Advanced Placement (AP) Biology Practice Exam

Which statement describes the behavior of hydrophobic substances in water?

1. They dissolve easily in water.

2. They do not form hydrogen bonds.

3. They have a strong affinity for water.

4. They are polar molecules.

The correct answer is: They do not form hydrogen bonds.

The behavior of hydrophobic substances in water is accurately described by the statement that they do not form hydrogen bonds. Hydrophobic substances, which are typically nonpolar, do not interact favorably with water molecules, which are polar and capable of forming hydrogen bonds. This interaction is crucial in understanding why hydrophobic substances tend to separate from water rather than dissolve in it. Since water molecules are more attracted to each other than to hydrophobic molecules, this results in an exclusion effect. Instead of incorporating hydrophobic substances into the water solution, water molecules will remain preferentially associated with each other. This characteristic also explains why many biological membranes, composed primarily of phospholipids, form bilayers; the hydrophobic tails face inward away from the water, while the hydrophilic heads interact with the aqueous environment, maintaining cellular structure and function. Understanding this concept is fundamental in the study of molecular interactions in biological systems.