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.