The ion gradients maintained across the plasma membranes of most cells play a significant role in cellular energetics. Ion gradients are often either generated by the hydrolysis of ATP or used to make ATP by the phosphorylation of ADP.

a) cite an example in which ATP is used to generate and maintain an ion gradient. What is another way that an iron gradient can be generated and maintained?

b) Cite an example in which an ion gradient is used to make ATP. What is another use for ion gradients?