Transcript Alternative Photosynthesis

Alternative Methods
of Carbon Fixation
Photorespiration & C3 Plants
 C4 Photosynthesis & Plants
 CAM & CAM Plants
 (pages 168-172)
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Photorespiration & C3 Plants
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Remember the STOMA?
Stomata allow for plants to take in CO2,
release O2 and H2O.
O2
Transpiration
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Transpiration: is the loss of H2O from the
plant.
It has a cooling effect that prevents plant
leaves from overheating and denaturing
enzymes required for photosynthesis.
When a molecule of water is lost from a
stoma, it creates a transpiration pull that
moves water, minerals and other
substances from the roots to leaves where
they are used.
Under Hot and Dry Conditions….
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Guard cells close the stomata (or
decrease its size) to conserve water.
→ H2O can’t get out.
→ O2 can’t get out.
→ CO2 can’t get in.
↓↓↓ CO2 and ↑↑↑ O2
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This leads to PHOTORESPIRATION.
Photorespiration
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C3 plants (i.e. soybeans, and sunflowers)
use the Calvin Cycle to fix carbon.
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CO2 is required for the Calvin Cycle.
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Photorespiration is a process in which O2
is used to produce CO2.
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HOW???
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Rubisco (the enzyme that binds RuBP to
CO2 in the Calvin Cycle) can also bind
RuBP to O2.
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When RuBP binds to O2 it produces a 3carbon PGA molecule and a 2-carbon
GLYCOLATE molecule.
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Some glycolate will leave the chloroplast
and go to the mitochondria to yield CO2.
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Some will be returned to the cycle as G3P
to regenerate RuBP
Photorespiration
Normal Conditions
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Sugars are produced
Cycle regenerated
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CO2 is produced
No (net) sugar
produced
RuBP is used up
No ATP produced
Photorespiration & C3 Plants
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The CO2 produced can be used for
photosynthesis BUT overall,
photorespiration decreases photosynthetic
output.
It siphons materials from the Calvin Cycle
and produces little CO2.
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***Note, photorespiration will regenerate
some RuBP but it will also use RuBP
reserves (because the RuBP is providing
the carbon to make sugar in
photorespiration)
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So, the plant will eventually use up all its
RuBP. If CO2 becomes available, there
will be no RuBP for the Calvin Cycle.
Photorespiration & C3 Plants
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Under normal conditions, 20% of fixed
carbon is lost to photorespiration.
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The optimum temperature for
photorespiration is 30ºC – 40ºC.
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The optimum temperature for
photosynthesis is 15ºC – 25ºC.
Why do Plants Undergo
Photorespiration?
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Hypothesis: Rubisco evolved when the
Earth’s atmosphere was rich in CO2 and
poor in O2, so it did not matter that rubisco
also had oxigenase activity.
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Over evolutionary time, as O2 levels
increased, plants did NOT evolve a
modified enzyme that would only bind to
CO2 and not O2.
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However, some plant species have
evolved alternative mechanisms of carbon
fixation that effectively suppress the rate of
photorespiration.
1. C4 photosynthesis
2. CAM (Crassulacean Acid
Metabolism)
C4 Plants
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C4 plants including sugar cane, corn, and many
grasses, undergo C4 photosynthesis.
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C4 plants have a unique leaf anatomy that
facilitates this form of photosynthesis.
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C4 plant leaves contain two types of photosynthetic
cells: bundle sheath cells and mesophyll cells.
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Chloroplasts in C4 plants are concentrated in the
bundle sheath cells.
C4 Plant Leaf
Substances can move from mesophyll to bundlesheath cells via plasmodesmata: cell-cell connections
C4 Photosynthesis
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In the cytoplasm (NOT the chloroplast) of
mesophyll cells, the enzyme PEP
carboxylase catalyzes the reaction of CO2
and PEP to form the 4-carbon molecule
oxaloacetate (OAA).
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OAA is converted into the 4-carbon acid
malate.
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Malate diffuses from the mesophyll cells
into bundle-sheath cells through
plasmodesmata.
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Malate converts into CO2 and 3-carbon
pyruvate.
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Pyruvate diffuses back into the mesophyll
to regenerate PEP, and CO2 enters the
Calvin cycle to be catalyzed by rubisco
and produce sugar.
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Since the Calvin Cycle is localized to the
bundle-sheath cells, CO2 is continuously
pumped into the bundle-sheath
chloroplasts from surrounding mesophyll
cells via malate and the C4 pathway.
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The concentration of CO2 is increased and
rubisco is saturated with CO2.
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Because there is CO2 available, rubisco
won’t bind to O2 and photorespiration is
minimized.
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Photorespiration is minimized
Sugar production is maximized.
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C4 photosynthesis uses almost TWICE the
amount of ATP (compared to C3
photosynthesis) BUT without it,
photorespiration would stress the plant.
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The process is called C4 photosynthesis,
because the first product of CO2 fixation is
a 4-carbon molecule (OAA)
CAM Plants
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CAM plants are water-storing plants
(succulents) such cacti and pineapples.
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To conserve water, they open their
stomata at night and close them during the
day – the REVERSE of other plants.
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Closing the stomata during the day
prevents water loss, but also prevents CO2
from entering the leaves.
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At night, the stomata open to allow the
intake of CO2.
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CO2 is converted into C4 organic acids
(such as malate) using PEP carboxylase.
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The 4-carbon organic acids are stored in
the vacuole until the morning.
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When the stomata close in the morning,
the organic acids release CO2 molecules
to enter the Calvin cycle.
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This process is called CAM –
Crassulacean Acid Metabolism because
it was first discovered in the crassulacean
family of plants.
In C4 plants, 1st part of
carbon fixation and the
Calvin cycle occur in
different compartments.
In CAM plants, the steps
occur in same
compartment, but at
different times of the day.
C4 and CAM
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The C4 and CAM pathways present
evolutionary solutions to the problem of
maintaining photosynthesis when stomata
close on sunny, hot, dry, days.
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Both methods produce organic acids that
eventually transfer CO2 to the Calvin
Cycle.