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)