Biochemistry - The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

Membranes and Cell Walls
Metabolism and Membrane Functions

What determines the vital role of membranes in Cells? First and foremost, membranes envelop cells and define their boundaries. At the same time, they act as natural aggregates of amphipathic molecules—that is, molecules with one hydrophobic end and one hydrophilic end. The way these molecules pack within the bilayer creates an ordered boundary layer between two liquid phases. Furthermore, membranes provide a natural "habitat" for a significant number of relatively non-polar compounds generated during metabolic processes. Many Proteins whose surfaces exhibit hydrophobic properties are embedded in membranes. Some proteins, such as cytochrome b5 (Chapter 10, Section B.5), feature specialized hydrophobic domains that anchor them to the membrane surface. Thanks to the semi-fluid state of the membrane's interior, proteins and low-molecular-weight components can freely move in and out of the bilayer in response to metabolic processes occurring in the adjacent Cytoplasm.

It has been suggested [36] that the structural stability of macromolecules and membranes is maintained primarily by hydrophobic interactions between hydrocarbon chains, enabling lipid, protein, and other molecules to form oligomeric aggregates and membranes within the aqueous cytoplasm. At the same time, the most active catalysts—namely, the majority of Enzymes—are Water-soluble. Thus, membranes function as relatively stable, thin films adjacent to the aqueous compartments of The Cell, where Chemical Reactions proceed readily and water-soluble polar molecules are contained.

It should also be noted that the Stability of the membrane surface facilitates closer contact between reactants and accelerates sequential biochemical reactions. Consequently, metabolic activity is highest at the boundary between the membrane and the cytoplasm.

Despite their relative stability, membrane components are not chemically inert. They undergo metabolic transformations mediated by oxidative enzymes localized within or On the surface of membranes. Membranes also harbor Quinones and other low-molecular-weight catalysts. Oxidation reactions play a crucial role in modifying the hydrophobic components of membranes. For instance, sterols, Prostaglandins, and other regulatory substances are initially synthesized as hydrophobic chains bound to water-soluble carriers (Chapter 12). Hydrophobic biosynthetic products can accumulate in membranes (for example, polyunsaturated Fatty acids derived from Phospholipids serve as precursors for prostaglandins). However, upon reacting with oxygen, hydroxyl groups are introduced into these molecules, leading to a gradual increase in their water solubility. As the hydrophilicity of a compound increases through successive hydroxylation, the hydrophobic Components of the membrane inevitably pass into aqueous solution and become fully integrated into METABOLISM. Another process leading to the active breakdown of Membrane Lipids is Hydrolysis catalyzed by phospholipases.