|
Biomembranes are not simply static inert boundaries to the cellular components, but dynamic structures intimately involved in many of the biochemical processes of the cell. Components of biomembranes are phospholipids, cholesterol, proteins and carbohydrates. Proper arrangement and orientation of these components appears to determine the biological functions and the physical characteristics of the membrane. Much of our understanding of biomembrane structures has been obtained through the investigation of models. Surfactant monolayers, bilayers, lipid vesicles and other membrane-mimetic systems have been used extensively as membrane models. Molecules are able to move in the biomembrane bilayer. This encompasses various translational, rotational and vibrational modes of movements, and the special property which phospholipids possess of undergoing the phase transition between liquid-crystalline and crystalline states. In fluid state, phospholipids have four permitted modes of mobility within the bilayer structure. These are fast lateral diffusion within the plane of the bilayer, "flip-flop" movement across the bilayer, intra-chain motion of fatty acyl chains and fast axial rotation of the fatty acyl chains. Mobility of molecules in biomembranes is as a whole expressed as membrane fluidity. It has been monitored by various methods including fluorometry, NMR and ESR. In this lecture, the concept of biomembrane fluidity and its implications in membrane physiology and pharmaceutical sciences will be introduced. Methods of measuring the microviscosity of the hydrocarbon region of bimembrane with the use of fluorescence probes will be also discussed.
|