The intermembrane space is the region between the inner membrane and the outer membrane of a mitochondrion or a chloroplast. Its main function is nucleotide phosphorylation . Channel proteins called porins in the outer membrane allow free movement of ions and small molecules into the intermembrane space. This means that it is essentially continuous with the cytosol in terms of solutes relevent for the functioning of these organelles. Enzymes destined for the mitochondrial matrix or the stroma can bypass the intermembrane space via transport through translocases . These are known as TOM and TIM in mitochondria and TOC and TIC in chloroplasts. It tends to have a low pH because of the proton gradient which results when protons are pumped from the mitochondrial matrix into the intermembrane space during electron transport. The structures responsible for this are coenzyme q , complex I , complex II , and complex III .