5.6.4 The Light-Dependent Stage UNFINISHED

Question 1

What is the light dependent stage?

Answer 1

• Photolysis (light breaks down water)
• Producing Hydrogen ions, Electrons and Oxygen in thylakoid lumen
• Proton gradient formed from photolysis of water
• High concentration of H+ in thylakoid lumen
• Electrons travel through electron transport chain of proteins within membrane
• Reduced NADP forms when H+ in stroma and electrons from electron transport chain combine with carrier molecule NADP
• ATP produced during photophosphorylation

Question 2

Photophosphorylation scribble

Answer 2

• Excited electrons passed along electron transport chain
• Alternatively reduced and oxidised
• Release energy
• Energy released used to actively transport H+ across thylakoid lumen (Proton pump transports protons across thylakoid membrane from stroma to thylakoid lumen)
• Proton gradient created
• High concentration of protons in thylakoid lumen, low in stroma
• Protons return to stroma moving across transmembrane ATP Synthase by facilitated diffusion, synthesising ATP (adding Pi to ADP)
• This is chemiosmosis
• Whole process is photophosphorylation
• Can be cyclic or non cyclic

Question 3

Cyclic Phosphorylation

Answer 3

• Light absorbed to PS1 only and passed to primary pigment P700
• Electron in primary pigment molecule excited to higher energy level in photoactivation
• Accepted by an electron acceptor then passed to electron transport chain then back to PS1 (hence cyclic)
• Electrons passing through electron transport chain provide energy to transport H+ from stroma to thylakoid lumen in proton pump
• H+ faciliatetly diffused back across thylakoid membrane by transmembrane ATP synthase
• ATP synthesised from ADP + Pi

Question 4

Non Cyclic Phosphorylation scribble

Answer 4

• Light absorbed by PS2 and passed to primary pigment P680
• 2 electrons in P680 photoactivated (excited to higher energy level) and emitted (called photoionisation)
• Excited electrons passed down electron transport chain and passed to PS1
• Chemiosmosis occurs during that
• PS2 contains oxygen-evolving complex catalysing photolysis
• H2O → 2H+ + 2e- + ½O2
• Excited electrons from photolysis leave PS2 and replace electrons passed onto PS1