ATP formation in chloroplast and mitochondrion is explained by

ATP formation in chloroplast and mitochondrion is explained by the Chemi-osmotic theory of Mitchell.
The chemiosmotic theory states that the transfer of electrons down an electron transport system through a series of oxidation-reduction reactions releases energy. This energy allows certain carriers in the chain to transport hydrogen ions (H⁺ or protons) across a membrane.
Mitchell's theory stated that the electron transport chain and ATP synthesis are embedded in the same membrane, that the membrane is impermeable to protons, that compounds involved in the electron transport chain and ATP synthesis are in contact with one another or the other side of the membrane, that there is an exchange of protons across the membrane, leading to the synthesis of ATP.