1. BIOL 205 :: Photosynthesis Lecture 1
Introduction and the light reactions

2. What is photosynthesis?
The process of converting solar energy into chemical energy.
Can use only water and carbon dioxide to create sugars = chemical energy.
Responsible for removal of ~ 200 billion tons of C from the atmosphere yearly.

3. What is photosynthesis?
6CO H2O + hv C6H12O6 +6O2 +6H2O
hv designates light
you should memorize this equation!! you will see it again O2
= oxygenic photosynthesis

4. What is photosynthesis?
2 stages of photosynthesis
Light reactions and Dark reactions
Light reactions convert sunlight into chemical energy (ATP + NADPH)
Dark reactions use those products to form sugars (stored chemical energy)

5. Overview of Photosynthesis
Dark Reactions
CO2
ATP
NADPH
Light Reactions
H2O +
CO2
Sugars O2

6. Where does photosynthesis take place?
Prokaryotes
Prokaryotes have both anoxygenic and oxygenic
Cyanobacteria have oxygenic
-Photosynthesis on thylakoids (from plasma membrane)
-Cyanobacteria are source of eukaryotic photosynthesis
Eukaryotes
Oxygenic only
Takes place in chloroplasts

7. Chloroplast Morphology

8. Chloroplast Morphology-Terms
Inner Membrane
Outer Membrane
Thylakoid
Thylakoid lumen
Stroma
Granum

9. How can light provide energy for plants?
Light is composed of particles: photons
Light behaves like a wave
Can e described w/ wavelength & frequency
Only a small portion of the electromagnetic spectrum.

10. The electromagnetic spectrum
PAR = photosynthetically available radiation

11. Pigments Pigment = a light absorbing molecule
Associated with the thylakoid membranes
Chlorophyll
Chl a and Chl b (Chl c in some algae)
Xanthophylls
Carotenoids
ß-carotene

12. Chlorophyll Chl a has a methyl group Chl b has a carbonyl group
Porphyrin ring
delocalized e-
Phytol tail

13. Different pigments absorb light differently

14. Different pigments absorb light differently

15. 2-minute quiz
1. What is the relationship between the structure of chlorophyll and its location in the chloroplast?
2. Why are plants green?

16. Light Reactions 1: Light capture and redox
2 spatially & functionally distinct units = Photosystems
Photosystem II = 1st stage
Photosystem I = 2nd stage
Named after order of discovery

17. Light capture 1
1. Most Chlorophyll is located in the Light Harvesting Complex
2. Sunlight is absorbed in the LHC and is passed from pigment to pigment
Chl a
Chl b
Remember the porphyrin ring?
ß-carotene
LHC
Reaction Center

18. Light capture - 2
3. Energy finally ends up in a pair of special chlorophyll a molecules: P680
4. e- in P680 Chl a goes to excited state and is shed = Charge separation
e-*
P680 chlorophylls *
<680 nm
LHC
Reaction Center

19. Light capture - 3
5. High-energy e- accepted by quinone Q = primary e- acceptor
6. Q has been reduced; P680 Chl a has been oxidized
Oxidation by light = photo-oxidation
e- transport chain
7. The excited e- is shunted into the electron transport chain
e-* Q +
LHC
Reaction Center

20. Chl a+ = most powerful biological oxidizing agent
Light capture - 4
8. The O2 evolving complex + Chl a+ strip e- from H2O and reduce Chl a+
Chl a+ = most powerful biological oxidizing agent + e-
O2 evolving complex
H2O
1/2 O2
4H+

21. Light capture - 5
9. The reaction center is reset and ready to go again

22. Light Capture Movie

23. PSII electron transport chainQ e- H+
1. Q accepts 2 e- from P680 and removes 2 H+ from the stroma
2. Q passes the e- to cytochrome b/f complex & pumps the 2H+ into thylakoid lumen
b/f
cytochrome
complex e-
3. As the e- moves through b/f more H+ are pumped into lumen

24. The PS II Electron transport chainQ e- e-
cytochrome
b/f
complex e-
2H+
plastocyanin H+

25. PS II to PS I
plastocyanin e-
4. e- end up on plastocyanin: a soluble electron carrier in the lumen
plastocyanin e-
5. Plastocyanin serves as e- donor for PSI reaction center Chl a

26. Photosystem I
e-*
1. Charge separation and photo-oxidation are similar to PSII
Optimal wavelength = 700nm +
plastocyanin e-
P700
2. Plastocyanin acts as reducing agent on P700 Chl a

27. PSI electron transport chain
Ferredoxin
Fe/S
1. Ferredoxin recieves e- from P700*
FAD
NADP+
FAD-NAPD
Reductase H+ e-
2. e- moves throuh FAD (flavin adenine dinucleotide)
3. e- plus stromal H+ are used to reduce NADP+ to NADPH
NADPH
= FINAL PRODUCT!

28. PSI electron transport chain
Ferredoxin
Fe/S e- e-
FAD H+ e-
FAD-NAPD
Reductase
NADP+
NADPH

29. ATP synthesis H+
1. O2 evolving complex liberates H+ into lumen from water
2. Q and Cyt b/f pump H+ from stroma into lumen.
3. NADP+ scavenges protons from the stroma
pH separation across membrane = Proton Motive Force! H+
stroma
lumen H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+

30. ATP synthesis F0F1 complex H+ ATP stroma H+ lumen
ADP + Pi
F0F1
complex H+
ATP
stroma H+
lumen
The energy released as protons travel down their concentration gradient is used to fuel an ATP synthase H+ H+ H+ H+ H+ H+ H+ H+ H+ H+

31. Light Reactions Products
In: CO2, H2O, sunlight
Out: O2, ATP, NADPH