functional biology of and limitations to photosynthesis lecture 17 03/29/05
TRANSCRIPT
Functional biology of and limitations to Photosynthesis
Lecture 1703/29/05
Last Time
Biomechanics and allometry
BiomechanicsEuler buckling equationHow the Euler equation has constrained and guided
plant evolutionDifferent ways to be a treeAllometryAllometric growth and biomass partitioning
Today
Photosynthesis Part II
-Carbon reactions
-What regulates photosynthesis (carbon reactions)-Controls over CO2 diffusion
-Light reactions review
-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
www.tiscali.co.uk/.../ hutchinson/m0030839.html
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users.rcn.com/.../ BiologyPages/L/Leaf.html
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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 mitochondriawere moving while the image was being recorded.
Chloroplasts are light green and essentially fill most of the cell. Fluorescently vital stained with Rhodamine-123.
Palisade cells are packed with chlorophyll and mitochondria
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Where do these reactionstake place??
http://www.daviddarling.info/images/chloroplast.jpg
Mem
bran
e
Harvest Light
3 Major Functions of Light Reactions
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 reactionsto reduce CO2 to sugars
Chloroplast inner membranes
Chloroplast
Initial product isA 3 carbon sugar (PGA)Phosphoglyceric acid
Carbon fixationcatalyzed by enzymeRibulose-bisphosphate carboxylase-oxygenase
Rubisco
Structure of Rubisco showing its four-fold symmetry. From the University of Hamburg site.
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http://www.biologie.uni-hamburg.de/b-online/fo24_1/e1rxoe.htm
Rate of Rubiscomediated PCR cycleis mediated by
Light concentration
Starch is achain of sugars
All reactions are enzyme mediated
[A] + [B] C + D[enzyme]Substrates
Catalyst
Products
Rate of production of A* will depend upon many things
-Most importantly on concentrations of players
If [enzyme] is low then reaction is enzyme (binding site) limited
If [A] and or [B] is low then reaction is substrate limited
-Efficiency of enzyme
Notable Features of Carbon Fixation Reactions
(i) Large nitrogen requirement for Rubisco and otherphotosynthetic enzymes (Nitrogen limitation)
(ii) Dependence on the products of the light-harvestingreactions (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 environmentplants must follow diffusion laws
Flow of certain resource per unit area per unit time
Diffusion coefficient (varies with temp and concentration)
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 vaporor CO2 per unit driving force (a given concentration gradient)
Elaborating Fick’s Law - Anatomy of the System
(since we identify CO2, we remove the diffusivity term
– but it will return when we begin to consider water versus CO2)
Conversion – Fick’s Law
JCO2 = Dx / r
JCO2 is the flux of CO2
Dx is the concentration gradient
r is the sum of all resistances to the flux of CO2 along that path
€
∂c j
∂x
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)
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
Resistance Pathway
Should a plant always minimize resistance to CO2 diffusion?
When plants reduce stomatal conductance (or Increase resistance) water is conserved BUTphotosynthesis declines.
Reduce the efficiency with which plantsconverts light energy to carbohydrates
Remember . . . .
Also, when water stress is too great stomates will close
(i) Large nitrogen requirement for rubisco and otherphotosynthetic enzymes (Nitrogen limitation)
(ii) Dependence on the products of the light-harvestingreactions (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
Notable Features of Carbon Fixation Reactions
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
Chloroplast
Production of sucrose
Triose Phosphate/ Inorganic Phosphate Translocator(TPT)
Transporters - exchange between Chloroplast and Cytosol
Outer membrane of chloroplast has pore-formingproteins (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]
Internal CO2 concentration (mol mol-1)
Ass
imil
atio
n r
ate
(m
ol
m-2 s
-1)
Internal CO2 concentration (mol mol-1)
Dem
and
Func
tion
Ass
imil
atio
n r
ate
(m
ol
m-2 s
-1)
RuB
P sa
tura
ted
regi
on
Ca
What sets this intercept?
*key feature that has driven evolutionary diversification in land plants!
Internal CO2 concentration (mol mol-1)
Dem
and
Func
tion
Ass
imil
atio
n r
ate
(m
ol
m-2 s
-1)
RuB
P sa
tura
ted
regi
on
Ca
Internal CO2 concentration (mol mol-1)
Dem
and
Func
tion
Ass
imil
atio
n r
ate
(m
ol
m-2 s
-1)
RuB
P sa
tura
ted
regi
on
Ca
?
Internal CO2 concentration (mmol mol-1)
Dem
and
Func
tion
Ass
imil
atio
n r
ate
(mm
ol
m-2 s
-1)
RuB
P sa
tura
ted
regi
on
Ca
Internal CO2 concentration (mmol mol-1)
Dem
and
Func
tion
Ass
imil
atio
n r
ate
(mm
ol
m-2 s
-1)
Supply Function
RuB
P sa
tura
ted
regi
on
Ci
Supply functionRate at which CO2 is supplied to rubisco sitesand is determined by [CO2] in atmosphere andStomatal conductance (1/resistance)
Plant Example #1
Ca
Rates of RuBP regeneration limiting to photosynthesis
Internal CO2 concentration (mmol mol-1)
Ass
imil
atio
n r
ate
(mm
ol
m-2 s
-1)
Supply FunctionCa
CiD
eman
d Fu
nctio
n
How does plant #2 differ from plant #1?
Plant Example #2
Controls over photosynthesis
Supply function – controlled by stomatal responses to:
What Controls Demand and Supply Functions?
Controls over photosynthesis
Demand function – controlled by:
Questions