Mitochondria are often described as the powerhouses of the cell because they generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of chemical energy. The organelle is composed of two membranes with distinct functions. The outer membrane is smooth and acts as a protective barrier, while the inner membrane is highly folded into structures called cristae. These folds are crucial because they significantly increase the surface area available for chemical reactions, specifically those involved in the electron transport chain. Within the inner membrane is the matrix, which contains ribosomes, mitochondrial DNA, and enzymes necessary for the citric acid cycle. The efficiency of ATP production is directly related to the surface area provided by the cristae, allowing for a high density of the proteins required for energy conversion.