Summary
Electrons cycle around photosystem I to make extra ATP without producing NADPH or oxygen.
In depth
PSI electrons return to the cytochrome complex via ferredoxin, pumping protons, making ATP only. It supplies extra ATP required by the Calvin cycle, especially in C4 plants.
Electrons from PSI return via the cytochrome complex, so no NADPH or O2 is made. The ratio of cyclic to non-cyclic flow adjusts the ATP:NADPH ratio to match demand.
Key idea: the Calvin cycle needs more ATP than NADPH (3:2), so extra ATP comes from cyclic flow.
In cyclic flow, only PSI is involved, so no NADPH or oxygen is formed, but ATP is made. It adjusts the ATP to NADPH ratio when the Calvin cycle needs more ATP (3 ATP to 2 NADPH per CO2 fixed, while linear flow gives only about 1.28), and in C4 bundle sheath cells that lack PSII. It also helps photoprotection by building a proton gradient that triggers non-photochemical quenching.
Two pathways exist: a PGR5/PGRL1-dependent route and an NDH complex route. Primitive photosynthetic bacteria (green sulfur and purple bacteria) use cyclic flow only. In mutants lacking cyclic flow, growth is poor under fluctuating light.
Example: in high light, cyclic flow increases and helps protect PSII from damage.
Frequently asked questions
What is Cyclic Photophosphorylation?
Electrons cycle around photosystem I to make extra ATP without producing NADPH or oxygen.
Which area of biology covers Cyclic Photophosphorylation, and how advanced is it?
Cyclic Photophosphorylation is one of the 56 topics in Plant Biology. It is rated Advanced, so it suits learners who want specialist depth. Related topics in this area include Light-Dependent Reactions, Z-Scheme of Electron Transport, Photosynthesis.
Can you give an example of Cyclic Photophosphorylation?
Photosystem I only operates in some bundle sheath cells.
What should I learn before Cyclic Photophosphorylation?
Work through the Beginner topics in Plant Biology first, such as Plant Kingdom, Photosynthesis, Chlorophyll, then return to Cyclic Photophosphorylation. Advanced topics assume the Intermediate vocabulary.
What topics are related to Cyclic Photophosphorylation?
The closest topics to Cyclic Photophosphorylation are Light-Dependent Reactions, Z-Scheme of Electron Transport, Photosynthesis, C4 Photosynthesis. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Cyclic Photophosphorylation?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Cyclic Photophosphorylation. To practise the whole of Plant Biology, use the Plant Biology quiz.
How is Cyclic Photophosphorylation different from Light-Dependent Reactions?
Cyclic Photophosphorylation: Electrons cycle around photosystem I to make extra ATP without producing NADPH or oxygen. Light-Dependent Reactions: In the thylakoid membranes, light splits water, releasing oxygen and making ATP and NADPH. Read both side by side and note what each one explains.
How should I study Cyclic Photophosphorylation step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Light-Dependent Reactions and explain the difference aloud. Finally, take the Plant Biology quiz and come back to this page for any question you miss.