This animation shows how the enzyme complexes of the electron transport chain harvest energy from cofactor molecules to pump protons across the mitochondrial membrane and establish a chemical gradient. It is the second of three animations about cellular respiration. These animations bring to life the molecular engines inside mitochondria that generate ATP, the main source of chemically stored energy used throughout the body.
 
Electrons harvested from cofactors generated by the citric acid cycle (shown in the first animation) travel through the enzyme complexes of the electron transport chain, powering the pumping of protons across the mitochondrial inner membrane. The resulting concentration gradient powers the synthesis of ATP (shown in the third animation). After the electrons have moved through the electron transport chain, they are accepted by oxygen.
 
The animation is appropriate for teaching advanced high school or college-level students the in-depth mechanisms of the electron transport chain. For more general audiences, the animation can be used as an example of molecular machines, enzymes, and chemical gradients in action. Depending on students’ background, it may be helpful to pause the animation at various points to discuss different reactions or molecules. 

All three cellular respiration animations are also available in a YouTube playlist.