Lecture #15-17.  Photosynthesis II: Carbon Reduction.

THREE MAIN STAGES OF C3 REDUCTION PATHWAY:  
   3 CO_{2 +} 3 RuBP + [6NADPH + 9ATP] ---> 1 G3P + 3 RuBP

1. CARBOXYLATION:
3 RuBP (5C-P) + --> 6 PG Acid (3 C-P)

2. REDUCTION OF P-GLYCERATE:
6 PGA --> 6 G3P --> 5 G3P + 1 G3P
   NADPH, ATP

3. REGENERATION OF CO₂ ACCEPTOR:
5 G3P --> ----> -->  3 RubP
                         ATP     CO₂ acceptor

REGULATION OF KEY ENZYMES BY LIGHT

C4 and CAM

PHOTORESPIRATION:

A process where O₂ is consumed and CO₂ is evolved in light by photosynthetic tissues.

C4 METABOLISM: A mechanism to concentrate CO₂ in the chloroplasts to reduce RUBP Oxygenase activity.
C4 PLANTS: Corn, SUGAR CANE, MANY WEEDS

Separate reactions in space:  MESOPHYLL CELL
1. CARBOXYLATION:
CO₂ + PEP (3C)---> OAA (C4) ---> MAL (C4)
TRANSPORT TO BUNDLE SHEATH CELLS

2. DECARBOXYLATION: MAL (C4) ---> CO₂ + PVA

3. CALVIN CYCLE: 3CO₂ --> G3P
TRANSPORT OF PVA BACK TO MESOPHYLL

4. REGENERATION OF PEP:   PVA ---> PEP

CAM (CRASSULACEAN ACID METABOLISM)

-CAM PLANTS MINIMIZE WATER LOSS BY OPENING STOMATA AT NIGHT AND CLOSING THEM IN THE DAY. ---THESE PLANTS FIX CO₂ AT NIGHT, AND RELEASE (BY DECARBOXYLATION) CO₂ IN THE DAY FOR REDUCTION BY THE CALVIN CYCLE.
-DAY: HIGH [CO₂] CONC.MINIMIZES PHOTORESPIRATION.

CAM SIMILAR TO C4 METABOLISM EXCEPT REACTIONS SEPARATED IN TIME.

NIGHT PEP Case
1. Carboxylation: PEP + CO₂ ---> OAA + MAL
2. Mal is stored in the vacuole

DAY
3. Mal is transported back to the cytoplasm
4. Decarboxylation:
   MAL + NADP⁺ -(NADP MALIC ENZYME) ->
   PVA + CO₂ + NADPH

5. Carbon Reduction Cycle: CO2 -> PGA -> HEXOSE

6. Regeneration of PEP:
PVA + ATP + P-(PYRUVATE DIKINASE)-> PEP +AMP + PP

EXPORT AND STORAGE OF PHOTOSYNTHETIC PRODUCTS

1. STARCH SYNTHESIS IN THE CHLOROPLAST: G3P-->---> Starch

2. SUCROSE SYNTHESIS IN THE CYTOPLASM:

G3P MOVES INTO CYTOPLASM.
G3P is exported via a triose-P translocator in exchange for Pi.

G3P--> --> SUC

FATE: USED FOR ENERGY AND SYNTHESIS
EXPORTED TO SINKS VIA PHLOEM

FATE OF SUCROSE IN SINKS:

1. -SUC. --> HEXOSES --> RESPIRATION & SYNTHESIS

2. -SUC. --> HEXOSES --> STARCH FOR STORAGE

Starch breakdown and export from chloroplast at night

Starch --[amylase]--> maltose (2 glu) ---> glucose ---> G3P

Glucose is exported via a glucose transporter.
Maltose is exported via MEX1 (maltose transporter)

Genetically-modified foods

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Exercises

I.   What is the signficance of generating NADPH, and ATP in the light reactions?
 

a.  What is NADPH used for?  Show the reaction and name the enzyme.  What is the importance of this reaction?
 
 

b.  What is ATP used for?  Show the reactions  (there are 2) and the names of the enzymes.  What is the significance of these reactions?
 
 

c.  What is meant by the 'carbon reactions'?
Where do these reactions occur (organelle and location) and can these reactions proceed in the night?   Why or why not?
 
 
 

d.  Fixation of 3 CO₂ by the C3 or Calvin cycle  results in ________ (the main product of the C3 cycle.   The product  is exported from the ___________ ( organelle) using _____________ (which transporter).    The transporter takes up ________ (a nutrient) which is need to make _____ in the light reactions.

e.  What is photorespiration?

The uptake of oxygen in photorespiration takes place in the _________(mitochondria, chloroplast).
Why?
 
 
 
 
 
 

II.  Starch accumulates in spinach leaves in the day (60 units) and its levels drop at night (2 units).  Spinach is a C3 plant.  Several enzymes in the leaf show higher activity in the day than at night.  This question examines how starch is made and used, and how the enzymes are regulated.

a) What reactions do these enzymes catalyze?
Rubilose bisphosphate carboxylase (RuBP Case):    Significance?
[Full credit for Chemical structures]
 
 
 
 

NADP-glyceraldehyde 3-phosphate dehydrogenase (NADP-G3P DH)
[Full credit for Chemical structures]
 
 
 
 

b)   How is starch synthesized from a triose phosphate and what reaction does Starch Synthase catalyze?
 
 
 
 
 
 

c)  [8] Starch accumulates in the day in part because RuBP Case and G3P DH activities increase in the day and drop at night.  What specific changes (at least 2) occur in the day that result in high enzyme activity? and why?

III.  C4 metabolism in corn or sugar cane
a.    what reaction is catalyzed by RuBP Case when [CO₂] is high and when [CO₂] is low?
 
 
 
 

b.  C4 plants contain the following enzymes.
What reaction do the following enzymes catalyse ?
  PEP Case:                                                   Active in ______[cell type ]_________[day/night]
 

 NADP-Malic enzyme                                   Active in _______[cell type]_________[day/night]
 

c.   Plants with C4 pathway survive in dry and hot climates better than C3 plants. Why?