Transcript CO 2

Rubisco
(Triose-phosphate)
CALVIN 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
FOR SUGAR OR STARCH - THE REMAINDER IS USED
FOR REGENERATION 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
FOR SUGAR OR STARCH - THE REMAINDER IS USED
FOR REGENERATION OF FIVE-CARBON ACCEPTERS
ATP, NADPH
3 CO2  6 PGA  6 Triose-P
3 RUBP
5 Triose-P
1Triose-P
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 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.
CARBON RECOVERY DURING
PHOTORESPIRATION
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.
Competition between Carboxylation and Oxygenation
Decreases the Efficiency of Photosynthesis
~25%
(Favored by High
Temperatures, High
O2, Low CO2)
~75%
CO2-CONCENTRATION MECHANISMS
 C4 PHOTOSYNTHESIS - Spatial separation of CO2fixation 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 REACTIONS
High Pi favors
sucrose synthesis;
Low Pi favors
starch synthesis