Mitochondria are potato-shaped objects floating around inside our cells. Each cell contains about 500 to 1,000 mitochondria, and each mitochondrion contains about 500 to 1,000 respiratory complexes, which produce the [[ATP]] our bodies need to survive. Because our muscle cells need more energy, they are more densely packed with mitochondria than fat cells, which need less energy. The rate at which our bodies produce ATP is our metabolic rate.