Biology · Ch 12 — Photosynthesis
Cyclic Photophosphorylation
Cyclic Photophosphorylation
In cyclic photophosphorylation, only Photosystem I is involved. When PS-I is illuminated, its reaction centre continuously ejects electrons that pass through a chain of carriers — ferredoxin, cytochrome b6, cytochrome f, and plastocyanin — before returning to the very same PS-I reaction centre they started from, forming a closed loop. As these electrons flow around the loop, their energy is used to phosphorylate ADP into ATP; because the whole process uses only PS-I and never touches water, no photolysis of water takes place and consequent …
What this figure shows. A closed loop beginning and ending at PS-I: light energy excites PS-I and ejects two electrons (2e-), which pass in sequence through ferredoxin, cytochrome b6, cytochrome f and plastocyanin before returning to PS-I; at two points along this loop, ADP + Pi are shown being converted to ATP, representing the phosphorylation events driven by the passage of t …