Photorespiration ACi curves What happens to carbon after photosynthesis

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PhotorespirationA/Ci curves What happens to
carbon after photosynthesis?
Carbon part 2
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Remember Rubisco?The name, Ribulose
bisphosphate carboxylase oxygenase implies that
this enzyme has two functions. Last week we saw
how Rubisco functions as a carboxylase. This is
normal photosynthesis. But Rubisco can also
function as an oxygenase.
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Photorespiration is initiated when RUBISCO acts
as an oxygenase instead of a carboxylase
Calvin cycle
Photorespiration
The phosphoglycolate is recycled to produce
more 3PGA, consuming energy and releasing more CO2
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The net rate of photosynthesis is higher in air
with low oxygen levels this is because
respiration (both mitochondrial respiration and
photorespiration), but not the Calvin Cycle,
requires oxygen
Rate of photosynthesis
net
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• Summary points of photorespiration
• The energy used to produce the RuBP acceptor is
  wasted
• It takes even more energy to recycle the 2-carbon
  glycollate
• High light, low CO2 and high O2 promote
  photorespiration
• C3 plants can lose up to ¼ of their newly fixed
  carbon through photorespiration
• Photorespiration may have a role in reducing
  photoinhibition and photooxidation

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The type of photosynthesis weve discussed so far
is often called C3 metabolism because the first
product of photosynthesis is a 3-carbon sugar
phosphate. Most plants, and virtually all woody
plants, have C3 metabolism. However, many
grasses and some succulents have another form of
metabolism C4. As you might guess, the first
product of carboxylation in C4 metabolism is a
4-carbon molecule.
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Kranz anatomy
Leaves of C4 plants have specialized Kranz
anatomy. The Rubisco enzymes are in special cells
called bundle sheath cells. CO2 is
concentrated in these cells. The high
concentration virtually eliminates the oxygenase
activity of Rubisco, and therefore eliminates
photorespiration, making photosynthesis more
efficient.
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Heres how it happens. First, CO2 diffuses into
the mesophyll cells and is converted to
bicarbonate ions by the enzyme carbonic
anhydrase. Then, an enzyme called
Pospho-enolpyruvate carbozylase (PEP carboxylase)
fixes the carbon to form a 4-carbon organic
acid. The organic acid is transported across
cell membranes into the bundle sheath cells. In
the bundle sheath cell, the 4 carbon organic acid
releases CO2 that is then used in the normal
way by Rubisco. The process requires a lot of
energy!
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C4 metabolism is found most frequently in
tropical and sub-tropical grasses (including corn
and sugar cane). It is especially advantageous
in situations of high light, high temperature and
low atmospheric CO2 concentrations.
C3 and C4 metabolism are of great interest to
ecologists who study the impacts of climate on
plant competition because C4 grasses compete with
C3 plants including shrubs and trees. What do
you think should happen to the relative
competitive ability of C3 and C4 plants as
atmospheric CO2 increases?
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• Rate limiting processes in CO2 fixation
• Enzyme activity (RUBISCO) depends on N, light,
  temperature.
• Regeneration of RUBP depends on the production
  of ATP and NADPH in electron transport
• CO2 supply depends on stomatal opening and
  ambient CO2 and O2 levels
• Metabolism of end-products (triose-phosphates)
• (otherwise there is a possibility for feedback
  inhibition, regulated by phosphate supply)

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CO2 and water vapor move into and out of a leaf
by diffusion
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The rate of diffusion can be described by Ficks
Law
Rate of diffusion difference in concentration
conductance
(or net gas exchange)
(the symbol g is usually used for conductance)
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The medium could be a membrane, or wood, or
soil, or many other materials In this application
it is the surface of a leaf so well call it
leaf
leaf
xo concentration of x at the source, or
outside
xi concentration of x at the destination,
or inside
Net flux of x difference in concentration of
x conductance to x
Fx (xo - xi) gleaf,x
Fx (Dx gleaf,x)
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In this illustration, leaf conductance would be
greater for the small yellow dots (you might
think of these as water vapor) than for the big
pink dots (you might think of these as CO2)

• gleaf,CO2 gleaf,H2O / 1.6
• (this is because water molecules are smaller and
  diffuse faster than CO2 molecules).

