The electron transport chain of a eukaryotic cell is located in the

Explanation

The cytochromes and other carriers are embedded in the inner membrane, whose folding into cristae provides the surface area, and the same membrane is impermeable enough to hold the proton gradient that chemiosmosis depends on. The outer membrane is freely permeable to small molecules and so could never maintain a gradient. In prokaryotes the equivalent chain sits on the plasma membrane.