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Determine how the plant utilizes the energy
from the light dependent reactions to build
sugars from CO2.
Identify some common adaptations to this
metabolic pathway which allow plants to
thrive in various environments.
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https://www.youtube.com/watch?v=hj_WKgn
L6MI
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The following reactions all take place in the
Stroma of the chloroplast.
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ATP and NADPH produced
from the light dependent
reactions are used to provide
energy for synthesis of sugar
from molecules of CO2. This is
termed carbon fixation.
This occurs in a cycle, whereby
the primary reactant, a 5carbon molecule termed
ribulose bisphosphate (RuBP),
is regenerated constantly.
For every three CO2 molecules
that are fixed, one 3-carbon
sugar is produced that can
leave the cycle.
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Rubisco catalyzes the addition of CO2
to RuBP, and a 6-Carbon intermediate
breaks into two three carbon
molecules called 3-Phosphoglycerate
(3PG).
Think of these molecules as being low
energy; they need to be energized to
be useful to the plant.
RuBisCO (Ribulose Bisphosphate
Carboxylase/Oxygenase) is an enzyme that
catalyzes the addition of CO2 to RuBP, the
primary reactant of the Calvin Benson Cycle.
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Each of the 3PG molecules is phosphorylated by
ATP, and then reduced by NADPH into a sugar
called Glyceraldehyde 3-phosphate (G3P).
If RuBisCO catalyzes three carboxylations, how
many G3P molecules would we have?
fun fact: RuBisCO is slow, and can only catalyze
about 3-10 molecules of CO2 per second per
enzyme.
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If the cell used all 6 molecules of G3P
produced from 3 CO2 molecules, it would
run out of RuBP very quickly.
Rather, 5 of the G3P molecules are used to
regenerate the 3 molecules of RuBP
necessary for the reaction.
This means, for every three CO2 molecules
fixed, only one G3P molecule is available to
be used for other purposes.
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RuBisCO not only catalyzes
the addition of CO2 to RuBP, it
can also catalyze the addition
of oxygen. No useable sugar
results from this reaction,
though energy is still used.
This is termed
photorespiration.
In this way, CO2 and O2 are
competing for the active sites
of RuBisCO.
What conditions do you think
could affect the rate of
photorespiration?
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The main factor that affects the rate at which
photorespiration occurs is O2 concentration
vs CO2 concentration.
O2 wins out in high temperatures, or in
periods of water stress for the plant.
Plants that undergo too much
photorespiration are not productive, and will
die if they can’t synthesize sugars.
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The “normal” form of
photosynthesis we just learned
is called C3 photosynthesis (the
first step produces a 3 Carbon
molecule).
C2 is another term for the
photorespiratory pathway (The
first step produces a 2 Carbon
molecule).
Some plants have evolved other
adaptations in order to reduce
photorespiration. Two
commonly studied adaptations
are:
C4 Photosynthesis
CAM Photosynthesis
some plants are constantly
exposed to high temperatures,
and minimal H2O. They require
adaptations to survive.
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C4 Plants have a different leaf anatomy, with
no spongy mesophyll.
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In C3 plants, Mesophyll
cells are responsible for
complete photosynthesis,
whereas in C4 plants,
Mesophyll cells sequester
Carbon in a 4 Carbon
molecule via a different
metabolic pathway.
This Carbon is then
released into Bundle Sheath
Cells where O2
concentrations are low and
the normal Calvin-Benson
Cycle can occur without
much photorespiration.
EP carboxylase is more
efficient at carboxylation in
high temperatures
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Amazingly, C4 Photosynthesis has
evolved on up to 40 independent
occasions, acting as a prime
example of convergent evolution.
Over 7500 species of plants utilize
C4 Photosynthesis (~3% of
terrestrial plant species). Corn is
an example, as are many grasses
and other plants exposed to large
amounts of sunlight and higher
temperatures.
The C4 Rice Project
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Crassulacean Acid
Metabolism (CAM) is a
mechanism of storing
Carbon Dioxide in a four
carbon acid in Vacuoles
overnight, so the plant can
have its stomata remain shut
during the day, and only
open it at night.
Plants that have evolved this
mechanism exist in arid
environments, and can avoid
rapid evaporation of H2O
from stomata.
Cactus are examples
of CAM Plant
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https://vimeo.com/7316737