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The units of conductance and resistance
In the 1980s and before, people commonly used
cm s-1 (or m s-1) for leaf conductance to water
vapor (and the inverse for resistance).
This is because of the units people used to
express flux and the driving force Flux (mol m2
s-1) driving force (mol/m3) conductance (m
s-1)
These units are still used in the hydrology
literature, but not in the plant physiology
literature.
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Today, the most common units for conductance are
mol m-2 s-1 (or mmol m-2 s-1 or mmol m-2 s-1)
This is because we use we use mole fractions to
describe gas concentration. A mole fraction is
the number of moles of a particular gas (CO2 for
example) in a mole of atmospheric gas. Therefore
the units are moles/moles i.e. the value is
unitless
Flux (mmol m2 s-1) driving force (mol/mol)
conductance (mmol m2 s-1)
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Applying Ficks Law to carbon assimilation

• Net C assimilation (ca-ci) leaf conductance
• Or A (ca-ci) gleaf-CO2

For more info see LCP pp 20-21 and/or the link
from the class web site to calculating important
parameters of gas exchange
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Supply and Demand

• The three rate-limiting processes we looked at
  last week (carboxylation, RUBP regeneration,
  triose phosphate utilization) regulate the DEMAND
  for CO2 in photosynthesis
• gleaf regulates the SUPPLY of CO2 for
  photosynthesis

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Now we are going to look at the kinetics of
carbon assimilation. In biochemistry, the
kinetics of enzyme-mediated reactions typically
look like this
Rate, or velocity, of the reaction
Concentration of a substrate, or reactant
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We can apply these ideas about kinetics to carbon
assimilation, with Ci as a substrate, to develop
an A/Ci curve
These are actual, measured data
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But the situation is a little more complicated,
because there are THREE rate-limiting reactions
that affect overall net C assimilation
Rubisco limitation (Wc) RUBP regeneration (Wj) T
PU (Wp)
?
Compensation point (?)
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Supply and Demand functions in A/Ci curves

• When you develop data for an A/Ci curve, you
  measure the assimilation rate at a series of
  different CO2 levels (i.e., different Ca), and
  at each Ca the corresponding Ci is calculated

Ca (ppm) (modified by user)
A (assimilation) measured
Ci (ppm) (calculated)
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The Demand Function is the plot of A vs. Ci
Note that here, Ci is plotted in units of Pascals
(the partial pressure of CO2) rather than ppm.
At 1 atm pressure, 10 ppm 1 Pa
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To construct the supply function, you first
need to find out from the data what value of Ci
corresponds to ambient CO2 concentration (about
355 ppm, or 35.5 Pascals)
Ca (ppm) (set by instrument)
A (assimilation) measured
Ci (ppm) (calculated)
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A 15.5 mmol m-2 s-1
If there were absolutely no leaf resistance at
all (I.e., infinite leaf conductance) Ca would
equal Ci. The red dot indicates the
assimilation rate that would occur when Ca35.5
Pa if leaf conductance were infinite.
Ci Ca 35.5 Pa if leaf resistance were zero
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A 10 mmol m-2 s-1
A 15.5 mmol m-2 s-1
But because of leaf conductance (gleaf) is not
infinite, Ci is significantly lower than Ca. In
this example, Ci 23 Pa when Ca35.5 Pa. The
reduction in A is the stomatal limitation to
photosynthesis. In this example, that is about
5.5 mmol m-2 s-1, or 35.
Ca 35.5 Pa
Ci 23 Pa
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The line connecting ambient Ca on the x-axis
(here, the point 0,35.5) to A at the Ci that
corresponds to ambient Ca (here, the point
23,10) is called the supply function
Note that the slope of this line is (A 0)/(Ci
Ca), or A/(Ci-Ca). Also recall that
according to Ficks Law A gleaf (Ca-Ci) So
the slope of the line is equal to the opposite of
leaf conductance (-gleaf) that occurred at
ambient CO2 concentrations (35 Pa)
So gleaf CO2 10/(35.5 23)
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As stomata close (making leaf conductance
smaller), as long as the demand function
doesnt change (i.e., Rubisco activity and
electron transport arent affected), Ci is
reduced. From the A/Ci curve you can predict the
reduction in A due to stomatal closure
Demand
Supply gleaf 150
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Or, here is what happens if demand function
changes (i.e., Rubisco activity or electron
transport are reduced, perhaps because of a light
limitation), and leaf conductance is not changed.
Again you can determine the impact of the change
on net A.
Demand
Supply gs 150
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WATER STRESS USUALLY AFFECTS BOTH THE SUPPLY AND
DEMAND FUNCTIONS, BUT ON DIFFERENT TIME SCALES
From your text -gt
Days of witholding water from bean plants
Stomatal closure causes a change in slope of the
supply function
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From your text -gt
Effects of water stress on the supply/demand
functions
Days of witholding water from bean plants
Down regulation of photosynthetic capacity
causes a change in slope of the demand function
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WHAT HAPPENS TO NEWLY-FIXED CARBON?
1. Storage Starch is synthesized for
short-term energy storage. Starch may be
synthesized and stored in the chloroplasts during
the day some or all is mobilized for export to
the cytosol at night.
2. Utilization Carbon in sugars, and the
energy stored in the sugars, may be used in
mitochondrial respiration to meet the energy and
biosynthetic needs of the mesophyll cells
3. Transport Fixed carbon is incorporated into
transport sugars and translocated through the
phloem to other parts of the plant where it is
used for energy and biosynthesis
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• Starch is synthesized in the chloroplast (and
  also in other organelles) for short-term energy
  storage

