2. Calvin cycle or C3 cycle is the carbon reduction
cycle
Calvin cycle named after its discoverer Melvin
Calvin
In 1961 Melvin Calvin was awarded the Nobel prize
in chemistry for this fundamental discovery
Process is powered by ATP and NADPH (formed
during the light reactions of photosynthesis)
The reaction of the light reaction is:
CO2 +H2O (CH2O) + O2
Calvin cycle takes place in the stroma of chloroplast
3. The Calvin cycle can be subdivided into three
sections:
1. The carboxylation of the C5 sugar ribulose 1,5-
bisphosphate leading to
the formation of two molecules 3-
phosphoglycerate;
2. The reduction of the 3-phosphoglycerate to
triose phosphate; and
3. The regeneration of the CO2 acceptor ribulose
1,5-bisphosphate from
triose phosphate
4. The purpose of the Carbon-fixation (Calvin Cycle)
Reactions
CO2 + NADPH + H+ + ATP
C6H12O6 + NADP+ + ADP + Pi
carbohydrate
Note: synthesis of carbohydrate from CO2 is favorable
only
because coupled to very favorable reactions
NADPH to NADP+
and
ATP to ADP + Pi
5. Carried out by the enzyme “rubisco”
(ribulose 1,5 bisphosphate carboxylase oxygenase)
Carbon Fixation
6. 6 ATP and 6 NADPH used
REDUCTION
1 G3P molecule produced
8. Overview of Carbon-Fixation Reactions
CO2
5 Carbon Sugar
with 2 phosphates on it
3 Carbon Aldehyde
with one phosphate on it
R-C-OH
=
O
R-C-H
Higher
Energy
Compound
=
O
Reduction
+ 3 Carbon Acid
with one phosphate on it
2
ATP
ADP + Pi
NADPH
NADP+
ATP
ADP + Pi
5 Carbon Sugar
with one phosphate on it
6 Carbon Sugar - Glucose
with NO phosphate on it
10. 10
Integration of Light-Dependent
and
Light-Independent Reactions
They generally occur AT THE SAME TIME
O2
CO2
H2O
Light
Light reaction Calvin cycle
NADP+
ADP
ATP
NADPH
+ P 1
RuBP 3-Phosphoglycerate
Amino acids
Fatty acids
Starch
(storage)
Sucrose (export)
G3P
Photosystem II
Electron transport chain
Photosystem I
Chloroplast
11.
12. Regulation of the Calvin
Cycle
Stromal Environment: Increases In Mg2+, NADPH, Fdred, and a decrease in
H+
Activation of RUBISCO by light: addition of CO2 to lysine (201), which is
favored by alkaline pH and increased Mg2+
Regulation of enzyme activities via thioredoxin
LIGHT
14. Photorespiration - “ the OXYGEN PROBLEM”
Oxygen is a competing substrate for the 1st enzyme
in the C3 cycle (Rubisco)
O2
5 Carbon Sugar
with 2 phosphates on it
3 Carbon Acid
with one phosphate on it
+
2 Carbon Acid
with one phosphate on it
Energy
Wasted
With NO
synthesis
of glucose
CO2
5 Carbon Sugar
with 2 phosphates on it
3 Carbon Acid
with one phosphate on it
+
3 Carbon Acid
with one phosphate on it
Net increase
in material
make
glucose with
“extra”
15. Rubisco
RuBp CO2
RuBp
Rubisco
P G A P G A
P G A
Later becomes glucose!
CO2
CO2
O2
Photorespiration – these CO2’s did not get
incorporated into glucose this time!
P G