Mitochondria, Cell Energy, ATP Synthase | Learn Science At Scitable

As previously mentioned, mitochondria contain two major membranes. The outer mitochondrial membrane fully surrounds the inner membrane, with a small intermembrane space in between. The outer membrane has many protein-based pores that are big enough to allow the passage of ions and molecules as large as a small protein. In contrast, the inner membrane has much more restricted permeability, much like the plasma membrane of a cell. The inner membrane is also loaded with proteins involved in electron transport and ATP synthesis. This membrane surrounds the mitochondrial matrix, where the citric acid cycle produces the electrons that travel from one protein complex to the next in the inner membrane. At the end of this electron transport chain, the final electron acceptor is oxygen, and this ultimately forms water (H20). At the same time, the electron transport chain produces ATP. (This is why the the process is called oxidative phosphorylation.)

During electron transport, the participating protein complexes push protons from the matrix out to the intermembrane space. This creates a concentration gradient of protons that another protein complex, called ATP synthase, uses to power synthesis of the energy carrier molecule ATP (Figure 2).

Figure 2: The electrochemical proton gradient and ATP synthaseAt the inner mitochondrial membrane, a high energy electron is passed along an electron transport chain. The energy released pumps hydrogen out of the matrix space. The gradient created by this drives hydrogen back through the membrane, through ATP synthase. As this happens, the enzymatic activity of ATP synthase synthesiszes ATP from ADP.© 2010 Nature Education All rights reserved. Figure Detail