Advanced Placement (AP) Biology Practice Exam

How do phospholipids behave in an aqueous environment?

1. They dissolve completely

2. They form a bilayer structure

3. They float on the surface

4. They remain dispersed as individual molecules

The correct answer is: They form a bilayer structure

Phospholipids behave in an aqueous environment by forming a bilayer structure. This behavior is primarily due to their amphipathic nature, which means that they possess both a hydrophilic (water-attracting) "head" and two hydrophobic (water-repelling) tails. When placed in water, the hydrophilic heads of the phospholipids orient themselves towards the water, while the hydrophobic tails move away from the water, seeking to minimize their exposure to the aqueous environment. This leads to the arrangement of phospholipids into a bilayer, where two layers of phospholipids are formed, with the hydrophobic tails facing inward, shielded from water, while the hydrophilic heads face outward towards the surrounding water. This bilayer structure is fundamental to the formation of cellular membranes, as it creates a barrier that separates the interior of the cell from the external environment, allowing for essential cellular functions and the maintenance of homeostasis. The fluidity of the bilayer also facilitates the movement of proteins and other molecules within the membrane, which is crucial for various cellular processes. By understanding this behavior, it is clear why the formation of a bilayer structure is the correct answer in describing how phosph