Fig. 8.2

1

An outline of C3 photosynthesis

• The Calvin Cycle
• (reductive
• pentose phosphate
• cycle)
• 3 Stages
• Carboxylation
• Reduction
• Regeneration

Fig. 8.2
A 3 carbon molecule
2

• Carboxylation
• The key initial step in C3 photosynthesis
• RUBP CO2 ---gt 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 -----gt 2 x 3C molecules
Fig. 8.3 (partial)
3

• Carboxylation
• Reduction
• Regeneration

Fig. 8.2
4
Reduction
Reduction steps of the Calvin Cycle use ATP and
NADPH to produce a carbohydrate, glyceraldehyde 3
phosphate. 3PGA ATP NADPH --gt G3P G3P can
be used to make sucrose or starch
5
Fig. 8.3 (partial) - the reduction steps
6

• Carboxylation
• Reduction
• Regeneration

Fig. 8.2
7
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.
8
Fig. 8.3 (partial) - the regeneration steps
9

3-PGA
RUBP
3 carbon molecules, hence C3 photosynthesis
Fig. 8.4
10

Reviewing the Calvin cycle and counting carbon
(C) atoms associated with one carboxylation.

• 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.

11

• 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

12
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 its
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 lt 1 CO2/8
photons 2) higher energy light (l lt 680nm) is
used
13
Fig 9.8 Typical light response of photosynthesis
for a C3 plant
Quantum yield CO2 fixed/photon absorbed
14
In low O2 air, 2.
In standard air, 21 O2.

15
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?
16

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

17
Plant of the day, Zea mays (Poaceae)
18
How does the photosynthetic response to light
compare in corn and beans?
19

• Corn vs. bean
• Corn has
• Lower QY
• 2. Higher max.
• photosynthesis
• 3. Higher light
• saturation
• 4. O2 insensitive

20
The first step in the Calvin cycle is the
carboxylation of RUBP by Rubisco. Remember
Rubiscos full name?
Ribulose 1,5 bisphosphate carboxylase-oxygenase
21

Rubisco can catalyze the oxygenation (O2) of RuBP
and the carboxylation (CO2) of RuBP.
Rubisco
Fig. 8.8
22
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.
23
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?
24
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
25
Oxygenation of RuBP causes a loss of CO2 and
reduces CO2 uptake.
In low O2 air, 2.
In standard air, 21 O2.

26
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
27

• 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

28
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

29

• 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
• PEPcase activity is not affected by O2.
• PEPcase uses HCO3-, not CO2.
• HCO3- gt CO2

30

C4 leaf anatomy model (Fig 8.8d)
31
C4 leaf anatomy (Fig. 8.9a)
32

C4 leaf anatomy differs from C3 Primary
carboxylation is spatially separated from the
Calvin cycle.
33
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.
34
Extra ATP cost of regenerating PEP means that C4
CO2 fixation requires more light energy. 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.
35

• Corn vs. bean
• Lower QY
• 2. Higher max.
• photosynthesis
• 3. Higher light
• saturation
• 4. O2 insensitive