The Question on the GRE is: Mitochondria isolated and placed in a buffered solution with a low pH begin to manufacture ATP. Which of the following is the best explanation for the effect of low external pH? The multiple choice questions are: ( A ) It increases the concentration of OH- causing the mitochondria to pump H + to the intermembrane space. ( B ) It increases the OH- concentration in the mitochondria matrix. ( C ) It increases the acid concentration in the mitochondrial matrix ( D ) It increases diffusion of H+ from the intermembrane space to the matrix. ( E ) It causes a decrease in H+ in the intermembrane space. I thought ( A ) was a good candidate for a correct answer (A) It increases the concentration of OH- causing the mitochondria to pump H + to the intermembrane space, because the outer mitochondrial membrane is permeable to small molecules like OH- and the question says ‘external pH.’ Furthermore, H+ is pumped into the inner membrane space as a result of electrons passing from one electronegative molecule to an even greater electronegative molecule in the inner mitochondrial membrane releasing energy that is used to pump H+ across the inner mitochondrial membrane into the intermembrane space which creates an electro chemical gradient resulting in the flow of H+ through ATP synthase creating ATP in the mitochondrial cortex. ( B ) It increases the OH- concentration in the mitochondria matrix. This one can be eliminated because the inner mitochondrial membrane is not permeable to OH- ( C ) It increases the acid concentration in the mitochondrial matrix This one can be eliminated because OH- is a base not an acid and the inner mitochondrial membrane is not permeable to OH- ( D ) According to the answer Key ( D ) is the correct answer It increases diffusion of H+ from the intermembrane space to the matrix. I didn’t think this one was correct because H+ flows from the matrix into the intermembrane space as a result of the electron transport chain, not the other way around. However ultimately it is the flow of H+ from the inner membrane space that results in an electro chemical proton gradient that drives the synthesis of ATP. However, firstly, H+ has to flow across the inner mitochondrial membrane, before ATP can be created. ( E ) It causes a decrease in H+ in the intermembrane space. This one can be eliminated because it’s a surplus of H+ in the intermembrane space not a deficit of H+ that creates the electrochemical gradient and the flow of H+ through ATP-synthase creating ATP.
