A Computer Evaluation of Equations for Predicting the Potential across Biological Membranes

The Goldman, Henderson, and Planck junction potential equations can be used to describe the potential across the resting giant squid axon. These equations are used to calculate the relative Na, K, and Cl permeabilities of the squid axon using the experimental measurements of Hodgkin and Katz. The equations all provide excellent agreement with the observed data and yield similar permeability values.     

生物立方膜

脂质立方液晶在药物载体方面有着较为广泛的应用.然而在生物体内,有一种与之类似的结构,称为立方膜.具体而言,立方膜就是指含有脂蛋白的三维周期性脂质双分子单层、双层或多层的纳米曲面结构.亚细胞器中的这种生物立方膜结构可能也能作为药物载体,同时有抗氧化、紫外滤光等潜在作用.本文将主要介绍立方膜的研究进展、形成机理,以及在自然界中的存在情况,及其功能和潜在的应用价值.     

A High-Resolution Luminescent Assay for Rapid and Continuous Monitoring of Protein Translocation across Biological Membranes

Protein translocation is a fundamental process in biology. Major gaps in our understanding of this process arise due the poor sensitivity, low time resolution and irreproducibility of translocation assays. To address this, we applied NanoLuc split-luciferase to produce a new strategy for measuring protein transport. The system reduces the timescale of data collection from days to minutes and allows for continuous acquisition with a time resolution in the order of seconds, yielding kinetics parameters suitable for mechanistic elucidation and mathematical fitting. To demonstrate its versatility, we implemented and validated the assay in vitro and in vivo for the bacterial Sec system and the mitochondrial protein import apparatus. Overall, this technology represents a major step forward, providing a powerful new tool for fundamental mechanistic enquiry of protein translocation and for inhibitor (drug) screening, with an intensity and rigor unattainable through classical methods.     

Mechanical Equilibrium of Biological Membranes

The nature of mechanical and electrical forces on biological membranes in relation to mechanical equilibrium is examined. The presence of a double layer of electric charge is shown to give rise to an effective pressure drop across a curved membrane of finite thickness. For certain geometric shapes of a membrane, the magnitude of the pressure drop due to electrostatic forces may set a limit on the hydrostatic pressure drop that the membrane can support without buckling. The results are applied to the equilibrium shape of the red blood cell.     

Ion-pair Movement across Bilayer Membranes

WE have used bilayer lipid membranes (BLM) as models for biological membranes to study the transport of metal halides as ion-pairs. In the presence of iodine (I₂) we find¹ that various monovalent and divalent halides can readily move across BLM along a concentration gradient. The rate of transport increases as the size of the cation increases. As the membrane interior is thought to be of a liquid hydrocarbon nature, these results can be related to those of solvent extraction studies² which showed that various metal halides could be extracted efficiently from aqueous solutions into organic solvents; the extraction efficiency increases as the cation size increases.     

Atomic Force Microscopy of Biological Membranes

Atomic force microscopy (AFM) is an ideal method to study the surface topography of biological membranes. It allows membranes that are adsorbed to flat solid supports to be raster-scanned in physiological solutions with an atomically sharp tip. Therefore, AFM is capable of observing biological molecular machines at work. In addition, the tip can be tethered to the end of a single membrane protein, and forces acting on the tip upon its retraction indicate barriers that occur during the process of protein unfolding. Here we discuss the fundamental limitations of AFM determined by the properties of cantilevers, present aspects of sample preparation, and review results achieved on reconstituted and native biological membranes.     

The Balance of Fluid and Osmotic Pressures across Active Biological Membranes with Application to the Corneal Endothelium

The movement of fluid and solutes across biological membranes facilitates the transport of nutrients for living organisms and maintains the fluid and osmotic pressures in biological systems. Understanding the pressure balances across membranes is crucial for studying fluid and electrolyte homeostasis in living systems, and is an area of active research. In this study, a set of enhanced Kedem-Katchalsky (KK) equations is proposed to describe fluxes of water and solutes across biological membranes, and is applied to analyze the relationship between fluid and osmotic pressures, accounting for active transport mechanisms that propel substances against their concentration gradients and for fixed charges that alter ionic distributions in separated environments. The equilibrium analysis demonstrates that the proposed theory recovers the Donnan osmotic pressure and can predict the correct fluid pressure difference across membranes, a result which cannot be achieved by existing KK theories due to the neglect of fixed charges. The steady-state analysis on active membranes suggests a new pressure mechanism which balances the fluid pressure together with the osmotic pressure. The source of this pressure arises from active ionic fluxes and from interactions between solvent and solutes in membrane transport. We apply the proposed theory to study the transendothelial fluid pressure in the in vivo cornea, which is a crucial factor maintaining the hydration and transparency of the tissue. The results show the importance of the proposed pressure mechanism in mediating stromal fluid pressure and provide a new interpretation of the pressure modulation mechanism in the in vivo cornea.     

