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What is cyclic photophosphorylation?
When light is not a limiting factor, the light-independent reactions may proceed more slowly than the light-dependent reaction, so the supply of NADP+ runs out. The electrons boosted from photosystem I have no acceptor available to take them, so they are sent back to photosystem II and rejoin the electron transport chain near the start, generating more ATP. It produces neither O2 nor NADPH.
What is non-cyclic photophosphorylation?
When ATP is produced using energy from excited electrons flowing from photosystem II through photosystem I and on to NADP+.
What happens in cyclic photophosphorylation?
Only one photosystem is involved, where an energized electron is ejected from the photosystem reaction center and passes through ferredoxin and the cytochrome b6-f complex back to the reaction center without generating reducing power.
What happens in non-cyclic photophosphorylation?
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What does the energy released during cyclic photophosphorylation generate?
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Why does cyclic photophosphorylation occur?
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Study Notes
Cyclic Photophosphorylation
- Occurs when light is abundant but NADP+ supply is low.
- Electrons from photosystem I are returned to photosystem II instead of forming NADPH.
- This process does not produce oxygen (O2) or NADPH+H+.
- Generates ATP through the electron transport chain.
Non-Cyclic Photophosphorylation
- Involves both photosystems II and I working sequentially.
- When light hits photosystem II, electrons are ejected and replaced by electrons from water splitting.
- The electrons travel through a series of carriers, establishing a proton gradient for ATP production.
- Electrons from photosystem I are used to generate NADPH.
- This process is characterized by the non-return of electrons to photosystem II, hence 'non-cyclic'.
Events in Cyclic Photophosphorylation
- Only one photosystem (usually photosystem I) functions in this process.
- Energized electrons are transferred to ferredoxin and then through the cytochrome b6-f complex.
- Ultimately, electrons return to the reaction center of photosystem I.
- No reducing power is generated, limiting the ability for biosynthetic processes.
Events in Non-Cyclic Photophosphorylation
- Sequential operation of both photosystems allows energy and reducing power production.
- Light energizes electrons from photosystem II, which are then replaced by electrons from water.
- The electron transport pathway involves plastoquinone, b6F complex, and plastocyanin.
- Creates a proton motive force for ATP synthesis.
- Photosystem I also generates reducing power (NADPH) from absorbed light energy.
Proton Gradient and ATP Generation
- Energy released during electron transport in cyclic photophosphorylation establishes a proton gradient.
- This gradient is crucial for the synthesis of ATP through chemiosmosis.
Reasons for Cyclic Photophosphorylation
- Occurs when NADP+ is unavailable for NADPH formation.
- If the Calvin cycle cannot keep up, cyclic photophosphorylation recycles electrons to maintain ATP production without generating NADPH.
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Description
This quiz covers the concepts of cyclic and noncyclic photophosphorylation as part of IBH Biology. Understand the processes involved in light-dependent reactions and how they relate to the electron transport chain in photosynthesis. Test your knowledge with flashcards designed for deeper comprehension.
