Where is ATP synthase located in the mitochondrion?

A. Cytosol

B. Electron transport chain

C. Outer membrane

D. Inner membrane

E. Mitochondrial matrix

D. Inner membrane ...

In mitochondria ATP synthase is located in the inner membrane, the hydrophilic catalytic F1 portion is sticking into matrix. In a way a mitochondrion is a bacterium "swallowed" by the eukaryotic cell: then the inner mitochondrial membrane corresponds to the bacterial cell membrane.

Explanation:

ATP synthase is an enzyme that creates the energy storage molecule adenosine triphosphate (ATP). ATP is the most commonly used "energy currency" of cells for all organisms. It is found from adenosine diphosphate (ADP) and inorganic phosphate (Pi). The overall reaction catalyzed by ATP synthase is:

ADP + Pi + 3H+out ⇌ ATP + H2O + 3H+in