Membranes of different origin differ also in their lipid compositions Different proteins cause different functions of membranes. Some enzyme activities can be used as markers for certain types of membranes. The orientation of some proteins is important, too. Different possibilities to fix integrated membrane proteins within the membrane . The N-terminus is always at the side that is turned away from the cytosol.
Membranes have numerous functions.
Permeability for one type of molecule and impermeability for another belong to the properties that hint at the presence of a membrane. All biological membranes are selectively permeable. Membranes separate reaction areas that can be distinguished due to their chemical composition, their pH value and their electric potential. Both the pH and the electric gradient can be measured . This transport is energy-consuming.