electron transport chain

electron transport chainA Duracell battery powers a simple circuit consisting of a wire, a small electric toy motor and the battery itself. The electrodes of the alkaline “Copper Top” battery correspond to zinc (negative terminal, or reluctant) and MnO2(positive terminal, or oxidant) which produces and EO value of 1.5 volts. The mitochondria are responsible for energizing eukaryotic organisms by performing oxidative phosphorylation through the electron transport chain. Oxidative phosphorylation describes the movement of electrons from chemical energy molecules, NADH and FADH2 to oxygen. The electron transport chain can be thought of as analogous to the circuit in a Duracell battery. The diagram below shows a cartoon depiction of the path of electrons accepted from NADH and FADH2 until it is accepted by the oxygen molecule, O2.

The system is somewhat complex because there are several proteins that act as proton pumps to generate a concentration gradient, very similar to the light reactions of photosynthesis. However before understanding the system on a molecular scale, we had already been using a similar model for many years in our batteries. A battery is composed of a wire linking together two terminals: positive and negative. The circuit is completed by a chemical reaction in which the negative battery terminal moves energy in the form of current to the positive terminal. A battery only lasts so long because eventually the reactants in the negative terminal are used up. This isn’t the case in the electron transport chain because our body consistently consumes food and generates new chemical energy in form of NADH and FADH2 via the Krebs Cycle. Additionally our positive terminal always exists because O2 is abundant in the atmosphere and we continuously breathe it in. The wire in the battery connecting to the two terminals can be thought of as the physical path of the electron in the mitochondrial membrane from the NADH or FADH2 to oxygen acceptor.

Typically we harness a battery by attaching it to a circuit to provide energy to some object. Only when the battery is connected is the circuit complete for the chemical reaction to take place. A battery can be used to power a toy motor for instance. Similarly the electron transport chain only can really function if it is powering a motor. The motor can be thought of as the membrane protein proton pumps.  If one were to block the electric motor in a circuit the motor could no longer turn and the current-flow would effectively stop. Similarly we can show this is the same for the electron transport chain. Using an inhibitor such as rotenone or antimycin we can effectively block a proton pump and also efficiently block the flow of electrons.

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