Liposomes biological functionality

A good experimental model for biomembranes should possess a lipid bilayer structure, onto as well as into which functional entities can be embedded. Thus, since the 1960s, the two most widely used model membranes have been BLMs, also referred to as planar lipid bilayers and spherical liposomes. Planar BLMs and spherical liposomes are complementary to each other, since both types are derived from common amphipathic lipids and related compounds. Both are excellent model membrane systems, and have been extensively employed for investigations into a variety of physical, chemical, and biological functions. In the remainder of this chapter on membrane electrochemistry, the focus will be mainly on planar BLMs, because they are easily and have been investigated electrochemically (for K-posomes, see Refs. [3, 12, 34]). As a result... 

Models for biological membranes have either been realized as planar lipid monolayers at the gas-water interface (3) or as bi-molecular lipid membranes (BLM) (4) and spherical liposomes (vesicles), respectively (5 6) (Figure 2.). All these models that are only composed of lipid molecules exhibit a diminished stability compared to natural cell membranes. Obviously the protein part besides being functionally important plays a role in terms of stability of biomembranes. This is the case not only for the integral but especially for the boundary proteins ( 7). 

The methods of remote release described here can be used for studying numerous processes relevant for biology, e.g., the cell-surface presentation of small peptides, transport of biomolecules, and cell functions in general. Also, such methods can be used to study the properties and release from other systems, e.g., liposomes [133, 146, 147],... 

Liposomes have aroused interest in a great variety of areas from biochemistry and molecular biology to cosmetics and food technology. One of the most salient applications of liposomes has been promoted by their high similarity to natural cell membranes, for which they are extensively used as substitutes in medical and pharmaceutical research. Since their inception, liposomes have often been used as models for studying the nature of cell membranes, the structure and functions of which they can mimic quite closely. One example is the determination of membrane distribution coefficients of drugs with a view to estimate their ability to penetrate cells. Interactions between analytes and phospholipid membranes depend on the characteristics of both the analytes and the membrane. 

Our aim in this article is to provide the reader with some flavor of the various structures and functions related to lipids. Rather than presenting an exhaustive description of the topic, however, we prefer a down-to-earth approach on a level in which we combine many relevant views and methods with several instructive examples and highlights of recent studies. The concise list of references at the end of this article follows this idea, in accord with the style of this review series. The emphasis will be on lipid membranes because of their abundance in numerous biological systems, which include cellular membranes, liposomes, and other delivery vehicles. 

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