Photosynthesis

Stages of photosynthesis:

Light-dependent stage occurs in the thylakoids and converts light energy into chemical energy (ATP+NADH)

Light-independent stage occurs in the stroma and uses the chemical energy to produce carbon compounds

1. Light-dependent stage 

1.1 Non-cyclic phosphorylation

• Chlorophyll in photosystems I and II (PSI and PSII) absorbs light which triggers the release of energised electrons (photoactivation) --> electrons are excited 
• Electrons from PSI reduce NADP to form NADPH
• Electrons from PSII pass through the electron transport chain before replacing electrons from PSI
• The transport chain produces ATP (photophosphorylation) 
  • 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 
• Electrons lost by PSII are replaced by water (photolysis)
  • H₂O --> 2H⁺+2e^-+1/2O₂

1.2 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…