The oxidation of reduced electron carriers in the respiratory chain drives the active transport of protons across the inner mitochondrial membrane, generating a proton-
learning outcomes
Describe how the proton motive force is established by the electron carriers and enzymes of the respiratory chain.
Analyze experimental results that relate to electron transport and chemiosmosis.
Predict results from experiments designed to explore aspects of electron transport and chemiosmosis.
How are protons transported from the mitochondrial matrix to the intermembrane space during electron transport?
A series of electron carriers in the inner mitochondrial membrane transports electrons by reduction–
How do the experiments described in Figures 9.9 and 9.10 demonstrate the chemiosmotic mechanism?
The experiment in Figure 9.9 shows that in the absence of electron transport, a gradient of protons across the membrane is sufficient to produce ATP if the ATP synthase is present in the membrane. The experiment in Figure 9.10 shows directly that ATP synthase can carry protons from a gradient, harnessing the potential energy from the gradient to make ATP.
Trace the sequence of changes in redox reactions that occur in mammalian tissue when the oxygen supply is cut off. The first change is that all of the cytochrome c becomes reduced, because electrons can still flow from cytochrome c, but there is no oxygen to accept electrons from cytochrome c oxidase. What happens after this?
If cytochrome c remains reduced and cannot accept electrons, the electron transport (respiratory) chain stays reduced and NADH and FADH2 remain reduced. This prevents oxidation reactions in the citric acid cycle and pyruvate oxidations, so pyruvate cannot be converted to acetyl CoA. Instead, pyruvate is converted to lactic acid, regenerating some NAD that can be used so that glycolysis can continue. Because the electron transport chain is not working, no proton gradient is set up in the mitochondria, and ATP is not made by oxidative phosphorylation.
The drug antimycin A blocks electron transport in mitochondria and chloroplasts. Explain what would happen if the experiment in Figure 9.9 were repeated in the presence of this drug.
If antimycin A were present, it would make no difference to the results of the experiment, since an artificial proton gradient was already set up.
Oxidative phosphorylation captures a great deal of energy in ATP. But it does not occur if O2 is absent. We will turn now to the metabolism of glucose in anaerobic conditions.