The correct answer is: In both systems, ATP is produced by chemiosmosis.
Both of the processes, photosynthesis and electron transport chain in mitochondria use chemiosmosis (movement of ions across a semipermeable membrane, down their electrochemical gradient) to produce energy or ATP (via ATP synthase). The movement of hydrogen ions across the thylakoid membrane in order to galvanize the production of ATP is equal to the movement of those ions across the inner mitochondria membrane. Electrons are accepted by NADPH in photosynthesis (but not FADH2 as in mitochondria).
