In which part of the mitochondria does the electron transport chain occur?

The electron transport chain occurs in the inner membrane of the mitochondria. This inner membrane is highly specialized and contains the necessary protein complexes and enzymes required for the chain's function. It is here that electrons are transferred through various carriers, ultimately leading to the pumping of protons from the mitochondrial matrix into the intermembrane space. This creates a proton gradient that is crucial for ATP synthesis through chemiosmosis, as protons flow back into the matrix through ATP synthase.

The architecture of the inner membrane, including its folds known as cristae, increases the surface area available for these reactions, making the process more efficient. The other parts of the mitochondria do not host the electron transport chain; for instance, the cytoplasm is where glycolysis occurs, the mitochondrial matrix contains enzymes for the Krebs cycle, and the outer membrane primarily serves as a barrier and has various porins that allow the passage of molecules.