General ---------Biology----------------------------------------------------- ------------------Online--------------------------------------- Ch. 9 Cellular Respiration: Harvesting Chemical Energy

Figure 9.15, Chemiosmosis couples the electron transport chain to ATP synthesis, shows five of the nine proteins in the inner mitochondrial membrane.  As the electrons pass from protein to protein, the energy given off changes the shape of the protein.  The change in shape causes the protein to pump a hydrogen ion (i.e. a proton) from the mitochondrial matrix (yellow area) across the membrane and into the inner membrane space (orange). 

This pumping is a form of active transport called a proton pump (see text Fig. 8.17 An electrogenic pump.)  Instead of using ATP as the energy, the electron transport proteins use the energy given off by a "falling" electron.

This pumping of positively charged protons creates a high concentration of positive charges in the inner membrane space and a resulting negative charge in the matrix.  This creates a diffusion gradient of hydrogen ions and a +/- gradient across the membrane, called electrochemical gradient, i.e. both chemical and electrical. 

The energy in this gradient is used by an enzyme, called ATP synthase, to phosphorylate ADP.  It is analogous to pumping water up behind a dam, and then using the energy of falling water to generate electricity.  As the protons come through a channel in ATP synthase, the energy they give off is the energy that drives ATP synthesis.  ATP synthase is the enzyme that harnesses the energy from "falling" hydrogen ions and uses it to phosphorylate ADP.  The nine proteins of the electron transport chain, that pass along "falling" electrons (oxidation-reduction-oxidation-reduction-etc...) and use the energy to pump protons, is illustrated in thisfigure.

For a close up of how ATP is made by the electron transport chain, click here.