Electron Transport Chain & Chemiosmosis

Final stage of aerobic cellular respiration which occurs in the cristae of the mitochondria Involves a series of electron carriers and multi-enzyme complexes which oxidize and reduce the NADH and FADH2 that have entered the cristae from various stages in the metabolic process These redox reactions liberate energy and account for ~90% of the ATP in a cell The electrons enter the system as they are donated from NADH and FADH2 and are then transferred along the components of the ETC

The components of the ETC are arranged in order of increasing electronegativity; the weakest attractor of electrons (NADH dehydrogenase) at the beginning of the chain and the strongest (cytochrome oxidase) at the end Oxygen is the final electron accept and together with H becomes H2O

Complex I: NADH dehydrogenase Complex II: succinate dehydrogenase Complex III: cytochrome b-c1 complex Complex IV: cytochrome oxidase complex Electron Carriers: Q = ubiquinone c = cytochrome C c Q

RECALL: During glycolysis and Kreb’s Cycle – ATP molecules are produced through substrate level phosphorylation However….

In the ETC, production of ATP involves redox reactions Since oxygen must be present to accept the electrons at the end of the ETC and helps create ATP when ADP adds a Pi, the process is called oxidative phosphorylation

Electron transport process is highly exergonic At various points, energy is liberated from the electrons The free energy is used to move protons (H+ ions) from the matrix to the intermembrane space 3 proton pumps embedded in the membrane Creates an electrochemical gradient High number of H+ ions in the intermembrane space compared to the matrix This electrochemical gradient is used to power ATP synthesis via Chemiosmosis

Chemiosmosis Since there is a high [ ] of protons outside compared to inside they naturally want to diffuse in, but are too big The only way they can enter is through special protein channels associated with the enzyme ATP synthase The free energy stored in the electrochemical gradient produces a proton-motive force which pushes the protons through the ATP synthase complex This energy then drives ADP to bind with an inorganic phosphate in the matrix to create ATP

NADH and FADH2 do not transfer electrons in the same way NADH: gives up its 2 electrons to complex I (NADH dehydrogenase) and its oxidation pumps 3 protons into the intermembrane space FADH2: transfers its 2 electrons to Q, since its electronegativity is too high to pass it to NADH dehydrogenase. Therefore, its oxidation only pumps 2 protons into the intermembrane space Result: 3 ATP for every NADH 2 ATP for every FADH2

Important notes: NADH molecules produced in glycolysis cannot diffuse into the mitochondria a shuttle system called glycerol-phosphate shuttle helps to push them through with the help of ATP The continual production of ATP is dependent on the H+ reservoir. Requires the continual movement of electrons through the ETC which is, in turn, dependent on oxygen availability We need to constantly breathe to keep the electrochemical gradient Ultimately, without glucose, there would be no electrons in the first place – we must continuously eat!!!

Homework Section 4.2 p. 177-181