Peter Mitchell, a British biochemist, in 1961 proposed the Chemiosmotic theory where he states that the protons pumped out due to the action of the electron transport system in to the intermembrane space of mitochondria generates an electrochemical gradient. The gradient of protons causes higher pH value in the internal matrix of mitochondria and lower pH value in the intermembrane space and when the protons flow back in, energy is dissipated which helps in synthesis of ATP by the ATP synthase.

Such proton gradient also develops in the chloroplast at the lumen of thylakoid and stroma. This occurs due to the photolysis of water releasing protons and movement of protons from stroma to thylakoid due to the action of NADP reductase enzyme. There is low pH value in thylakoid lumen and high pH value in stroma and the flow of proton due to the gradient again results in formation of ATP.