Bilayer lipid membranes general discussion

In this chapter, the authors describe the composition, structural organization, and general functions of biological membranes. After outlining the common features of membranes, a new class of biomolecules, the lipids, are introduced in the context of their role as membrane components. The authors focus on the three main kinds of membrane lipids—the phospholipids, glycolipids, and cholesterol. The amphi-pathic nature of membrane lipids and their ability to organize into bilayers in water are then described. An important functional feature of membranes is their selective permeability to molecules, in particular the inability of ions and most polar molecules to cross membrane bilayers. This aspect of membrane function is discussed next and will be revisited when the mechanisms for transport of ions and polar molecules across membranes is discussed in Chapter 13. 

From this discussion, one could logically deduce that the lipids that constitute biological membranes are all cylindrical. Indeed, bilayer structures are generally represented by a regular arrangement of a xmique species of lipids (presumably PC) with a typical cylindrical shape (Fig. 2.11). This is a misleading oversimplification. 

Aqueous Interfaces Environment of a Membrane Protein Force Fields A General Discussion Molecular Dynamics Studies of Lipid Bilayers Molecular Dynamics Techniques and Applications to Proteins. 

The earliest general model of adaptation to temperature in membrane lipids focused on the physical state ( static order or viscosity [= 1 / fluidity ]) of the bilayer. The finding that the physical state of membrane lipids from Escherichia coli cultured at different temperatures was similar at the different growth temperatures led to the homeoviscous adaptation hypothesis, which states that lipid composition is modified during thermal acclimation to facilitate retention of a relatively stable membrane physical state (Sinensky, 1974). At the outset of any discussion of homeoviscous adaptation, it is important to examine carefully what is meant by physical state (or the related terms static order, viscosity, and fluidity ). In such an analysis, one must also consider the physical methods that are used to make such measurements—and the limitations of these techniques. 

From studies of lipid-water mixtures and isolated membranes the general functional features of the bilayer are known barrier properties, lateral diffusion, acyl chain disorder and protein association. To vmderstand the mechanisms behind a wide spectrum of membrane functions, a detailed picture at the level of local curvature is needed. Examples are fusion processes, cooperativity in receptor/ligand binding or transport through the bilayer of the proteins that are constantly synthesised for export from the endoplasmic reticulum. Some preliminary discussions of the possibilities of curved, rather than flat, membremes follow. 

Most of the properties attributed to living organisms (e.g., movement, growth, reproduction, and metabolism) depend, either directly or indirectly, on membranes. All biological membranes have the same general structure. As previously mentioned (Chapter 2), membranes contain lipid and protein molecules. In the currently accepted concept of membranes, referred to as the fluid mosaic model, membrane is a bimolecular lipid layer (lipid bilayer). The proteins, most of which float within the lipid bilayer, largely determine a membrane s biological functions. Because of the importance of membranes in biochemical processes, the remainder of Chapter 11 is devoted to a discussion of their structure and functions. 

MEMBRANE TRANSPORT Membrane transport mechanisms are vital to living organisms. Ions and molecules constantly move across cell plasma membranes and across the membranes of organelles. This flux must be carefully regulated to meet each cell s metabolic needs. For example, a cell s plasma membrane regulates the entrance of nutrient molecules and the exit of waste products. Additionally, it regulates intracellular ion concentrations. Because lipid bilayers are generally impenetrable to ions and polar substances, specific transport components must be inserted into cellular membranes. Several examples of these structures, referred to as transport proteins or permeases, are discussed. 

After Chapter 1 on non-mediated transport of lipophilic compounds. Chapters 2 and 3 are devoted to the passive transport of water and other small polar molecules and to that of ions. Chapter 4 discusses the insertion of ionophores in lipid bilayers as model systems for carriers and channels in biological membranes. Chapter 5 treats the general principles of mediated transport. Chapters 6, 7 and 8 are devoted to the ATPases, which are involved in the primary active transport of Na, Ca and H, respectively. After Chapters 9 and 10 on specific transport systems in mitochondria and bacteria, the book concludes with Chapters 11 and 12 on secondary active transport, the coupling of the transport of metabolites and water to that of ions. 

In the lipid bilayer systems, since the membrane molecules are arranged in such a way that the charged groups face a water phase and the interior of the membrane is a hydrocarbon phase, the contribution of surface potential to the membrane potential is important. It should be mentioned that the contribution of surface potential to the membrane potential, as discussed above, is generally a transient one in these systems. However, since the electrical conductance due to ion permeation across the lipid bilayer membrane is very low, we can observe the transient potential difference as a quasi-steady state phenomenon. However, if a constant ion distribution is restored by a transport process with a nonelectrical current (active transport) and maintained continuously, the above membrane potential process could become a steady state process. 

Although phosphatidylserine is in general asymmetrically distributed in cell membranes with the bulk of this lipid in the cytoplasmic leaflet of the bilayer, some phosphatidylserine appears to reside in the outer lipid monolayer of the axonal membrane. Furthermore, this phosphatidylserine is involved in the nerve action potential. Treatment of an axon with extracellular serine decarboxylase converts phosphatidylserine to -ethanolamine, which results in a decrease in the action potential spike height. Catalysis of the reversed reaction by this enzyme in the presence of excess L-serine converts phosphatidylethanolamine to -serine. This produces an average of 28% increase in the action potential amplitude. It is worth noticing that several anaesthetic compounds have been shown to bind phosphatidylserine in vitroThe role of phosphatidylserine phase behavior in the nerve action potential will be discussed in somewhat more detail in Section 7. 

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