Mitochondria are constructed in a way that maximizes output. It consists of two membranes. The outer membrane surrounds and protects the organelle the same way as skin does. Multiple folds in the inner membrane generate cristae, which are stacked structures. The mitochondria are surrounded by a fluid called the matrix. Mitochondria are unique because they have ribosomes and DNA, free to move about in the matrix. Another sort of structure that can regulate ion concentrations is granules. Cell biologists are still investigating granule activity.