I. Photorespiration

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• I. Photorespiration
• II. CO2 concentrating mechanisms - variation on
  the
• C3 photosynthetic metabolism.

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Plant of the day, Zea mays (Poaceae)
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How does the photosynthetic response to light
compare in corn and beans?
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• Corn vs. bean
• Corn has
• Lower QY
• 2. Higher max.
• photosynthesis
• 3. Higher light
• saturation
• 4. O2 insensitive

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The first step in the Calvin cycle is the
carboxylation of RUBP by Rubisco. Remember
Rubiscos full name?
Ribulose 1,5 bisphosphate carboxylase-oxygenase
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Rubisco can catalyze the oxygenation (O2) of RuBP
and the carboxylation (CO2) of RuBP.
Rubisco
Fig. 8.8
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The set of reactions that begins with
Rubisco oxygenation of RUBP is called
photorespiration. When Rubisco oxygenates RUBP,
a CO2 is lost from the leaf, reducing the net
uptake of CO2.
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CO2 Carbon gain
RuBP O2 Carbon loss, photorespiration
What determines the rate of carboxylation vs.
oxygenation? What determines the reaction rates
for any two competing substrates in an
enzyme-catalyzed reaction?
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Determinants of carboxylation vs. oxygenation. 1.
Concentration of CO2 O2 2. Rubisco specificity
for CO2 vs. O2

Rubisco
CO2
O2
Concentration of O2 gtgt CO2, but Rubisco
specificity favors CO2 binding.
Chloroplast stroma
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Oxygenation of RuBP causes a loss of CO2 and
reduces CO2 uptake.
In low O2 air, 2.
In standard air, 21 O2.

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So why does Rubisco have this inefficient
property? Consider Earths atmosphere 3 billion
years ago. High CO2/low O2 20 CO2 no
O2 Oxygenation was not a problem CO2/O2
ratio has decreased greatly over Earths
history 0.04 CO2 (and rising) 21 O2
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• The O2 inhibition of CO2 uptake represents a huge
  selective
• pressure for plant characteristics to prevent
  carboxylation.
• How to avoid oxygenation?
• 1. Develop new Rubisco thats insensitive to O2
• 2. Reduce O2 concentration in chloroplast
• 3. Increase CO2 concentration in chloroplast

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Plants like corn show no effect of O2
concentration apparently no oxygenation by
Rubisco. They also have different initial
products 14C label shows up first in 4 carbon
organic acids - malic acid, aspartic acid. These
are called C4 plants.

• C4 plants have Rubisco, so how do they avoid
  oxygenation?
• a) Initial carboxylation is not by Rubisco in
  C4 plants
• b) C4 leaf anatomy differs

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• How does C4 biochemistry differ from C3?
• Primary carbon fixation step uses different
  substrates and enzymes.
• HCO3- PEP --------gt 4 carbon organic
  acids
• PEP
• carboxylase
• Phosphenol pyruvate PEP
• Phosphenol pyruvate carboxylase PEPcase
• Two important differences between PEPcase and
  Rubisco
• 1. PEPcase activity is not affected by O2.
• 2. PEPcase uses HCO3-, not CO2.
• HCO3- gt CO2

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C4 leaf anatomy model (Fig 8.8d)

• Two distinct cell types
• Mesophyll
• (PEPcase)
• 2. Bundle sheath
• (Rubisco)

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C4 leaf anatomy (Fig. 8.9a)
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C4 leaf anatomy relates to its biochemistry Initi
al carboxylation is in mesophll cells and is
spatially separated from the Calvin cycle in the
bundle sheath cells
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The C4 biochemistry and anatomy concentrates CO2
in the b.s. cells at Rubisco. This is
advantageous in warm environments because 1)
the solubility of CO2 decreases more with
temperature than the solubility of O2, so
photorespiration is a bigger problem in warmer
environments. 2) C4 plants can operate with
lower stomatal aperture (conductance), thereby
losing less water.
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CO2 and O2 solubilities
Web Topic 8.3
CO2/O2
Temp..
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Theres no energetically free biochemical
lunch!! The CO2 concentrating mechanism requires
extra energy.
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2
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Extra ATP is needed to regenerate PEP, meaning
that CO2 fixation by C4 plants requires more
light energy than C3 photosynthesis. Symptoms
of this added cost 1. Quantum yield of C4 lt
C3 Extra ATP (light) cost is not a problem in
high light environments, but is in low light
environments. Few C4 shade plants.
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• Corn vs. bean
• Lower QY
• 2. Higher max.
• photosynthesis
• 3. Higher light
• saturation
• 4. O2 insensitive

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• C4 has two features that are advantages in warm,
  dry
• environments.
• Suppression of photorespiration (more C gain)
• 2. Lower stomatal conductance (less water loss)
• C4 plants can achieve high photosynthetic rates
  at
• lower stomatal conductance than C3 plants. How?

C4
C3
Photo- synthesis
Stomatal conductance
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C4 plants Because of the CO2 concentrating
mechanism, the CO2 at Rubisco is much higher
than in the leaf internal air spaces. A
saturating level of CO2 at Rubisco can be
achieved at low stomatal aperture and current
atmospheric CO2 C3 plants The CO2 decreases
from the leaf internal air spaces to the
chloroplast, and photosynthesis is not saturated
at current CO2 levels.