Summary OCR A-Level Biology 5.6.3 Light Dependent Stage

Light Dependent Stage
Z-Scheme (Non-cyclic Photophosphorylation)
 A photon of light reaches photosystem II and activates chlorophyll a P680
 An electron is released from the Magnesium in the chlorophyll which becomes excited
and increases in energy
 Magnesium becomes positively charged (Mg²⁺) due to an electron deficit
 To fill this electron deficit, water is hydrolysed by photolysis
 The electron is accepted by an electron acceptor and enters an electron transport
system (ETS)
 As the electron moves along the ETS, it loses energy which is used to form a molecule of
ATP via photophosphorylation
 The electron arrives at photosystem I where chlorophyll a p700 is activated by another
photon of light
 The electron is excited again and increases in energy and movers to another ETS where
another molecule of ATP is produced by photophosphorylation as the electron loses
energy
 The electron arrives at NADP which is reduced by protons (H⁺ ions) from photolysis to
form NADPH, catalysed by the enzyme NADP reductase



Cyclic Photophosphorylation
 When photons excite electrons at photosystem I, some enter cyclic
photophosphorylation instead of continuing in the Z-scheme to an electron acceptor
 These electrons enter a short ETS which ends back at photosystem I
 A molecule of ATP is produced in each cycle



Photolysis
 Photolysis occurs in cyclic photophosphorylation to fill the electron deficit at
photosystem II
 Water (2H₂O) is split into 4H⁺ + 4e⁻ + 2O
 The electron are used to fill the electron deficit
 The protons (hydrogen ions) are used to reduce NADP
 The oxygen is released into the atmosphere