Photosynthesis Part II:
The Calvin Cycle,
Environmental Conditions,
& Preventing
Photorespiration
Photosynthesis: An Overview

The net overall equation for
photosynthesis is:
6 CO2 + 6 H2O

light
C6H12O6 + 6 O2
Photosynthesis occurs in 2 “stages”:
1.
2.
The Light Reactions (or Light-Dependent
Reactions)
The Calvin Cycle (or Calvin-Benson Cycle
or Dark Reactions or Light-Independent
Reactions)
2
Photosynthesis: An Overview

To follow the energy in photosynthesis,
Light
Reactions
light
thylakoids
light
ATP
NADPH
Calvin
Cycle
stroma
Organic
compounds
(carbs)
3
Phase 2: The Calvin Cycle


In the Calvin Cycle, chemical energy (from the
light reactions) and CO2 (from the atmosphere)
are used to produce organic compounds (like
glucose).
The Calvin Cycle occurs in the stroma of
chloroplasts.
4
Phase 2: The Calvin Cycle

The Calvin Cycle involves the process of
carbon fixation.
•
This is the process of assimilating carbon from a
non-organic compound (ie. CO2) and
incorporating it into an organic compound (ie.
carbohydrates).
CARBON FIXATION
5
Phase 2: The Calvin Cycle
Step 1: Carbon Fixation
 3 molecules of CO2 (from the atmosphere)
are joined to 3 molecules of RuBP (a 5-carbon
sugar) by Rubisco (an enzyme also known as
RuBP carboxylase)
C
C
Rubisco
C
3 carbon dioxide
molecules
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
This forms 3
molecules
which each
have 6 carbons
(for a total of 18
carbons!)
3 RuBP molecules
6
Phase 2: The Calvin Cycle
Where did the NADPH and
ATP come from to do this?
Step 2: Reduction
The three 6-carbon molecules (very unstable)
split in half, forming six 3-carbon molecules.
These molecules are then reduced by gaining
electrons from NADPH.
ATP is required for this molecular
P
C C C
C C C
ADP
ATP
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
NADPH
NADP+
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Phase 2: The Calvin Cycle
Where did these 3 extra
carbons come from?


There are now six 3-carbon molecules, which
are known as G3P or PGAL.
Since the Calvin Cycle started with 15 carbons
(three 5-carbon molecules) and there are now
18 carbons, we have a net gain of 3 carbons.
• One of these “extra” 3carbon G3P/PGAL
molecules will exit the
cycle and be used to form
½ a glucose molecule.
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
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Phase 2: The Calvin Cycle

Once the Calvin Cycle “turns” twice (well,
actually 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 3 turns of
the Calvin Cycle)
C
C
C
G3P
(from 3 turns of
the Calvin Cycle)
C
C
C
C
C
C
glucose
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Phase 2: The Calvin Cycle
Where does the ATP
Step 3: Regeneration of RuBP come
from to do this?
Since this is the Calvin Cycle, we must end
up back at the beginning.
The remaining 5 G3P molecules (3-carbons
each!) get rearranged (using ATP) to form 3
RuBP molecules (5-carbons each).
C
C
C
C
C
C
C
C
C
C
C
C
C
5 G3P molecules
Total: 15 carbons
C
C
ATP
ADP
P
3 RuBP molecules
Total: 15 carbons
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Phase 2: The Calvin Cycle
CO2
RuBP
NADPH
ATP
NADP+
ADP
P
ORGANIC
COMPOUND
Phase 2: The Calvin Cycle
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Phase 2: The Calvin Cycle
Quick recap:
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).

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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.
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Photosynthesis: A Recap
The photosynthetic equation:
Provides the carbon to
produce organic
compounds during the
Calvin Cycle
Based on this equation,
how could the rate of
photosynthesis be
measured?
The organic compound
ultimately produced
during the Calvin Cycle
light
6 H2O
Split during the
light reactions
to replace
electrons lost
from
Photosystem II
6 CO2
6 O2
Excites
electrons
during the
light
reactions
C6H12O6
Produced as a
byproduct of the
splitting of
water during the
light reactions
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Photosynthesis: A Recap
Photosynthesis Animation
(click on “Animation” after clicking the link)

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Environmental Factors & Photosynthesis

