The electron transport chain is the first stage of oxidation phosphorylation. During this phase, Complex 1 strips the electrons off of NADH and Complex 2 strips the electrons of FADH2. These electrons are then donated to coenzyme Q, which in turn donates the electrons to complex 3. Complex 3 then proceeds to donate its electrons to complex 4 which donates to the final electron receptor, oxygen, producing H2O. During this process, complex 1, 3, and 4 pump protons into the intermembrane space and create a concentration gradient. This concentration gradient is used to power ATP synthase, as the protons will naturally flow to an area of lower concentration on the interior of the mitochondrial matrix. To do this, the protons pass through ATP synthase, which rotates its mechanism and powers the synthesis of ATP. For every 3 protons which passes through ATP synthase 1 ATP is generated.