Photosystem : It is a group of pigment molecules and electron acceptors which take part in oxidation and reduction reaction in photosynthesis for producing ATP and NADPH.

Reaction Centre : Chl_a molecules  act as reaction centres. The Chl_a of speacial types form different reaction centres like P₆₈₀ in PS II and P₇₀₀ in PS I.

Interaction of PS I and PS II :

PS I and PS II are interlinked in following two ways :

Ist way : P₆₈₀ reaction centre of PS II takes energy from sunlight. Then, it emits electron which pass through number of electron acceptors and then go to P₇₀₀, a reaction centre of PSI.

IInd way : The P₇₀₀ takes energy from sunlight via harvesting molecule and its electron is emitted. This electron passes through primary acceptor x and then to ferrodoxin and after this to NADP reductase.

During photolysis of H₂O, the H₂O splits into H⁺ and OH^–. The OH^– changes into H₂O, O₂ and electrons are released which go to P₆₈₀ molecule. The H⁺ goes to NADP reductase which combines NADP, H⁺ and e^– form assimilatory power called NADPH