The rate (or speed) of photosynthesis can
vary, based on environmental conditions.
•
•
•
Light intensity
Temperature
Oxygen concentration
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Environmental Factors & Photosynthesis

Light intensity
•
As light intensity increases, so too does the rate of
photosynthesis.
light
• This occurs due to increased
excitation of electrons in the
photosystems.
saturation
point
• However, the photosystems
will eventually become
saturated.
• Above this limiting level, no
further increase in
photosynthetic rate will occur.
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Environmental Factors & Photosynthesis

Temperature
•
•
The effect of temperature on the rate of
photosynthesis is linked to the action of enzymes.
As the temperature increases up to a certain
point, the rate of photosynthesis increases.
• Molecules are moving faster &
colliding with enzymes more
frequently, facilitating chemical
reactions.
• However, at temperatures
higher than this point, the rate
of photosynthesis decreases.
• Enzymes are denatured.
19
Environmental Factors & Photosynthesis

Oxygen concentration
•
•
As the concentration of oxygen increases, the
rate of photosynthesis decreases.
This occurs due to the phenomenon of
photorespiration.
20
Photorespiration

Photorespiration occurs when Rubisco (RuBP
carboxylase) joins oxygen to RuBP in the first
step of the Calvin Cycle rather than carbon
dioxide.
•
•
Whichever compound (O2 or CO2) is present in higher
concentration will be joined by Rubisco to RuBP.
Photorespiration prevents the synthesis of glucose AND
utilizes the plant’s ATP.
More CO2
More O2
Rubisco joins
CO2 to RuBP
Photosynthesis
occurs; glucose is
produced
Rubisco joins
O2 to RuBP
Photorespiration
occurs; glucose is
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NOT produced
Photorespiration

Photorespiration is primarily a problem for
plants under water stress.
•
•
When plants are under water stress, their stomata
close to prevent water loss through transpiration.
However, this also limits gas exchange.

O2 is still being produced (through the light reactions).
• Thus, the concentration of O2
is increasing.
• CO2 is not entering the leaf since
the stomata are closed.
• Thus, as the CO2 is being used
up (in the Calvin Cycle) and not
replenished, the concentration
of CO2 is decreasing.
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Photorespiration


As the concentration of O2 increases and the
concentration of CO2 decreases (due to the
closure of the stomata to prevent excessive water
loss), photorespiration is favored over
photosynthesis.
Some plant species that live in hot, dry climates
(where photorespiration is an especially big
problem) have developed mechanisms through
natural selection to prevent photorespiration.
•
•
C4 plants
CAM plants
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C3 Plants

C3 plants, which are “normal” plants,
perform the light reactions and the Calvin
Cycle in the mesophyll cells of the leaves.
• The bundle sheath cells of
C3 plants do not contain
chloroplasts
palisade mesophyll
spongy mesophyll
bundle sheath cells
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C3 Photosynthesis/Plants






Called C3 because the CO2 is first
incorporated into a 3-carbon compound.
Stomata are open during the day.
RUBISCO, the enzyme involved in
photosynthesis, is also the enzyme
involved in the uptake of CO2.
Photosynthesis takes place throughout
the leaf.
Adaptive Value: more efficient than
C4 and CAM plants under cool and
moist conditions and under normal light
because requires less machinery (fewer
enzymes and no specialized anatomy)..
Most plants are C3.
C4 and CAM Plants


C4 plants and CAM plants modify the process
of C3 photosynthesis to prevent
photorespiration.
Overview:
•
•
•
C4 plants perform the Calvin Cycle in a different
location within the leaf than C3 plants.
CAM plants obtain CO2 at a different time than
C3 plants.
Both C4 and CAM plants separate the initial
fixing of CO2 (carbon fixation) from the using of
CO2 in the Calvin Cycle.
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Leaf Anatomy C3 vs C4

The C4 plants have two rings of cells surrounding their vascular bundles.
The inner ring is called the Bundle Sheath cell which contains starch rich
chloroplasts that do not have grana that is in the outer layer of mesophyll
cells. This particular anatomy is called the kranz anatomy. The function of
it is to provide an area where the CO2 can be concentrated around the
Rubsico, and by doing this photoresperation is reduced.
C4 Plants: Preventing Photorespiration


