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Calvin Cycle
Photosynthesis: Carbon Reactions
1. Calvin cycle
1.
2. C2 Oxidative Photosynthetic Carbon Cycle or
Photorespiration
3.
3. CO2 Concentrating Mechanisms
CO2 Pumping
C4 Photosynthesis
Crassulacean Acid Metabolism (CAM)
Carboxylation = CO2 Fixation
2.
Reduction of 3-phosphoglycerate
Rubisco
1. ATP phosphorylates
3-phosphoglycerate
Ribulose-1,5-bisphosphate carboxylase/oxygenase
1. 40% of chloroplast protein
2. 500:1 ratio of enzyme to CO2
2. NADPH reduces
1,3-phosphoglycerate to
Glyceraldehyde 3-phosphate
Regeneration of Ribulose 1,5-bisphosphate
Unraveling the Calvin Cycle
Rubulose 1,5-bisphosphate
3
5
6
5
5
6
3
4
5
7
5
3
7
5
1
Calvin Cycle Summary
1. Autocatalytic – makes more substrate (RuBP) than consumed
2. Fate of triosphophate: 5/6 Regeneration of RuBP
1/6 exported for sucrose synthesis
3. To fix 6 molecules of CO2 requires
18 ATP
12 NADPH
4. Energy efficiency of the Calvin Cycle based on free energies
1 mole hexose = 2804 kJ (2016 cal)
12 NADPH = (12 X 217) = 2804 kJ (624 cal) from NADPH
18 ATP
= (18 X 29) = 522 kJ (126 cal) from ATP
= 3126 kJ (750 cal) from NADPH & ATP
Efficiency = energy stored / energy used
= 2804 / 3126 ≈ 90%
5. Sources of Energy: Energy:
6. Thermodynamic efficiency:
END Calvi
n Cycle
83% from NADPH
17% from ATP
= 2804 kJ hexose / 8400 kJ light energy
≈ 33%
C2 Oxidative Photosynthetic Carbon Cycle &
Photorespiration
Photorespiratory Cycle
1. Three organelles involved
1. Rubisco catalyzes oxygenation and carboxylation of RuBP
A. When [CO2] = [O2]… CO2 fixed 80X faster than O2
B. In air [CO2] << [O2]… CO2 fixed 3X faster than O2
2. Photorespiration is oxygen consumption and oxygenation of RuBP
3. C2 oxidative photosynthetic cycle results in partial recovery of fixed
Carbon
Photorespiratory Cycle
Chloroplast
Photorespiratory Cycle
2 Glycolates from Chloroplast
Peroxisome
2 Glycolates leave Chloroplast
2 Glycines Leave Peroxisome
2
Photorespiratory Cycle
Photorespiratory Cycle
αketoglutarate Out to Chloroplast
Ammonium Nitrogen
returns to Chloroplast
2 Glycines from
Peroxisome
Glycerate Out
to Chloroplast
Serine Leave
Mitochondrion
Mitochondrion
One CO2 Lost
Two 2-Carbon
Amino Acid
Glycines
One 3-Carbon
Amino Acid Serine
Peroxisome
Serine In
Photorespiratory Cycle
Photorespiratory Cycle
Chloroplast
NH4+ from
mitochondrion
Glutamate to
Peroxisome
αkGlutarate
Glycerate
From Peroxisome
Consequences of Photorespiration
CO2 Concentration Mechanisms
1. Loss of 25% carbon fixed in photosynthesis
2 glycine (2c amino acid) Æ 1 serine (3c amino acid) + 1 CO2
2. Photorespiration lowers efficiency of photosynthesis from
90% to 50%.
1. CO2 Pumps
2. C4 Photosynthetic Carbon Fixation
3. Crassulacean Acid Metabolism (CAM)
3. Warmer temperature reduce CO2 solubility faster than O2.
Implications:
A. Increasing temp Æ Increasing Photorespiration
4. Function of Photorespiration
A. Dissipation of excess energy under high light
B. Recapture 75% carbon potentially lost (from glycolate).
3
Carbon Dioxide and Bicarbonate Pumps
C4 Photosynthetic Carbon Fixation
In cyanobacteria and algae
Suppresses photorespiration
1. Adaptation to minimize Photorespiration in high
light, low water conditions
2. Requires two CO2 fixations spatially separated
in the leaf
CO2 gas
channel
3. Four stages
1. Fixation of CO2 into a 4 Carbon acid in mesophyll by PEP
2. Transport of C4 acid to bundle sheath cells
3. Decarboxylation of C4 acid &
CO2 generation &
Calvin Cycle fixation of CO2
4. Transport of C3 acid back to mesophyll cells & PEP synthesis
C4 Photosynthetic Carbon Fixation
C4 Photosynthetic Carbon Fixation
Leaf Anatomy
Mesophyll Cells
C4 Photosynthetic Carbon Fixation
Consequences & Occurrence
1. Suppresses photorespiration
a. O2 does not compete with PEP carboxylase
b. Concentrated CO2 in Bundle Sheath cells
2. Minimizes water loss due to reduced stomatal
aperture and steeper CO2 gradient
3. Energy Cost: 2 ATP per CO2 fixed.
4. C4 plants often found in hotter, drier climates
Found in 1% of all known species…
…Tropical grasses, sugarcane, maize (corn)
Bundle Sheath
CAM
1. Crassulacean Acid Metabolism
2. Observed in
Crassulaceae (Jade plant, Kalanchoe)
Cacti
Euphorbia
Pineapple, vanilla (orchid), agave
3. Function: Enhanced water use efficiency
Stomata open at night & remain closed during daylight
C3 Plant: loses 400-500g H2O per gram C fixed
CAM Plant: loses 50-100 g H2O per gram C fixed
4. Temporal and spatial separation of two CO2
fixations
4
CAM
during
the Day
CAM at
Night
3C
4C
3C
4C
4C
CAM Photosynthetic Carbon Fixation
Consequences & Occurrence
END Photosynthetic Carbon Reactions
Photorespiration & C2 Pathway
C4 & CAM
1. Adaptation to hot, dry conditions often for
extended periods
2. May be a facultative adaptation
to stressful conditions of
heat or water
e.g. Ice plant
Unstressed Æ C3
Stressed Æ CAM
5
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