Chapter 1
Biological Membranes

1.1 Introduction

Biological membranes maintain the spatial organization of life. The first living cells felt the need to be enveloped by a selectively permeable barrier to protect and shield the set of their life-sustaining chemical transformations from changes in the environment. This envelope, which encloses all living cells, is the plasma membrane. The plasma membrane prevents undesirable agents from entering the cell, while keeping needed molecules on its inside. To function effectively, it must also selectively pass molecules, ions, and signals from one side to the other. Its permeability properties ensure that essential molecules such as glucose, amino acids, and lipids readily enter the cell, metabolic intermediates remain in the cell, and waste compounds leave the cell. The organization of the metabolic activity of the cell requires an additional compartmentalization in its inside, via specialized subunits called organelles, which are again enveloped by a membrane and carry out specific functions. The aqueous solution inside the organelles often contains solutes that are different from those of the solution directly contained in the cell (the cytosol), and the organelle membranes maintain such a difference.

Since all membranes are interposed between two aqueous media, the strategy underlying their function involves creating a hydrophobic barrier, which is formed by a thin lipid layer. The water-soluble compounds present both within cells and organelles as well as outside of them are not soluble in the lipid medium of the membrane, and pass slowly or not at all through it. The lipid material is particularly convenient for a number of reasons. Thus, it assembles spontaneously into two juxtaposed lipid monolayers that generate a highly hydrophobic region in their interior, while exposing a relatively hydrophilic surface to the aqueous solutions bathing the two sides of the membrane. Moreover, the lipid bilayer so formed is highly fluid and allows an easy incorporation of different biomolecules capable of spanning it and of transferring molecules, energy, or information across it in a selective way. Finally, the hydrophobic interior of the lipid bilayer, thanks to its low dielectric constant, concentrates by far the majority of the electric potential difference between the two sides of the membrane, that is, the transmembrane potential. The change of the transmembrane potential over time, often induced by some external stimulus, modulates the function of many biomolecules incorporated in the membrane.

Life, similar to all other processes in our universe, obeys the laws of physics and chemistry. Consequently, all biological processes may occur only if they are accompanied by a decrease in the corresponding Gibbs energy, in accordance with the second law of thermodynamics. Some biological processes, when taken separately and out of their context, may seem to proceed with an increase in Gibbs energy. However, an attentive examination of their context reveals unavoidably that they are intimately coupled to some other process that proceeds with a decrease in Gibbs energy, such that the combination of the two is still characterized by a decrease in Gibbs energy. This coupling finds its justification in the thermodynamics of irreversible processes.

The thickness of the plasma membrane is much smaller than the radius of a cell, allowing us to treat the membrane surface as if it were planar; this simplifies the mathematics to a significant extent. To understand in depth the structure and function of biological membranes, it is also essential to understand and apply the principles of physical chemistry. In particular, the fundamental role played by the transmembrane potential in modulating the function of the biomolecules incorporated in the membrane allows us to consider and treat the membrane as a proper electrified interface. Hence, to understand the function of biological membranes and the properties of their experimental models, called biomimetic membranes, a knowledge of some basic principles of electrochemistry and of the most significant electrochemical techniques is required. The purpose of this work is to construct a coherent thermodynamic and electrochemical framework to achieve this goal. The role of the electrochemical foundations and techniques for the investigation of processes of biological relevance was first recognized in the 1980s, when Gutmann and Keyzer (1986) coined the name of bioelectrochemistry to denote this area of science.

In addition to membrane processes, bioelectrochemistry deals with the investigation of the properties and functions of water-soluble biomolecules, usually by adsorbing them on surface-modified and derivatized electrodes and by studying electron transfer reactions between them and the electrode (Bartlett, 2008; Alkire et al., 2011). However, the electrochemical behavior of biomolecules at electrodes does not necessarily pertain to bioelectrochemistry: this is true only if it provides some useful piece of information on the role played by these molecules in biological processes. These aspects of bioelectrochemistry are beyond the scope of this book. Bioelectrochemistry has also many applications in practical devices such as biosensors and biofuel cells.

1.2 The Biological Membranes

To stay alive, all living things need biological membranes (briefly, biomembranes). Biomembranes are thin layers that form the outer boundary of living cells, separating their inside (the cytoplasm) from their outside (the extracellular fluid). The cytoplasm comprises the cytosol (a gel-like substance enclosed within the cell membrane) and a number of substructures called organelles, which are also enclosed by a membrane. The membrane enclosing a cell is called plasma membrane. One important component of biological membranes consists of two monolayers (leaflets) of lipid molecules (Fig. 1.1). Lipid molecules are amphiphilic, that is, they have a hydrophobic section (the hydrocarbon tail) and a hydrophilic section (the polar head or headgroup). In biomembranes, the two lipid monolayers are oriented with the hydrocarbon tails directed toward each other and the polar heads turned toward the aqueous solutions that bath the two sides of the membrane. The resulting lipid bilayer, about 6 nm thick, is a matrix that incorporates different proteins performing a variety of functions.

Figure 1.1 Schematic picture of a plasma membrane, showing the bimolecular layer of lipid molecules (including cholesterol), integral proteins spanning the lipid bilayer, peripheral proteins, filaments of cytoskeleton (the cellular "scaffolding" present in the cytoplasm), as well as glycoproteins, which expose their covalently attached oligosaccharide chains (glycans) to the extracellular fluid.

Source: https://commons.wikimedia.org/wiki/File:Cell_membrane_detailed_diagram_de.svg.

Biomembranes form a highly selective barrier between the inside and the outside of living cells. They are highly insulating to inorganic ions, and large ion concentration gradients can be maintained across them. The permeability and structural properties of biological membranes are sensitive to the chemical nature of the membrane components and to events that occur at the interface or within the bilayer. For example, biomembranes provide the environmental matrix for proteins that specifically transport certain ions and other molecules, for receptor proteins and for signal transduction molecules. The lipid and protein portions of biomembranes are also sensitive to the presence of lipophilic perturbants, that is, molecules with a high affinity for lipids. Anesthetics, for example, readily partition into lipid membranes, altering their electrical and permeability characteristics. The various responses observed in biomembranes are concentration dependent, usually very rapid and reversible, and frequently dependent upon the transmembrane potential.

Proteins are composed of linear chains of polymers of amino acids (polypeptide chains) linked by amide bonds, called peptide bonds. Some of the proteins (the structural proteins) simply support the texture of the membrane. A more important group of proteins (the functional proteins) participates directly in membrane processes such as flow of matter, energy, or information. Some proteins (the integral proteins) are embedded in the lipid bilayer with the hydrophobic section of their polypeptide chains, and protrude from the bilayer surface into one or both the adjacent aqueous solutions with the extrinsic, more hydrophilic section of the chains. Other proteins (the peripheral proteins) are weakly bound to the surface of the bilayer by electrostatic interactions or by hydrogen bonds, and interact with the polar heads of the lipid or with the integral membrane proteins. These are globular proteins, namely proteins in which the polypeptide chain folds spontaneously in a way that removes the hydrophobic sections from contact with the aqueous solution, burying them in the interior of the protein; conversely, the hydrophilic sections remain on the surface of the protein, where they form hydrogen bonds with water and between themselves. These proteins are water soluble. Some of them (e.g., cytochrome c, plastocyanin, ferredoxin) contain electrophilic metal ions and exchange electrons with the integral proteins. The majority of redox proteins, namely proteins containing one or more redox sites, have no biological function when taken...