Mitochondria

Without special staining, mitochondria are hard to make out under the microscope. How many a cell holds depends on how physiologically active that cell is.

They are typically sausage-shaped or cylindrical, about 0.2–1.0 μm in diameter (average 0.5 μm) and 1.0–4.1 μm long, though shape and size vary a lot.

A mitochondrion has two membranes, which split its interior into two aqueous compartments, an outer one and an inner one; the inner compartment holds the matrix, a dense and uniform substance.

The outer membrane forms a continuous boundary. The inner membrane folds into the matrix as cristae, which greatly increase its surface area.

Each membrane carries its own specific enzymes linked to mitochondrial function.

Mitochondria are the sites of aerobic respiration and make energy in the form of ATP, hence the name 'power houses' of the cell.

Inside the matrix are 70S ribosomes, one circular DNA molecule, a few RNA molecules and the components required for protein synthesis.

Mitochondria divide by fission.