Plant lecture 3 - Light reactions of photosynthesis

Question 1

Light vs Dark reaction

Answer 1

• Dark reaction = CO2 fixation, E located in stroma

- Light reaction = energy harvesting, in ↑ structured thylakoids

Question 2

Electron transport chain

Answer 2

• PSI absorbs photos that excite e-, e- donated to e- carriers, hole in PSI is filled by PSII and hole in PSII is filled by oxidation in water
• PSII starts with strong oxidant, generates weak reductant
• PSI starts with weak oxidant, generates strong reductant

Question 3

LHCII structure

Answer 3

• Has core complex D1 + D2
• Surrounding core complex = peripheral antennas encoded by 6 genes Lhcb1-6
• Lhcb1-3 = variable numbers, Lhcb4-6 usually = single q

Question 4

Evidence for coordination btw PSI + PSII

Answer 4

• Illuminate plant w/ PSII light
• Initial ↑ chlorophyll fluorescence
• Turn on PSI light, ↑ in photosynthesis + small ↓ in fluorescence (PSI pulls e-s from PSII)
• Turn off PSI ↓ photosynthesis, ↑ in fluorescence
• Inverse relationship btw ↑ in PSI photosynthesis + ↓ of fluorescence in PSII = redistribution

Question 5

PsaH, L + O

Answer 5

• Structure to 3.3A
• PsaH,L+O form interface btw LHCI+LHCII
• Particularly, PsaL interacts w/ phosphate on residue 3 threonine
• For lHCII to share excitation E w/ PSI, need ↑ ordered chlorophyll.
• PsaO = associated w/ 2 chlorophyll molecules positioned in orientations + at distances that allow transfer of E

Question 6

Transcriptional redox control of PSI + PSII

Answer 6

• CSK responds to amount of oxidised PQ through identification of BS for PQ on CSK
• Autophosphorylated CSK → activation.
• Facilitates phosphorylation of sigma factor associated w/ plastid-encoded RNA pol
• In phosph. form, RNA pol can transcribe PSII, PSI transcribed weakly
• Reduced PQ = PSI + II genes both transcribe