In IB Biology curriculum, there are two ideas of this stage:

1. It occurs in the thylakoids and converts light energy into chemical energy (ATP+NADH)

2. It occurs in the stroma and uses the chemical energy to produce carbon compounds

1. Chlorophyll in photosystems I and II (PSI and PSII) absorbs light which triggers the release of energised electrons (photoactivation) --> electrons are excited
2. Electrons from PSI reduce NADP to form NADPH
3. Electrons from PSII pass through the electron transport chain before replacing electrons from PSI
4. The transport chain produces ATP (photophosphorylation) are listed out in IB Biology curriculum:
   • PSII electrons pass through the electron chain, lose energy, translocates H+ ions to thylakoid
   • this creates an electrochemical gradient or proton motive force
   • Chemiosmosis: H+ ions return to the stroma via ATP synthase
   • This catalyzes ATP
5. Electrons lost by PSII are replaced by water (photolysis)
   • H2O --> 2H++2e-+1/2O2

Usually in IB Biology, there are five key points of cyclic phosphorylation:

• Only chlorophyll in Photosystem I are activated by light
• The electrons move through an electron transport chain before returning to their original location (i.e. cyclic)
• The transport chain produces ATP (photophosphorylation)
• Cyclic photophosphorylation does not produce NADPH
• Hence, while cyclic photophosphorylation can produce usable energy (ATP), it cannot produce organic molecules

That's all of part 1! (remember to read part 2)

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