And what this basically means is for the electron transport chain to actually take place and take place effectively and efficiently inside the matrix of the mitochondria we must have a high enough level of ADP molecules because the electron transport chain more specifically ATP synthase of the electron transport chain actually generates those ATP molecules by using the ADP molecules. So for the electron transport chain to be effective, ATP levels in the mitochondrial membrane must be appropriately high to ensure that complex five ATP synthase of the electron transport chain can actually use the proton motive force that is generated by complexes one, three and four to generate those ATP molecules. Now, it also actually means that once we form these ATP molecules, we don't want to keep those ATP molecules in the matrix of the mitochondria. We want to actually move those ATP molecules out of the matrix and into the cytoplasm of the cell so that the ATP can become readily available to actually power all the different types of biological processes that exist within the cytoplasm of the cell. Now, the problem with actually transporting ADP and ATP molecules across the inner mitochondrial membrane is the membrane is impermeable to these two molecules. Why?