A Level Biology Practice Exam

What structure do phospholipids form in cell membranes?

1. A lipid monolayer

2. A lipid bilayer

3. A lipid trilayer

4. A carbohydrate layer

The correct answer is: A lipid bilayer

Phospholipids form a lipid bilayer in cell membranes due to their unique amphipathic nature. Each phospholipid molecule consists of a hydrophilic (water-attracting) "head" and two hydrophobic (water-repelling) "tails." When phospholipids are placed in an aqueous environment, they spontaneously arrange themselves in such a way that the hydrophilic heads face outward toward the water, while the hydrophobic tails face inward, away from the water. This arrangement creates a bilayer structure, which serves as the fundamental framework of biological membranes. The bilayer allows for the formation of a semi-permeable membrane, enabling various substances to pass in and out of the cell while maintaining the internal environment. This structural configuration is critical for cellular organization, signaling, and function.