Transcript Photosynthesis - University of Arizona

Lecture 17
03/29/05
Functional biology of and
limitations to Photosynthesis
Last Time
Biomechanics and allometry
Biomechanics
Euler buckling equation
How the Euler equation has constrained and guided
plant evolution
Different ways to be a tree
Allometry
Allometric growth and biomass partitioning
Today
Photosynthesis Part II
-Carbon reactions
-Light reactions review
-What regulates photosynthesis (carbon reactions)
-Controls over CO2 diffusion
-Carbon fixation limits
-Biochemical control over carbon fixation
-Triose-P transporter
-A/Ci Curves
Central Question
• What have been the important constraints which
have shaped the evolution of plant form and
function?
• Last Lecture (2nd lecture in class)
– Evolution of Photoautotrophy
– Basic form of the photosynthetic apparatus
• This Lecture
– Controls over photosynthesis - evolutionary implications /
important variation
QuickTime™ and a
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QuickTime™ and a
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are needed to see this picture.
www.tiscali.co.uk/.../ hutchinson/m0030839.html
QuickTime™ and a
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users.rcn.com/.../ BiologyPages/L/Leaf.html
Palisade cells are packed with chlorophyll and mitochondria
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A group of palisade mesophyll cells in a spinach leaf, in confocal stereo.
The numerous mitochondria (light green) lie between the chloroplasts (light green)
(which are only very dimly fluorescent under the optical conditions used here).
Some are round particles, others long branched filaments. Some are
slightly blurred because the cells were still alive and the mitochondria
were moving while the image was being recorded.
Chloroplasts are light green and essentially fill most of the cell. Fluorescently vital
stained with Rhodamine-123.
Where do these reactions
take place??
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www.kensbiorefs.com/ cellstructure.html
QuickTime™ and a
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are needed to see this picture.
http://www.daviddarling.info/images/chloroplast.jpg
3 Major Functions of Light Reactions
Membrane
Harvest Light
Where does the ATP and NADPH go
from the light reactions?
The Calvin cycle (PCR cycle)
Turns out there is plenty of light energy, most of
the time, what regulates photosynthesis is
carboxylation!
Calvin Cycle or Photosynthetic Carbon Reduction (PCR Cycle)
Overview - Photosynthetic Apparatus
• Light harvesting reactions
• Resources: Light (photons) and Water
(electrons)
• Products: ATP (energy), H+ (potential energy)
and NADPH (reducing power)
• Carbon reactions
Carbon fixation reactions (C3 Photosynthesis)
Use ATP and NADPH from light harvesting reactions
to reduce CO2 to sugars
Chloroplast inner membranes
Initial product is
A 3 carbon sugar (PGA)
Phosphoglyceric acid
Carbon fixation
catalyzed by enzyme
Ribulose-bisphosphate
carboxylase-oxygenase
Rubisco
Chloroplast
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
http://www.biologie.uni-hamburg.de/b-online/fo24_1/e1rxoe.htm
Structure of Rubisco showing its
four-fold symmetry. From the University of Hamburg site.
Rate of Rubisco
mediated PCR cycle
is mediated by
Light concentration
Starch is a
chain of sugars
All reactions are enzyme mediated
Catalyst
Substrates
[A] + [B]
[enzyme]
Products
C+D
Rate of production of A* will depend upon many things
-Most importantly on concentrations of players
-Efficiency of enzyme
If [enzyme] is low then reaction is enzyme (binding site) limited
If [A] and or [B] is low then reaction is substrate limited
Notable Features of Carbon Fixation Reactions
(i) Large nitrogen requirement for Rubisco and other
photosynthetic enzymes (Nitrogen limitation)
(ii) Dependence on the products of the light-harvesting
reactions (which depends on amount of light hitting leaf)
(iii) Dependent upon CO2 supply to the chloroplast
What influences CO2 supply?
Ability of CO2 to reach reactions. . .
Limits to Photosynthesis
Consider one important limit
- diffusion of CO2 into chloroplasts from atmosphere
How can we make sense of CO2 limitation of photosynthesis?
Fick’s Law of Diffusion
In order to exchange ‘resources’ with the environment
plants must follow diffusion laws
Diffusion coefficient
(varies with temp and concentration)
Flow of certain resource
per unit area per unit time
Concentration Gradient
(change in resource concentration, cj
with distance, x)
Resistance Pathway
Plants regulate CO2
uptake and water loss by
changing the size of stomatal opening
(which regulates Stomatal conductance - the flux of water vapor
or CO2 per unit driving force (a given concentration gradient)
Elaborating Fick’s Law - Anatomy of
the System
Conversion – Fick’s Law
JCO2 = Dx / r
JCO2 is the flux of
CO2
Dx is the concentration gradient
c j
x
r is the sum of all resistances to the flux of CO2 along that path
(since we identify CO2, we remove the diffusivity term
– but it will return
 when we begin to consider water versus CO2)
Elaborating Fick’s Law - Concentration
gradient
• Ca is the external CO2 concentration (external to
the leaf)
• Ci is the concentration at the site of
carboxylation (we call this “intercellular CO2
concentration)
Resistance Pathway
Resistances to CO2 diffusion
1.
