1. Light Independent Reactions of
Photosynthesis
Dr. Mark A. McGinley
Honors College and Department of
Biological Sciences
Texas Tech University

2. Results of the Light Dependent
Reactions
• The potential energy in the ATP and NADPH
produced during the light dependent
reactions of photosynthesis can only be stored
for a fraction of a second
– Thus this energy must be used immediately or the
energy will be lost

3. Calvin Cycle
• The Calvin Cycle is the chemical process that
helps to convert potential energy from ATP
and NADPH into potential energy stored in
glucose
• The Calvin Cycle occurs in the stroma

4. Calvin Cycle
First Reaction
C02 reacts with RUBP to form unstable 6 C
intermediate

5. Where Does the CO2 Come From?
• The CO2 comes from the environment
– Terrestrial plants pick up CO2 from the air
– Marine and aquatic plants pick up CO2 from the
water

6. RuBP Carboxylase
• The reaction of CO2 +
RUBP is calatlyzed by
the enzyme RuBP
carboxylase
– rubisco

7. RuBP Carboxylase
• The most abundant protein on the planet
– More than50% of protein in some plant cells
– 40 million Kg of RuBP carboxlyase created each
year
• 1000 Kg per second!!

8. RuBP Carboxylase
• Protein with quaternary structure
– 8 large subunits
– 8 small subunits
• Large subunits coded for by chloroplast DNA
• Small subunits coded for by nuclear DNA

9. Calvin Cycle
CO2 + RuBP => unstable 6 carbon intermediate
2 molecules of PGA
(3 phosphoglycerate)

10. Calvin Cycle

11. Calvin Cycle
Important Points
• Converting PGA to Glyceraldehyde phosphate
(G3P) requires the addition of energy released
by ATP and NADPH
– (this is the same ATP and NADPH that were just
produced in the Light Dependent Reactions)

12. Calvin Cycle
Important Points
• Glyceraldehyde 3 Phospate (G3P)
– Can be used to produced glucose
• Potential energy stored in chemical bonds of glucose
– Can be used to make RUBP
• Converting G3P to RUBP requires the input of energy
from ATP (also created by the light dependent
reactions)

13. Light Independent Reactions
• Please don’t call these the “Dark Reactions”
(you will sometimes see this term, but try to
avoid it)
– Light independent reactions must take place in the
light because light is required to put potential
energy in the bonds of ATP and NADPH

14. Light Independent Reactions
• Potential energy in ATP and NADPH is
converted to potential energy held in glucose
• CO2 => => glucose