Organic Chemistry MCAT Practice Exam

How does the amphipathic nature of fatty acids influence their behavior in water?

1. They form aggregates

2. They dissolve freely

3. They precipitate out

4. They emit gas

The correct answer is: They form aggregates

The amphipathic nature of fatty acids plays a crucial role in how they interact with water. Fatty acids consist of a hydrophilic (water-attracting) carboxylic acid head and a hydrophobic (water-repelling) hydrocarbon tail. When fatty acids are introduced to water, their structure causes them to self-associate in a way that minimizes the unfavorable interactions between the hydrophobic tails and water. This leads to the formation of aggregates such as micelles or lipid bilayers, where the hydrophilic heads orient towards the water, while the hydrophobic tails are sequestered away from the water, facing inward. This behavior is energetically favorable and reduces the system's overall free energy. In contrast, options suggesting that fatty acids dissolve freely or precipitate out do not accurately reflect the amphipathic property's influence on their behavior in aqueous environments. Moreover, the idea of emitting gas is unrelated to the fundamental chemistry of fatty acids in water. The formation of aggregates is thus the defining behavior of fatty acids when they are introduced into an aqueous solution.