5.4 Mitochondria
Mitochondria are the sites of aerobic respiration. They release most of the cell's energy as ATP, which is why they are often called the powerhouses of the cell.
Each mitochondrion is bounded by two membranes: a smooth outer membrane and a highly folded inner membrane. The inner membrane folds into cristae, which increase the surface area available for the electron transport chain.
The matrix contains the enzymes of the Krebs cycle, along with circular DNA and 70S ribosomes. This allows mitochondria to make some of their own proteins.
Because their DNA and ribosomes resemble those of bacteria, most biologists think mitochondria were once free-living bacteria that were taken in by a larger cell. This idea is called the endosymbiotic theory.
The electron transport chain on the cristae produces most of the ATP made during respiration.
5.5 Where mitochondria are found
Very active cells, such as muscle cells and sperm, contain many mitochondria. Mature red blood cells have none.
The number of mitochondria in a cell can change. Regular endurance training, for example, increases the number of mitochondria in skeletal muscle cells.
| Cell type | Mitochondria |
|---|---|
| Muscle cell | Many |
| Sperm cell | Many, in the midpiece |
| Liver cell | Up to 2,000 |
| Red blood cell | None |