Rubisco (Triose-phosphate)

CALVIN CYCLE: NET REACTION 6CO H NADPH + 18ATP  Fructose-6-phosphate + 12 NADP + + 6H ADP 17 P i

ONLY ONE-SIXTH OF THE TRIOSE PHOSPHATE IS USED FOR SUGAR OR STARCH - THE REMAINDER IS USED FOR REGENERATION OF FIVE-CARBON ACCEPTERS C 3 + C 3  C 6 C 6 + C 3  C 5 + C 4 C 4 + C 3  C 7 C 7 + C 3  C 5 + C 5 NET: 5C 3  3C

ONLY ONE-SIXTH OF THE TRIOSE PHOSPHATE IS USED FOR SUGAR OR STARCH - THE REMAINDER IS USED FOR REGENERATION OF FIVE-CARBON ACCEPTERS ATP, NADPH 3 CO2  6 PGA  6 Triose-P 1Triose-P 5 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

PHOTORESPIRATION  RUBISCO can also react with O 2 : RUBP + O 2  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 CO 2.  The salvage pathway is called “photorespiration” because it uses O 2, gives off CO 2, 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 CO 2. 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 Decreases the Efficiency of Photosynthesis ~25% ~75% (Favored by High Temperatures, High O 2, Low CO 2 )

CO 2 -CONCENTRATION MECHANISMS  C 4 PHOTOSYNTHESIS - Spatial separation of CO 2 - fixation and the Calvin Cycle; Kranz leaf anatomy.)  CRASSULACEAN ACID METABOLISM (CAM) - Temporal separation of CO 2 -fixation and the Calvin Cycle; stomata open at night.  CO 2 PUMPS ON PLASMA MEMBRANE - Only in algae and cyanobacteria

POA, A C 3 MONOCOT

SUGAR CANE, A C 4 MONOCOT

FLAVERIA, A C 4 DICOT

THE SYNTHESIS OF SUCROSE AND STARCH ARE COMPETING REACTIONS

High P i favors sucrose synthesis; Low P i favors starch synthesis