STAGE 4: ELECTRON TRANSPORT AND CHEMIOSMOSIS (Figs – P ) NADH and FADH 2 eventually pass H-atom electrons to the electron transport chain (ETC) which is composed of (mostly) proteins imbedded in the inner mitochondrial membrane the ETC is arranged in order of increasing electronegativity, each component alternately reduced (by pulling electron pairs away from the component before it in the chain) and oxidized (by having electron pairs pulled away by components after it in the chain)

ultimately, cytochrome oxidase catalyzes the reaction between the electron pair, a pair of H+ protons, and molecular oxygen, to produce a molecule of water in the matrix release of free energy as electron pairs move successively closer to associated nuclei is used to pump H+ protons from the matrix, through each of three imbedded protein complexes, to the intermembrane space as each e- pair crosses the inner membrane, chemical potential energy is converted to electrochemical potential energy which creates an electrochemical gradient across the membrane that is used to power ATP synthesis in chemiosmosis u/animations/etc/m ovie-flash.htm

NADH and FADH 2 transfer electron pairs to the ETC in different ways NADH passes its electrons to the first protein complex (NADH dehydrogenase) FADH 2 bypasses the first component, passes its electrons to the second component (ubiquinone = Q), thus only pumping 2 H + protons (= 2 ATP) vs. 3 for NADH (= 3 ATP) NADH produced from glycolysis in the cytoplasm (cytosolic NADH) is able to cross the outer mitochondrial membrane into the intermembrane space, but is unable to cross the inner membrane into the matrix (to access the ETC) 2 separate shuttle systems transport electron pairs from cytosolic NADH in the intermembrane space to either FAD or NAD+ in the matrix: glycerol-phosphate shuttle FAD reduced to FADH2 2 ATP via chemiosmosis aspartate shuttle NAD+ reduced to NADH 3 ATP via chemiosmosis oxidized coenzymes (NAD+ and FAD) are recycled to pick up more electron pairs in glycolysis, pyruvate oxidation, or the Krebs cycle

H + protons are forced to pass through proton channels imbedded in the inner mitochondrial membrane associated with the enzyme ATP synthase ATP synthase catalyzes the synthesis of ATP from ADP and P I in the matrix, powered by the reduction in the proton-motive force (PMF = free energy of the electrochemical gradient) as the H + proton passes through the ATPase complex Chemiosmosis and Oxidative ATP Synthesis the intermembrane space becomes a H+ proton reservoir as the inner mitochondrial membrane is (virtually) impermeable to them electrochemical gradient creates a potential difference across the inner mitochondrial membrane radient/movie-flash.htm

An Overview of Oxidative Phosphorylation (Fig. 23, P. 108) the success of ATP synthesis depends on a continual supply of H + protons, which is dependent on the continual movement of electron pairs through the ETC, which is dependent on a continual supply of oxygen as the final electron acceptor if the last protein is not freed up, the chain becomes clogged with stationary electrons H + protons cannot be pumped into the intermembrane space, and NADH and FADH 2 are unable to give up their electron pairs to the ETC chemiosmosis stops, and no more electrons can be removed from glucose at the other end ATP synthesis grinds to a halt ATP molecules are transported through both mitochondrial membranes by facilitated diffusion into the cytoplasm where they are used to drive endergonic processes

ATP synthesis is coupled with electron transport, and both are dependent on the availability of electrons (from glucose) and a final electron acceptor (oxygen) electrons flow downhill in oxidative phosphorylation The Energetics of Oxidative Phosphorylation (Fig. 24) the formation of water at the end of the ETC differs from process that forms water as a result of the combustion of H 2 gas, as glucose is the source of hydrogen and the process is mediated by enzymes that remove electrons in steps to progressively more electronegative substances, capturing much of the released free energy in ATP and ultimately turning them over to oxygen in an already relatively stable state for their reunion with H + protons

The Aerobic Respiration Energy Balance Sheet 1.the inner mitochondrial membrane is not completely impermeable to H + protons, reducing the number that go through the ATPase complex to synthesize ATP 2.some of the H + protons in the intermembrane space are used for other energy- requiring activities the actual yield is 30 ATP/glucose molecule 30 x 31kJ/mol ATP 2870 kJ/mol glucose = 32% efficiency the theoretical yield of 36 ATP is not actually achieved because:

Controlling Aerobic Respiration (Fig. 28) ATP inhibits the enzyme phosphofructokinase in glycolysis while ADP activates it citrate accumulation in the Krebs cycle inhibits phosphofructokinase, while a deficit reduces inhibition and increases the rate of glycolysis a high concentration of NADH allosterically inhibits pyruvate dehydrogenase which reduces acetyl-coA, which reduces the production of NADP Chapter 2 Review: P , #1-10, 16, 17, 21, 26 Chapter 2 Self-Quiz – P. 133 aerobic respiration is regulated by various feedback inhibition and product activation loops