CO2 concentrating mechanisms

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• Topics
• CO2 concentrating mechanisms
• Starch and Sucrose synthesis

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RuBis Carboxylase/Oxygenase
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The cost of photorespiration
Why do they do this?
3x O2 needs 2x ATP and 2x Ferredoxin
AND high temperature increases photorespiration
Modifies Rubiscos kinetics oxygenation more
favorable Decreases the CO2/O2 ratio in solution
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What do plants do?
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CO2 Concentrating Mechanisms

1. CO2 and HCO3- Pumps
2. C4 Photosynthesis
3. Crassulacean Acid Metabolism (CAM)

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CO2 and HCO3- Pumps Aquatic Organisms
Cyanobacteria Thylakoid
Carboxysomes Rubisco, Carbonic anhydrase
Nevo et al., 2007
(Tanaka et al., 2008 Yeates Lab, UCLA)
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CO2 Concentrating Mechanisms

1. CO2 and HCO3- Pumps
2. C4 Photosynthesis
3. Crassulacean Acid Metabolism (CAM)

PEP-Carboxylase
CH2 II C-OPO32- HCO3- I COO-
COO- I CH2 HPO42- I CO I COO-
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Could plants just use PEP-carboxylase instead of
Rubisco?
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HCO3-
CO2
Fixation/carboxylation
C3 HCO3-
C3-recycling
C3
C4
C4 transport
C3 CO2
Decarboxylation
RUBISCO
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The C4 carbon cycle Spatial separation

1. Different Cells Bundle Sheath cells/ Kranz
   anatomy
2. Within one cell

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Kranz Anatomy
Bundle sheath cells
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The C4 carbon cycle Spatial separation
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The C4 carbon cycle Modifications
Convergent evolution
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(V
Single Cell C4 Photosynthesis
Voznesenskaya et al., 2002
Bienertia
Borszczowia
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CO2 Concentrating Mechanisms

1. CO2 and HCO3- Pumps
2. C4 Photosynthesis
3. Crassulacean Acid Metabolism (CAM)

PEP-Carboxylase
CH2 II C-OPO32- HCO3- I COO-
COO- I CH2 HPO42- I CO I COO-
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CAM temporal separation Minimizing water loss
H20 loss/CO2 gained (g) CAM
50-100g C4 250-300g C3 400-500
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CAM Day/Night switch
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CO2 Concentrating Mechanisms

1. CO2 and HCO3- Pumps
2. C4 Photosynthesis
3. Crassulacean Acid Metabolism (CAM)

PEP-Carboxylase
CH2 II C-OPO32- HCO3- I COO-
COO- I CH2 HPO42- I CO I COO-
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PEPC regulation
CAM
C4
Metabolites Salt stress Circadian clock
PEPC-Kinase
ATP
ADP
ATP
ADP
PEPC-Kinase
PEPC-Kinase
PEPC
PEPC
PEPC
PEPC
P
P
P
P
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• Topics
• CO2 concentrating mechanisms
• Starch and Sucrose synthesis

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Saccharide Synthesis Overview
Saccharides
Pi
Triose-P
Glc-1-P
Glc-NtDP
NTP (ATP/UTP)
PPi
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Plastids Starch Synthesis
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Starch is a branched polymer
Remember Cellulose b-D-1,4-glucosyl
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PHOSPHOROLYTIC
HYDROLYTIC
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Saccharide Synthesis Overview
Saccharides
Pi
Triose-P
Glc-1-P
Glc-NtDP
NTP (ATP/UTP)
PPi
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Pi
Sucrose
Suc-6-P
UDP-Glc
PPi
UTP
SPS
Glc-1-P
Glc-6-P
Fru-6-P
ATP
Pi
Pi
PPi
ADP
Fru-1,6-bisP
Triose-P
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Regulation of Starch and Sucrose Synthesis
UDP-Glc Fru-6-P
Suc-6P
SPS
Glc-6-P
Pi
SPS-P
Glc-1-P
Pi
ATP
Sucrose-P Synthase (SPS)
3PGA
PPi
2xPi
Ferredoxin Red.
Fru-6-P
Fru-2,6-bisP
ADP-Glc
Pi
ADP-Glc Pyrophosphorylase (AGPase)
Fru-1,6-bisP
Fru-1,6-bisP-Phosphatase
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Balance Starch vs Sucrose Synthesis
Cytosol
Plastid
Sucrose Synthesis
Triose-P
Triose-P
Pi
Pi
Starch Synthesis