And so what happened is these mitochondrial porins found on the outer membrane basically allowed the movement of anion molecules. So small molecules, which are anions. So things like chloride ions, things like ATP molecules, can actually move across the outer mitochondrial membrane from the cytoplasm into the intermembrane space without much difficulty. But the inner membrane doesn't actually contain these mitochondrial porns. So that means it is impermeable to charged molecules and it's impermeable to polar molecules. So molecules such as citrate molecules or pyruvate molecules or ATP molecules actually depend on special protein membranes or membrane proteins to actually shuttle those molecules across the matrix, across the matrix and into the intermembrane space.