CALVIN CYCLE

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

7. Regenerate RuBP
Use 3 more ATP

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

9. Acid -COOH
Aldehyde
-C=O
H
Priming
Step
Input of
energy
Regenerate
What started
with
Carbon
Fixation
Reduction
Rubisco

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

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

13. Enzymes regulated by thioredoxin

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