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
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.
Editor's Notes
Photosynthesis is an endergonic reaction because it requires an input of energy to occur; that energy comes in the form of light.
It cannot be overemphasized that energy is not MADE, but is instead transformed through the processes of photosynthesis and cellular respiration. That’s why I previously mentioned the 1st law of thermodynamics. Photosynthesis is a process that converts energy from an “un-usable form” (light) into a “usable form” (organic compounds), and requires an intermediate step (ATP/NADPH).
The Calvin Cycle- Calvin Cycle was explained by Sir Melvin Calvin. We also remember the law of Conservation of Mass i.e. “Matter can change form through physical and chemical changes, but through any of these changes, matter is conserved.”
The 2nd phase of photosynthesis i.e. The Calvin Cycle Within which the 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.
- Only the carbons are shown in this diagram for clarity, though oxygen and hydrogen are also present. Also, the carbon atoms shown in red and those shown in black are identical, but are color-coded to show where they come from (red are CO2 from the atmosphere, black are the carbons in RuBP).
- The Answer is it is produced during the light reactions.
6 extra carbons came from the carbon dioxide, which was obtained through the stomata from the atmosphere; these 6 additional carbons are denoted in red.
- We say that the Calvin Cycle turns twice to make one molecule of glucose (6-carbons), but really it turns 6 times; each entering carbon dioxide molecule represents one “turn” of the cycle, and 6 carbon dioxide molecules must be incorporated into organic compounds in order for one 6-carbon glucose molecule to be produced. This PowerPoint (along with many textbooks) shows 3 carbon dioxide molecules entering together for clarity (it’s hard to show 1/3 of a G3P molecule as the product of each turn of the cycle).
- Glucose is what we usually think of as being the major product of photosynthesis; however, G3P (also known as PGAL) is the real product, and though it is often used to make glucose it can also be used as a carbon skeleton to form other organic molecules.
The ATP required to do this rearranging comes from the ATP generated during the light reactions.
- This simple schematic diagram gives a basic overview of what occurs during the Calvin Cycle. Carbon dioxide enters the cycle from the atmosphere and is joined to RuBP by Rubisco. NADPH and ATP are used to “turn” the cycle, and organic compounds (such as G3P/PGAL) are produced.
- Though the AP curriculum framework very clearly states that students do not need to memorize the steps in the Calvin cycle, the structure of the molecules and the names of the enzymes involved (except for ATP synthase), some students may find this diagram helpful in understanding the cyclical nature of the Calvin cycle.
Emphasize to students the importance of understanding how and when each component of the photosynthetic equation is used; this is much more valuable (and less intimidating!) than simply having them memorize the equation!
Most realistically, the rate of photosynthesis could be measured by using the:
Decrease in environmental CO2 (in a closed system)
Increase in environmental O2 (in a closed system)
Increase in glucose (perhaps measured using radioactive carbon)