Rubisco

(Triose-phosphate)

CALVIN CYCLE: NET REACTIONCALVIN CYCLE: NET REACTION

6CO2 + 11H20 + 12 NADPH + 18ATP

Fructose-6-phosphate + 12 NADP+ + 6H+ + 18ADP 17 Pi

ONLY ONE-SIXTH OF THE TRIOSE PHOSPHATE IS USED ONLY ONE-SIXTH OF THE TRIOSE PHOSPHATE IS USED FOR SUGAR OR STARCH - THE REMAINDER IS USED FOR SUGAR OR STARCH - THE REMAINDER IS USED FOR FOR REGENERATIONREGENERATION OF FIVE-CARBON ACCEPTERS OF FIVE-CARBON ACCEPTERS

C3 + C3 C6

C6 + C3 C5 + C4

C4 + C3 C7

C7 + C3 C5 + C5

NET: 5C3 3C5 --------------------------------------------------------

ONLY ONE-SIXTH OF THE TRIOSE PHOSPHATE IS USED ONLY ONE-SIXTH OF THE TRIOSE PHOSPHATE IS USED FOR SUGAR OR STARCH - THE REMAINDER IS USED FOR SUGAR OR STARCH - THE REMAINDER IS USED FOR FOR REGENERATIONREGENERATION OF FIVE-CARBON ACCEPTERS OF FIVE-CARBON ACCEPTERS

ATP, NADPH

3 CO2 6 PGA 6 Triose-P

1Triose-P5 Triose-P

3 RUBP

THE FERREDOXIN-THIOREDOXIN SYSTEM ACTIVATES FIVE SPECIFIC CALVIN CYCLE ENZYMES IN THE LIGHT

RUBISCO

NADP:glyceraldehyde-3-phosphate dehydrogenase

Fructose-1,6-bisphosphatase

Sedoheptulose-1,7-bisphosphatase

Ribulose-5-phosphate kinase

PHOTORESPIRATIONPHOTORESPIRATION

RUBISCO can also react with O2 :

RUBP + O2 3-PGA + 2-phosphoglycolate

3-PGA can be used in the Calvin Cycle.

One of the carbons in 2-phosphoglycolate can be recovered via a salvage pathway. The other carbon is lost as CO2.

The salvage pathway is called “photorespiration” because it uses O2, gives off CO2, and only occurs in the light (because RUBP is only produced in the light).

Occurs in three organelles: chloroplast, peroxisome, and mitochondrion.

OVERALL, TWO MOLECULES OF PHOSPHOGLYCOLATE (FOUR CARBON ATOMS), LOST FROM THE CALVIN CYCLE BY OXYGENATION OF RUBP, ARE CONVERTED INTO ONE MOLECULE OF GLYCERATE (A THREE CARBON COMPOUND) AND ONE CO2.

IN OTHER WORDS, 75% OF THE CARBON LOST BY THE OXYGENATION OF RUBP IS RECOVERED BY THE PHOTORESPIRATORY PATHWAY AND RETURNED TO THE CALVIN CYCLE.

CARBON RECOVERY DURING PHOTORESPIRATION

Competition between Carboxylation and Oxygenation Competition between Carboxylation and Oxygenation Decreases the Efficiency of PhotosynthesisDecreases the Efficiency of Photosynthesis

~25%

~75%

(Favored by High Temperatures, High O2, Low CO2)

COCO22-CONCENTRATION MECHANISMS-CONCENTRATION MECHANISMS

C4 PHOTOSYNTHESIS - Spatial separation of CO2-fixation and the Calvin Cycle; Kranz leaf anatomy.)

CRASSULACEAN ACID METABOLISM (CAM) - Temporal separation of CO2-fixation and the Calvin Cycle; stomata open at night.

CO2 PUMPS ON PLASMA MEMBRANE - Only in algae and cyanobacteria

POA, A C3 MONOCOT

SUGAR CANE, A C4 MONOCOT

FLAVERIA, A C4 DICOT

THE SYNTHESIS OF SUCROSE AND STARCH ARE COMPETING REACTIONSTHE SYNTHESIS OF SUCROSE AND STARCH ARE COMPETING REACTIONS

High Pi favors sucrose synthesis; Low Pi favors starch synthesis