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 (H+)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 (ΔpH) and electric potential (Δψ) across the membrane.