron exposure. Both ferric and ferrous ions could significantly enhance the activity of MAO-B, instead of MAO-A, in SH-SY5Y cells. In addition, iron-induced increase in MAO-B probe fluorescence could be prevented by pargyline and other newly developed MAO-B inhibitors, suggesting that it was MAO-B activity-dependent. These findings may suggest MAO-B is an important sensor in iron-stressed neuronal cells. 相似文献
990.