Plants that use C4 photosynthesis include corn,
sugar cane, and sorghum.
In this process, CO2 is transferred from the
mesophyll cells into the bundle-sheath cells,
which are impermeable to CO2.
• This increases the concentration of
CO2.
• Thus, the Calvin Cycle is favored
over photorespiration.
• The bundle-sheath cells of C4
plants do contain chloroplasts.
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C4 Plants: Preventing Photorespiration

C4 plants use the Hatch-Slack
pathway prior to the Calvin
Cycle:
•
•
PEP carboxylase adds carbon dioxide
to PEP, a 3-carbon compound, in the
mesophyll cells.
 This produces a 4-carbon
compound (which is why it’s
known as C4 photosynthesis).
This 4-carbon molecule then moves
into the bundle-sheath cells via
plasmodesmata.
• In the bundle sheath cells, the CO2 is released and the
Calvin Cycle begins.
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C4 Plants: Preventing Photorespiration
If the Hatch-Slack
pathway helps to
prevent
photorespiration,
why wouldn’t ALL
plants have this
adaptation?
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C4 Photosynthesis/Plants






Called C4 because the CO2 is first incorporated into a 4-carbon compound.
Stomata are open during the day.
Uses PEP Carboxylase for the enzyme involved in the uptake of CO2. This enzyme
allows CO2 to be taken into the plant very quickly, and then it "delivers" the
CO2 directly to RUBISCO for photsynthesis.
Photosynthesis takes place in inner cells (requires special anatomy called Kranz
Anatomy)
Adaptive Value:
 Photosynthesizes faster than C3 plants under high light intensity and high
temperatures because the CO2 is delivered directly to RUBISCO, not allowing it
to grab oxygen and undergo photorespiration.
 Has better Water Use Efficiency because PEP Carboxylase brings in CO2 faster
and so does not need to keep stomata open as much (less water lost by
transpiration) for the same amount of CO2 gain for photosynthesis.
C4 plants include several thousand species in at least 19 plant families. Example:
fourwing saltbush pictured here, corn, and many of our summer annual plants.
CAM Plants: Preventing Photorespiration


Plants that use CAM photosynthesis include
succulent plants (like cacti) and pineapples.
In CAM (crassulacean acid metabolism)
photosynthesis, plants open their stomata at
night to obtain CO2 and release O2.
•
This prevents them from drying out by keeping
their stomata closed during the hottest & driest
part of the day.
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CAM Plants: Preventing Photorespiration



When the stomata are opened at night, the CO2 is
converted to an organic acid (via the C4 pathway) and
stored overnight.
During the day – when light is present to drive the
Light Reactions to power the Calvin Cycle – carbon
dioxide is released from the organic acid and used in
the Calvin Cycle to produce organic compounds.
Remember:
• Even though the CO2 is
taken in at night, the Calvin
Cycle cannot occur because
the Light Reactions can’t
occur in the dark!
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35
CAM Photosynthesis/Plants




Called CAM after the plant family in which it was first found (Crassulaceae) and
because the CO2 is stored in the form of an acid before use in photosynthesis.
Stomata open at night (when evaporation rates are usually lower) and are usually
closed during the day. The CO2 is converted to an acid and stored during the night.
During the day, the acid is broken down and the CO2 is released to RUBISCO for
photosynthesis
Adaptive Value:
 Better Water Use Efficiency than C3 plants under arid conditions due to
opening stomata at night when transpiration rates are lower (no sunlight, lower
temperatures, lower wind speeds, etc.).
 May CAM-idle. When conditions are extremely arid, CAM plants can just
leave their stomata closed night and day. Oxygen given off in photosynthesis is
used for respiration and CO2given off in respiration is used for photosynthesis.
This is a little like a perpetual energy machine, but there are costs associated
with running the machinery for respiration and photosynthesis so the plant
cannot CAM-idle forever. But CAM-idling does allow the plant to survive dry
spells, and it allows the plant to recover very quickly when water is available
again (unlike plants that drop their leaves and twigs and go dormant during dry
spells).
CAM plants include many succulents such as cactuses and agaves and also some
orchids and bromeliads
Avoiding Photorespiration

Both C4 and CAM plants – which are primarily found in hot,
dry climates – have evolutionary adaptations which help
prevent photorespiration.

C4 plants perform the Calvin Cycle in the bundlesheath cells.

CAM plants open their stomata at night and store the CO2 until
morning.
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