What are the steps in oxidative phosphorylation?

Two components of the process can be distinguished:

  1. Electron transport. The electrons from NADH and FADH2 pass through the respiratory chain, a series of membrane-associated electron carriers. The flow of electrons along this pathway results in the active transport of protons out of the mitochondrial matrix and across the inner mitochondrial membrane, creating a proton concentration gradient.

  2. Chemiosmosis. The protons diffuse back into the mitochondrial matrix through a channel protein, ATP synthase, which couples this diffusion to the synthesis of ATP. As we mentioned in the chapter opening, the inner mitochondrial membrane is normally impermeable to protons, so the only way for them to follow their concentration gradient is through the channel.

Before we proceed with the details of these pathways, let’s consider an important question: Why should the respiratory chain be such a complex process? Why don’t cells use the following single step?

2 NADH + 2 H+ + O2 → 2 NAD+ + 2 H2O

The answer is that this reaction would simply release too much energy to be efficiently trapped to make ATP. Oxidizing NADH to NAD+ is extremely exergonic—doing it in one step would be like setting off a stick of dynamite in the cell. There is no biochemical way to harvest that burst of energy efficiently and put it to physiological use (that is, no single metabolic reaction is so endergonic as to consume a significant fraction of that energy in a single step). To control the release of energy during the oxidation of glucose, cells have evolved a lengthy respiratory chain: a series of reactions, each of which releases a small amount of energy, one step at a time.