The Krebs cycle and electron transport chain take place in the inner membrane of the mitochondria, an organelle in the cell cytoplasm.
The mitochondria consists of an outer membrane, an inner membrane and a gel-like material called the matrix. The outer membrane covers the organelle like a skin. The inner membrane folds over many times and creates layered structures called cristae. It is where the Krebs cycle and the electron transport chain take place.
The folding of the inner membrane serve to increase surface area inside the mitochondria.
