Advanced Placement (AP) Biology Practice Exam

How do membranes become semipermeable to polar molecules?

1. Through lipid diffusion

2. By active transport

3. Via protein channels

4. Through endocytosis

The correct answer is: Via protein channels

Membranes achieve semipermeability to polar molecules primarily through protein channels. These protein structures embedded in the lipid bilayer create specific pathways that allow the passage of polar substances that cannot freely diffuse through the hydrophobic core of the membrane. Unlike small nonpolar molecules that can move through the lipid bilayer easily, polar molecules, due to their charge and hydrophilicity, require assistance to cross the membrane. Protein channels are selective, meaning they can facilitate the transport of certain ions and polar molecules while excluding others, effectively maintaining the semipermeable nature of the membrane. This transport can occur passively, where molecules move down their concentration gradient without energy expenditure, or can also be involved in more complex processes that may include signaling or regulation of substance flow. By utilizing these protein channels, the cell is able to regulate its internal environment by controlling the entry and exit of specific polar molecules crucial for various biological functions.