Mitochondria are organelles in eukaryotic cells that are about the size of a bacterial cell and have a double membrane. The outer membrane is smooth, while the inner membrane has many folds called cristae. The folded cristae increases the surface area for cellular respiration to cccur. The matrix is the space contained within the inner membrane. 10. In what type of cells are mitochondria found? 11. Why is it important that the cristae is foided? Crate ari ier mnane Outer membrane