I. Photorespiration
II. CO2 concentrating mechanisms - variation on the
“C3” photosynthetic metabolism.
Plant of the day, Zea mays (Poaceae)
How does the photosynthetic response to light
compare in corn and beans?
Corn vs. bean
Corn has:
1. Lower QY
Corn
Bean
2. Higher max.
photosynthesis
3. Higher light
saturation
4. O2 insensitive
The first step in the Calvin cycle is the carboxylation of
RUBP by Rubisco.
Remember Rubisco’s full name?
Ribulose 1,5 bisphosphate carboxylase-oxygenase
Rubisco can catalyze the oxygenation (O2) of RuBP and
the carboxylation (CO2) of RuBP.
Rubisco
Fig. 8.8
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.
CO2
O2
+
+
Carbon gain
RuBP
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?
Rubisco
CO2
O2
Determinants of carboxylation vs. oxygenation.
1. Concentration of CO2 & O2
2. Rubisco specificity for CO2 vs. O2
Concentration of O2 >> CO2, but Rubisco specificity favors CO2 binding.
Chloroplast stroma
Oxygenation of RuBP causes a loss of CO2 and
reduces CO2 uptake.
In low
O2 air, 2%.
In standard
air, 21% O2.
So why does Rubisco have this inefficient property?
Consider Earth’s 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 Earth’s history
0.04% CO2 (and rising)
21% O2
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 that’s insensitive to O2
2. Reduce O2 concentration in chloroplast
3. Increase CO2 concentration in chloroplast
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
How does C4 biochemistry differ from C3?
•
Primary carbon fixation step uses different substrates
and enzymes.
HCO3- + PEP --------> 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-] > [CO2]
C4 leaf anatomy model (Fig 8.8d)
Two distinct cell types:
1. Mesophyll
(PEPcase)
2. Bundle sheath
(Rubisco)
C4 leaf anatomy (Fig. 8.9a)
C4 leaf anatomy relates to its biochemistry
Initial carboxylation is in mesophll cells and is spatially
separated from the Calvin cycle in the bundle sheath cells
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.
CO2 and O2 solubilities
Web Topic 8.3
CO2/O2
Temp..
There’s no energetically free biochemical lunch!!
The CO2 concentrating mechanism requires extra energy.
2
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 < C3
Extra ATP (light) cost is not a problem in high light
environments, but is in low light environments.
Few C4 “shade” plants.
Corn vs. bean
1. Lower QY
Corn, a C4 plant
Bean, a C3 plant
2. Higher max.
photosynthesis
3. Higher light
saturation
4. O2 insensitive
C4 has two features that are advantages in warm, dry
environments.
1. 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
Photosynthesis
Stomatal conductance
C3
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.