Final answer:
The hydroxyl head of cholesterol, which is the hydrophilic -OH group, interacts with the polar heads of fatty acids in the biological membrane, aiding in membrane structure and fluidity.
Step-by-step explanation:
The hydroxyl head of cholesterol is defined as the -OH group, which is the hydrophilic part of the molecule. This contrasts with the hydrophobic tail of the cholesterol, which consists of a tetracyclic ring structure and an alkyl side chain that avoids water. Within the context of a fatty acid, it's important to understand that the hydroxyl head of cholesterol can interact with the polar heads of fatty acids in a biological membrane.
Fatty acids typically have a hydrophilic carboxylic acid group (-COOH) at one end and a hydrophobic alkyl chain. The interactions that occur between cholesterol and fatty acids mainly involve the polar, hydrophilic components aligning with one another, because like dissolves like. Therefore, the hydroxyl head of cholesterol would be in proximity to and interact with the carboxylic acid group of fatty acids in the membrane, aiding the membrane's structural integrity and fluidity.
Cholesterol's amphiphilic nature allows it to embed within the lipid bilayer of cell membranes. Here, the hydroxyl group interacts with the polar heads of phospholipids and other lipids such as fatty acids and helps to regulate the fluidity and permeability of the membrane.