7.1: Introduction

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We have seen that glycolysis generates two pyruvate molecules per glucose molecule, and that the subsequent oxidation of each pyruvate generates two Ac-S-CoA molecules. After the further oxidation of each Ac-S-CoA by the Krebs cycle, aerobic cells have captured about 30 Kcal out of the 687 Kcal potentially available from a mole of glucose in two molecules of ATP. Not much for all that biochemical effort! However, a total of 24 H+ (protons) pulled from glucose in redox reactions have also been captured, in the form or the reduced electron carriers NADH and FADH2. We begin here with a look at electron transport and oxidative phosphorylation, the linked (“coupled”) mechanism that transfers much of nutrient free energy into ATP. We will see that the free energy released by the transport of electrons from the reduced electron carriers is captured in a proton (H+) gradient. Then we’ll see how dissipation of this gradient releases free energy to fuel ATP synthesis by oxidative phosphorylation. Next, we will contrast mitochondrial oxidative phosphorylation with the substrate-level phosphorylation we saw in glycolysis and the Krebs cycle. After presenting an energy balance sheet for respiration, we look at how cells capture of free energy from alternate nutrients. Then we discuss photosynthesis (overall, the opposite of respiration) and conclude by comparing photosynthesis and respiration.

learning objectives

When you have mastered the information in this chapter, you should be able to:

1. explain the centrality of the Krebs Cycle to aerobic metabolism.

2. identify sources of electrons in redox reactions leading to and within the Krebs cycle.

3. illustrate the path of electrons from the Krebs cycle to and through the electron transport chain.

4. trace the evolution of the electron transport chain from its location on an aerobic bacterial membrane to its location in eukaryotic cells.

5. list the expected properties of a proton gate and a proton pump.

6. interpret experiments involving redox reactions, ATP synthesis and ATP hydrolysis conducted with intact mitochondria and separated mitochondrial membranes.

7. distinguish between the pH, H+ and electrical gradients established by electron transport.

8. explain the chemiosmotic mechanism of ATP synthesis and contrast it with substrate- level phosphorylation.

9. compare and contrast the role of electron transport in respiration and photosynthesis and discuss the evolution of each.

10. trace and explain the different paths that electrons can take in photosynthesis.

11. explain the presence of similar (or even identical) biochemical intermediates in respiration and photosynthesis.