Chapter 6: Membranes - Introduction

Although not all researchers agree on the fundamental nature of membranes, all agree that membranes take on properties fundamental to the very existence of life. Following the theory of Albert Szent-Gyorgyi, they believe that the understanding of life can be achieved through an understanding of the mechanisms for electron flow through basic organic systems (photosynthesis and respiration) and that these systems in turn exist only through compartmentalization within a cell. Without it, there would be no organelles, and indeed, no cell.

While membranes can be studied within living cells, membrane composition requires isolation and subfractionation of the membrane components. Once isolated, the membranes can be solubilized through the use of detergents and analyzed for proteins, lipids and carbohydrates. The study of biomembranes can be done with techniques requiring minimal equipment, such as osmotic shrinking or swelling of erythrocytes. Some techniques can be so complex as to require a complete organic and physical chemistry lab. The labs in this exercise can be completed with the equipment in the average college biochemistry lab.

The earliest investigations of membranes were those concerned with chemical analysis and were based on the observation that lipid solvents readily permeated cells. Membranes were known to contain protein, and from these early studies, it was concluded that lipids were also a significant factor in membrane composition. The specific nature of the proteins and lipids has been the subject of much research since these early days. As an example of the protein and lipid composition of membranes, as well as the percentage of some other substances, Table 6.1 presents the composition of myelin membranes from the central nervous system and the brain.

Myelin is one of the most often studied biomembranes. It is readily available in quantity, and has a relatively simple composition when compared to membranes from other cells. There are three major types of protein found associated with myelin, basic protein, Folch-Lees proteolipid, and Wolfgram protein. There is a high percentage of lipid composition and the lipids are reasonably uncomplicated.

Of course, this is a simple view of myelin based on early chemical extractions. Anatomists have long recognized the distinction between myelin from the central nervous system (CNS) and that from the peripheral nervous system (PNS). CNS myelin is formed by oligodendrocytes, while PNS myelin is formed by the Schwann Cells. The PNS system is the most often illustrated because of the ease of identification of the laminar membranes of the system. Biochemical analysis of myelin further indicates that there may be subtle differences even within the broad categories of myelin, with, for example, clear distinction between that found in the brain and the spinal cord of the CNS, additional distinctions among species.

Often the chemical composition is of interest by itself, but more intriguing are the dynamic characteristics of membranes, (such as semi-permeability) and functions (such as active transport and facilitated transport). To study these properties, a relatively new approach has been developed: the synthesis of artifical membranes in the form of lipid bilayers (also referred to as bilayer lipid membranes or BLM's). The lipid bilayers are composed of natural or synthetic lipids that are artificially held between two aqueous environments. Table 6.2 presents a list of the major lipids used for lipid bilayer formation, while Table 6.3 indicates some of the membranes these systems have modeled. Artificial membrane research has been significantly advanced by the addition of membrane components extracted from biological systems. The components have come from brain extracts, chloroplast or mitochondrial membranes, and have been augmented with oxidized cholesterol and any number of surfactants. Researchers have been able to create fairly complex artificial membranes and which can mimic nearly all of the qualities of a cell membrane. They have formed the lipid membranes as sheets spread across tiny apertures, or as small droplets within an aqueous environment. Sheets are more relevant to membrane permeability studies, while droplets (known as liposomes) are useful for analyzing cell fusions and membrane flow.

Yet another approach to membrane analysis has been the use of markers or probes. The work utilizes either enzymes or immunofluorescenct reagents. More recently, fluorescent dyes have been covalently attached to specific membrane proteins, which can in turn be micro-injected into cells and their paths through the cell and monitored by computer aided video systems. With this new advancement, quantitative as well as qualitative analysis of membrane flow in a dynamic living cell is possible.

These chemical studies augment the abundant data available from the work of virologists on such phenomena as capping, and membrane flow related to viral reproduction. They are also supplemented by studies of vessicle/vacuole dynamics during cell endocytosis.

Finally, physiologists have studied ion flux across membranes. No view of membranes would be complete without discussion of osmosis, diffusion, active transport and facilitated diffusion. These processes of flux are mathematically defined by the movement of ions across the membrane. While virtually every type of cell has been studied, the major work has involved the erythrocyte and various components of nerve and muscle function.

Through all of these approaches, a coherent theory for membrane structure is emerging. Most membrane analysts believe that essentially membranes are lipids in fluid suspension between two aqueous phases (inside and outside the cell), while the proteins are then attached to this lipid bilayer. Some researchers, however, feel that the protein matrix of the membrane is set and gives substance to the membrane. Lipids are then attached to a protein substrate due to their hydrophobic or hydrophyllic tendencies. This latter view would account for the more structured nature of polar cellular membranes, which do not appear to behave strictly as fluids.

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