USA Biology Olympiad (USABO) Practice Exam

What characterizes an amphipathic molecule?

1. Two hydrophilic ends

2. A hydrophilic end and a hydrophobic end

3. Complete hydrophobic nature

4. No polarity

The correct answer is: A hydrophilic end and a hydrophobic end

An amphipathic molecule is characterized by having both a hydrophilic (water-attracting) end and a hydrophobic (water-repelling) end. This unique structure allows amphipathic molecules to interact with both polar and nonpolar substances, which is crucial in biological systems. For instance, phospholipids, which are key components of cell membranes, have a hydrophilic "head" and hydrophobic "tails." This amphipathic nature enables them to form bilayers, with the hydrophilic heads facing the aqueous environment and the hydrophobic tails shielded from water. This property plays a vital role in the formation of cellular membranes, influencing the behavior of molecules in aqueous environments, and facilitating processes like emulsification and micelle formation. In contrast, the other options describe properties that do not represent an amphipathic structure. Molecules with two hydrophilic ends lack the hydrophobic component essential for amphipathicity, while completely hydrophobic molecules do not interact favorably with water, and non-polar molecules are devoid of the characteristics required for amphipathic behavior.