<P> Electrochemical gradients also play a role in establishing proton gradients in oxidative phosphorylation in mitochondria. The final step of cellular respiration is the electron transport chain . Four complexes embedded in the inner membrane of the mitochondrion make up the electron transport chain . However, only complexes I, III, and IV pump protons from the matrix to the intermembrane space (IMS). In total, there are ten protons translocated from the matrix to the IMS which generates an electrochemical potential of more than 200mV . This drives the flux of protons back into the matrix through ATP synthase which produces ATP by adding an inorganic phosphate to ADP . Thus, generation of a proton electrochemical gradient is crucial for energy production in mitochondria . The total equation for the electron transport chain is: </P> <P> N A D H + 11 H + (m a t r i x) + 1 / 2 O 2 ⟶ N A D + + 10 H + (I M S) + H 2 O (\ displaystyle NADH + 11H ^ (+) (matrix) + 1 / 2 \ O_ (2) \ longrightarrow NAD ^ (+) + 10H ^ (+) (IMS) + H_ (2) O) </P> <P> Similar to the electron transport chain, the light - dependent reactions of photosynthesis pump protons into the thylakoid lumen of chloroplasts to drive the synthesis of ATP by ATP synthase . The proton gradient can be generated through either noncyclic or cyclic photophosphorylation . Of the proteins that participate in noncyclic photophosphorylation, photosystem II (PSII), plastiquinone, and cytochrome b f complex directly contribute to generating the proton gradient . For each four photons absorbed by PSII, eight protons are pumped into the lumen . The total equation for photophosphorylation is shown: </P> <P> 2 H 2 O + 6 H + (s t r o m a) + 2 N A D P + ⟶ O 2 + 8 H + (l u m e n) + 2 N A D P H (\ displaystyle 2H_ (2) O + 6H ^ (+) (stroma) + 2NADP ^ (+) \ longrightarrow O_ (2) + 8H ^ (+) (lumen) + 2NADPH) </P>

Where does the energy come from to make a proton gradient