Select the statement that describes why energy generated from the catabolism of sugars and other macromolecules is ultimately harnessed to generate ATP.

a. ATP is required to generate the proton gradient in the intermembrane space of mitochondria
b. ATP can be used by cells to drive endergonic reactions
c. ATP can be used to make RNA, which is an energy storage molecule in the cell
d. ATP synthesis is an exergonic reaction