In mitochondria, protons accumulate in the

During the passage of two electrons from one an electron carrier (NADH) to another on ETC, enough energy is liberated that pumps out three pairs of protons $\left(\mathrm{H}^{+}\right)$from matrix (inner chamber) to outer chamber (inter membrane space) of mitochondria. The accumulation of protons in inter membrane space results their higher concentration than in the matrix creating a proton gradient $(\Delta \mathrm{pH})$ and electric potential $(\Delta \psi)$ across the membrane.