Functional biology of and limitations to Photosynthesis Lecture 17 03/29/05.

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



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

www.kensbiorefs.com/ cellstructure.html QuickTime™ and aTIFF (LZW) decompressor




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.



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)


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!


Dem

and

Func

tion

Ass

imil

atio

n r

ate

(m

ol

m-2 s

-1)

RuB

P sa

tura

ted

regi

on

Ca


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


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


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

Functional biology of and limitations to Photosynthesis Lecture 17 03/29/05.

Documents

Transcript of Functional biology of and limitations to Photosynthesis Lecture 17 03/29/05.