Advanced Placement (AP) Biology Practice Exam

What type of molecules are primarily transported through protein channels in the plasma membrane?

1. Lipid-soluble molecules

2. Large carbohydrate molecules

3. Polar and charged molecules

4. Small uncharged molecules

The correct answer is: Polar and charged molecules

Protein channels in the plasma membrane specifically facilitate the transport of polar and charged molecules. This is primarily due to the hydrophilic nature of the proteins that make up these channels, which allows them to interact positively with the polar and charged molecules while providing a pathway for these substances to cross the hydrophobic lipid bilayer of the membrane. Lipid-soluble molecules generally pass through the plasma membrane via simple diffusion, as they can easily navigate through the lipid bilayer without the need for assistance from protein channels. Large carbohydrate molecules, on the other hand, often require more complex mechanisms, such as endocytosis or transport through specialized proteins, since they are too large to pass easily through channels. Small uncharged molecules can also diffuse through the membrane, but to a lesser extent compared to polar and charged molecules that rely more heavily on channels for efficient transport. Thus, the role of protein channels is crucial for the transport of substances that cannot permeate the membrane freely, with polar and charged molecules being the primary candidates for this mode of transport.