Final answer:
ABC superfamily proteins function as ATP-dependent flippases, actively transporting lipophilic drugs out of mammalian cells, using ATP to move substances against their concentration gradient.
Step-by-step explanation:
The ABC superfamily proteins are thought to act as ATP-dependent flippases, which are proteins that facilitate the active transport of various substances across cellular membranes. A main function of these proteins is to transport lipophilic drugs out of mammalian cells.
Unlike passive transport mechanisms, such as diffusion or facilitated diffusion, active transport involves the use of ATP to move substances against their concentration gradient. These ABC transporters typically function to expel foreign substances, including lipophilic drugs, out of cells, thus contributing to the cellular defense mechanism against potentially harmful compounds. Therefore, the correct answer is A. lipophilic drugs out of mammalian cells.