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Summary OCR A-Level Biology 5.6.3 Light Dependent Stage
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Revision summary of key knowledge from OCR A-Level Biology 5.6.3 Light Dependent Stage.
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Light Dependent StageZ-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
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