Dark Reaction (Calvin Cycle)

1. PHOTOSYNTHESIS
Plant Physiology

2.  Photosynthesis is a chemical process in which light
energy is converted into chemical energy.
 Photosynthesis equation:
CO2 + H2O+ light energy →Sugar (glucose) + O2

3.  Leaves of plants have chloroplasts (found in
mesophyll cell) that are filled with chlorophyll, which
capture the energy from the sunlight).

4. Chloroplast is consists of:
1. Outer membrane.
2. Innermembrane.
3. Thylakoid membrane (in which light reaction take place).
4. Granum ( stack of thylakoid).
5. Stroma ( in which dark reaction take place).

5. Photosynthesis has to main phases:
1. Light Reaction
2. Dark Reaction ( Light Independent Reaction)

6. Light Reaction
 Required light and water (H2O).
 ATP & NADPH molecules are form which utilize in
Calvin Cycle.
 O2 is released as a by product through stomata.

7. Calvin Cycle (Dark Reaction)
Discovered by Melvin Calvin & other scientists.
Takes place in stroma.
Doesn’t required light.
Other names C3 cycle or light independent
reaction.
Use CO2 to form Glucose.
ATP use as energy.
NADPH use as reducing power.
6 turn of cycle make one molecule of Glucose.

8. Continue…
Involves 3 steps:
1. Carboxylation ( Carbon
Fixation).
2. Reduction.
3. Regeneration.

9. 1. Carboxylation ( Carbon Fixation )
 Begins with 5 carbon sugar call ribulose bisphosphate (RuBP).
 CO2 combines with RuBP to form 2 molecules of 3-
Phosphoglycerate (3-PGA or PGA).
 First CO2 converts to a very unstable 6 carbon intermediate that is
immediately converted to 2 molecules of 3-phosphoglycerate or
3-phosphoglyceric acid (3-PGA).
 Carboxylation catalyzed by an enzyme called ribulose
1,5-bisphosphate carboxylase (RuBisCO= most abundant enzyme
found on earth).
3RuBP + 3CO2 → 6 glycerate-3-phosphate

11. 2.Reduction
 ATP and NADPH are used to reduce 3-PGA
(synthesized in phase 1) into glyceraldehyde 3-
phosphate (G3P or PGAL), then ATP and NADPH are
converted to ADP and NADP+, respectively.
 An enzyme 3-phosphoglycerate kinase adds a
phosphate from ATP to 3-PGA to make 1,3-
bisphosphoglycerate.

12.  Then another enzyme glyceraldehyde-3-phosphate
dehydrogenase removes hydrogen from NADPH and adds
it to the 1,3-bisphosphoglycerate to make glyceraldehyde-
3-phosphate (G3P or PGAL)
 The resulting NADP+, ADP, and Pi can be recycled to the
light reactions.
2 3-PGA + 2ATP + 2NADPH 2G3P + 2ADP + NADP++ 2Pi
Continue…

14. 3.Regeneration
 Regeneration of RuBP: G3P is converted back to RuBP to keep the Calvin cycle going.
 A complex series of reactions rearranges the carbon skeletons of 5 glyceraldehyde-3-
phosphate molecules (G3P) into 3 RuBP molecules (using ATP).
 G3P serves as the starting material for the synthesis of glucose and fructose.
 One molecule of G3P leaves the Calvin Cycle and is used to make other organic
compounds including amino acids, lipids, and carbohydrates.
 RuBP is thus regenerated to begin the cycle again.
5 G3P + 3 ATP 3 RuBP + 3 ADP + 2 Pi

16. ThankYou 