The term respiration is used to refer to the process in which cellular energy is generated through the oxidation of nutrient molecules, with O2 as the ultimate electron acceptor. This type of respiration is also called cellular respiration to distinguish it from the respiration of breathing.

Figure 14.2 illustrates the three stages of the metabolic oxidation of organic substrates:

1. Generation of an activated two carbon fragment (the acetyl group of acetyl-CoA ) from pyruvate (cytoplasm), fatty acids (mitochondria), or amino acids (cytoplasm/mitochondria).

2. Oxidation of the carbons of the acetyl fragment by the citric acid cycle (mitochondria).

3. Passage of electrons from the oxidation in step 2 through the electron transport system to yield ATP from oxidative phosphorylation.

The processes in step 3 occur in the mitochondria of eukaryotic organisms. Oxidation involves the loss of electrons, but these electrons do not appear freely in the cell. Instead, they are carried by electron acceptors, such as FADH2 and NADH, which transport the electrons to the electron transport system.