Download Bio3460-9 Photosynthesis

04/02/2015
Variation in Photosynthetic Pathways
Two major variations on the basic C3 photosynthetic pathway
exist:
Do you know any examples of:
Local Native C4 plants?
Agricultural C4 plants?
Local Native CAM plants?
Agricultural CAM plants?
1) C4 photosynthesis
2) CAM photosynthesis (Crassulacean Acid Metabolism)
In both C4 and CAM plants – they have all the components
of the ancestral C3 photosynthetic pathway - but have additional
“add-on” characteristics that act as a CO2-concentrating
mechanism
C4 photosynthesis occurs in ~1% of plant species and
contributes about 15% of global productivity
CAM plants contribute about 4% of global productivity
Abbreviations used for molecules in biochemical reactions of photosynthesis
Variation in Photosynthetic Pathways
Symbol
Definition
C4 photosynthetic pathway
i) Initial Reaction (mesophyll cells)
RUBISCO
RuBP
PGA
GAP
DHAP
Ribulose-1,5-bisphosphate Carboxylase/Oxygenase (enzyme)
Ribulose-1,5-bisphosphate
Phosphoglycerate or Phosphoglyceric Acid
Glyceraldehyde Phosphate
DiHydroxyacetone Phosphate
PEP
PEPcase
Phosphoenol Pyruvate
Phosphoenol Pyruvate Carboxylase
PEP carboxylase
CO2 + PEP
(3C)
Oxaloacetate >> Malate
(4C)
(4C)
ii) Subsequent Reaction (bundle sheath cells)
Malate
(4C)
CO2 + Pyruvate >> PEP
(3C)
(3C)
The CO2 released in the bundle sheath cells is
used by RUBISCO in the normal Calvin (C3) cycle
Variation in Photosynthetic Pathways
Atmospheric CO2
PEP carboxylase
CO2 + PEP
(3C)
Oxaloacetate >> Malate
(4C)
(4C)
PEP Carboxylase Enzyme:
•
•
•
•
has a high affinity for CO2 (HCO3-)
fast reaction rates
no oxygenase activity
small enzyme – does not need much Nitrogen
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Figure 8.11 The C4 photosynthetic carbon cycle involves five successive stages
Atmosphere
400 ppm CO2
Mesophyll
150 ppm CO2
Bundle Sheath
2000 ppm CO2
Atmosphere
400 ppm CO2
Mesophyll
150 ppm CO2
Mesophyll Cell
Bundle Sheath
2000 ppm CO2
Bundle Sheath Cell
C4 plants have Kranz (wreath) anatomy:
Kranz anatomy occurs because:
1) the different types of chloroplasts present in mesophyll and
bundle sheath cells
2) the thick, suberized (waxy) cell walls of the bundle sheath cells
- which help to prevent leakage of CO2 out of the bundle sheath
Note: Plasodesmata connections between mesophyll and bundle sheath
C4 plants have Kranz Anatomy
Kranz - a German word for Wreath
C4 Plant4
Mesophyll Cell
Bundle Sheath Cell
C3 Plant4
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See Fig. caption on next slide
S von Caemmerer et al. Science (2012) 336:1671-1672
Variation in Photosynthetic Pathways
(A) C3 photosynthesis fixes atmospheric CO2 into C3 acids with Rubisco
in single cells.
(C) Two-cell C4 photosynthesis requires spatial separation of fixation of
atmospheric CO2 into C4 acids and the donation of CO2 from these C4
acids to Rubisco.
CAM photosynthesis is similar to C4 photosynthesis
… BUT
CAM photosynthesis has temporal (day-night) separation of:
Also shown are light microscopy images of transverse sections of leaves
of:
(i) initial CO2 fixation, and
(ii) subsequent de-carboxylation of malate
(B) rice, a C3 plant, and (D) sorghum, a C4 plant.
While C4 photosynthesis has spatial separation
(mesophyll- bundle sheath cells) of these two activities
The rice section shows vascular bundles with few chloroplasts and large
numbers of mesophyll cells between the vascular bundles typical for C3
species. The sorghum leaf section shows chloroplasts in bundle sheath
and only two or three mesophyll cells in between the vascular tissue
typical of a C4 species.
Keeping stomata closed during the day reduces water loss
and is advantageous for plants of deserts or epiphytes
Figure 8.13 Crassulacean acid metabolism (CAM)
transpiration
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CAM photosynthetic Pathway
Diurnal variation in CO2 and H2O exchange in a strict CAM plant
Figure 9.23 Photosynthetic carbon assimilation, evaporation, and stomatal
conductance
Variation in Photosynthetic Pathways
Both C4 and CAM photosynthesis act as CO2 concentrating
mechanisms which improve RUBISCO function by
reducing photorespiration
There is a cost to these variations in photosynthetic pathway
- an extra ATP used to regenerate PEP from pyruvate
Extra ATP is used
in C4 and CAM
photosynthesis
This extra cost must be considered relative to the benefits of
of the CO2 concentrating mechanism
The balance between the costs and benefits determines where
C4 and CAM plants are successful
– which ecological niches they occupy
Physiological Benefits of C4 Photosynthesis:
1) Increased water-use efficiency (ratio of carbon gain to water loss)
- C4 plants have lower stomatal conductance
- lower CO2 concentration in mesophyll cells
- PEP carboxylase fixes CO2 efficiently and saves water
2) Increased nitrogen-use efficiency (ratio of carbon gain to leaf N)
- C4 plants have low RUBISCO contents
- PEP carboxylase is a small enzyme with low N requirements
Physiological Benefits of CAM Photosynthesis:
1) Increased water-use efficiency
- CAM plants open stomata at night when VPD is low
- water loss in transpiration is much reduced
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Physiological Costs of C4 Photosynthesis:
1) Lower quantum yield (light-use efficiency) at low/moderate
temperatures
- the extra ATP cost in C4 plants means less CO2 is fixed per
equivalent amounts of absorbed light energy
- reduces ecological competitiveness in cool environments
Physiological Costs of CAM Photosynthesis:
1) Slow rate of carbon fixation and slow growth rate
- but high survivorship in chronic water stressed environments
- most CAM plants are desert succulents or tropical epiphytes
Figure 9.8 The quantum yield of photosynthetic carbon fixation in a C 3 plant and a
C4 plant
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Hot Environments
Figure 9.17 The relative rates of photosynthetic carbon gain
Cool Environments
C3 plants are more abundant at higher (cooler) latitudes
C4 plants are more abundant at lower (warmer) latitudes
Comparison of patterns of CO2 and H2O exchange
in plants with different photosynthetic pathways
C3 plants are more abundant at higher (cooler) altitudes
C4 plants are more abundant at lower (warmer) altitudes
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