THE CALVIN CYCLE: REDUCING CO2TO SUGAR
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7.10 The Calvin cycle generates sugar within chloroplast
GOAL - Reduce CO2 to produce the 3-Carbon sugar G3P.
Reactants: CO2 enter from stomata, ATP and NADPH from light reactions
A plant cell may then use G3P to make glucose and other organic molecules.
Equation for Calvin Cycle:
3 CO2 + 9 ATP + 6 NADPH
G3P + 9 ADP + 6 NADP+ + P
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Input
Output: G3P
CalvinCycle
CO2
ATPNADPH
7.10 The Calvin Cycle
The steps of the Calvin cycle include
– carbon fixation,
– reduction,
– release of G3P, and
– regeneration of the starting molecule ribulose bisphosphate (RuBP).
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1
1
PPP3
3
63-PGARuBP
CO2
Rubisco
Input:
Step Carbon fixation
CalvinCycle
Rubisco = enzyme responsible for Carbon fixation: RuBP + CO2 2 3-PGA
Figure 7.10B_s2
2
2
1
1
P
P
PPP
ATP
ADP
3
3
6
6
6
6
6
6
NADPH
NADP
G3P
3-PGARuBP
CO2
Rubisco
Input:
Step Reduction
Step Carbon fixation
CalvinCycle
Figure 7.10B_s3
2
2
1
1
3
3
Glucoseand othercompounds
P
P
P
P
P
PP
ATP
ADP
3
3
5
1
6
6
6
6
6
6
NADPH
NADP
G3P
G3PG3P
3-PGARuBP
CO2
Rubisco
Input:
Output:
Step Release of onemolecule of G3P
Step Reduction
Step Carbon fixation
CalvinCycle
Figure 7.10B_s4
2
2
1
1
3
4
4
3
Glucoseand othercompounds
P
P
P
P
P
PP
ATP
ATP
ADP
3
ADP3
3
3
5
1
6
6
6
6
6
6
NADPH
NADP
G3P
G3PG3P
3-PGARuBP
CO2
Rubisco
Input:
Output:Step Regeneration of RuBP
Step Release of onemolecule of G3P
Step Reduction
Step Carbon fixation
CalvinCycle
CO2
G3P
3-PGA
RuBP
Sugars
CalvinCycle
(in stroma)
Stroma
Cellularrespiration
Other organiccompounds
Cellulose
Starch
The Fate of G3P50% of carbs made by photosynthesis are used as fuel for plants
–Cellular respiration in the mitochondria!!!
G3P serves as starting material for making other organic molecules:
–proteins, lipids, and cellulose.
Excess carbs stored as starch in roots, stems, etc.
Photosynthesis and Cellular Respiration Form a Cycle!!
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sun
ATP
Photosynthesis(chloroplasts)
Respiration(mitochondria)
O2glucosesugarCO2 H2O+ +
photoautotrophs
Heterotrophs and autotrophs
USED FOR CELLULAR WORK!!
ATP produced by light reactions is ONLY used by Calvin cycle - never leaves chloroplast!
Factors Affecting the Rate of Photosynthesis
Photorespiration decreases energy yields in plants!!
Under water stress or dehydration,
– Stomata close due to decreased turgor pressure of guard cells
– Gas exchange reduced -- O2 levels increase, CO2 levels decrease
– Rubisco adds O2 to RuBP instead of CO2
– This process is called photorespiration because it occurs in the light, consumes O2, releases CO2, wastes ATP resources!!!
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Photorespiration = Reduced energy yields due to high O2 levels!!
Typical Plants are C3 Plants - Fix CO2 by Rubiscoforming 3-PGA
C3 plants are more affected by Photorespiration!
3-PGA
Fig. 6.15a
Alternative methods of Carbon Fixation to Prevent Photorespiration
C4 plants have evolved a means of
– Limiting water loss while optimizing the Calvin cycle.
C4 plants first fix CO2 into a four-carbon compound.
© 2012 Pearson Education, Inc.
CalvinCycle
Sugarcane
Mesophyllcell
Bundle-sheathcell
CO2
4-C compound
CO2
3-C sugar
C4 plant
Fig. 6.15a
Adaptations to Prevent Photorespiration, cont’d
CAM plants conserve water by opening their stomata and admitting CO2 only at night.
CO2 is fixed into a four-carbon compound,
– which banks CO2 at night and
– releases it to the Calvin cycle during the day.
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CalvinCycle
Pineapple
CO2
3-C sugar
CAM plantDay
CO2
4-C compound
Night
You should now be able to
1. Define autotrophs, heterotrophs, producers, and photoautotrophs.
2. Describe the structure of chloroplasts and their location in a leaf.
3. Explain how plants produce oxygen.
4. Describe the role of redox reactions in photosynthesis and cellular respiration.
5. Compare the reactants and products of the light reactions and the Calvin cycle.
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You should now be able to
6. Describe the properties and functions of the different photosynthetic pigments.
7. Explain how photosystems capture solar energy.
8. Explain how the electron transport chain and chemiosmosis generate ATP, NADPH, and oxygen in the light reactions.
9. Compare photophosphorylation and oxidative phosphorylation.
10. Describe the reactants and products of the Calvin cycle.
© 2012 Pearson Education, Inc.
You should now be able to
11. Compare the mechanisms that C3, C4, and CAM plants use to obtain and use carbon dioxide.
12. Review the overall process of the light reactions and the Calvin cycle, noting the products, reactants, and locations of every major step.
13. Describe the greenhouse effect.
14. Explain how the ozone layer forms, how human activities have damaged it, and the consequences of the destruction of the ozone layer.
© 2012 Pearson Education, Inc.