ATP synthetase is located in the _ of the mitochondrion.

Explanation

The inner mitochondrial membrane is the site where ATP synthase is located. This membrane is impermeable to most ions and molecules and plays a crucial role in establishing the proton gradient necessary for ATP synthesis. The electron transport chain, which is part of oxidative phosphorylation, takes place on the inner membrane. As electrons pass through the electron transport chain, protons are pumped from the matrix to the intermembrane space, creating a proton gradient. ATP synthase is embedded in the inner membrane and utilizes this proton gradient to convert ADP (adenosine diphosphate) and inorganic phosphate (Pi) into ATP.