AQA Unit 4 Photosynthesis

Light dependent and light independent reactions

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Light dependent reaction

When light strikes the chlorophyll molecules, a pair of electrons are excited and so are raised to a higher energy level.
These electrons are lost from the chlorophyll molecule (oxidation).
The pair of electrons are then picked up by the electron carrier molecules (reduction).
Each carrier molecule has a slightly higher electron affinity than the one before it, to ensure the electrons keep moving down the chain.
Each carrier molecule is at a slightly lower energy level than the carrier previous to it, and so energy is lost at each carrier.
This energy is used to used to combine ADP and Pi to form ATP in photophosphorylation ("photo"-light is required, "phosphorylation"-the addition of phosphate to a molecule).

Two molecules of water is split using light energy to give 4 H+ protons, 4 electrons and a single molecule of oxygen (O2).
The hydrogen protons are taken up by and reduce NADP (for use in the light independent reaction).
The oxygen is either used in respiration or diffuses out of the leaf as a waste product.
The electrons replace the electrons lost in the formation of ATP.

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Light-independent reaction

Glucose and RuBP is formed in the light-independent reaction

Carbon dioxide from the atmosphere diffuses into the leaf through the stomata and ultimately into the stroma of the chloroplast.
Within the stroma, the CO2 molecule combines with the 5-carbon molecule RuBP( ribulose biphosphate) using the enzyme rubisco.
The combination of CO2 with RuBP produces two molecules of the 3-carbon GP (glycerate-3-phosphate).
ATP and NADPH from the light independent reduce the activated GP to TP molecules (triose phosphate).
The NADP used is reformed and returns to the light dependent reaction.
Some of the TP formed is used to generate organic substances such as glucose (requires six turns of the cycle to form one molecule of glucose).
Most of the TP molecules are used to regenerate RuBP, using ATP from the light dependent reaction.

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