Light Dependent Stage

Question 1

Where does it occur?

Answer 1

Thylakoids

Question 2

What is a summary of what happens in the light dependent stage?

Answer 2

• Photolysis occurs (H+, e- and oxygen produced)
• High concentration of H+ in thylakoid lumen causes proton gradient
• Electrons travel through electron transport chain in thylakoid membrane
• NADP combine with H+ from stroma and e- in electron transport chain to form NADPH
• ATP produced during photophosphorylation

Question 3

What happens during photophosphorylation?

Answer 3

• Excited electrons passed along ELTC
• Electron carriers are alternately reduced and oxidised as electrons are passed from carrier to carrier
• Excited electrons release energy as they are passed through ELTC
• Energy released used to actively transport H+ across thylakoid membrane from stroma to thylakoid lumen via a proton pump
• Proton gradient created, high conc of H+ in thylakoid lumen, low conc in stroma
• H+ diffuses by facilitated diffusion across TRANSMEMBRANE ATP SYNTHASE enzymes back into the stroma
• Releases energy for the synthesis of ATP from ADP + Pi
• Can be cyclic or non-cyclic
• Cyclic involves PSI, Non-Cyclic involves PSI and PSII

Question 4

What are photosystems?

Answer 4

• PSI and PSII
• Collection of pigments that absorb light energy and transfer energy to electrons
• Different pigment molecules are arranged in funnel like structures in thylakoid membrane
• Each pigment molecule passes energy down to the next pigment molecule in the cluster until it reaches the primary pigment reaction centre

Question 5

What is PSI?

Answer 5

• Contains primary pigment P700
• Absorbs light of wavelength 700nm
• In the middle of the electron transport chain

Question 6

What is PSII?

Answer 6

• Contains primary pigment P680
• Absorbs light at a wavelength of 680nm
• At beginning of ELTC
• Where photolysis takes place

Question 7

What is cyclic phosphorylation?

Answer 7

• Light absorbed at PSI and passed to P700 (primary pigment of PSI)
• Electron in primary pigment molecule becomes excited to a higher energy level
• Photoactivation occurs
• Excited electron released from chlorophyll molecule and accepted by an electron acceptor
• Electron acceptor transports electrons to ELTC
• As electrons pass through ELTC, they provide energy, chemiosmosis occurs, releasing energy for the synthesis of ATP
• Electrons then transported BACK TO PSI

Question 8

What is non-cyclic phosphorylation?

Answer 8

• Light absorbed in PSII, passed to P680
• 2 electrons in primary pigment molecule excited to higher energy level
• Emitted from chlorophyll in process called photoionisation
• Each excited electron passed down ELTC
• Chemiosmosis occurs
• THEN ELECTRONS PASSED TO PSI
• PSII contains water-splitting enzyme called oxygen-evolving complex
• Catalyses photolysis
• H2O → 2H+ + 2e- + ½O2
• Excited electrons leave PSII to go to PS1
• They are replaced by electrons from photolysis of water
• So as photoactivation of electrons in PSII, photoionisation of electrons in PSI occurs
• Excited electrons from PSI pass along an ELTC
• These electrons combine with H+ produced by photolysis of water, and carrier molecule NADP to form NADPH
• 2H+ + 2e- + NADP → reduced NADP
• NADPH passes to light-independent reaction to be used in synthesis of carbohydrates