What side of the membrane do H ions build up?

H⁺ ions, or protons, typically build up on the intermembrane space side of the mitochondrial membrane during cellular respiration, particularly during the electron transport chain process. This accumulation creates a proton gradient, which is essential for ATP synthesis as protons flow back into the mitochondrial matrix through ATP synthase. In other cellular contexts, such as in chloroplasts during photosynthesis, a similar buildup occurs in the thylakoid lumen.