Biochemistry and Molecular Biology - Belyasova N.A. 2002

Structure and Functions of Cellular Components
Biomembranes

Introduction/36.html">Biological Membranes include Plasma Membranes of Cells, the nuclear envelope, membranes of the Endoplasmic reticulum and Golgi apparatus, lysosomal, peroxisomal, mitochondrial, and chloroplast membranes, viral supercapsids, as well as certain specialized membranes of individual organisms. All of them share a classical Structure—a lipid bilayer interspersed with protein molecules—and it can be said that their fundamental Organization is uniform, which makes it possible to examine these membranes, belonging to various organisms and Organelles, within a general framework.

Biological membranes perform such a vast array of Functions essential to living organisms that their appearance can be considered a decisive milestone in THE ORIGIN OF life. The Main Functions of membranes are as follows:

1) limiting function: biological membranes enclose all prokaryotic and Eukaryotic cells, thereby separating the living from the non-living or individual cells within a multicellular Organism. Membranes also confine the contents of individual organelles in eukaryotic cells, enabling specific processes to take place within each of them;

2) barrier function: biological membranes act as a highly selective barrier for most substances attempting to enter or leave a Cell (or organelle);

3) cell surface individuality: membranes determine the individuality of cell surfaces, which is particularly crucial for Multicellular animals lacking cell walls, as this allows cells to interact with one another and form Organs and Tissues;

4) energy storage: such indispensable processes for many cells as Oxidative Phosphorylation AND Photophosphorylation can only take place on membranes;

5) enzymatic regulation: many enzymatic reactions occur exclusively within membranes, and biological membranes modulate The activity of numerous Enzymes;

6) signaling and reception: the GENERATION AND PROPAGATION of nerve impulses are only possible on membranes, and biological membranes possess receptor functions. The operation of highly developed Sensory Organs in higher animals and humans relies directly on membrane properties;

7) Metabolic Regulation: many functions vital to The Cell require the obligatory participation of membranes. These primarily include Protein Synthesis, METABOLISM/36.html">DNA Replication, protein modification and secretion, and metabolic regulation based on hormonal responses.

Biological membranes are natural films 5–7 nm thick, composed mainly of Lipids—which primarily fulfill a structural function—and Proteins, which determine The Diversity of membranes and the uniqueness of their properties. In addition, membranes contain CARBOHYDRATES, which are present not in a free state, but as components of complex compounds: Glycolipids and Glycoproteins. To explain the organization of biological membranes, several models have been proposed, of which the fluid-mosaic model by S.J. Singer and G.L. Nicolson, put forward in 1971, is currently universally accepted. According to this model, The basis of membranes is a fluid lipid bilayer in which the fatty acid residues of Phospholipids are in a liquid-crystalline state. Protein molecules capable of movement are embedded and integrated into the bilayer. Since its inception, the Singer-Nicolson model has undergone certain modifications and refinements, but its conceptual foundation remains unchanged.

To study The structure of biological membranes, researchers predominantly use Electron Microscopy of freeze-fracture preparations and X-Ray Diffraction Methods.