Physics

The balance between stretching and bending deformations characterizes shape transitions of thin elastic sheets. While stretching dominates the mechanical response in tension, bending dominates in compression after an abrupt buckling transition. Recently, experimental results in suspended living epithelial monolayers have shown that, due to the asymmetry in surface stresses generated by molecular motors across the thickness e of the epithelium, the free edges of such tissues spontaneously curl out-of-plane, stretching the sheet in-plane as a result. This suggests that a competition between bending and stretching sets the morphology of the tissue margin. In this study, we use the framework of non-euclidean plates to incorporate active pre-strain and spontaneous curvature to the theory of thin elastic shells. We show that, when the spontaneous curvature of the sheet scales like 1/e , stretching and bending energies have the same scaling in the limit of a vanishingly small thickness and therefore both compete, in a way that is continuously altered by an external tension, to define the three-dimensional shape of the tissue.

The synthesis of ATP, life's 'universal energy currency', is the most prevalent chemical reaction in biological systems, and is responsible for fueling nearly all cellular processes, from nerve impulse propagation to DNA synthesis. ATP synthases, the family of enzymes that carry out this endless task, are nearly as ubiquitous as the energy-laden molecule they are responsible for making. The F-type ATP synthase (F-ATPase) is found in every domain of life, and is believed to predate the divergence of these lineages over 1.5 billion years ago. These enzymes have therefore facilitated the survival of organisms in a wide range of habitats, ranging from the deep-sea thermal vents to the human intestine. In this review, we present an overview of the current knowledge of the structure and function of F-type ATPases, highlighting several adaptations that have been characterized across taxa. We emphasize the importance of studying these features within the context of the enzyme's particular lipid environment: Just as the interactions between an organism and its physical environment shape its evolutionary trajectory, ATPases are impacted by the membranes within which they reside. We argue that a comprehensive understanding of the structure, function, and evolution of membrane proteins -- including ATP synthase -- requires such an integrative approach.

We investigated whether we can detect enhanced magnetic nanoparticle uptake under application of a large magnetic force by tagging the particles with a fluorescent dye. Human liver cells were cultured in a micro-channel slide and exposed to two types of magnetic nanoparticles with a diameter of 100 nm at a concentration of 10000 particles/cell for 24 hours. Even though we achieved a magnetic force that exceeded the gravitational force by a factor of 25, we did not observe a statistically significant increase of immobilised particles per cell.

A low-cost device for registration-free quantitative phase microscopy (QPM) based on the transport of intensity equation (TIE) of cells in continuous flow is presented. The method uses acoustic focusing to align cells into a single plane where all cells move at a constant speed. The acoustic focusing plane is tilted with respect to the microscope's focal plane in order to obtain cell images at multiple focal positions. As the cells are displaced at constant speed, phase maps can be generated without the need to segment and register individual objects. The proposed inclined geometry allows for the acquisition of a vertical stack without the need for any moving part, and it enables a cost-effective and robust implementation of QPM. The suitability of the solution for biological imaging is tested on blood samples, demonstrating the ability to recover the phase map of single red blood cells flowing through the microchip.

Upon stimulation, plants elicit electrical signals that can travel within a cellular network analogous to the animal nervous system. It is well-known that in the human brain, voltage changes in certain regions result from concerted electrical activity which, in the form of action potentials (APs), travels within nerve-cell arrays. Electrophysiological techniques like electroencephalography, magnetoencephalography, and magnetic resonance imaging are used to record this activity and to diagnose disorders. In the plant kingdom, two types of electrical signals are observed: all-or-nothing APs of similar amplitudes to those seen in humans and animals, and slow-wave potentials of smaller amplitudes. Sharp APs appear restricted to unique plant species like the "sensitive plant", Mimosa pudica, and the carnivorous Venus flytrap, Dionaea muscipula. Here we ask the question, is electrical activity in the Venus flytrap accompanied by distinct magnetic signals? Using atomic optically pumped magnetometers, biomagnetism in AP-firing traps of the carnivorous plant was recorded. APs were induced by heat stimulation, and the thermal properties of ion channels underlying the AP were studied. The measured magnetic signals exhibit similar temporal behavior and shape to the fast de- and repolarization AP phases. Our findings pave the way to understanding the molecular basis of biomagnetism, which might be used to improve magnetometer-based noninvasive diagnostics of plant stress and disease.

