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
Photosynthesis - overview
Photosynthesis - overview
Overall purpose:
• photosynthesis –
light chemical energy
• complements
respiration
Energy for all life on
earth ultimately comes
from photosynthesis
Cellular Respiration vs. Photosynthesis
Cellular Respiration:
(Exergonic)
Photosynthesis:
(Endergonic)
Photosynthesis – chloroplast recap
Outer membrane
Inner membrane
Thylakoid membrane
Stroma
Thylakoid space
Intermembrane space
Photosynthesis - overview
• Photosynthesis 1. light rxn: store
energy & split
water – “photo”
NADPH & ATP
2. dark rxn: “fix” CO2
& make sugars –
“synthesis”
Calvin cycle
Redox Reactions
6CO2 + 12H2O + light energy
C6H12O6 + 6O2 + 6H2O
Equation for photosynthesis
Photosynthesis - overview
• light reactions:
– thylakoid membrane
H2O
– thylakoid space
• dark reactions:
– stroma
CO2
Light
NADP+
ADP
+ P
Calvin
Cycle
Light
Reactions
ATP
NADPH
Chloroplast
O2
[CH2O]
(sugar)
Photosynthesis – light absorption
• visible light ~380 to 750 nm
• chloroplast pigments – abs blue-violet & red
- transmit and reflect green
Photosynthesis – light absorption
• pigments:
• chlorophyll a
-energy-absorbing ring
-hydrocarbon tail
• accessory pigments
- chlorophyll b
- carotenoids
- photoprotective
Photosynthesis – light absorption
• 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
Pigments have two states:
ground & excited
• chlorophyll abs light
• e- excited
• more energy
• energy transferred
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)
Photosynthesis – energy transfer
STROMA
Photosystem I
Photosystem II
Light
Light
THYLAKOID SPACE
Thylakoid
membrane
• Photosystem I (PS I) & PS II
• Difference – light wavelength, proteins,  where e- from
Photosynthesis – energy transfer
• PSII: absorbs 680 nm, splits water, powerful ETC, ATP made
• PS I: absorbs 700 nm, e- from PSII, short ETC, NADPH made
(less energy)
Photosynthesis – energy transfer
• e- in PS II, from split H20
• e- from PS II
electron transport chain (ETC)
• e- from PS I
2nd ETC
PS I
e- carrier: NADP+  NADPH
Photosynthesis – chemiosmosis
• How is ATP produced?
Chemiosmosis
• e- down ETC, H+
to thylakoid space
• H+ conc. gradient
• H+ down gradient,
ATP synthase
photophosphorylation
Light reaction - summary
• inputs: light energy, H2O
• PS II, ETC, PS I, ETC
• outputs:
ATP
NADPH
O2 (waste)
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! 
Photosynthesis – energy transfer
“Dark” reaction (Light-independent Reaction)
6CO2 + 12H2O + light energy
• “Dark” reaction:
Calvin cycle
• regenerative
C6H12O6 + 6O2 + 6H2O
H2O
Light
CO2
NADP+
ADP
+ Pi
RuBP 3-Phosphoglycerate
Calvin
Cycle
• anabolic
ATP
• CO2 in, sugar out
NADPH
G3P
Starch
(storage)
Chloroplast
• during daylight
O2
Sucrose
(export)
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)
Reduction
• 3 stages of
Calvin-cycle:
• #2 – reduction
• 3-C reduced
• e- from NADPH
• reduced 3-C: G3P
• 3 stages of
Calvin-cycle:
Regeneration of C-acceptor
• #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
Self-Check
Step of
Photosynthesis
Light reaction
overall
PSII
PSI
“Dark” reaction
overall
Location IN
chloroplast
Inputs
Outputs
ATP
produced
(don’t need #)
e- carriers
loaded
Alternate methods of C fixation
Alternate methods of C fixation
• CO2 in  stomata
H2O
Light
• open, lose
water
CO2
NADP+
ADP
+ Pi
• hot, dry – open stomata less;
lowers water loss, lowers CO2
RuBP 3-Phosphoglycerate
Calvin
Cycle
ATP
NADPH
• O2 fixed – photorespiration – inefficient
G3P
Starch
(storage)
Chloroplast
O
2
• fix CO2 into 4-C molecules
Sucrose
(export)
Photosynthesis – summary
• light reaction: Light energy + H2O
O2, NADPH, ATP
Thylakoids
• light-independent:
CO2, NADPH, ATP
G3P (sugar), RuBP
Stroma
Photosynthesis – summary
Where do photosynthetic products go?
Photosynthesis – in context of big picture
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