Photosynthesis: Dark cycle reactions,

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Photosynthesis Dark cycle reactions, variatio
n 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.
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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.
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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.
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
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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.
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?
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.)
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Field measurement of photosynthesis and its
control by environmental conditions
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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
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Basics of foliage photosynthesis
Increasing CO2 concentration in the atmosphere
can increase the maximum rate of photosynthesis
in the short term
Any questions?
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Measured light response curve of Abies amabilis
first year foliage.
Shade foliage with low maximum value and low
compensation point.
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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.
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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.
Any questions?
Another important measure is called Water Use
Efficiency the ratio of photosynthesis achieved
per unit of water lost.
milli m 0.001 (a thousandth) micro µ 0.000
001 (a millionth)
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Wind River Canopy Crane Research Facility
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Notice the difference in branch structure between
the species
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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
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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!
This becomes a  problem when photosynthesis rates
are high, i.e. photosystem II produces lots of 
O2 .
This is called Photorespiration
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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.
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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.
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Anatomical separation of the C4 photosynthesis
component processes
Xylem
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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
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C3, majority of species C4, e.g., sugar cane, corn CAM, e.g., cacti
Leaf structure

Typical habitatcharacteristics
Productivity
Optimum Temperature
Efficiency in light
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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.