Answer:
C. passive transport by facilitated diffusion with the help of transport proteins
Step-by-step explanation:
The electron transport chain (shown in the second animation) establishes a proton gradient across the inner mitochondrial membrane. ATP synthase uses the energy stored in the proton gradient to catalyze the synthesis of ATP. The molecular machinery involved in this process converts chemical energy to mechanical energy and provides a great example of the relationship between the structure and function of macromolecules.