Biochemistry and Molecular Biology - Belyasova N.A. 2002

Structure and Functions of Cellular Components
Biomembranes

Introduction/36.html">Biological Membranes include the Plasma Membranes of Cells, the nuclear membrane, the membranes of the Endoplasmic reticulum and Golgi apparatus, lysosomal, peroxisomal, mitochondrial, and chloroplast membranes, viral supercapsids, as well as several specialized membranes found in specific organisms. All of them share a classical Structure consisting of a lipid bilayer embedded with protein molecules. Because their fundamental Organization is largely identical, these membranes—despite belonging to diverse organisms and Organelles—can be studied within a unified framework.

Biological membranes perform such a vast array of vital Functions essential to living organisms that their emergence can be considered a decisive milestone in THE ORIGIN OF life. The primary functions of membranes include the following:

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

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

3) cell surface individuality: membranes define the unique character of cell surfaces, which is particularly important for Multicellular animals that lack cell walls, enabling cells to interact with one another and form Tissues and Organs;

4) energy storage: essential energy-storing processes, such as Oxidative Phosphorylation AND Photophosphorylation, can only take place on membranes;

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

6) signal generation and reception: the GENERATION AND PROPAGATION of nerve impulses are exclusively membrane-dependent processes, and biomembranes possess receptor functions. The operation of highly developed Sensory Organs in higher animals and humans is fundamentally based on membrane properties;

7) core cellular processes: many vital cellular functions strictly require the involvement of membranes, most notably Protein Synthesis, METABOLISM/36.html">DNA Replication, protein modification and secretion, and Metabolism regulation driven by hormonal responses.

Biomembranes are natural films 5–7 nm thick, composed primarily of Lipids—which predominantly serve a structural role—and Proteins, which account for the diverse types of membranes and the uniqueness of their properties. In addition, membranes contain CARBOHYDRATES, which do not exist in a free state but are part of complex compounds such as Glycolipids and Glycoproteins. Several models have been proposed to explain the organization of biomembranes, of which the fluid-mosaic model, introduced by S.J. Singer and G.L. Nicolson in 1971, is currently the universally accepted paradigm. According to this model, the foundation of the membrane is a fluid lipid bilayer in which the fatty acid residues of Phospholipids are in a liquid-crystalline state. Protein molecules are embedded and integrated into the bilayer and are likewise capable of lateral movement. Since its inception, the Singer-Nicolson model has undergone certain modifications and refinements, yet its conceptual foundation remains unchanged.

The structure of biomembranes is studied primarily using freeze-fracture Electron Microscopy and X-Ray Diffraction Analysis.