Photosynthesis:
Dark cycle reactions,
variation in the dark cycle system,
protection of the photosynthesis system,
control of photosynthetic rate.
Objectives of the lecture:
1. Describe the dark cycle reactions of photosynthesis.
2. Illustrate field measurements of photosynthesis.
3. Discuss how dark cycle reactions can limit the rate
of photosynthesis.
4. Describe photorespiration.
5. Describe C4 and CAM photosynthesis.
Text book pages:
213-219.
Recap and importance:
The photochemical reactions produce ATP and NADH at sites in the stroma.
The Dark Cycle (Calvin Cycle), or more descriptively, the carbon reactions of photosynthesis
~200 billion tons of CO2 are converted to biomass each year
The enzyme ribulose biphosphate carboxylase/oxygenase, Rubisco, that
incorporates CO2 is 40% of the protein in most leaves.
The Calvin cycle proceeds in three stages: carboxylation, reduction, and regeneration
Carboxylation of the CO2 acceptor,
ribulose-1, 5-biphosphate, forming
two molecules of 3-phosphoglcerate.
Rubisco – the enzyme ribulose
biphosphate carboxylase/oxygenase
Reduction of 3-phosphoglycerate to
form glyceraldehyde-3-phosphate
which can be used in formation of
carbon compounds that are
translocated.
Regeneration of the CO2 acceptor
ribulose-1, 5-biphosphate from
glyceraldehyde-3-phosphate
RuBP
The affinity of Rubisco for CO2 is sufficiently high to
ensure rapid carboxylation at the low concentration
of CO2 found in photosynthesizing cells
The negative change in free energy associated with
carboxylation of RuBP is large so the forward
reaction is favored.
Rubisco will also take O2 rather than CO2 and
oxygenate RuBP – called photorespiration.
The rate of operation of the Calvin Cycle can be
enhanced by increases in the concentration of its
intermediates. That is the cycle is autocatalytic.
Also, if there are insufficient intermediates
available, for example when a plant is transferred
from dark to light, then there is a lag, or induction
period, before photosynthesis reaches the level
that the light can sustain. (There can also be
enzyme induction.)
Rubisco is notoriously inefficient as a
catalyst for the carboxylation of RuBP
and is subject to competitive inhibition
by O2, inactivation by loss of
carbamylation, and dead-end inhibition
by RuBP. These inadequacies make
Rubisco rate limiting for photosynthesis
and an obvious target for increasing
agricultural productivity. Really?
Field measurement of
photosynthesis and its control by
environmental conditions
LI-COR 6400
The chamber is enclosed over
the leaf. Light and
temperature are measured
while photosynthesis is being
measured.
Infra-red Gas Analyzer measures the
concentration of CO2 in the air stream before
and after it flows across the leaf in the
chamber
Photosynthesis rate calculated from gas
flow rate and CO2 concentration difference
Basics of foliage photosynthesis
Light Reaction
Limiting
Dark Reaction
Limiting
Increasing CO2
concentration in the
atmosphere can
increase the maximum
rate of photosynthesis
in the short term
Saturation level.
sometimes called
photosynthetic
capacity.
Photosynthetic efficiency:
Increase in photosynthesis
per increase in irradiance
0
0
Any questions?
Compensation point
The irradiance at which CO2 uptake is zero
Measured light response curve of Abies amabilis first year foliage.
Shade foliage with low maximum value and low compensation point.
Observed assimilation rates
(µmolCO2/m2s) of Tsuga
heterophylla and Abies amabilis in
response to periods of 10 minutes
high light (1500µmol/m2s PPFD),
with 5 minutes intervals of
darkness (shaded parts in the
diagram) in between. Values
measured using 200 mol/s flow rate.
Species differences in leaf photosynthesis
A has the highest
photosynthetic rate at
light saturation
B has the highest
photosynthetic efficiency
and the lowest
compensation point.
Units: μmol/m2/s
micro mols of CO2 per square
meter foliage per second
Any questions?
Another important measure is called Water Use Efficiency:
the ratio of photosynthesis achieved per unit of water lost.
