Advanced Placement (AP) Biology Practice Exam

Phospholipids are characterized as amphipathic molecules, which means they have both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions. This dual nature is key to their role in the formation of the lipid bilayer structure found in biological membranes.

In an aqueous environment, the hydrophilic heads of the phospholipids face outward toward the water, while the hydrophobic tails point inward, away from the water. This arrangement minimizes the exposure of the hydrophobic tails to water, thereby stabilizing the membrane structure. The result is a bilayer that serves as a barrier, separating the interior of the cell from the external environment, and it allows for the compartmentalization of cellular processes.

The other properties mentioned do not contribute to this bilayer formation. For instance, if phospholipids were all hydrophilic, they would not form a bilayer because there would be no hydrophobic interactions to drive the formation. The notion of being rigid and impermeable does not apply to phospholipids, as they need to be fluid to allow for the movement and function of membrane proteins. Lastly, while cholesterol is important for membrane fluidity, it is not a property of phosph