The infoldings of the inner mitochondrial membrane are called

Explanation

Cristae greatly increase the area available for the electron transport chain and the ATP synthase particles, so cells with a high energy demand such as cardiac muscle have mitochondria packed with them. The matrix is the fluid enclosed by that membrane and holds the Krebs cycle enzymes. Thylakoids and grana are the equivalent membrane stacks inside a chloroplast, not a mitochondrion.