Photosynthesis II

February 25, 2009

 Pp. 157-162



Summary Figures

5.  The Calvin Cycle

ATP and NADPH created in the "light reactions" now used to reduce CO2 to carbohydrate

Pathway discovered by Calvin using radiotracer 14-C

Three steps: 1) carboxylation of a 5-carbon sugar (ribulose bisphosphate = RUBP)
                    2) reduction of attached CO2
                    3) regeneration of RUBP

These reactions occur in stroma of the chloroplast

Enzyme RuBP Carboxylase/Oxidase (abbreviated RuBisCO) has both a carboxylating AND an
                 oxygenating function - will cause problems (later)

RuBisCO is slow - plants needs a lot.  ~50% of all leaf protein is RuBisCO.  Most abundant protein
                on earth

RuBisCO carboxylates RUBP (RUBP + CO2) to 2 - 3 carbon phosphoglycerate (PGA)

PGA + PGAKinase + ATP ---> 1,3 - bi - phosphoglycerate + ADP

1,3 - bi - phosphoglycerate + NADPH ---> Glyceraldehyde - 3- phosphate (G3P) + NADP + Pi

some G3P to storage and ultimate formation of sugars

some G3P regenerated RUBP:  G3P + ATP ---> RUBP + ADP

Calvin cycle overview

Complete Calvin cycle needs to turn 6 times to produce 1 glucose molecule

Formation of 1 molecule of glucose requires 18 ATP and 12 NADPH

Since first intermediate of Calvin cycle is a 3 - carbon PGA, often called C3 photosynthesis
 

6. Photorespiration

Recall that RuBisCO also has an oxygenating function - catalysed on the same site as carboxylation

two reactions compete:  RUBP + CO2 ---> 2 PGA   OK, but
                                      RUBP + O2 ---> phosphoglycolic acid + CO2 + PGA

consumes O2, released CO2 previously fixed.

Looks like respiration, but NO ATP or NADPH derived = photorespiration

at 25oC, carboxylation is 4X oxidation - so, 20% fixed carbon lost to photorespiration

photorespiration increases with temperature, and with increasing O2:CO2

25 - 50% fixed carbon lost to photorespiration

Many plants that live in high temperature environments have adopted a strategy to  minimize
               photorespiration.
 

7. C4 Photosynthesis

Many tropical plants (corn, sugar cane, sorghum, etc.) have enzyme phosphoenolpyruvate carboxylase
                 (PEPcase) that carboxylates PEP

PEP + CO2 ---> oxaloacetate (a 4-carbon compound).  Called C4 photosynthesis

Oxaloacetate converted to malate and moved into bundle sheath cells

In bundle sheath cells, malate decarboxylated into CO2 and pyruvate

CO2 fed into Calvin cycle, pyruvate moved back to mesophyll - PEP

C4 photosynthesis is a spatial partitioning: no oxidative function of PEPcase
                                                                  high concentration of CO2 in bundle sheath

Costs higher - C4 requires 30 ATP per molecule of glucose, but, avoids photoresp. losses

C4 plants have a unique leaf anatomy.  Compare C3 leaves with C4 leaves

Kranz (German for "wreath") anatomy.  Mesophyll cells clustered around bundle sheath cells.

C4 plants ~ 100 genera in 18 families.  Some genera with both C3 and C4 species.
 

8. CAM Photosynthesis

Another strategy used in desert succulent plants (Red Barrel Cactus, Sempervivum tectorum)

First discovered in species of Crassula (C. falcata, C. portulaceae, C. in habitat)

CAM = Crassulacean Acid Metabolism

Stoma open at NIGHT.  CO2 carboxylated using PEPcase.  Oxaloacetate formed

Oxaloacetate converted to malate - stored in large vacuoles.

In day, stoma close.  Malate decarboxylated - CO2 fed into Calvin cycle

Occurs within same cell.  A temporal partitioning

Stoma open at night when R.H. higher, close in day to prevent water loss.

Decarboxylation of malate creates low O2:CO2 - minimized photorespiration
 

Review Questions:
1) What is the first product of the Calvin cycle (also the reason for the name "C3" photosynthesis)?
2) Ribulose-bis-phosphate carboxylase/oxygenase creates problems for the plant.  How?
3) The Calvin cycle needs 6 molecules of CO2 to make one molecule of glucose.  How many ATP and NADPH molecules are required?
4)What is the fate of glyceraldehyde-3-phosphate (G3P)? (two answers)
5) What is photorespiration?  Does it have anything to do with the light reactions (photo) or respiration?
6) How do C4 plants reduce photorespiration, and what is the cost of doing so?
7) Contrast the spatial partitioning of C4 and the temporal partitioning of CAM.
8) What is Kranz anatomy?  Where would you expect to find it?
9) Explain how the C4 pathway is conservative with water loss.  Do the same for the CAM pathway.
10) How do we know that C4 and CAM evolved independently several times?
 
  Key Terms:  Calvin cycle, Ribulose bisphosphate carboxylase/oxygenase (RuBISCO), RUBP, PGA, Glyceraldehyde 3-phosphate (G3P), photorespiration, Phosphoenol pyruvate carboxylase (PEPCase), C4 photosynthesis, oxaloacetate, malate, bundle sheath, Kranz anatomy, Crassulacean acid metabolism (CAM photosynthesis)