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Topics:
1.Regulation of the Calvin Cycle2.Photorespiration3.CO2 concentrating mechanisms4.Sucrose and starch synthesis
Regulation of the Calvin cycle
a.RuBP “activase” b.Light induction of Calvin cycle gene expressionc. Enzyme activites regulated by redox state of the
chloroplast
c: Redox state of stroma: The Ferredoxin-Thioredoxin System
NADPH
Topics:
1.Regulation of the Calvin Cycle2.Photorespiration3.CO2 concentrating mechanisms4.Sucrose and starch synthesis
RUBISCO has a higher affinity for CO2 compared to O2 (lower Km)
Rubisco :
Km (CO2)= 15 μM
Km (O2)= 550 μM
But concentration of O2 is much higher:
Atmophere: 20% O2 and only 0.03% CO2
In solution: CO2 = 12 μM, O2= 265 μM
NET RESULT: a lot of O2 gets “fixed” instead of CO2
This process is called photorespiration.
“The Problem with Oxygen”: RUBISCO reacts with oxygen as well as CO2
(oxygenase/carboxylase)
RuBis Carboxylase/Oxygenase
P-Glycolate (C2)
Glycolate (C2)
Glycine (C2-N)
Serine (C3-N)
Hydroxypyruvate (C3)
Glycerate (C3)
3-P-Glycerate (C3)
Calvin Cycle
Glycine (C2-N) CO2
Rib15bisP (C5) + O2
Glycolate (C2)
Serine (C3-N)
Glycerate (C3)
NH4
NH4Glycolate (C2)
O2
H2O2
Chl.
Per.
Mit
ATP
3xATP
2x NADPH
ATP
Fd
The cost of photorespiration
3x O2 needs 2x ATP and 2x Ferredoxin
AND high temperature increases photorespiration:*Modifies Rubisco’s kinetics: oxygenation more favorable*Decreases the CO2/O2 ratio in solution
What do plants do?
CO2 Concentrating Mechanisms
a) CO2 and HCO3- Pumps: aquatic organisms
b) CO2 concentrating mechanisms: higher plants
CO2 Concentrating Mechanisms
Clicker question: Is there only one type of CO2 concentrating mechanisms in higher plants?
A. YesB. No, there are two. C. N, there are many
CO2 Concentrating Mechanisms
Clicker question: Did these different mechanisms evolved from one common ancestor?
A. Yes, modifications occurred later. B. No, there are two independent origins.C. No , there were many independent origins.
CO2 concentrating mechanisms evolved many times independently: Convergent evolution
CO2 Concentrating Mechanisms
PEP-Carboxylase
CH2
IIC-OPO3
2- + HCO3-
ICOO-
COO-
ICH2 + HPO4
2-
IC=O ICOO-
a) CO2 and HCO3- Pumps: aquatic organisms
b) CO2 concentrating mechanisms: higher plants
PhosphoenolpyruvateOxaloacetate
Could plants just use PEP-carboxylase instead of Rubisco?
C3 + HCO3-
C4
C3 + CO2
HCO3- CO2
RUBISCO
C3
Fixation/carboxylation
C4 transport
Decarboxylation
C3-”recycling”
Principles of CO2 concentration mechanisms
C3 + HCO3-
C4
C3 + CO2
HCO3- CO2
RUBISCO
C3
Fixation/carboxylation
C4 transport
Decarboxylation
C3-”recycling”
CO2 Concentrating Mechanisms
a) C4 Photosynthesis: spacial separationb) Crassulacean Acid Metabolism (CAM):
temporal separation
The C4 carbon cycle: Spatial separation
a. Different Cells: Bundle Sheath cells/ Kranz anatomyb. Within one cell
Kranz (=Wreath) Anatomy
Bundle sheath cells
(V
Single Cell C4 Photosynthesis
Borszczowia
CAM: temporal separationMinimizing water loss
H20 loss/CO2 gained (g)CAM 50-100gC4 250-300gC3 400-500g
CAM: Day/Night switch
Topics:
1.Regulation of the Calvin Cycle2.Photorespiration3.CO2 concentrating mechanisms4.Sucrose and starch synthesis
UDP-Glucose
Triose-P Glc-1-P Glc-NtDP
NTP(ATP/UTP)
PPi
Saccharides
Saccharide Synthesis: Overview
Pi
Plastids: Starch Synthesis
Remember: Cellulose = -D-1,4-glucosyl
Starch is a branched polymer
Regulation of starch and sucrose biosynthesis
Triose-P
Fru-1,6-bisP
Fru-6-P
Glc-6-P
Glc-1-P
UDP-Glc
Suc-6-P
PiPi
PPi
ATP
ADP
PPi
UTP
PiSucrose
SPS Phosphate is generated in the cytosol during sucrose synthesis
Cytosol Plastid
Triose-PTriose-P
Pi Pi
Sucrose Synthesis
Starch Synthesis
Balance: Starch vs Sucrose Synthesis
Regulation of Starch and Sucrose Synthesis
UDP-Glc + Fru-6-P Suc-6P
Sucrose-P Synthase (SPS)
Glc-6-P
SPS-P
SPS
Pi
Glc-1-P
ADP-Glc
ATP
PPi2xPi
Pi
3PGAFerredoxinRed.
ADP-Glc Pyrophosphorylase (AGPase)
