Biochemistry and Molecular Biology - Belyasova N.A. 2002

Structure and Functions of Cellular Components
Biomembranes
Characteristic Properties of Biomembranes

Despite the fact that membranes vary significantly in the composition of their constituent components, several features of their Organization and properties can be identified and characterized that point to the structural universality of all Biomembranes.

Membrane fluidity. This property is driven by the constant motion of membrane components. Membranes are dynamic structures in which both Lipids and Proteins undergo various types of movement at different speeds. These properties characterize the liquid-crystalline state of the membrane. When membranes transition into a gel phase, their fluidity drops by two orders of magnitude, drastically disrupting the organization and proper functioning of membrane components (especially proteins). Therefore, living organisms strive to maintain their membranes in a liquid-crystalline state.

Membrane components exhibit the following types of movement:

1) rotational movement (around the longitudinal axis)—observed primarily in lipid molecules and occurring very rapidly, with a frequency of 10⁹—10¹⁰ s^-1. Rotational motion is also characteristic of most Membrane Proteins, albeit at a much lower speed, as proteins are larger and frequently form aggregates;

2) lateral diffusion (within the plane of the membrane). The lateral diffusion coefficient for lipids is 10^-9—10^-7 cm²-s^-1, meaning that in 1 s a phospholipid molecule makes from 1,000 to 100,000 jumps with a step size equal to its diameter (~ 1 nm). For proteins, The rate of lateral diffusion is restricted, which may be due to the large size of protein molecules, their association with other proteins, or their attachment to the Cytoskeleton;

3) migration of molecules from one side of the membrane to the other (flip-flop transition). This is the slowest mode of movement for membrane components, with a characteristic timescale of several days for Phospholipids. The reason for this slow movement lies in its energetic unfavorability—a lipid with a polar HEAD must cross the hydrophobic region of the bilayer. However, the flip-flop transition can be accelerated in the presence of certain integral proteins, by perturbations in the bilayer, or in specialized membranes. For instance, in The Endoplasmic reticulum membrane of rat Liver Cells, this rate can reach a few minutes. It is hypothesized that the increased rate of transmembrane lipid transfer is associated with the presence of molecules in the membrane that facilitate lipid Transport Across the bilayer.

For proteins in native membranes, the flip-flop transition has not yet been observed. It is believed that proteins, while residing within the plane of the bilayer, do not alter their topological orientation. They are inserted into the membrane in a strictly defined orientation (relative to both of its surfaces) and remain in that position throughout their lifespan.

Membrane Asymmetry. All natural Membranes are asymmetric, which is primarily driven by differences in the COMPOSITION OF THE fluid environments bathing them. For example, the outer surface of Cell/30.html">The Plasma Membrane contacts the surrounding aqueous solution, whereas its inner surface contacts The Cell interior. Furthermore, the membrane surfaces differ in their curvature: the outer surface is convex, while the inner surface is concave. Consequently, both lipid and protein molecules are distributed asymmetrically between the two layers of the bilayer. For instance, Glycolipids are always embedded in the outer leaflet of the bilayer, with their carbohydrate moieties facing the extracellular environment. In Erythrocyte membranes, phosphatidylcholine and sphingomyelin are predominantly located in the outer leaflet of the bilayer, whereas phosphatidylethanolamine and phosphatidylserine reside mostly in the inner leaflet. Integral and peripheral proteins exhibit an even greater degree of asymmetry within membranes.

One of the primary factors determining membrane asymmetry is the interaction between lipids and the cytoskeleton. Transmembrane asymmetry dictates the membrane's sensitivity to environmental changes on both of its sides and is critically important for cellular function.

A continuous redistribution of lipid molecules takes place between the internal membranes of eukaryotes—a process of intermembrane lipid transport facilitated by specific proteins. Phospholipid transfer occurs intensively among mitochondrial membranes, the endoplasmic reticulum, the plasma membrane, Lysosomes, and other Organelles. This phenomenon enables the cell to regulate the rates of processes taking place within membranes.