Lipid bilayer membranes are flexible thin laterally fluid films consisting of two unimolecular layers of lipids. On spatial scales much larger than the bilayer thickness, the membrane elasticity is well determined by its shape and adequately described by the classical Helfrich Hamiltonian. However, various local membrane heterogeneities can result in a lipids tilt relative to the membrane surface normal. On the basis of the classical elasticity theory of 3D bodies, Hamm and Kozlov [Eur. Phys. J. E 3, 323 (2000)] derived the most general energy functional, taking into account the tilt and bending. Recently, Terzi and Deserno [J. Chem. Phys. 147, 084702 (2017)] showed that Hamm and Kozlov's derivation was incomplete because the tilt-curvature coupling term had been missed. However, the energy functional derived by Terzi and Deserno appeared to be unstable, thereby being invalid for applications. Here, we derive a stable elastic energy functional, showing that the squared gradient of the curvature was missed in both of these works. This change in the energy functional arises from a more accurate consideration of the transverse shear deformation terms and their influence on the membrane stability. We also consider the influence of the prestress terms on the stability of the energy functional, and we show that the effective Gaussian curvature should be neglected because of the stability requirements. We further generalize the theory, including the stretching-compressing deformation modes, and we provide the geometrical interpretation of the terms that were previously missed by Hamm and Kozlov. The physical consequences of the new terms are analyzed in the case of a membrane-mediated interaction of two amphipathic peptides located in the same monolayer. We also provide the expression for director fluctuations, comparing it with that obtained by Terzi and Deserno.

We build a simulation platform for Airy beam photoacoustic microscopy based on K-Wave simulation toolbox in MATLAB. The sample was located in the middle of z direction in the environment. The propagation of Airy beam irradiated on the sample was simulated by solving the Schrodinger equation. And the intensity of Airy beam was simulated by Airy wave pocket. The simulation result showed that: in A-scan, the intensity of light waved over time, reached peak at 49ns; in C-scan, the image agrees with the sample. This research showed that the Airy beam with large depth of field and no diffraction can realize the large depth of field imaging of the photoacoustic microimaging system. The establishment of the simulation platform has a significance for the theoretical research of photoacoustic microscopy and its application in biomedicine.

Spatio-temporal analysis typically performed in botany and in statistical physics reveals persistent correlations of olive yields which range depend on the size of the averaging region. Mapping spatially correlated regions reveals areas which mimic historical spread of Olea europaea. These yield patterns are remarkable given the intensive nature of modern agriculture, and cannot be attributed to weather due to inability of weather variables to predict yields. Long-range correlations between olive trees may indicate long-range communications.

An improved analysis for single particle imaging (SPI) experiments, using the limited data, is presented here. Results are based on a study of bacteriophage PR772 performed at the AMO instrument at the Linac Coherent Light Source (LCLS) as part of the SPI initiative. Existing methods were modified to cope with the shortcomings of the experimental data: inaccessibility of information from the half of the detector and small fraction of single hits. General SPI analysis workflow was upgraded with the expectation-maximization based classification of diffraction patterns and mode decomposition on the final virus structure determination step. The presented processing pipeline allowed us to determine the three-dimensional structure of the bacteriophage PR772 without symmetry constraints with a spatial resolution of 6.9 nm. The obtained resolution was limited by the scattering intensity during the experiment and the relatively small number of single hits.

In this paper we derive a model to describe the effective motion of a protein laterally diffusing in a lipid membrane. By postulating that the lipid membrane is a linear viscoelastic fluid and the protein a rigid body, we derive a continuum description of the system. Within this framework, the lipid membrane is modeled through the linearized Navier-Stokes equations complemented by a non Newtonian constitutive equation, and the protein by a (modified) Euler's law, where a noise term is introduced to account for the stochasticity of the system. Then, by eliminating the lipid membrane degrees of freedom, we obtain a Generalized Langevin Equation (GLE) describing the diffusive motion of the protein with a memory kernel that can be related to the response of the membrane encoded in the solution of the constitutive equation. By representing the viscoelastic behavior through the Prabhakar fractional derivative, one generates a memory kernel containing a three-parameter Mittag-Leffler function. An additional Dirac delta-function term in the memory kernel accounts for the instantaneous component of the system response. A comparison between the Mean Squared Displacement (MSD) derived in the framework of this model and the MSD of a protein diffusing in a membrane calculated through molecular dynamics (MD) simulations shows that the proposed model accurately describes all regimes of the diffusive process, namely, the ballistic, the subdiffusive (anomalous) and the Brownian one, as well as the transitions from one regime to another. The model further provides an estimation of the membrane viscosity, which is of the order of magnitude of the values found by rheology experiments on analogous systems.