Photosynthesis uses light energy, carbon dioxide, and water to produce oxygen and energy-rich organic compounds like glucose. It occurs in two stages - the light-dependent reactions where light energy is captured to make ATP and NADPH, and the light-independent reactions where CO2 is incorporated into organic compounds through the Calvin cycle. Chloroplasts contain chlorophyll and other pigments that absorb light for use in the photosystems. The energy from light drives electron transport and chemiosmosis to produce ATP, then electrons are transferred to NADP+ to form NADPH. These products fuel the Calvin cycle to reduce CO2 into glucose using the energy from ATP and NADPH.

1. Photosynthesis
By:
Muhammad Iqbal Khan
SS Biology
Email ID mikhan1313@yahoo.com
Contact: 0923135448175

2. Lecture 3 Outline (Ch. 8)
I. Photosynthesis overview
A. Purpose
B. Location
II. The light vs. the “dark” reaction
III. Chloroplasts pigments
A. Light absorption
B. Types
IV. 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. Overall purpose:
Photosynthesis - overview
• photosynthesis –
light chemical energy
• complements
respiration
Energy for all life on
earth ultimately comes
from photosynthesis

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

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

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

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

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

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

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

12. • chlorophyll a – abs
blue-violet, red
400-450, 650-700 nm
• chlorophyll b &
carotenoids – abs
broadly blue-violet
mid-400s
• more wavelengths used for photosynthesis =
more light energy absorbed
Photosynthesis – light absorption

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

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

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

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

17. Photosynthesis – energy transfer
• e- from PS II electron transport chain (ETC) PS I
 NADPH
• e- from PS I 2nd ETC e- carrier: NADP+
• e- in PS II, from split H20

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

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

20. 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
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
• regenerative
• anabolic
• CO2 in, sugar out
• during daylight
CO2
NADP+
ADP
Pi
+
RuBP 3-Phosphoglycerate
Calvin
Cycle
G3P
ATP
NADPH
Starch
(storage)
Sucrose
(export)
Chloroplast
Light
H2O
O2

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

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

26. 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
Pi
+
RuBP 3-Phosphoglycerate
Calvin
Cycle
G3P
ATP
NADPH
Starch
(storage)
Sucrose
(export)
Chloroplast
Light
H2O
O2
• CO2 in  stomata
• open, lose water
• O2 fixed – photorespiration – inefficient
• hot, dry – open stomata less;
lowers water loss, lowers CO2
• fix CO2 into 4-C molecules

29. Photosynthesis – summary
• light reaction: Light energy + H2O
• light-independent:
CO2, NADPH, ATP
O2, NADPH, ATP
Thylakoids
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