Photoreactions
The light dependent reactions use photosynthetic pigments (organised into photosystems) to convert light energy into chemical energy
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The intermediate products of the light dependent reactions (ATP and NADPH) are then utilised by the light independent reactions
The light dependent reactions occur within specialised membrane discs called thylakoids
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In plants, the thylakoid discs are organised into stacks called grana (singular = granum) within the chloroplasts
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In cyanobacteria and single-celled algae, the thylakoids do not form grana (individual thylakoids are free-floating instead)
Thylakoids / Grana
Transmission EM
Scanning EM
When a photosystem absorbs light energy, delocalised electrons within the pigments become energised or ‘excited'
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The excited electrons are transferred to an electron transport chain within the thylakoid membrane
Electron transport chains consist of several electron-shuttling carrier proteins and the transmembrane enzyme ATP synthase
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As energised electrons from the photosystems are passed through the chain they lose energy, which is used to translocate protons from the stroma into the thylakoid
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This build up of protons within the thylakoid creates an electrochemical gradient, or proton motive force
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The protons return to the stroma (along the proton gradient) via the transmembrane enzyme ATP synthase (this movement is called chemiosmosis)
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ATP synthase uses the passage of protons to catalyse the synthesis of ATP (from ADP + Pi) – similar to how a waterwheel generates electricity from the flow of water
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This process is called photophosphorylation – as light provided the initial energy source for ATP production
Chemiosmosis
The production of ATP by the light dependent reactions is called photophosphorylation, as it uses light as an energy source
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Photophosphorylation may be either a cyclic process or a non-cyclic process
Cyclic Photophosphorylation
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Cyclic photophosphorylation involves the use of only one photosystem (PS I) to generate ATP
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The energised electrons released from photosystem I are recycled after passing through the electron transport chain
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This cyclic process can be used to produce a steady supply of ATP in the presence of sunlight, but cannot be used for organic synthesis
Non-Cyclic Photophosphorylation
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Non-cyclic photophosphorylation involves the use of both photosystems (PS I and PS II) to generate ATP and NADPH
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The energised electrons from photosystem I are used to reduce NADP to form NADPH (loaded coenzyme)
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The lost electrons are replaced by electrons from photosystem II that have passed through the electron transport chain
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The electrons lost from photosystem II are replaced by electrons generated by the photolysis of water
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This process is non-cyclic, as the reduction of NADP requires the oxidation of a water molecule