Units: mmol/mol
milli mols of CO
2
per mol of water transpired
milli [m] 0.001 (a thousandth)
micro [µ] 0.000 001 (a millionth)
Wind River Canopy Crane
Research Facility
Old-growth species:
Tsuga
heterophylla
Pseudotsuga
menziesii
Thuja plicata
Notice the
difference in
branch structure
between the
species
Abies grandis
Upper Canopy
Phot. Cap.
Douglas-fir
Pseudotsuga
Western
hemlock
Tsuga
13.1
9.0
Phot. Cap.
Water Use Eff.
6.2 mmol/mol
8.8
3.5
4.9
3.2
4.8
Water Use Eff.
2
μmol/m /s
Lower Canopy
The problem of photorespiration
and the evolution of photosynthesis
When the enzyme Rubisco uses oxygen to breakdown
carbohydrate to CO2 rather than using CO2 to synthesize
carbohydrate
How some grasses have evolved a C4 metabolic
process and some desert plants have evolved
Crassulacean Acid Metabolism
Although Rubisco acts like a carboxylase in photosynthesis,
it can also act as an oxygenase when O2 is available.
O2 and CO2 compete for the same active site!
P
P
C-C-C-C-C
O2
P
C-C-C +
RuBisCO
Ribulose 1, 5-biphosphate
3-phospho
glycerate
P
C-C
2-phospho
glycerate
Enzyme
CO2
This becomes a problem when photosynthesis rates are
high, i.e. photosystem II produces lots of O2 .
This is called
Photorespiration
275 ppm CO2
73 ppm CO2
In the presence of higher O2 levels,
photosynthesis rates are lower.
The inhibition of photosynthesis
by O2 was first noticed by the
German plant physiologist, Otto
Warburg, in 1920, and called
the "Warburg effect".
It is believed that photorespiration in plants has increased over
geologic time due to increasing atmospheric O2 concentration -the
product of photosynthetic organisms themselves.
C4 Photosynthesis
The first product of CO2 fixation is
malate (C4) in mesophyll cells, not
PGA as it is in C3 plants. This is
transported to bundle sheath cells
CO2 is released from malate in bundle
sheath cells, where it is fixed again by
Rubisco and the Calvin cycle proceeds.
PEP is recycled back to mesophyll
cells.
Decarboxylation of malate (CO2 release)
creates a higher concentration of CO2 in
bundle sheath cells than found in
photosynthetic cells of C3 plants.
This enables C4 plants to sustain higher
rates of photosynthesis. And, because
the concentration of CO2 relative to O2 in
bundle sheath cells is higher,
photorespiration rates are lower.
Anatomical separation of the
C4 photosynthesis
component processes
Parenchyma filled
with chloroplasts
Bundle sheath
cells filled with
chloroplasts.
CALVIN
REACTION SITE
Xylem
Phloem
Carbon
skeleton
compounds
return to
parenchyma
C4 acids synthesized in the
parenchyma move to the bundle
sheath
Crassulacean Acid Metabolism (CAM)
First discovered in succulents of the
Crassulacea: e.g.,sedums
Uses C4 pathways, but segregates CO2
assimilation and Calvin cycle between
day and night
CAM plants open their stomates at
night. This conserves H2O. CO2 is
assimilated into malic acid and stored
in high concentrations in cell vacuoles
During the day, stomates close, and
the stored malic acid is gradually
recycled to release CO2 to the Calvin
cycle
C3, majority of
C4, e.g., sugar
CAM,
species
cane, corn
e.g., cacti
Leaf structure
Bundle sheath
cells lack
chloroplasts
Bundle sheath
cells have
chloroplasts
Mesophyll cells
have large
vacuoles
Efficiency in
light
Can be sun
or shade plants
Ineffective
in shade
CO2 capture
at night
Typical habitat
characteristics
Requires
relatively
moist habitats
Arid or
tropical
regions
Arid
environments
Moderate
High
Low
15-25oC
30-40oC
35oC
Productivity
Optimum
Temperature
Things you need to know ...
1. The basic reactions of the Calvin Cycle with the names and basic
structures of the principal reactants but not their detailed
chemical formulea.
2. The characteristics of Rubisco.
3. The method of field measurement of photosynthesis by gas exchange
4. The light saturation curve of leaf photosynthesis and its
important features
5. Water use efficiency. Calculation and value as a physiological
measure
6. Why photorespiration is important and the processes of C4
and CAM photosynthesis.