1. Structure of chlorophyll molecule
Long, non-polar hydrophobic “tail” anchors in lamellar membranes
Bound to membranes
“head” has a complex ring with Mg2+ at center
Where light energy is trapped

2. Accessory Pigments Absorb wavelengths that chlorophyll can’t
Pass along energy into photosynthetic process  plants more efficient this way
Do not waste wavelengths
Note that chlorphyll a, b, c each have slightly different spectrum, so each can use a different part of the light

3. Accessory Pigments Phycobilins
Found in red algae & blue-green bacteria
Phycoerythrin: (red) max 550 nm
Phycocyanin: (blue) max 620 nm

4. Accessory Pigments Carotenoids
Protect chlorophylls from damage by bright light
Take in certain λ, transfer energy to chlorophyll a

6. Light Reactions Overview
System I responds to λ of 700 nm
Molecule is called P700
System II responds to λ of about 680 nm
Molecule is called P680
BOTH reactions (both λ) are necessary for photosynthesis

7. Notice that Photosystem I is the SECOND molecule in the chain…
This is only because that was the first one discovered, and they noticed that the electrons were being brought to P700 from somewhere else… the search begins!

8. Electron Transport Chain Light Reactions
Photosystem I
700 nm light strikes P700
2e- released
NADP+ reduced to NADPH (stored in stroma)
NADPH contains energy
NADP = nicotinamide adenine dinucleotide phosphate

9. Electron Transport Chain Light Reactions
Photosystem II
680 nm light strikes P680
H2O splits, O2 released
2e- released
e- goes to plastiquinone
e- transferred to ETC
Pumps H into lumen
NADP = nicotinamide adenine dinucleotide phosphate

10. Photosystem I
Light strikes P700
Electron released
NADP reduced to NADPH (stored I stroma)
Photosystem II
Light strikes P680
Electron goes to plastiquinone (Pq), carrying H into lumen
Electron goes into ETC then eventually back into Photosystem I

11. Electron Transport Chain Light Reactions
Manganese (Mn)-complex give electrons back to P680 by splitting 2 water molecules
4 photons need to split 2 water molecules

12. Summary: Light Reactions
NADPH stored in stroma
H+ pumped into thylakoid reservoir (lumen)
Creates electrochemical gradient for ATP synthesis

13. Chemiosmotic Theory of ATP formation (noncyclic flow)
NADP+
PSII
PSI
NADPH
lumen
ATP
synthase
thylakoid
membrane
stroma
ATP
ADP

14. Cyclic flow
Note that only PSI is used. The electrons cycle back from Fd to the cytochrome complex and from there continue on to a P700 chlorophyll. The purpose is to make more ATP