The Light-Dependent Stage

Question 0

Explain photophosphorylation (cyclic)

Answer 0

• light energy hits PSI (p700) exciting 2 electrons
• the electrons are passed along the ETC, transporting energy to the proteins in the chain, which use the energy to make ATP by photophosphorylation (ADP + Pi –> ATP)
• the 2 electrons are returned to PSI (p700)

Question 1

Explain photophosphorylation (non-cyclic)

Answer 1

• light energy hits and is absorbed by PSII (p680)
• 2 electrons become excited and gain energy
• electrons are passed along the ETC, the proteins use the energy to make ATP by photophosphorylation
• an enzyme associated with PSII uses the light energy to split water (photolysis) H2O –> 2H+ + 2e- +1/2O2
- the 2 electrons replace the electrons lost from PSII
• light energy hits PSI (p700) exciting 2 electrons. These electrons are replaced by the electrons lost from PSII
• the 2e- and 2H+ (protons) from photolysis of water combine with NADP catalysed by NADP reductase NADP + 2e- + 2H+ –> reduced NADP (NADPH, NADPH+, H+)
- this is used in the light independent reaction

Question 2

Explain the formation of ATP during non-cyclic photophosphorylation

Answer 2

• as electrons are passed along the ETC they decrease in energy, transferring the energy to the proteins in there
- the proteins use the energy to pump H+ ions (protons) from the stroma into the thylakoid space
• the membrane is impermeable to H+ ions, so the H+ ions accumulate, setting up a concentration, electrochemical and pH gradient
• the protons (H+ ions) flow down the gradient through ATP synthase (chemiosmosis)
• the proton motive force generated by the flow of protons causes the head of ATP synthase to rotate, catalysing the phosphorylation of ADP to ATP (ADP + Pi –> ATP)

Question 3

Where does it take place?

Answer 3

Thylakoid membrane