Advanced Placement (AP) Biology Practice Exam

Passive transport is a process that allows substances to move across cell membranes without the necessity of energy input from the cell. This movement occurs along a concentration gradient, meaning substances will naturally move from areas of higher concentration to areas of lower concentration until equilibrium is reached. This reliance on concentration differences is a defining feature of passive transport, as it utilizes the intrinsic energy of the concentration gradient rather than cellular energy (like ATP) to drive the movement.

While it’s true that passive transport can involve various types of molecules, such as small nonpolar molecules or ions, it is not limited to just those classes. In fact, larger or charged molecules can also undergo passive transport through specific channels or facilitated diffusion, which further illustrates that passive transport is broadly applicable beyond just small nonpolar molecules. Thus, the description of passive transport as relying on concentration differences to facilitate movement captures the essence of this fundamental biological process.