The intermembrane space (IMS) is the region between the inner membrane and the outer membrane of a mitochondrion or a chloroplast. The main function of mitochondrial intermembrane space is oxidative phosphorylation. Channel proteins called porins in the outer membrane allow free movement of ions and small molecules into the intermembrane space. This does not mean that it is essentially continuous with the cytosol in terms of the solutes relevant for the functioning of these organelles. Enzymes destined for the mitochondrial matrix or the stroma can pass through the intermembrane space via transport through 1s. These are known as translocase of the outer mitochondria membrane ( TOM) and translocase of the inner mitochondrial membrane ( TIM) in mitochondria and translocase of the outer chloroplast membrane (TOC) and translocase of the inner chloroplast membrane (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, NADH coenzyme Q oxidoreductase complex ( complex I), succinate-coenzyme Q oxidoreductase complex ( complex II), and coenzyme Q- cytochrome c oxidoreductase complex ( complex III).