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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
Carboxylation
•The key initial step in C3 photosynthesis
•RUBP + CO2 ---> 3-PGA
•Catalyzed by “Rubisco”: ribulose 1,5-bisphosphate carboxylase-oxygenase
• binds the 5C RUBP molecule and 1C CO2, making two 3C molecules.
5 C + 1 C -----> 2 x 3C molecules
Fig. 8.3 (partial)
Fig. 8.2
•Carboxylation
•Reduction
•Regeneration
Reduction
Reduction steps of the Calvin Cycle use ATP and
NADPH to produce a carbohydrate, glyceraldehyde 3
phosphate.
3PGA + ATP + NADPH --> G3P
G3P can be used to make sucrose or starch
Fig. 8.3 (partial) - the reduction steps
Fig. 8.2
•Carboxylation
•Reduction
•Regeneration
Regeneration
The regeneration steps of the Calvin Cycle
use ATP to regenerate RUBP from some of
the glyceraldehyde-3-P so the cycle
can continue.
Some of the carbohydrate is converted back
into ribulose 1,5 bisphosphate, the initial CO2
receptor molecule.
Fig. 8.3 (partial) - the regeneration steps
3-PGA
RUBP
Fig. 8.4
3 carbon molecules,
hence “C3” photosynthesis
Reviewing the Calvin cycle and counting carbon
(C) atoms associated with one carboxylation.
1. Carboxylation. 1 CO2 binds to 1 RuBP (5C) producing
two molecules of 3-PGA (total of 6 C).
2. Reduction. The two 3-PGA (3 C each) are reduced to two
glyceraldehyde 3 phosphate (G3P, 3 C each) using ATP and
NADPH produced by the light reactions (still 6 C).
3. Regeneration. 5 of the 6 C in the 2 molecules of G3P are
used to regenerate one RuBP (5C) using ATP.
A total of 6 turns of the Calvin cycle are required to make one
hexose (6C). This requires 18 ATP + 12 NADPH.
6 turns (6 CO2) of the Calvin cycle are required to make one
hexose (6C). This requires 18 ATP + 12 NADPH.
How much light energy is required to produce hexose?
•Minimum of 8 (often 9 to 10) photons required per CO2
fixed (remember quantum yield?)
•Red light (680nm) = 175kJ/mol photons (from E = hn)
•6 CO2/hexose x 8photons/CO2 x 175 kJ/photon =
8400 kJ/mole hexose
What is the energy efficiency of hexose production?
8400 kJ/mole hexose (for the red light example!)
One mole of hexose (e.g. glucose or fructose) yields about
2800 kJ when it’s oxidized. (The heat of combustion)
Efficiency = energy output/energy input
= 2800kJ/8400kJ
= 33%
This is the maximum overall thermodynamic efficiency
of photosynthesis.
Actual efficiency is much lower because:
1) quantum yield is < 1 CO2/8 photons
2) higher energy light (l < 680nm) is used
Quantum yield
=CO2 fixed/photon absorbed
Fig 9.8
Typical light
response of
photosynthesis
for a C3 plant
In low
O2 air, 2%.
In standard
air, 21% O2.
Why does decreasing the O2 concentration around
a C3 leaf increase the uptake of CO2?
Why is this effect not seen in some plants such as
corn, sugar cane, and many grasses common in
warm environments?
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
PEPcase activity is not affected by O2.
PEPcase uses HCO3-, not CO2.
[HCO3-] > [CO2]
C4 leaf anatomy model (Fig 8.8d)
C4 leaf anatomy (Fig. 8.9a)
C4 leaf anatomy differs from C3
Primary carboxylation is spatially separated from the
Calvin cycle.
The C4 system concentrates CO2 at Rubisco.
This is particularly useful in warm environments because
1) the solubility of CO2 decreases more with temperature
than the solubility of O2.
2) Allows C4 plants to operate with lower stomatal
aperture (conductance), thereby losing less water.
Extra ATP cost of regenerating PEP means that
C4 CO2 fixation requires more light energy.
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