What is Mitochondrion?

A mitochondrion is an organelle $0.5\text{ to }1\,\text{µ}\mathrm{m}$ across and a few micrometres long, bounded by two membranes: a smooth outer membrane permeable to small molecules, and an inner membrane folded into cristae that is impermeable to ions and carries the respiratory chain and the ATP synthase (Chapter 15). Inside is the matrix, holding the enzymes of the Krebs cycle, ribosomes of the bacterial type and several copies of a small circular DNA. Mitochondria divide by fission, move along the cytoskeleton, and number from a few hundred to a few thousand per cell.