Post on 12-Feb-2016
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Photosynthesis: Dark cycle reactions,variation 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.
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.
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
Rubisco – the enzyme ribulose biphosphate carboxylase/oxygenase
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.
RuBP
Rubisco will also take O2 rather than CO2 and
oxygenate RuBP – called photorespiration.
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.)
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?
Field measurement of photosynthesis and its control by
environmental conditions
Infra-red Gas Analyzer measures the concentration of CO2 in the air stream before and after it flows across the leaf in the chamber
The chamber is enclosed over the leaf. Light and temperature are measured while photosynthesis is being measured.
Photosynthesis rate calculated from gas flow rate and CO2 concentration difference
LI-COR 6400
Basics of foliage photosynthesis
00
Saturation level. sometimes called photosynthetic capacity.
Compensation pointThe irradiance at which CO uptake is zero2
Photosynthetic efficiency:Increase in photosynthesis per increase in irradiance
Any questions?
Increasing CO2 concentration in the atmosphere can increase the maximum rate of photosynthesis in the short term
Light Reaction Limiting
Dark Reaction Limiting
Measured light response curve of Abies amabilis first year foliage.
Shade foliage with low maximum value and low compensation point.
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.
Species differences in leaf photosynthesisA has the highest photosynthetic rate at light saturation
B has the highest photosynthetic efficiency and the lowest compensation point.
Another important measure is called Water Use Efficiency:the ratio of photosynthesis achieved per unit of water lost.
Units: mmol/mol milli mols of CO per mol of water transpired
2
Units: μmol/m /smicro mols of CO per square meter foliage per second
22
milli [m] 0.001 (a thousandth) micro [µ] 0.000 001 (a millionth)
Any questions?
Wind River Canopy Crane Research Facility
Thuja plicata
Abies grandis
Pseudotsuga menziesii
Tsuga heterophylla
Old-growth species:
Douglas-firPseudotsuga
Western hemlockTsuga
Upper Canopy Lower CanopyPhot. Cap.
13.1
9.0
μmol/m /s2
Water Use Eff.
6.2
4.9
mmol/mol
Phot. Cap.
8.8
3.2
Water Use Eff.
3.5
4.8
Notice the difference in branch structure between the species
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
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 is called Photorespiration
3-phosphoglycerate
2-phosphoglycerate
This becomes a problem when photosynthesis rates are high, i.e. photosystem II produces lots of O2 .
P P PPC-C-C-C-C C-C-C + C-C
RuBisCORibulose 1, 5-biphosphate
Enzyme
O2
CO2
It is believed that photorespiration in plants has increased over geologic time due to increasing atmospheric O2 concentration -the
product of photosynthetic organisms themselves.
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".
275 ppm CO2
73 ppm CO2
Decarboxylation of malate (CO2 release) creates a higher concentration of CO2 in bundle sheath cells than found in photosynthetic cells of C3 plants.
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.
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.
C4 Photosynthesis
Xylem
Bundle sheath cells filled with chloroplasts. CALVIN REACTION SITE
Phloem
Parenchyma filled with chloroplasts
C4 acids synthesized in the parenchyma move to the bundle sheath
Carbon skeleton compounds return to parenchyma
Anatomical separation of the C4 photosynthesis component processes
Crassulacean Acid Metabolism (CAM)
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
First discovered in succulents of the Crassulacea: e.g.,sedums
C3, majority of
speciesC4, e.g., sugar
cane, cornCAM, e.g., cacti
Leaf structure
Typical habitatcharacteristics
Productivity
Optimum Temperature
Efficiency in light
Bundle sheathcells have
chloroplasts
Bundle sheath cells lack
chloroplasts
Mesophyll cells have large vacuoles
Can be sun or shade plants
Ineffective in shade
CO2 capture at night
Requires relatively
moist habitats
Arid or tropicalregions
Arid environments
Moderate High Low
15-25oC 30-40oC 35oC
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.