Energy-producing organelles are called mitochondria. Animal cells normally have 1000 to 2000 mitochondria. Cells with higher energy needs, such as fat and muscle cells, have more. Mitochondria are capsule-shaped organelles measuring between 1 and 10 micrometers in length.
Mitochondria are made up of two membranes. The outer layer allows small molecules, such as nutrients, ATP, oxygen and carbon dioxide, to pass through to the center. The inner membrane consists of folds called cristae. It is less permeable than the outer membrane, allowing only oxygen, carbon dioxide and water through. Mitochondria make energy in the form of adenosine triphosphate, or ATP, in a process called cellular respiration. ATP supports aerobic respiration, which uses oxygen to power cells.