全文获取类型
收费全文 | 2015篇 |
免费 | 155篇 |
国内免费 | 269篇 |
专业分类
2439篇 |
出版年
2024年 | 6篇 |
2023年 | 20篇 |
2022年 | 50篇 |
2021年 | 55篇 |
2020年 | 43篇 |
2019年 | 49篇 |
2018年 | 66篇 |
2017年 | 58篇 |
2016年 | 70篇 |
2015年 | 62篇 |
2014年 | 94篇 |
2013年 | 150篇 |
2012年 | 81篇 |
2011年 | 113篇 |
2010年 | 98篇 |
2009年 | 109篇 |
2008年 | 108篇 |
2007年 | 117篇 |
2006年 | 99篇 |
2005年 | 102篇 |
2004年 | 62篇 |
2003年 | 63篇 |
2002年 | 43篇 |
2001年 | 42篇 |
2000年 | 38篇 |
1999年 | 34篇 |
1998年 | 44篇 |
1997年 | 35篇 |
1996年 | 35篇 |
1995年 | 37篇 |
1994年 | 37篇 |
1993年 | 35篇 |
1992年 | 39篇 |
1991年 | 31篇 |
1990年 | 17篇 |
1989年 | 21篇 |
1988年 | 18篇 |
1987年 | 19篇 |
1986年 | 17篇 |
1985年 | 28篇 |
1984年 | 28篇 |
1983年 | 22篇 |
1982年 | 26篇 |
1981年 | 19篇 |
1980年 | 35篇 |
1979年 | 15篇 |
1978年 | 11篇 |
1977年 | 8篇 |
1976年 | 7篇 |
1973年 | 5篇 |
排序方式: 共有2439条查询结果,搜索用时 0 毫秒
61.
62.
Katharina L. Dürr Neslihan N. Tavraz Susan Spiller Thomas Friedrich 《Journal of visualized experiments : JoVE》2013,(72)
Whereas cation transport by the electrogenic membrane transporter Na+,K+-ATPase can be measured by electrophysiology, the electroneutrally operating gastric H+,K+-ATPase is more difficult to investigate. Many transport assays utilize radioisotopes to achieve a sufficient signal-to-noise ratio, however, the necessary security measures impose severe restrictions regarding human exposure or assay design. Furthermore, ion transport across cell membranes is critically influenced by the membrane potential, which is not straightforwardly controlled in cell culture or in proteoliposome preparations. Here, we make use of the outstanding sensitivity of atomic absorption spectrophotometry (AAS) towards trace amounts of chemical elements to measure Rb+ or Li+ transport by Na+,K+- or gastric H+,K+-ATPase in single cells. Using Xenopus oocytes as expression system, we determine the amount of Rb+ (Li+) transported into the cells by measuring samples of single-oocyte homogenates in an AAS device equipped with a transversely heated graphite atomizer (THGA) furnace, which is loaded from an autosampler. Since the background of unspecific Rb+ uptake into control oocytes or during application of ATPase-specific inhibitors is very small, it is possible to implement complex kinetic assay schemes involving a large number of experimental conditions simultaneously, or to compare the transport capacity and kinetics of site-specifically mutated transporters with high precision. Furthermore, since cation uptake is determined on single cells, the flux experiments can be carried out in combination with two-electrode voltage-clamping (TEVC) to achieve accurate control of the membrane potential and current. This allowed e.g. to quantitatively determine the 3Na+/2K+ transport stoichiometry of the Na+,K+-ATPase and enabled for the first time to investigate the voltage dependence of cation transport by the electroneutrally operating gastric H+,K+-ATPase. In principle, the assay is not limited to K+-transporting membrane proteins, but it may work equally well to address the activity of heavy or transition metal transporters, or uptake of chemical elements by endocytotic processes. 相似文献
63.
