AS VIEWED FROM EXPERIMENTAL BILAYER
(Planar Lipid Bilayers and Spherical Liposomes)
Published by Elsevier, Amsterdam and New York, 2000, 648 pp.
Membrane Biophysics: An Overview
1.1 What is biophysics ?
1.2 What is life and its Origin ?
1.3 What are the relative sizes of biological molecules and cells ?
1.4 What is a biomembrane ?
1.5 What topics are covered in this book ?
1.6 An idealized living cell
1.7 Signal transduction
The cell is the smallest functional unit of tissues and organs. Thermodynamically speaking, a living cell is an open system capable of energy transformation and material transport. Both the cell and its organelles (e.g., mitochondria and chloroplasts) are separated from their environment by membranes, which is believed to be crucial in the evolution of life. Structurally, the cell membrane is a lipid bilayer matrix modified by functional proteins, carbohydrates and their complexes. The formation of a lipid bilayer in water is a self-assembling, 'downhill' process involving rearrangements of water and lipid molecules such that the overall free energy change for the reaction is at a optimum. That is for the reactants involved, they must attain a state of minimum energy and maximum entropy. In order to accomplish this, the hydrocarbon chains of the lipids are sequestered away from water whereas the polar groups are in contact with water to attain maximal interactions. In other words, the chemist's rule of 'like dissolves like' is obeyed. Functionally, the cell membrane plays a pivotal role in energy conversion, material translocation, signal transduction and information processing. Most membranes are semipermeable, meaning they are permeable to water but selectively to solutes (e.g., glucose, ions, etc.). Transmembrane activities are, therefore, driven by gradients of chemical and electrochemical potentials. To counter these spontaneous activities or processes, metabolic energy must be spent in order to make the living state of the cell viable.
The area chosen for our study is membrane biophysics; it is motivated by the desire to understand as well as to interpret living organisms in physical and biochemical terms at the membrane level. This presupposes that a basic understanding of the physics and chemistry of the universe will result in an understanding of biological processes Fig.1.1. Thus, the proper study of living cell is membrane. Since the plasma membrane of erythrocytes, one of the simplest membranes, is still too complex to be characterized in simple physico-chemical terms, we have resorted to study reconstituted model membrane systems. At the membrane level, most cellular activities involve some kind of lipid bilayer-based receptor-ligand contact interactions. Outstanding examples among these are ion-sensing, molecular recognition (e.g., antigen-antibody binding and enzyme-substrate interaction), light conversion and detection, gated channels, and active transport. Our approach to study these physiological happenings is facilitated by in vitro lipid bilayers. The development of self-assembled bilayer lipid membranes (BLMs and liposomes) has made it possible to investigate directly the electrical properties and transported phenomena across a 5 nm (nanometer) thick cell membrane element separating two aqueous phases. A modified or reconstituted BLM is viewed as a dynamic structure that changes in response to environmental stimuli and changes as a function of time. The so-called 'dynamic membrane hypothesis' is invoked to explain the membrane function. According to this hypothesis, the self-assembled lipid bilayer, the fundamental moiety of biomembranes, is in a dynamic and liquid-crystalline state. A functional biomembrane should be considered in electronic and molecular terms; it can support ion or/and electron transport and is the site of cellular activities in that it functions as a 'device' for either energy conversion or signal transduction. Such a system, as we know it intuitively, must act as some sort of a transducer capable of gathering information, processing it, and then delivering a response based on this information. In the past, we were limited by our lack of sophistication in manipulating and monitoring such a system. Today, membrane biophysics is a matured field of research as a result of applications of many elegant techniques including spectroscopy, membrane bioelectrochemistry, patch-clamp, acoustics, and membrane reconstitution. In the chapters that follow, the area of membrane biophysics will be perused, from the origin of the lipid bilayer concept to membrane transport, electrochemistry, membrane reconstitution, bioenergetics, and to membrane photobiology. In the final chapter some potential applications of self-assembled lipid bilayers as biosensors will be presented. This is owing to the fact that recent advances in microelectronics coupled with membrane research of the past decades have come of age and are poised for biotechnological exploitation. The connecting thread among these topics is the experimental bilayer lipid membranes (mainly planar BLMs and spherical liposomes). The basic principles of thermodynamics, kinetics, the Nernst-Planck equation, and physical chemistry will be applied to the afore-mentioned area of membrane biophysics. The best starting point of discussion on the biomembrane is, perhaps, to begin with an overview of the cell. Fig.1.2 and Fig.1.3 show schematically the plasma membrane of a highly idealized eukaryotic cell, together with its organelle membrane systems. It may be seen that, even in this highly simplified drawing, the plasma membrane is an extremely complex and elaborate supramolecular structure. The most important feature is that the plasma membrane of cells is consisting of a two-dimensional liquid-crystalline lipid bilayer in which various functional entities (e.g., enzymes, receptors, channels) are embedded. In order to comprehend and to explain the structure and function of natural cell membranes in physical and biochemical terms, resorting to model systems has a long tradition in natural sciences. Thus, insofar as experimental model cell membranes are concerned, there are planar bilayer lipid membranes (BLMs) and spherical liposomes, both of which are also depicted in Fig.1.2 and Fig.1.3 and will be discussed in detail in the subsequent chapters. It should be pointed out, however, that planar BLMs and liposomes are more than mimetic biomembrane models; they are self-assembled lipid bilayers in vitro, which constitute the fundamental structure of all biomembranes.
