THE CALVIN CYCLE: REDUCING CO2TO SUGAR

© 2012 Pearson Education, Inc.

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

© 2012 Pearson Education, Inc.

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).

© 2012 Pearson Education, Inc.

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!!

© 2012 Pearson Education, Inc.

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!!!

© 2012 Pearson Education, Inc.

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.

© 2012 Pearson Education, Inc.

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.

© 2012 Pearson Education, Inc.

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.