This is an electron micrograph of a leaf cell in
corn that shows huge deposits of starch in the
chloroplasts
This is a chloroplast
This is a starch grain
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Starch is a polymer of glucose units
Starch is present in almost all plants (animals
use a very similar molecule, glycogen, instead of
starch for short-term energy storage)
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• Starch has low solubility in water and low
  osmotic potential. These properties, along with
  the large number of C-C bonds (a form of stored
  energy) make starch an excellent energy-storage
  compound.

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Energy for growth and tissue maintenance comes
from respiration of the sugars fixed by
photosynthesis.
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Respiration is often subdivided into Growth,
Maintenance and Transport costs
Growth respiration (a.k.a. construction
respiration) a fixed cost that depends on
the tissues or biochemicals that are synthesized.
Often described in terms of glucose equivalents
Maintenance respiration The cost of maintaining
existing tissues and functions, (Protein
turnover is the largest cost of maintenance
respiration)
Respiration for transport Moving material
across cell membranes often requires energy (e.g.
ATP). Transport of nutrients into cells of roots
can be especially expensive.
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• Mitochondrial respiration involves 3 steps
• 1. glycolysis,
• Citric acid cycle (also called Krebs cycle or
  the tricarboxylic acid (TCA) cycle)
• electron transport

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The whole cell
Glycolysis takes place in the cytosol
The citric acid cycle ( TCA Krebs Cycle) and
respiratory electron transport take place in
mitochondria
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Mitochondrial electron transport is controlled by
both supply (availability of carbohydrates and
organic acids) and demand (energy requirements
for growth, maintenance and transport processes)
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Demand regulation low amounts of ADP
dramatically reduce the rate of mitochondrial
respiration (when energy demand for growth,
maintenance and transport processes is high, ATP
is rapidly consumed, producing ADP, which in turn
increases the rate of respiration)
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Respiration and Plant Carbon Balance
On a whole-plant basis, mitochondrial respiration
consumes from 30 to 70 of total fixed
carbon Leaves account for about half of the
total roots account for much of the rest.
Respiration of woody tissues is relatively low.
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The amount of photosynthate consumed in
respiration varies with tissue type and with
environmental conditions. When nutrients are
limiting, respiration rates in roots increase
dramatically.
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Mitochondrial Respiration (like photorespiration)
increases rapidly with temperature. The Q10 for
respiration of plant tissues is typically around
2.
Q10 the multiplicative change in respiration
over a 10 degree C change in temperature
When plants are grown in higher temperatures, the
respiration rate typically acclimates and will
be lower at any given temperature compared with
plants grown at low temperatures. This is mostly
due to changes in enzyme amount and activity.
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From Ryan and Waring 1992, in LCP p. 130
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• Net photosynthesis
• Gross photosynthesis
• photorespiration
• mitochondrial respiration
• (also known as dark respiration
• well talk about this later)