Cyclic photophosphorylation

• Only needs PSI, when light is absorbed by pigments, electrons are excited, accepted by an electron acceptor molecule and transferred down an electron transport chain; final acceptor is chlorophyll so electrons are recycled
• Electrons are transferred down ETC to synthesise ATP from ADP + Pi, ATP can be transferred to light-independent stage but without NADPH, it does not significantly contribute to organic molecule synthesis
• Guard cells only contain PSI, can only make ATP, which is needed for stomatal opening (active process); no organic molecule synthesis