The Calvin cycle occurs in two stages: carbon fixation and reduction. During carbon fixation, the enzyme RuBisCO incorporates carbon from CO2 into RuBP to form unstable 6-carbon compounds. These 6-carbon compounds are then split into two 3-carbon molecules which are reduced using electrons from NADPH and energy from ATP. The 3-carbon molecules are then rearranged to regenerate RuBP, completing the cycle. The Calvin cycle uses energy from the light reactions in the form of ATP and NADPH along with CO2 to produce organic compounds like glucose.

1. SARDAR PATEL MAHAVIDYALAYA CHANDRAPUR
DEPARTMENT OF MICROBIOLOGY
(Session 2020-2021)
A SEMINAR ON
CALVIN CYCLE
PRESENTED BY
DEEPAK CHAWHAN
(M.Sc. I Yr. Semester I)

2. Energy can neither be created nor
destroyed; Energy can only be transferred or changed
from one form to another....

3. 3
Photosynthesis: An Overview
 The net overall equation for
photosynthesis is:
 Photosynthesis occurs in 2 “stages”:
1. The Light Reactions (or Light-
Dependent Reactions)
2. The Calvin Cycle (or Calvin-Benson
Cycle or Dark Reactions or Light-
Independent Reactions)
6 CO2 + 6 H2O C6H12O6 + 6 O2
light
Is photosynthesis an
ENDERGONIC or
EXERGONIC reaction?

4. 4
• To follow the energy in photosynthesis,
light
light ATP
NADPH
Light
Reactions
thylakoids
Calvin
Cycle
stroma
Organic
compounds
(carbs)
Photosynthesis: An Overview

5. 5
The Calvin Cycle
Sir Melvin Calvin
“Matter can change
form through physical
and chemical changes,
but through any of
these changes, matter
is conserved.”

6. 6
Phase 2: The Calvin Cycle
 Within the Calvin Cycle, chemical energy (from
the light reactions i.e. ATP/NADPH) and CO2
(from the atmosphere) are used to produce
organic compounds (like glucose).
 The Calvin Cycle occurs in the stroma of
chloroplasts.
 The Calvin Cycle occurs in the 3 steps: Carbon
Fixation, Reduction, and Regeneration of RUBP.

7. 7
Phase 2: The Calvin Cycle
Step 1 - Carbon Fixation.
 This is the process of assimilating carbon
from a non-organic compound (i.e. CO2)
and incorporating it into an organic
compound (i.e. carbohydrates).
CARBON FIXATION

8. 8
Phase 2: The Calvin Cycle
Step 1: Carbon Fixation
 Consider only 1 stroma 1 molecules of CO2
(from the atmosphere) are joined to 1
molecules of RuBP (a 5-carbon sugar) by
Rubisco (an enzyme also known as RuBP
carboxylase)
 Simulataneously Consider other 5 stroma
5 molecules of CO2 (from the atmosphere)
are joined to other 5 molecules of RuBP
(a 5-carbon sugar) by Rubisco (an enzyme
also known as RuBP carboxylase)
C C
C C
C
C C
C C
C
C C
C C
C
C
C
C
6 carbon dioxide
molecules
6 RuBP molecules
Rubisco
Point to Note: -One Cycle binds One Co2 with One RUBP within one Stroma
C
C
C
C C
C C
C
C C
C C
C
C C
C C
C

9. 9
Phase 2: The Calvin Cycle
Step 2: Reduction
 Each of the 6-carbon molecules within stroma
are very unstable which gets split in half,
forming Two 3-carbon molecules (i.e. a Total Six
6- Carbon molecules produces Twelve 3- Carbon
molecules).
 These molecules are then reduced by gaining
electrons from NADPH.
 ATP is required for this molecular rearranging
C C
C C
C C
C C
C C
C C
C C
C C
C C
C C
C
C C
C
C C
C
C
C C
C
C C
C
C C
NADPH
NADP+
ATP ADP P
Where did the NADPH and
ATP come from to do this?
C C
C C
C C
C C
C C
C C
C C
C C
C C C C
C
C C
C
C C
C
C
C C
C
C C
C
C C

10. 10
Phase 2: The Calvin Cycle
 There are now twelve 3-carbon
molecules, which are known as G3P or PGAL
(Glyceraldehyde 3 Phosphate).
 Since the Calvin Cycle started with 30 carbons {(i.e. 6
x 5-carbon molecules (RUBP) = 30 Carbons} and
there are now 36 carbons {(i.e. 12 x 3-carbon
molecules (G3PL) = 36 Carbons}, we have a net gain
of 6 carbons.
C C
C
C C
C
C C
C
C
C C
C
C C
C
C C
Where did these 6 extra
carbons come from?
• Two of these “extra” 3-
carbon G3P/PGAL
molecules will exit the
cycle.
C C
C
C C
C
C C
C
C
C C
C
C C
C
C C

11. 11
Phase 2: The Calvin Cycle
 The Calvin Cycle “turns” 6 times, those 2
molecules of G3P (a 3-carbon
carbohydrate) will combine to form 1
molecule of glucose (a 6-carbon
carbohydrate molecule) OR another
organic compound.
C
C C
G3P
(from 6 turns of
the Calvin Cycle)
C C
C
G3P
(from 6 turns of
the Calvin Cycle)
C
C C
C C
C
glucose

12. 12
Phase 2: The Calvin Cycle
Step 3: Regeneration of RuBP
• Since this is the Calvin Cycle, we must end up
back at the beginning.
• The remaining 10 G3P molecules (3-carbons
each) get rearranged (using ATP) to form 6
RuBP molecules (5-carbons each).
10 G3P molecules
Total: 30 carbons
6 RuBP molecules
Total: 30 carbons
Where does the ATP
come from to do this?
ATP ADP
P
C
C
C
C
C C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C

13. Phase 2: The Calvin Cycle
ORGANIC
COMPOUND
NADPH
NADP+
ATP
ADP
P
RuBP
CO2

14. 14
Phase 2: The Calvin Cycle

15. 15
Phase 2: The Calvin Cycle
Summary:
In the Calvin Cycle, energy and electrons from
the Light Reactions (in the form of ATP and
NADPH) and carbon dioxide from the atmosphere
are used to produce organic compounds.
The Calvin Cycle occurs in the stroma inside the
chloroplasts (inside the cells…).
Carbon dioxide, ATP, and NADPH are required
(reactants).
Organic compounds (G3P) are produced
(products).

16. 16
Photosynthesis: A Recap
 So, as a broad overview of
photosynthesis,
• The Light Reactions (Phase 1) capture the
energy in sunlight and convert it to chemical
energy in the form of ATP and NADPH through
the use of photosystems, electron transport
chains, and chemiosmosis.
• The Calvin Cycle (Phase 2) uses the energy
transformed by the light reactions along with
carbon dioxide to produce organic compounds.

17. 17
Photosynthesis: A Recap
Based on this equation,
how could the rate of
photosynthesis be
measured?
The photosynthetic equation:
light
Excites
electrons
during the
light
reactions
6 H2O
Split during the
light reactions
to replace
electrons lost
from
Photosystem II
6 CO2
Provides the carbon to
produce organic
compounds during the
Calvin Cycle
Produced as a
byproduct of the
splitting of
water during the
light reactions
6 O2 C6H12O6
The organic compound
ultimately produced
during the Calvin Cycle

18. References: -
1. Principles of Biochemistry (6th Edition), by Leninger ;
Page No. 810-812.
2. Biology of Microorganisms (14th Edition), by Brock; Page
No. 390-392.
3. Plant Physiology (3rd Edition), by Taiz & Zeiger; Page No.
146-150.