1.7: Mitochondria
Mitochondria are the major site of ATP and energy production in plants and animals. Numbers vary from 20 to 100,000 per cell and they vary in form and activity.
Mitochondria are double-membraned organelles (like the nucleus) so are said to be surrounded by an envelope. The outer membrane ( matrix ) is very elastic, the inner is folded many times ( christae ) and protrudes into the internal cavity.