B. Brenig 《Animal genetics》2020,51(4):590-594
Autochthonous cattle breeds are an important part of cultural heritage and reservoir of genetic diversity. Usually, such breeds have been selected over centuries and reflect adaptation to a specific local environment and human demands. However, owing to low effective population size in conjunction with increased inbreeding and genetic drift, many of these lineages are threatened with extinction. The Jochberger Hummel (‘Jochberger Bumblebee’) is such an endangered subtype of the Pinzgauer cattle originating from a single polled female calf. To evaluate the suspected uniqueness of this subtype and to assess whether it should be kept separate or managed together with the Pinzgauer cattle as one population, I examined the genetic diversity in a set of 844 cattle (Angus, Charolais, Holstein, Jochberger Hummel, Pinzgauer, Uckermaerker and Tux-Zillertaler) using principal component and biogeographical ancestry analysis. The analysis showed that the Jochberger Hummel is still a distinct subtype of Pinzgauer cattle with less than 5% admixture and a low inbreeding coefficient. 相似文献
64.
Termites from the genus Odontotermes are known to contain numerous species complexes that are difficult to tell apart morphologically or with mitochondrial DNA sequences. We developed markers for one such cryptic species complex, that is, Odontotermes srinakarinensis sp. nov. from Maxwell Hill Forest Reserve (Perak, Malaysia), and characterised them using a sample of 41 termite workers from three voucher samples from the same area. We then genotyped 150 termite individuals from 23 voucher samples/colonies of this species complex from several sites in Peninsular Malaysia. We analysed their population by constructing dendograms from the proportion of shared-alleles between individuals and genetic distances between colonies; additionally, we examined the Bayesian clustering pattern of their genotype data. All methods of analysis indicated that there were two distinct clusters within our data set. After the morphologies of specimens from each cluster were reexamined, we were able to separate the two species morphologically and found that a single diagnostic character found on the mandibles of its soldiers could be used to separate the two species quite accurately. The additional species in the clade was identified as Odontotermes denticulatus after it was matched to type specimens at the NHM London and Cambridge Museum of Zoology. 相似文献
65.
Aims/hypothesis
Patients treated with metformin exhibit low levels of plasma vitamin B12 (B12), and are considered at risk for developing B12 deficiency. In this study, we investigated the effect of metformin treatment on B12 uptake and distribution in rats.Methods
Sprague Dawley rats (n = 18) were divided into two groups and given daily subcutaneous injections with metformin or saline (control) for three weeks. Following this, the animals received an oral dose of radio-labeled B12 (57[Co]-B12), and urine and feces were collected for 24 h. Plasma, bowel content, liver, and kidneys were collected and analyzed for B12, unsaturated B12-binding capacity, and 57[Co]-B12.Results
Three weeks of metformin treatment reduced plasma B12 by 22% or 289 [47-383] pmol/L (median and [range]) (p = 0.001), while no effect was observed on unsaturated B12-binding capacity. Compared with controls, the amount of B12 in the liver was 36% (p = 0.007) higher in metformin-treated rats, while the B12 content in the kidney was 34% (p = 0.013) lower. No difference in the total amount of absorbed 57[Co]-B12 present in the tissues and organs studied was found, suggesting that metformin has no decreasing effect on the B12 absorption.Conclusions/interpretation
These results show that metformin treatment increases liver accumulation of B12, thereby resulting in decreases in circulating B12 and kidney accumulation of the vitamin. Our data questions whether the low plasma B12 observed in patients treated with metformin reflects impaired B12 status, and rather suggests altered tissue distribution and metabolism of the vitamin. 相似文献66.
The binary structure II hydrogen–tetrahydrofuran (THF) hydrate was studied with molecular dynamics simulation. The simulations were carried out at 300, 310 K and 10.1 MPa, and with various contents of hydrogen and THF. The migrations of hydrogen molecules from cage to cage were observed. The migration process of hydrogen was also analysed, and the diffusion coefficients of hydrogen in the hydrate were calculated. The calculated diffusion coefficients qualitatively agreed with the experimental data. Double and quintet occupancies of hydrogen molecules were observed in the small and large cages, respectively, without changing the hydrate structure. 相似文献
67.
