Photosynthesis
Life from Light and Air
Obtaining the
materials
– sunlight
• leaves = solar collectors
– CO2
• stomates = gas exchange
– H2O
• uptake from roots
– nutrients
• N, P, K, S, Mg, Fe…
• uptake from roots
Concurrent Reactions
Light-Dependent Reactions
*Key to these rxns is light absorption
Success lies in photosystems – 2 part complex
1. Antenna complex – site of
absorption; excites electrons &
passes them on
2. Reaction center – redox-rxns
occur here; excited ele- transferred
to electron acceptor
reaction
center
antenna
pigments
To Calvin Cycle!!!
ATP!
NADPH
ADP + P
PS 680
PS 700
NADP + + 2H +
2H+ + ½ O2 +2 2
- pheophytin
- plastoquinone
- Cytochrome
complex
- H + ion
- ferredoxin
- ele -
- NADP+
Reductase
- ATP
Synthase
Step 1: Light absorbed in PS II excites ele(from H2O splitting) → passed to electron
acceptor (pheophytin)
Step 2: Plastoquinone(PQ) shuttles high energy elethrough series of redox rxns (ETC 1); passes
through cytochrome complex, driving in H+, ↑
[H+] = ↓ pH in the ETC
Step 3: De-energized ele- are passed off to protein
plastocyanin (PC) → PS I re-excites ele- and ferredoxin
shuttles through ETC 2
Step 4: a) Ele- reduce NADP+ to NADPH at NADP+ reductase
b) H+ pumped out via ATP Synthase – activates
chemiosmosis → production of ATP from ADP
Regulation of the Light Reaction
– Calvin cycle uses more ATP
than NADPH
– If there is not enough
electrons from PS1, cycles back
to PS11 and NADPH
production is turned off
ATP
18 ATP +
12 NADPH
 1 C6H12O6
Review
Animations
• Light dependent reactions
Light-INdependent Reactions
•Want to make C6H12O6 → synthesis
**How? From what?
What raw materials are available?
CO2
NADPH
carbon fixation
NADP
C6H12O6
reduces CO2
NADP
From CO2  C6H12O6
• CO2 has very little chemical energy
– fully oxidized
• C6H12O6 has a LOT of chemical energy
– highly reduced
• Synthesis = endergonic process
– put in a lot of energy
• Reduction of CO2  C6H12O6 proceeds in many
small uphill steps
– each catalyzed by specific enzyme
– uses energy stored in ATP & NADPH ←from light rxns!
Calvin cycle
C
C
1C
C C C C C
C C C C C
3. Regeneration
of RuBP
C C C C C
RuBP
ribulose bisphosphate
starch,
sucrose,
cellulose
& more
C= C= C
H H H
| | |
C– C– C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
RuBisCo
ribulose
bisphosphate
carboxylase
3 ADP
used
to make
glucose
CO2
5C
glyceraldehyde-3-P
G3P aka PGAL
C C C
3C
6 NADP
C C C C C C
6C
PGA
phosphoglycerate
C C C C C C
3C
C
C
C
C
C
C
C
C
C
C
C
C
6 ATP
2. Reduction
6 NADPH
1. Carbon fixation
C C C C C C
5C
3 ATP
C
3C
6 ADP
C
C
C
C
C
C
H
|
H
|
H
|
A little more about RuBisCo…
•Enzyme which fixes carbon from air
•ribulose bisphosphate carboxylase
• the most important enzyme in the world!
**It makes life out of air!**
• most abundant enzyme
“Banking”
• Keeping the books straight…
– 1 G3P = 3C  each turn of the cycle uses 1 CO2
– 3 turns of Calvin cycle = 1 G3P
– 3 CO2  1 G3P (3C)
– 6 turns of Calvin cycle = 1 C6H12O6 (6C)
– 6 CO2  1 C6H12O6 (6C)
– 18 ATP + 12 NADPH  1 C6H12O6
– Left over ATP from light reactions used elsewhere by
cell
Sum it Up: Light Reactions
H2O + light

energy
H 2O
+
NADPH
+ O2
 produces ATP &
NADPH
 releases O2 as waste
sunlight
Energy Building
Reactions
NADPH
ATP
O2
ATP
Calvin Cycle
CO2 +
ATP
+
NADPH

CO2
ADP
NADP Sugar
Building
Reactions
NADPH
ATP
sugars
C6H12O6
+
ADP
+
NADP
 builds sugars
 uses ATP &
NADPH
 recycles ADP &
NADP+
Supporting a
biosphere
• Big Picture: photosynthesis is THE
most important process for continuation of life
on Earth
• Each year photosynthesis…
• captures 121 billion tons of CO2
• synthesizes 160 billion tons of carbohydrates
• heterotrophs depend on plants for
food, fuel, & raw materials
Light Depedent Review:
• Where did the energy come from?
• Where did the electrons come from?
• Where did the H2O come from?
• Where did the O2 come from?
• Where did the O2 go?
• Where did the H+ come from?
• Where did the ATP come from?
• What will the ATP be used for?
• Where did the NADPH come from?
• What will the NADPH be used for?
Light Independent
Review II:
6CO2 + 6H2O + light
 C6H12O6 + 6O2
energy
• Where did the CO2 come from?
• Where did the CO2 go?
• Where did the H2O come from?
• Where did the H2O go?
• Where did the energy come from?
• What’s the energy used for?
• What will the C6H12O6 be used for?
• Where did the O2 come from?
• Where will the O2 go?
• What else is involved…not listed in this equation?
Adaptations in
Alternative
Environments
C4 Path
• CO2 stored
in mesophyll
(enzyme has
an affinity for
CO2 and can
fix carbon) &
released in
vascular
tissue
Adaptations in
Alternative
Environments
CAM Path
• Stomata open at
night to preserve
water– CO2 stored
in vacuoles for
later use
Combating
Photorespiration
Photorespiration – rxn between RuBP &
O2 when [O2] is high → uses ATP &
produces CO2 instead of making ATP
Favored ONLY when
[O2] exceeds [CO2]
Store
CO2
C4 Path
CAM Plants
Evolution!
Atmosphere had less
O 2 than today. Rubisco
retains some affinity
for O2