23.2 Electron Transport and ATP The enzymes and electron carriers for electron transport are located along the inner membrane of the mitochondria. Learning Goal Describe the transfer of hydrogen ions and electrons in electron transport and the process of oxidative phosphorylation in ATP synthesis.

Electron Transport The reduced coenzymes NADH and FADH2 produced from glycolysis, oxidation of pyruvate, and the citric acid cycle are oxidized to provide the energy for the synthesis of ATP. In electron transport or the respiratory chain, hydrogen ions and electrons from NADH and FADH2 are passed from one electron acceptor or carrier to the next until they combine with oxygen to form H2O. energy released during electron transport is used to synthesize ATP from ADP and Pi during oxidative phosphorylation.

Glycolysis, Citric Acid Cycle Results

Electron Transport System In the electron transport system, there are five protein complexes, which are numbered I, II, III, IV, and V. two electron carriers, coenzyme Q and cytochrome c, attached to the inner membrane of the mitochondrion, carry electrons between these protein complexes bound to the inner membrane.

Electron Transport Chain In electron transport, the oxidation of NADH and FADH2 provides hydrogen ions and electrons that eventually react with oxygen to form water.

Complex I In complex I, electron transport begins when hydrogen ions and electrons are transferred from NADH to complex I. loss of hydrogen from NADH regenerates NAD+ to oxidize more substrates in oxidative pathways such as the citric acid cycle. hydrogen ions and electrons are transferred to the mobile electron carrier CoQ, forming CoQH2. CoQH2 carries electrons from complexes I and II to complex III.

Complex I, Electron Transfer During electron transfer, H+ ions are pumped through complex I into the intermembrane space, producing a reservoir of H+ (hydrogen ion gradient). for every two electrons that pass from NADH to CoQ, 4H+ are pumped across the mitochondrial membrane, producing a charge separation on opposite sides of the membrane.

Complex II Complex II consists of the enzyme succinate dehydrogenase from the citric acid cycle. In complex II, CoQ obtains hydrogen and electrons directly from FADH2. This produces CoQH2 and regenerates the oxidized coenzyme FAD, which becomes available to oxidize more substrates.

Complex III Complex II consists of the enzyme succinate dehydrogenase from the citric acid cycle. In complex II, CoQ obtains hydrogen and electrons directly from FADH2 and becomes CoQH2. two electrons are transferred from the mobile carrier CoQH2 to a series of iron-containing proteins called cytochromes. electrons are then transferred to two cytochrome c, which can move between complexes III and IV.

Complex III, Cytochrome c contains Fe3+/Fe2+, which is reduced to Fe2+ and oxidized to Fe3+. generates energy from electron transfer to pump 4H+ from the matrix into the intermembrane space, increasing the hydrogen ion gradient.

Complex IV At complex IV, four electrons from four cytochrome c are passed to other electron carriers. electrons combine with hydrogen ions and oxygen (O2) to form two molecules of water. energy is used to pump H+ from the mitochondrial matrix into the intermembrane space, further increasing the hydrogen ion gradient.

Oxidative Phosphorylation Energy is coupled with the production of ATP in a process called oxidative phosphorylation. In 1978, Peter Mitchell theorized about a chemiosmotic model, which links the energy from electron transport to a hydrogen ion gradient that drives the synthesis of ATP. allows complexes I, III, and IV to act as hydrogen ion pumps, producing a hydrogen ion gradient. equalizes pH and electrical charge between the matrix and intermembrane space that occurs when H+ must return to the matrix.

Oxidative Phosphorylation, ATP In the chemiosmotic model, H+ cannot move through the inner membrane but returns to the matrix by passing through a fifth protein complex in the inner membrane called ATP synthase (also called complex V). the flow of H+ from the intermembrane space through the ATP synthase generates energy that is used to synthesize ATP from ADP and Pi. This process of oxidative phosphorylation couples the energy from electron transport to the synthesis of ATP.

Electron Transport and ATP Synthesis When NADH enters electron transport at complex I, the energy transferred can be used to synthesize 2.5 ATP. When FADH2 enters electron transport at complex II, it provides energy for the synthesis of 1.5 ATP. Current research indicates that the oxidation of one NADH yields 2.5 ATP and one FADH2 yields 1.5 ATP.

Regulation of Electron Transport and Oxidative Phosphorylation is regulated by the availability of ADP, Pi, oxygen (O2), and NADH. decreases with low levels of any of these compounds and decreases the formation of ATP. When a cell is active and ATP is consumed rapidly, the elevated levels of ADP will activate the synthesis of ATP. The activity of electron transport is strongly dependent on the availability of ADP for ATP synthesis.

Study Check Match each with its function: CoQ cyt c A. a mobile carrier between complexes II and III carries electrons from complexes I and II to complex III C. accepts H and electrons from FADH2

Solution Match each with its function: CoQ cyt c A. a mobile carrier between complexes II and III Cyt C carries electrons from complexes I and II to complex III CoQ C. accepts H and electrons from FADH2 CoQ

Study Check Classify each as a product of the 1. CO2 A. citric acid cycle B. electron transport chain 2. FADH2 3. NAD+ 4. NADH 5. H2O

Solution 1. CO2 A. citric acid cycle 2. FADH2 A. citric acid cycle Classify each as a product of the 1. CO2 A. citric acid cycle 2. FADH2 A. citric acid cycle 3. NAD+ B. electron transport chain 4. NADH A. citric acid cycle 5. H2O B. electron transport chain