Topics: 1.Regulation of the Calvin Cycle 2.Photorespiration 3.CO2 concentrating mechanisms 4.Sucrose and starch synthesis

Regulation of the Calvin cycle a.RuBP “activase” b.Light induction of Calvin cycle gene expression c.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 Cycle 2.Photorespiration 3.CO2 concentrating mechanisms 4.Sucrose and starch synthesis

RUBISCO has a higher affinity for CO 2 compared to O 2 (lower K m ) Rubisco : K m (CO 2 )= 15 μM K m (O 2 )= 550 μ M But concentration of O 2 is much higher: Atmophere: 20% O 2 and only 0.03% CO 2 In solution: CO 2 = 12 μ M, O 2 = 265 μ M NET RESULT: a lot of O 2 gets “fixed” instead of CO 2 This process is called photorespiration. “The Problem with Oxygen”: RUBISCO reacts with oxygen as well as CO 2 (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 Glycolate (C2) O2 H2O2 Chl. Per. Mit ATP 3xATP 2x NADPH ATP Fd

The cost of photorespiration 3x O 2 needs 2x ATP and 2x Ferredoxin AND high temperature increases photorespiration: *Modifies Rubisco’s kinetics: oxygenation more favorable *Decreases the CO 2 /O 2 ratio in solution

What do plants do?

CO 2 Concentrating Mechanisms a)CO 2 and HCO 3 - Pumps: aquatic organisms b)CO 2 concentrating mechanisms: higher plants

CO 2 Concentrating Mechanisms Clicker question: Is there only one type of CO2 concentrating mechanisms in higher plants? A. Yes B. No, there are two. C. N, there are many

CO 2 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

CO 2 Concentrating Mechanisms PEP-Carboxylase CH 2 II C-OPO HCO 3 - I COO - I CH 2 + HPO 4 2- I C=O I COO - a)CO 2 and HCO 3 - Pumps: aquatic organisms b)CO 2 concentrating mechanisms: higher plants Phosphoenolpyruvate Oxaloacetate

Could plants just use PEP-carboxylase instead of Rubisco?

C3 + HCO 3 - C4 C3 + CO 2 HCO 3 - CO 2 RUBISCO C3 Fixation/carboxylation C4 transport Decarboxylation C3-”recycling” Principles of CO2 concentration mechanisms

C3 + HCO 3 - C4 C3 + CO 2 HCO 3 - CO 2 RUBISCO C3 Fixation/carboxylation C4 transport Decarboxylation C3-”recycling”

CO 2 Concentrating Mechanisms a)C 4 Photosynthesis: spacial separation b)Crassulacean Acid Metabolism (CAM): temporal separation

The C4 carbon cycle: Spatial separation a.Different Cells: Bundle Sheath cells/ Kranz anatomy b.Within one cell

Kranz (=Wreath) Anatomy Bundle sheath cells

(V Single Cell C4 Photosynthesis Borszczowia

CAM: temporal separation Minimizing water loss H20 loss/CO2 gained (g) CAM g C g C g

CAM: Day/Night switch

Topics: 1.Regulation of the Calvin Cycle 2.Photorespiration 3.CO2 concentrating mechanisms 4.Sucrose and starch synthesis

UDP-Glucose

Triose-P Glc-1-P Glc-NtDP NTP (ATP/UTP) PPi S a c c h a ri d e s 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 Pi PPi ATP ADP PPi UTP Pi Sucrose SPS Phosphate is generated in the cytosol during sucrose synthesis

Cytosol Plastid Triose-P Pi Sucrose Synthesis Starch Synthesis Balance: Starch vs Sucrose Synthesis

Regulation of Starch and Sucrose Synthesis UDP-Glc + Fru-6-PSuc-6P Sucrose-P Synthase (SPS) Glc-6-P SPS-P SPS Pi Glc-1-P ADP-Glc ATP PPi 2xPi Pi 3PGA Ferredoxin Red. ADP-Glc Pyrophosphorylase (AGPase)