Biology CLEP Prep Practice Exam

Cells are able to exchange materials through active transport because it requires energy (usually in the form of ATP) to move molecules across a membrane against their concentration gradient. This process is necessary for cells to obtain nutrients, get rid of waste products, and maintain homeostasis. Diffusion and osmosis, on the other hand, are passive processes that do not require energy. While diffusion does play a role in the movement of molecules across a membrane, it is not the primary mechanism. Similarly, while cells do synthesize proteins, this is not the main way in which they exchange materials. Thus, the most correct answer is A, as it describes the main mechanism by which cells are able to exchange materials.