FIGURE 11-14 Effect of V-class proton pumps on H⁺ concentration gradients and electric potential gradients across cellular membranes. (a) If an intracellular organelle contains only V-class pumps, proton pumping generates an electric potential across the membrane (the cytosolic face becomes negative and the luminal face positive), but no significant change in the intraluminal pH. (b) If the organelle membrane also contains Cl⁻ channels, anions passively follow the pumped protons, resulting in an accumulation of H⁺ and Cl⁻ ions in the lumen (low luminal pH) but no electric potential across the membrane.