Photosynthesis

Question 1

Describe the stages of the light dependant Reaction (non-cyclic)

Answer 1

• photons of light strike photosystem 2
• light is absorbed by photosynthetic pigments
• excites 2 high energy electrons from chlorophyll A in the reaction centre
• 2e- are passed down the electron transport chain
• Energy from these electrons are used to power the proton pumps
• proton pumps move H+ ions from the stroma into the thylakoid lumen
• water is small so enters the membrane via diffusion
• hydrolysed by photolysis
• H2O -> 2H+ + 0.5O2 +2e-
• O2 diffuses out of the stoma or is used by the plant in aerobic respiration
• H+ accumulates in the lumen
• H+ diffuses through ATP synthase to form ATP (energy from protons)
  ADP + Pi -> ATP
• electrons from PS1 lost energy to the proton pump
• PS1 absorbs more light - so excites more electrons 2e- transferred to NADP along with 2H+ from the stroma
  to form 2x reduced NADP