1. Lecture Oct 10, 2005
Photosynthesis II. Calvin Cycle

2. Lecture Outline The Calvin Cycle fixes carbon
makes reduced carbon compounds
Reactions of the Calvin Cycle – anabolic pathway
input of NADPH + H+, input of ATP
Regulation of the Calvin Cycle
The problem with oxygen – Photorespiration
Tricks some plants use to limit photorespiration
- C4 anatomy, C4 metabolism – division of labor
- CAM plants, the difference is night and day

3. DARK REACTIONS
energy utilization
The Calvin Cycle

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
energy released is greater than
it costs to make carbohydrate

5. The Calvin cycle has three phases
Carbon fixation
Reduction (energy input, reducing equiv input)
Regeneration of the CO2 acceptor
(energy input – “priming step”)

6. + 2 CO2 3 Carbon Acid 5 Carbon Sugar 5 Carbon Sugar 3 Carbon Aldehyde
Overview of Carbon-Fixation Reactions
R-C-OH = O 12 24
CO2 +
R-C-H
Higher
Energy
Compound = O
Reduction
3 Carbon Acid
with one phosphate on it 2
5 Carbon Sugar
with 2 phosphates on it
ATP
ADP + Pi
ATP
ADP + Pi
NADPH
NADP+ 12 24
20/24
5 Carbon Sugar
with one phosphate on it
3 Carbon Aldehyde
with one phosphate on it
6 Carbon Sugar - Glucose
with NO phosphate on it
4/24 2

7. Carbon Fixation Carried out by the enzyme “rubisco”
(ribulose 1,5 bisphosphate carboxylase oxygenase)
Do need to know this enzyme
Key regulatory enzyme

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

9. 1st Enzyme in Calvin Cycle
Regulation of Rubisco
1st Enzyme in Calvin Cycle
Substrate/Product availability
Allosterically regulated by NADPH and ATP
Very Narrow pH optimum
pH 8 / pH7
pH 8 or above, inactive at 7
Enzyme must be in
reduced form

10. Electron transport chain
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
Figure 10.21 4

11. + + PhotoRespiration - “ the OXYGEN PROBLEM”
Oxygen is a competing substrate for the 1st enzyme
in the C3 cycle (Rubisco)
CO2
5 Carbon Sugar
with 2 phosphates on it
3 Carbon Acid
with one phosphate on it +
Net increase
in material
make
glucose with
“extra” O2
5 Carbon Sugar
with 2 phosphates on it
3 Carbon Acid
with one phosphate on it +
2 Carbon Acid
Energy
Wasted
With NO
synthesis
of glucose 5

12. DIVISION OF LABOR BETWEEN CELLS C4 Plants
Some Plants Deal with this problem by a
DIVISION OF LABOR BETWEEN CELLS
C4 Plants
Mesophyll cells perform “usual”
noncyclic Light-Dependent Reactions
make oxygen, ATP and NADPH
Do NOT perform the C3 (Calvin cycle) reactions
Bundle Sheath cells perform “UNusual”
cyclic Light-Dependent Reactions
a lot of ATP but
very little NADPH
and very little O2
PERFORM the usual C3 (Calvin Cycle) reactions 6

13. C4 leaf anatomy and the C4 pathway
Mesophyll
cell
Mesophyll
cell
Photosynthetic
cells of C4 plant
leaf
PEP carboxylase
CO2
CO2
Bundle-
sheath
cell
Oxaloacetate (4 C)
PEP (3 C)
NADPH
used
ADP
Vein
(vascular tissue)
Malate (4 C)
ATP
C4 leaf anatomy
Malate (4 C)
Oxaloacetate (4 C)
Pyruate (3 C)
NADPH
regenerated
CO2
Mesophyll
Produce NADPH and ATP
PEP carboxylase insensitive to O2
Malate brings across reducing equivalents
Stomata
Bundle-
Sheath
cell
Cyclic
e- flow
Little O2
CALVIN
CYCLE
Sugar
Vascular
tissue
Figure 10.19

14. Mesophyll Cell C4 Metabolism
PEP
carboxylase
CO2
NADPH + H+
NADP+ OH /
C=O
CH2
C=O + Pi \
Oxaloacetate
4C acid OH /
C=O
CH2
H-C-O-H \
malate
4C acid
CH3
C=O \
O-Pi
“PEP”
ATP
ADP
CH3
C=O \
O-H
pyruvate
“carries”
Reducing
equivalents
Needs ATP and NAPDPH + H+
Non-cyclic electron flow

15. Bundle Sheath - Cell C4 Metabolism
CO2
CH3
C=O \
O-H
pyruvate OH /
C=O
CH2
H-C-O-H \
malate
4C acid OH /
C=O
CH2 \
Oxaloacetate
4C acid
NADPH + H+
NADP+
“carries”
Reducing
equivalents
ATP
Calvin
Cycle
glucose
NADPH
Needs ATP Only
Cyclic electron flow
Very low O2

16. Who cares as long as the sun is shining ?
Mesophyll cells provide a means for bundle sheath
cells to acquire NADPH + H+ reducing power
Mesophyll cells provide carbon dioxide to bundle sheath
cells at higher concentration than in air
Bundle Sheath cells not making oxygen, so very little
competitor with C3 reactions
Costs more energy to do business this way…
but has the advantage when CO2 is limiting
(when stomates are closed - like on hot days)
Who cares as long as the sun is shining ?
ATP is not limiting 7

17. CAM Plants Cacti, pineapple
Open their stomata only at night, too hot during day – survive very adverse (dry) conditions
NIGHT
Perform PEP carboxylase reaction at night (CO2 assimilation)
accumulate malate to high concentration in central vacuole
use sugar oxidation/catabolism to power (NADH and ATP)
carbon fixation
DAY
Perform “light” reactions during the day
mostly cyclic e- flow to produce ATP (low O2)
decarboxylate malate to yield CO2 and NADPH + H+
perform C3 reactions (Calvin Cycle) to produce
sugars and starch 2

18. Generally Slow growing Sugar Oxidation Sunlight Powers Powers One Both
Spatial separation
of steps. In C4 plants, carbon fixation and the Calvin cycle occur in different
types of cells.
(a)
Temporal separation
of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cells at different times.
(b)
Pineapple
Sugarcane
Bundle-
sheath cell
Mesophyll Cell
Organic acid
CALVIN CYCLE
Sugar
CO2 C4
CAM
CO2 incorporated
into four-carbon
organic acids
(carbon fixation)
Night
Day 1 2
Organic acids
release CO2 to
Calvin cycle
Figure 10.20
Generally
Slow
growing
Sugar
Oxidation
Powers
One
Phase
Sunlight
Powers
Both
phases

19. Summary Photosynthetic “light” reactions produce
ATP and reducing potential NADPH + H+
2. Dark reactions use ATP and reducing potential
to synthesize carbohydrates
- powers reduction of 3-carbon acid
to 3-carbon aldehyde
- powers regeneration of starting material
5-carbon di-phosphate (priming step for CO2 fixation)
3. Rubisco enzyme regulated tightly by allosteric modulators
pH, and reducing status of stroma
4. O2 interferes with carbon fixation by Rubisco enzyme
5. Metabolic “tricks” to avoid photorespiration
- C4 metabolism - CAM metabolism