2. Lecture 3 Outline (Ch. 8) Photosynthesis overview
A. Purpose
B. Location
The light vs. the “dark” reaction
Chloroplasts pigments
A. Light absorption
B. Types
Light reactions
A. Photosystems
B. Photophosphorylation
V. The light independent reaction (“dark” reaction)
A. Carbon “fixation”
B. Reduction
C. Regeneration
VI. Alternative plants

3. Photosynthesis - overview

4. Photosynthesis - overview
Overall purpose:
• photosynthesis –
light chemical energy
• complements respiration
Energy for all life on earth ultimately comes from photosynthesis

5. Cellular Respiration vs. Photosynthesis
(Exergonic)
Photosynthesis:
(Endergonic)

6. Photosynthesis – chloroplast recap
Outer membrane
Inner membrane
Thylakoid membrane
Stroma
Thylakoid space
Intermembrane space

7. Photosynthesis - overview
1. light rxn: store energy & split water – “photo”
NADPH & ATP
2. dark rxn: “fix” CO2 & make sugars – “synthesis”
Calvin cycle

8. Redox Reactions 6CO2 + 12H2O + light energy C6H12O6 + 6O2 + 6H2O
Equation for photosynthesis

9. Photosynthesis - overview
• light reactions:
• dark reactions:
– thylakoid membrane
– stroma
Light
H2O
Chloroplast
Reactions
NADP+ P
ADP +
ATP
NADPH O2
Calvin
Cycle
CO2
[CH2O]
(sugar)
– thylakoid space

10. Photosynthesis – light absorption
• visible light ~380 to 750 nm
• chloroplast pigments – abs blue-violet & red
- transmit and reflect green

11. Photosynthesis – light absorption
• pigments:
• chlorophyll a
-energy-absorbing ring
-hydrocarbon tail
• accessory pigments
- chlorophyll b
- carotenoids
- photoprotective

12. Photosynthesis – light absorption
• chlorophyll a – abs blue-violet, red
, nm
• chlorophyll b & carotenoids – abs broadly blue-violet
mid-400s
• more wavelengths used for photosynthesis = more light energy absorbed

13. Photosynthesis – light absorption
Pigments have two states: ground & excited
• chlorophyll abs light
• e- excited
• more energy
• energy transferred

14. Photosynthesis – light absorption
Pigments are held by proteins in the thylakoid membranes
light harvesting complex
• energy absorbed from light - to pigments
• to reaction center
- two special chlorophyll a
- proteins
- 1° electron acceptor
• light harvesting complex & reaction center = photosystem (PS)

15. Photosynthesis – energy transfer
Light
Thylakoid
membrane
THYLAKOID SPACE
STROMA
Photosystem II
Photosystem I
• Photosystem I (PS I) & PS II
• Difference – light wavelength, proteins,  where e- from

16. Photosynthesis – energy transfer
• PSII: absorbs 680 nm,
splits water, powerful ETC, ATP made
• PS I: absorbs 700 nm, (less energy)
e- from PSII, short ETC, NADPH made

17. Photosynthesis – energy transfer
• e- in PS II, from split H20
• e- from PS II electron transport chain (ETC) PS I
• e- from PS I nd ETC e- carrier: NADP+
 NADPH

18. Photosynthesis – chemiosmosis
• How is ATP produced?
Chemiosmosis
• e- down ETC, H+ to thylakoid space
• H+ conc. gradient
• H+ down gradient, ATP synthase
photophosphorylation

19. Light reaction - summary
• inputs: light energy, H2O
• PS II, ETC, PS I, ETC
• outputs:
ATP
NADPH
O2 (waste)

20. Self-Check Know figures of chloroplast reactions/locations! 
Step of Photosynthesis
Location IN chloroplast
Inputs
Outputs
ATP produced?
(don’t need #)
e- carriers loaded?
Light reaction overall
PSII
PSI
“Dark” reaction overall
Know figures of chloroplast reactions/locations! 

21. Photosynthesis – energy transfer

22. “Dark” reaction (Light-independent Reaction)
6CO2 + 12H2O + light energy C6H12O6 + 6O2 + 6H2O
• “Dark” reaction: Calvin cycle
CO2
NADP+
ADP P i +
RuBP
3-Phosphoglycerate
Calvin
Cycle
G3P
ATP
NADPH
Starch
(storage)
Sucrose
(export)
Chloroplast
Light
H2O O2
• regenerative
• anabolic
• CO2 in, sugar out
• during daylight

23. Carbon fixation • 3 stages of Calvin-cycle: • #1 – carbon fixation
• CO2 link to 5-C
• 5-C: ribulose bisphosphate (RuBP) - enzyme: Rubisco
abundant
• 6-C unstable – split  2(3-C)

24. Reduction • 3 stages of Calvin-cycle: • #2 – reduction • 3-C reduced
• e- from NADPH
• reduced 3-C: G3P

25. Regeneration of C-acceptor
• 3 stages of Calvin-cycle:
• #3 – regenerate C-acceptor
• still 5 G3P  3 RuBP
• multiple steps
• uses ATP
• every 3 cycles:
1 G3P made
3 RuBP regenerated
• C3 plants – CO2 fixed into 3-C

26. Step of Photosynthesis Location IN chloroplast
Self-Check
Step of Photosynthesis
Location IN chloroplast
Inputs
Outputs
ATP produced
(don’t need #)
e- carriers loaded
Light reaction overall
PSII
PSI
“Dark” reaction overall

27. Alternate methods of C fixation

28. Alternate methods of C fixation
CO2
NADP+
ADP P i +
RuBP
3-Phosphoglycerate
Calvin
Cycle
G3P
ATP
NADPH
Starch
(storage)
Sucrose
(export)
Chloroplast
Light
H2O O2
• CO2 in  stomata
• open, lose water
• hot, dry – open stomata less; lowers water loss, lowers CO2
• O2 fixed – photorespiration – inefficient
• fix CO2 into 4-C molecules

29. Photosynthesis – summary
• light reaction: Light energy + H2O
O2, NADPH, ATP
Thylakoids
• light-independent:
CO2, NADPH, ATP
G3P (sugar), RuBP
Stroma

30. Photosynthesis – summary
Where do photosynthetic products go?

31. Photosynthesis – in context of big picture

32. Lecture 3 Summary 1. Photosynthesis Overview (Ch. 8) Purpose
Redox reactions
Electron carriers & sugars
2. Light (Ch. 8)
Absorption pigments
Light spectra/wavelengths
3. Locations of steps, inputs/outputs, purpose, description (Ch. 8)
PSI vs. PS II
Whole light reaction [includes chemiosmosis]
“dark” reaction/Calvin cycle [3 steps]
4. Alternate modes of photosynthesis (Ch. 8)
5. Photosynthesis context (Ch. 8)
- Uses for products
Relationship of cell respiration and photosynthesis