The presence of cholesterol molecules in a biomembrane plays a significant role in modulating the surface tension and overall properties of the membrane. Biomembranes are primarily composed of a lipid bilayer, which consists of phospholipids, glycolipids, and cholesterol molecules. The interactions between these components determine the membrane's fluidity, permeability, and mechanical stability.Cholesterol molecules are amphipathic, meaning they have both hydrophilic water-loving and hydrophobic water-repelling regions. In a biomembrane, the hydroxyl group of cholesterol interacts with the polar head groups of phospholipids, while the hydrophobic steroid ring structure intercalates between the fatty acid chains of the phospholipids.The presence of cholesterol affects the surface tension of a biomembrane in the following ways:1. Condensing effect: Cholesterol molecules can fill the gaps between phospholipids, leading to a more tightly packed lipid bilayer. This condensing effect increases the surface tension of the membrane, making it less fluid and more mechanically stable.2. Modulation of membrane fluidity: Cholesterol can both increase and decrease membrane fluidity, depending on the lipid composition and temperature. At high temperatures, cholesterol restricts the movement of phospholipids, reducing fluidity and increasing surface tension. At low temperatures, cholesterol prevents the fatty acid chains of phospholipids from crystallizing and becoming too rigid, thus maintaining fluidity and reducing surface tension.3. Decreased permeability: The presence of cholesterol in the biomembrane reduces the permeability of small polar molecules and ions, as it creates a more tightly packed lipid bilayer with higher surface tension. This property is essential for maintaining the integrity of the cell and its selective permeability.4. Domain formation: Cholesterol can promote the formation of lipid rafts or microdomains within the biomembrane. These domains are enriched in cholesterol and specific types of phospholipids, leading to regions with distinct surface tension and properties compared to the surrounding membrane.In summary, the presence of cholesterol molecules in a biomembrane significantly affects its surface tension by modulating membrane fluidity, permeability, mechanical stability, and domain formation. These properties are essential for maintaining the proper function and integrity of the cell.