Boundary layer air-phase resistance
2.
Stomatal resistance (=1/conductance)
3.
Internal air spaces aqueous-phase resistance
4.
Diffusional resistance across mesophyll cell wall
5.
Resistance at the site of carboxylation (chloroplast
resistance
Should a plant always minimize resistance to CO2 diffusion?
Remember . . . .
When plants reduce stomatal conductance (or
Increase resistance) water is conserved BUT
photosynthesis declines.
Also, when water stress is too great stomates will close
Reduce the efficiency with which plants
converts light energy to carbohydrates
Notable Features of Carbon Fixation Reactions
(i) Large nitrogen requirement for rubisco and other
photosynthetic enzymes (Nitrogen limitation)
(ii) Dependence on the products of the light-harvesting
reactions (which depends on amount of light hitting leaf)
(iii) CO2 supply to the chloroplast
(iv) Phosphate Translocator – mediates the ‘supply’
and ‘demand’ for triose-P in the cell
–“Source-Sink” theory and carbohydrate backup
Summary - Biochemical regulation of
photosynthesis
• Light Reactions – mediate inputs of energy
• Rubisco (Ribulose bisphosphate carboxylase
/ oxygenase) – mediates inputs of carbon
– Extensive research has shown that Rubsico
controls the reaction rates of the whole Calvin
cycle reaction complex
• Phosphate Translocator – mediates the
‘supply’ and ‘demand’ for triose-P in the cell
– “Source-Sink” theory and carbohydrate backup
Triose Phosphate/ Inorganic Phosphate Translocator
(TPT)
Production of
sucrose
Chloroplast
Transporters
- exchange between Chloroplast and Cytosol
Outer membrane of chloroplast has pore-forming
proteins (porins)
- allow substances to diffuse freely
Inner chloroplast membrane is the permeability barrier
- transport is usually by specific translocators
Rate of transport depends upon demand for Sucrose
Take-home from the Pi regulation
• The translocator is ultimately controlled by sink
strength of the plant (the ability of plant’s sinks to
utilize sucrose)
• If sucrose builds up in the leaf due to lack of sink
strength, it negatively feeds back on reactions in the
cytosol (which in turn slows down the generation of
Pi)
• Pi must be available to exchange with triose-P out of
the chloroplast.
• Lack of triose-P transport results in triose-P
converted into starch in the chloroplasts.
– (slows RuBP regeneration)
Sharkey Table – limitations of
photosynthesis
• Rubisco activity limits photosynthesis under high
light, low Ci conditions, so there is more RuBP
(ribulose 1,5 bisphosphate) than binding sites on
Rubisco.
• RuBP regeneration limits photosynthesis under low
light and high CO2, so there are open binding sites on
Rubisco because electron transport capacity is
inadequate to regenerate enough RuBP
• Triose-P utilization limits photosynthesis under high
light and high CO2, also resulting in more RuBP than
available Rubisco binding sites.
How do we make sense of the
Limits to Photosynthesis?
The A/Ci Curve
A = CO2 Assimilation Rate
Ci = Internal [CO2]
Assimilation rate (mmol m-2 s-1)
Internal CO2 concentration (mmol mol-1)
Assimilation rate (mmol m-2 s-1)
Ca
Internal CO2 concentration (mmol mol-1)
What sets this intercept?
*key feature that has driven evolutionary
diversification in land plants!
Assimilation rate (mmol m-2 s-1)
Ca
Internal CO2 concentration (mmol mol-1)
Assimilation rate (mmol m-2 s-1)
?
Ca
Internal CO2 concentration (mmol mol-1)
Assimilation rate (mmol m-2 s-1)
Ca
Internal CO2 concentration (mmol mol-1)
Assimilation rate (mmol m-2 s-1)
Plant Example #1
Rates of RuBP regeneration
limiting to photosynthesis
Ci
Ca
Supply Function
Internal CO2 concentration (mmol mol-1)
Supply function
Rate at which CO2 is supplied to rubisco sites
and is determined by [CO2] in atmosphere and
Stomatal conductance (1/resistance)
Assimilation rate (mmol m-2 s-1)
Plant Example #2
Ci
Ca
Supply Function
Internal CO2 concentration (mmol mol-1)
How does plant #2 differ from plant #1?
Controls over photosynthesis
What Controls Demand and Supply Functions?
Supply function – controlled by stomatal
responses to:
Controls over photosynthesis
Demand function – controlled by:
Questions