Mitochondria are membrane-bound organelles found in most eukaryotic cells that generate energy through oxidative phosphorylation. They contain their own DNA and ribosomes. Mitochondria have an outer and inner membrane, and contain an intermembrane space and matrix. The inner membrane is highly folded into cristae to increase surface area. Mitochondria play a key role in cellular respiration by producing ATP from nutrients, and are often described as the "powerhouses" of the cell.