The right answer is Mitochondria.
Mitochondria are limited by two membranes with very different properties:
* The outer membrane is low in protein and contains a transmembrane protein, porin, which allows the passage of ions and water-soluble metabolites of molar mass <10,000 Da.
* In contrast, the inner membrane is very rich in proteins but it is almost impermeable to ions and water-soluble metabolites. These substances can only cross the membrane using transport membrane proteins (ATP, ADP and Pi are transported by this type of protein) or by more sophisticated mechanisms called shuttle.
* The space between these two membranes is called the intermembrane space.