The intermolecular potential energy surface (IPS) in the mixtures of CH4–Kr gases from ab initio calculations has been explored. The ab initio calculation was performed at the second-order Møller–Plesset perturbation theory (MP2), with the 6-311+G(2df,2pd) basis set, for three relative orientations of two CH4–Kr molecules as a function of CH4–Kr separation distance. In this work, the IPS, U(r), of the CH4–Kr complex has been investigated, where the vertex (V), edge (E) and face (F) of CH4 approaches to Kr have been considered. Then, adjustable parameters of the Lennard-Jones and Buckingham potential energy function are fitted to the ab initio MP2/6-311+G(2df,2pd) interaction energies for three different orientations. Assuming a given set of parameters, we theoretically obtained second virial coefficients for the CH4–Kr system, and compared with the experimental data at different temperatures. Trivial differences can be observed between the experimental and computational results. 相似文献
68.
The interaction of (−)-reboxetine, a non-tricyclic norepinephrine selective reuptake inhibitor, with muscle-type nicotinic acetylcholine receptors (AChRs) in different conformational states was studied by functional and structural approaches. The results established that (−)-reboxetine: (a) inhibits (±)-epibatidine-induced Ca2+ influx in human (h) muscle embryonic (hα1β1γδ) and adult (hα1β1εδ) AChRs in a non-competitive manner and with potencies IC50 = 3.86 ± 0.49 and 1.92 ± 0.48 μM, respectively, (b) binds to the [3H]TCP site with ∼13-fold higher affinity when the Torpedo AChR is in the desensitized state compared to the resting state, (c) enhances [3H]cytisine binding to the resting but activatableTorpedo AChR but not to the desensitized AChR, suggesting desensitizing properties, (d) overlaps the PCP luminal site located between rings 6′ and 13′ in the Torpedo but not human muscle AChRs. In silico mutation results indicate that ring 9′ is the minimum structural component for (−)-reboxetine binding, and (e) interacts to non-luminal sites located within the transmembrane segments from the Torpedo AChR γ subunit, and at the α1/ε transmembrane interface from the adult muscle AChR. In conclusion, (−)-reboxetine non-competitively inhibits muscle AChRs by binding to the TCP luminal site and by inducing receptor desensitization (maybe by interacting with non-luminal sites), a mechanism that is shared by tricyclic antidepressants. 相似文献
69.
70.
Fang Zhang Yan Zhang Man Chen Mark C.M. van Loosdrecht Raymond J. Zeng 《Biotechnology and bioengineering》2013,110(7):1884-1894
A modified metabolic model for mixed culture fermentation (MCF) is proposed with the consideration of an energy conserving electron bifurcation reaction and the transport energy of metabolites. The production of H2 related to NADH/NAD+ and Fdred/Fdox is proposed to be divided in three processes in view of energy conserving electron bifurcation reaction. This assumption could fine‐tune the intracellular redox balance and regulate the distribution of metabolites. With respect to metabolite transport energy, the proton motive force is considered to be constant, while the transport rate coefficient is proposed to be proportional to the octanol–water partition coefficient. The modeling results for a glucose fermentation in a continuous stirred tank reactor show that the metabolite distribution is consistent with the literature: (1) acetate, butyrate, and ethanol are main products at acidic pH, while the production shifts to acetate and propionate at neutral and alkali pH; (2) the main products acetate, ethanol, and butyrate shift to ethanol at higher glucose concentration; (3) the changes for acetate and butyrate are following an increasing hydrogen partial pressure. The findings demonstrate that our modified model is more realistic than previous proposed model concepts. It also indicates that inclusion of an energy conserving electron bifurcation reaction and metabolite transport energy for MCF is sound in the viewpoint of biochemistry and physiology. Biotechnol. Bioeng. 2013; 110: 1884–1894. © 2013 Wiley Periodicals, Inc. 相似文献