life cycle, biomembranes, their interrelationship and methods of inquiry
The pivotal concept of biomembranes is the lipid bilayer. The life cycle, driven by the solar energy, is represented by an infinity sign shown at the bottom of the figure. The two-coupled circles depict a steady-state energy transducing system of life.
idealized cell together with its organnelle membrane systems
An idealized plasma membrane of the cell possesses an elaborate array of transmembrane entities embedded in the lipid bilayer. Also shown are its organelle membrane systems.
(1) The Lipid Bilayer: An universal element of all cell membranes.
(2) Membrane Proteins: Manners by which they are associated with the lipid bilayer: (a) span the bilayer, (b) partially immerse in the bilayer, (c) held by non-covalent interactions with other proteins, and (d) attach to fatty acid chains that help to anchor the protein in one surface of the bilayer.
(3) Permeability and Transport: The passive transport occurs either by the simple diffusion or by the facilitated diffusion. The active transport needs an input of the metabolic energy.
(4) The Electrochemical Potential /u: The driving force for the membrane transport, oxidative- and photo-phosphorylation.
(5) The Na+/K+ Pump: Virtually all animal cells contain a Na+/K+ pump that operates as an antiport, actively pumping K+ into the cell and Na+ out against their electrochemical gradients.
(6) Ion Channels: There are ligand-gated and voltage-gated channels made of proteins, which open in response to the ligand interaction and voltage change. For example, a Ca2+ channel may be depicted as a port containing negatively charged sites, density and dimensions appropriate to act as "selectivity filter" to distinguish among different cations, with voltage-sensors conferring voltage dependence on channel opening and closing.
(7) The Nucleus: It is where the genetic material (DNA) is located. Its nuclear membrane has numerous pores ( up to ~ 80 nm in diameter) which probably transport large particles selectively.
(8) Energy-transducing Organelle Membranes: The mitochondrion converts foods into usable energy (ATP), whereas the chloroplast, in plants and other photosynthesizing organisms, transduces the electromagnetic radiation (solar energy) into other forms of energy.
(9) Experimental Bilayer Lipid Membranes: They can be prepared. Since their inception in the early 1960s, such lipid bilayer systems, either in the form of a planar BLM or of a vesicular liposome, have been used extensively as models of biomembranes (Chapter 4). The advantage of the planar lipid bilayer (BLM) system is that both sides of the membrane can be easily altered and probed by electrodes. For long-term investigations and technological applications, planar BLMs can also be made on either gel or metallic supports (Chapter 10).
bilayer lipid membrane (BLM) and signal transduction across the cell membrane
A. An enlarged view of the above lipid bilayer-bound entities and ligand (L) in the bathing solution.
B. A highly schematic diagram of a cell, illustrating one of the most common mechanisms of information transfer based on ligand-receptor interactions. The plasma membrane of the cell is depicted consisting of the following: a voltage-gated channel (VGC), a ligand-gated channel (LGC), a store-operated channel (SOC), G-proteins (G), amplifier enzyme (AE); all of these are lipid bilayer-bound. Upon receiving a signal (e.g., ligand, L) by the receptor (R), the resulting contact causes an enzyme to catalyze the production of second messengers from a phosphorylated precursor (cAMP, for example) which in turn triggers enzyme kinases through an intracellular effector leading ultimately to the cell response.
C. This diagram illustrates the molecular organization of the BLM separating two aqueous solutions. To mimic different biomembranes, a host of compounds (modifiers = circle, square, triangle) have been embedded in the BLM in order to impart unique functional characteristics.
D. Experimental arrangement used for the measurement of electrical properties of BLMs. Also shown is a lipid bilayer-based sensor using electrical methods of detection.
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