The electron transport system is a chain of electron acceptors embedded in the inner membrane of the mitochondrion. Hydrogens are passed from NADH to flavin mononucleotide (FMN), the first acceptor in the chain. As these hydrogens are transferred from one to another of the electron acceptor molecules, the hydrogen protons become separated from their electrons. When the hydrogen protons (H+) separate from their electrons, they are released into the surrounding medium. Hydrogens, or their electrons, pass down the electron transport chain in a series of redox reaction. The electrons entering the electron transport system have a relatively high energy content. As they pass along the chain of electron acceptors, they lose much of their energy, some of which is used to pump the protons across the inner mitochondrial membrane. This sets up an electrochemical gradient across the inner mitochondrial membrane; this gradient provides the energy for ATP synthesis. The electron acceptors in the electron transport chain include FMN, ubiquinone (CoQ), and a group of closely related proteins called cytochromes. Cytochrome molecules accept only the electron from each hydrogen, not the entire atom. The several types of cytochromes hold electrons at slightly different energy levels. Electrons are passed along from one cytochrome to the next in the chain, losing energy as they go. Finally, the last cytochrome in the chain, cytochrome a3, passes two electrons to molecular oxygen. Simultaneously, the electrons reunite with protons to form hydrogen, and the chemical union of the hydrogen and oxygen produces water. Thus, oxygen is the final hydrogen acceptor in the electron transport system, which explains why organisms that respire aerobically require oxygen. When no oxygen is available to accept the hydrogens, the last cytochrome in the chain is stuck with its electrons. When that occurs, each acceptor molecule in the chain remains stuck with electrons, and the entire system is halted all the way back to NADH. As a result, no further ATPs are produced by the way of the electron transport system. Most cells of complex organisms cannot live long without oxygen because the amount of energy they produce in its absence is insufficient to sustain life processes. Lack of oxygen is not the only factor that interferes with the electron transport system. Some poison or drugs, including cyanide, inhibit the normal activity of the cytochrome system. Cyanide binds tightly to cytochrome a3 so that it cannot transport electrons on to oxygen. This blocks the further passage of electrons through the chain, halting ATP production and life.