1. Metabolic Processes: Photosynthesis II The Light-Independent Reactions
SBI4U – Ms. Richardson

2. Light-Independent Reactions
Purpose:
To convert carbon dioxide into carbohydrates
Location:
Stroma of chloroplasts
Independent of light because they are fueled by ATP and NADPH produced in light-dependent reactions

3. The Calvin Cycle
1960, Melvin Calvin – Identified reactions of the cycle
3 phases:
Carbon fixation
Reduction reactions
RuBP regeneration
Occurs 6 times
(6 CO2 molecules for
every 2G3P made)

4. The Calvin Cycle: Carbon Fixation
CO2 from atmosphere is used to convert 5C ribulose -1,5-bisphosphate to two 3C molecules of 3-phosphoglycerate
“C3 photosynthesis”

5. The Calvin Cycle: Reduction Reactions
The two 3C PGA molecules are phosphorylated by ATP to form two1,3-bisphosphoglycerate (1,3-BPG)
Electron from NADPH reduces 1,3-BPG to two 3C glyceraldhyde-3-phosphate
The two G3P exit the cycle as a final product

6. The Calvin Cycle: RuBP Regeneration
Cycle continues 5 more times producing 10 more G3P which are used to regenerate 6 RuBP
Uses 6 ATP
RuBP regeneration allows for cycle to continue

7. Light-Independent Reactions

8. Light Independent Reactions

9. The Calvin Cycle Net equation: 6CO2 + 18ATP + 12NADPH + H2O 
2G3P + 16Pi + 18 ADP + 12 NADP+
The G3P produced is used to make other molecules required by plants

10. The Problem With C3 Plants
Need CO2 which requires opening stoma during day when O2 levels are higher
Rubisco binds easily to O2 as well as CO2 leading to photorespiration instead of fixation
Oxidation of RuBP by rubisco and O2 to form glyoclate, which later releases CO2
Carbohydrate production decreases
Photorespiration accounts for 20% loss of fixed carbon
Most prominent on hot, dry days

11. Adaptations: C4 Plants Include grasses (corn, sugar cane)
Reduces amount of photorespiration by continually pumping CO2 molecules (via malate) from mesophyll into bundle sheath cells
Rubisco will then bring them into the C3 Calvin cycle

12. Adaptations: CAM Plants
Found in extreme conditions (hot, dry, desert)
Include cacti, pineapples, jade plants
Stomata open at night only to conserve water
Fix carbon as C4 organic acids & stored in vacuoles until daylight when they enter the Calvin cycle

13. Photosynthesis & Aerobic Respiration