Final answer:
The mitochondria maintain a proton gradient using the inner mitochondrial membrane, which is impermeable to protons and allows them to enter the matrix exclusively through ATP synthase. This process generates ATP through chemiosmosis.
Step-by-step explanation:
The outer membrane of the mitochondria is indeed permeable to ions and molecules, but it is the inner mitochondrial membrane that is crucial in maintaining a proton gradient. While the outer membrane allows passage of molecules into the intermembrane space, the inner membrane is impermeable to free diffusion of protons. This distinction is essential for the organelle's function.
The proton gradient is generated by active transport of protons from the mitochondrial matrix into the intermembrane space. This process is part of the electron transport chain (ETC), where energy from electrons is used to pump protons against their concentration gradient, establishing what is known as the proton motive force (PMF). Due to this electrochemical gradient, protons cannot passively diffuse through the inner membrane; instead, they can only re-enter the matrix through specialized ATP synthase channels.
The ATP synthase acts as a molecular generator turned by the force of hydrogen ions diffusing back into the matrix down their electrochemical gradient. This movement is referred to as chemiosmosis, which is vital for the synthesis of ATP. The structure and function of mitochondrial membranes are therefore pivotal in the production of cellular energy.