Cell Structure and Function

Most questions about mitochondria are really questions about where something sits. A mitochondrion is bound by two membranes, the outer smooth and the inner folded inward into the matrix as cristae, with small knob-like $F_1$ particles on the inner surface of the cristae facing the matrix; one item asked where those particles sit and keyed the inner membrane, not the outer one. The matrix holds the enzymes that run the Krebs cycle, aerobic respiration and fatty acid metabolism, banking the energy of food as ATP.

The geography matters more than the vocabulary: oxidative phosphorylation runs in the cristae, photophosphorylation in the thylakoid membrane of chloroplasts, and substrate level phosphorylation is the third route, all forming ATP from ADP and inorganic phosphate at $7.3$ kcal per mole. Mitochondria also carry their own DNA and ribosomes, which makes them semi-autonomous and self-replicating, and that is why the endosymbiotic theory describes them, and chloroplasts, as ancient bacteria living inside larger cells. The chloroplast is the plant counterpart, double-membrane and discoid, with a fluid stroma and thylakoids.