Passive Asymmetric Transport through Biological Membranes

The magnitude of passive diffusional solute transfer through artificial membranes is usually considered to be independent of the direction of the concentration gradient driving force. It can be shown, however, that a composite membrane, having as one component a membrane with a chemical reaction-facilitated diffusion transport mechanism, can result in an asymmetrical flux. An asymmetric flux caused by this type of structural heterogeneity may be one mechanism contributing to the asymmetric properties of biological membranes. Similar vectorial fluxes can be generated in interfacial solute transfer through membranes if hydrodynamic boundary layers occur at the membrane interface and reversible chemical reactions with the permeant species are involved in either phase.     

Osmotic Flow of Water across Permeable Cellulose Membranes

Direct measurements have been made of the net volume flow through cellulose membranes, due to a difference in concentration of solute across the membrane. The aqueous solutions used included solutes ranging in size from deuterated water to bovine serum albumin. For the semipermeable membrane (impermeable to the solute) the volume flow produced by the osmotic gradient is equal to the flow produced by the hydrostatic pressure RT ΔC, as given by the van't Hoff relationship. In the case in which the membrane is permeable to the solute, the net volume flow is reduced, as predicted by the theory of Staverman, based on the thermodynamics of the steady state. A means of establishing the amount of this reduction is given, depending on the size of the solute molecule and the effective pore radius of the membrane. With the help of these results, a hypothetical biological membrane moving water by osmotic and hydrostatic pressure gradients is discussed.     

Asymmetrical Lipid Bilayer Structure for Biological Membranes

IT is generally accepted that the matrix of cellular membranes is a bimolecular leaflet of phospholipid molecules in which the phospholipids are oriented so that their polar heads reside on the outer surfaces of the bilayer, in contact with the aqueous environment, the interior of the sandwich being composed of hydrophobic lipid chains^1–5. To this basic structure proteins cholesterol, glycolipids and other molecules are usually inserted in such a way as to confer on the bilayer the functional properties appropriate for the particular membrane.     

Fluctuations and Fractal Noise in Biological Membranes

Our understanding of cell structure and function derives from applications of a variety of physical and life science disciplines, methods and models to an important physiological process, namely, the exchange and transport of ions and molecules across biological membranes. We know that ion transport through membranes arises from a diversity of interrelated and interactive physical and chemical phenomena over a wide range of spatial and temporal scales. Among these phenomena common to all cellular structure and function include metabolism, kinetics of molecules, chemically mediated alteration of cell membrane electrical potential, membrane ion conductance, electrical signal propagation, and modulation by chemo- and mechanoreceptive mechanisms. This review focuses on the unique information contained in fluctuations in electrical properties associated with cell membrane ion transport. Received: 19 May 2000/Revised: 10 July 2000     

Molecular Mechanisms of Raft Organization in Biological Membranes

Russian Journal of Bioorganic Chemistry - This review analyzes and summarizes some actual models of raft organization as dynamic structural units in lipid membranes emphasizing the discrimination...     

Further Observations on Asymmetrical Solute Movement across Membranes

The permeability of frog skin under the influence of urea hyperosmolarity has been studied. Flux ratio asymmetry has been demonstrated again for tracer mannitol. The inhibitors DNP, CN^-, and ouabain have been used to eliminate active sodium transport and it was found that urea hyperosmolarity produces asymmetrical mannitol fluxes on frog skins having no short-circuit current. These findings suggest that flux ratio asymmetry is due to solute interaction and is unrelated to sodium transport. Studies with a synthetic membrane show clearly that bulk flow of fluid can produce a "solvent drag" effect and change flux ratios. When bulk flow is blocked and solute gradients allowed their full expression, then solute interaction "solute drag" is easily demonstrable in a synthetic system.     

Effect of Surfactants on Diffusion of Drugs across Membranes

THE effect of surfactants on the diffusion of drugs across biological membranes and the extent of pharmacological response observed in whole organisms have been much studied¹. In a variety of organisms, including worms, goldfish and men, surfactants can both increase and decrease the rate, of drug diffusion across membranes^2–4 and although at low concentrations they enhance biological response, at higher concentrations they have the reverse effect². In addition to many naturally occurring surface active compounds, such as bile salts⁵ and phospholipids⁶, a wide variety of pharmaceutical preparations contain surfactants and, as Swisher pointed out, the vast industrial production of these compounds has resulted in their widespread occurrence throughout the biosphere⁷.