The mitochondrion is a double membrane-bound organelle found in most eukaryotic organisms. Some cells in some multicellular organisms lack mitochondria. A number of unicellular organisms, such as microsporidia, parabasalids, and diplomonads, have reduced or transformed their mitochondria into other structures. An analogy for mitochondria is that the mitochondria is like a power plant, because they both produce energy. The matrix of a mitochondrion is the mitochondrion internal spaces enclosed by the inner membrane. Several of the steps in cellular respiration occur in the matrix due to its high concentration of enzymes. As previously mentioned, mitochondria contain two major membranes. The outer mitochondrial membrane fully surrounds the inner membrane, with a small intermembrane space in between. The outer membrane has many protein-based pores that are big enough to allow the passage of ions and molecules as large as a small protein.