Transcript Document

The Calvin Cycle
In the light, acidification of the lumen
creates a pH gradient across thylakoid membranes.
Fig 7.22
Fig 7.33
ATP-synthase
is a protein motor
Driving force is
chemiosmotic
gradient
(Mitchell 1960s)
Relating the Light and Dark Reactions
Products and substrates of Light and Dark reactions
Substrate Energy Products Location
source
Light
H2 O
light
NADPH Thylakoids
reactions
ATP
Dark
reactions
CO2
NADPH CarboATP hydrates
Stroma
Carbon Fixation by the
Calvin Cycle
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Second set of reaction in
photosynthesis involves a biochemical
pathway known as the Calvin Cycle
Pathway produces organic compounds,
using energy stored in ATP and NADPH
during the light reactions
 Named after Melvin Calvin
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Carbon Fixation by the
Calvin Cycle
Atoms from CO2 are bonded or “fixed”
into organic compounds
 Incorporation of CO2 into organic
compounds is known as carbon fixation
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An outline of C3 photosynthesis
Fig. 8.2
A 3 carbon molecule
The Calvin Cycle
(reductive
pentose phosphate
cycle)
3 Stages
•Carboxylation
•Reduction
•Regeneration
3 major steps to Calvin Cycle
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Occurs within the stroma of the chloroplast
STEP 1:
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CO2 diffuses into the stroma from the surrounding
cytosol
Enzyme combines a CO2 molecule with a fivecarbon carbohydrate called RuBP
Product is a six-carbon molecule that splits
immediately into a pair of three-carbon molecules
known as PGA
STEP 2:
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PGA is converted into another three-carbon
molecule called PGAL in a 2-part process:
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1.) each PGA molecule receives a phosphate group from
a molecule of ATP.
2.) The resulting compound then receives a proton from
NADPH and releases a phosphate group producing
PGAL
In addition to PGAL, these reactions produce ADP,
NADP+ and phosphate
STEP 3:
 Most of the PGAL is converted back into RuBP in a
complicated series of reactions.
 Require a phosphate group from another molecule of
ATP, which is changed into ADP.
 By regenerating the RuBP that was consumed in
Step 1, the reactions of Step 3 allow the Calvin Cycle
to continue
 Some PGAL molecules are not converted into RuBP
– they leave the Calvin Cycle and can be used by the
plant to make other organic compounds
Balance Sheet for Photosynthesis
Each turn of the Calvin Cycle fixes one
CO2 molecule…since PGAL is a threecarbon compound, it takes three turns of
the cycle to produce each molecule of
PGAL.
 For each turn of the cycle 2 ATP, and
2 NADPH molecules are used in Step 2,
and 1 ATP molecule used in Step 3
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Therefore 3 turns of the Calvin cycle uses 9
molecules of ATP and 6 molecules of NADPH
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Simplest overall equation for photosynthesis,
including both the light reactions and the
Calvin Cycle can be written as:
CO2 + H2O + light energy  (CH2O)n + O2
The (CH2O)
represents the general formula for a
carbohydrate.
CO2 + H2O + light energy  (CH2O) + O2
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The (CH2O) represents the general
formula for a carbohydrate. Often
replaced in this equation by the
carbohydrate glucose, C6H12O6 giving
this equation:
6CO2 + 6H2O + light energy  C6H12O6 + 6O2
Alternative Pathways

Calvin Cycle is the most common pathway for
the carbon fixation
 Plant species that fix carbon exclusively
through the Calvin Cycle are known as C3
plants because of the three-carbon
compound PGA that is initially formed
 Other plant species fix carbon through
alternative pathways and then release it to
enter the Calvin Cycle
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Alternative pathways are found in plants
that are in hot, dry climates
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Can rapidly lose water to the air
Most of the water loss from a plant
occurs through small pores called
stomata which are usually located on
the undersurface of leaves
Types of Photosynthesis
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C3 Photosynthesis
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C4 Photosynthesis
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CAM
Photosynthesis
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Stomata are the major passageways through
which CO2 enters and O2 leaves a plant
 When stomata are partly closed, the level of
CO2 in the plant falls as CO2 is consumed in
the Calvin cycle
 At the same time, the level of O2 in the plant
rises as the light reactions split water and
generate O2
C3 Photosynthesis : C3 plants.
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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 Pathway
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Allows certain plants to fix CO2 into FOURCarbon Compounds.
 During the hottest part of the day, C4 plants
have their stomata partially closed.
 C4 plants include corn, sugar cane and
crabgrass.
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Such plants lose only about half as much water as
C3 plants when producing the same amount of
carbohydrates
C4 Photosynthesis : C4 plants.
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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.
The CAM Pathway
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Cactus, pineapples and certain other plants have
different adaptations to hot, dry Climates.
They fix carbon through a pathway called CAM.
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Plants that use the CAM Pathway by opening their Stomata
at night and Closing it during the day, the opposite of what
other plants do.
At night, CAM plants take in CO2 and fix it into organic
compounds.
During the day, CO2 is released from these compounds and
enters the Calvin Cycle.
Because CAM Plants have their Stomata open at night, they
grow very slowly, But they lose less water than C3 or C4
plants.
CAM Photosynthesis : CAM plants. CAM stands for
Crassulacean Acid Metabolism
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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:
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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 CO2 given 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
Rate of Photosynthesis
The rate at which a plant can carry out
photosynthesis is affected by the plant’s
environment.
 Three things in the plant’s environment
affect the rate of photosynthesis:
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1. LIGHT INTENSITY
2. CO2 LEVELS
3. TEMPERATURE
3 things affecting Photosynthesis
1. LIGHT INTENSITY - One of the most
important. As light intensity
INCREASES, the rate of photosynthesis
initially INCREASES and then levels off
to a plateau
3 things affecting Photosynthesis
2. CO2 LEVELS AROUND THE PLANT Increasing the level of CO2 stimulates
photosynthesis until the rate reaches a
plateau
3 things affecting Photosynthesis
3. TEMPERATURE - Raising the temperature
ACCELERATES the Chemical Reactions
involved in Photosynthesis.
The rate of Photosynthesis increases as
temperature increases. T
The rate of Photosynthesis generally PEAKS at
a certain temperatures, and photosynthesis
begins to decrease when the temperature is
further increased