Animation 9.1: Electron Transport and ATP Synthesis

INTRODUCTION

An organism that lives in the presence of oxygen can extract a great deal of energy from glucose by running it through two main metabolic pathways: glycolysis and cellular respiration. By the end of these pathways, glucose has been completely oxidized and the cell has gained 32 molecules of ATP—a versatile energy carrier that fuels most kinds of cellular work.

In glycolysis, enzymes in the cytosol split glucose into two molecules of pyruvate. Pyruvate then enters a mitochondrion, where cellular respiration occurs. Cellular respiration occurs in three main phases: pyruvate oxidation, the citric acid cycle, and the respiratory chain. In the accompanying animation, we focus on the respiratory chain, the final phase of cellular respiration, and the phase in which the cell makes the bulk of its ATP.

CONCLUSION

During the early phases of glycolysis and cellular respiration, glucose is completely broken down. CO₂ is liberated into the atmosphere, and the hydrogen atoms (H⁺ + e^–) from glucose are donated to the energy carriers NAD⁺ and FAD to form NADH + H⁺ and FADH₂. In order for glycolysis and cellular respiration to continue to operate on additional glucose molecules, these energy carriers must be recycled.

The work of the respiratory chain is, in part, to recycle these carriers. The carriers donate their extra hydrogen atoms to the respiratory chain and thereby convert back into NAD⁺ and FAD. NADH donates electrons to the first complex in the chain. FADH₂ donates electrons to the second complex.

The other work of the respiratory chain is to transform the chemical energy of the hydrogen atoms (specifically, their electrons) into potential energy. In a series of redox reactions, electrons jump from one complex to another and, in the process, release energy. The chain uses the released energy to pump protons across the membrane, from a region of low concentration inside the mitochondrion to a region of high concentration within the intermembrane space. This concentration gradient represents potential energy.

The cell taps the potential energy of the gradient when protons flow back across the membrane through a pore in the ATP synthase complex. As the protons flow, they release energy, which the complex uses to convert ADP and inorganic phosphate to ATP. The production of ATP from energy derived from the flow of electrons through the respiratory chain is referred to as oxidative phosphorylation. Chemiosmosis is another term for ATP synthesis, referring to the use of a proton gradient to fuel the production of ATP.

Textbook Reference: Key Concept 9.3 Oxidative Phosphorylation Forms ATP