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Photosynthesis
Photosynthesis:
CO2 + H2O + Energy CH2O + O2 + H2O
Photosynthesis is a balance between competing processes
C3
Photosynthesis
RUBISCO
CH20
CH20
|
|
HO-C-H
HO-C-H
|
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COOH
COOH
Two molecules of
3-phosphoglyceric acid
(3PG)
C3 photosynthesis is dominant pathway
…photosynthesis converts light energy to chemical energy
chloroplast
From Chapin et al. (2003)
2 H2O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + 3 H2O + O2
Photosynthesis
C3 Photosynthesis: dominant photosynthetic pathway for land plants
CO2 + RuBP
RUBP Carboxylase Oxygenase
Energy (ATP, NADPH)
2 PGA
(3C sugar)
Ribulose Bisphosphate
5 Carbon sugar
Calvin-Benson Cycle:
-- PGA forms sugars and starches
-- RuBP, NADP and ADP are regenerated
Photosynthesis
Rubisco binds CO2 and O2; if binds O2 then no C fixation
High CO2 favors C3 photosynthesis
High Temperature / Dry environments  C3 becomes inefficient
Photorespiration…inefficiency in the Ps process
1. Stomata relatively closed to conserve water
2.  CO2 diffusion into leaf and leaf [CO2]
3.  photorespiration [O2 “fixation”]
C4 Photosynthesis has evolved in warm & dry
3 Major differences w.r.t. C3 Ps
-- Spatially segregated
-- CO2 fixed into a 4-C sugar
-- new enzymes & substrates
C4 photosynthesis can dominate in grasslands
C4 Photosynthesis
Very high affinity for CO2, no oxygenase function
Leaf
Mesophyll
PEP Carboxylase
CO2 + PEP
Phosphoenol Pyruvate
3 Carbon sugar
OAA
Energy
Oxaloacetic Acid
4 Carbon sugar
OAA
Returned to
mesophyll cells
Pyruvate + CO2
Bundle
Sheath
Cells
C3 Photosynthetic
Pathway
Now, RuBP and
carboxylase are present
under high [CO2]
Photosynthesis
3 Distinct Ecological Features of C4 Photosynthesis (relative to C3):
1.
C4 acid (OAA) transferred to BS where decarboxylated thereby ↑
[CO2] relative to [O2], ↑ rubisco efficiency
2.
PEP carboxlase efficiently draws down internal leaf [CO2], ↑
concentration gradient with atmosphere, absorb CO2 w/more tighly
closed stomata
3.
PEP regeneration ↑ energy requirement by 30% relative to C3
Distribution of C3 vs C4 photosynthetic pathways is highly sensitivity
to atmospheric CO2, temperature and precipitation
J. Ehleringer
Photosynthesis
CAM Photosynthesis (Crassulacean Acid
Metabolism)
Based on C4 photosynthetic pathway
Desert Species: Extremely hot and dry
At night: Stomates open
CO2 mesophyll cells  CO2 + PEP 
OAA, stored in vaculoes
During Day: Stomates closed
OAA + enzymes CO2 (enters Calvin
Cycle) + Pyruvate
Photosynthesis
Favored Ps
Pathway
C3
Ecosystem
Type
forests, arctic, tundra
Temperature
cold-warm
Precipitation
wet
C4
warm – hot
dry
semi-aridgrasslands,
tropical savanna
CAM
cold - hot
extremely dry
desserts
Stable Isotopes of Carbon:
13C ~ 1.1% of total C
12C
is ~98.9% of total C
Two classes of C-isotope ratios in plants:
δ 13C approximately –12‰ = C4 Ps pathway
δ 13C approximately –28‰ = C3 Ps pathway
Plants are ‘depleted’ in 13C
2 sources of isotope discrimination:
A. Physical: 13CO2 diffusion slower than 12CO2
B. Enzymatic: Biology ‘favors’ light isotopes 12CO2 b/c of greater enzyme conformity
C3 Isotope Discrimination (greater)
-- Stomata more open, internal CO2 concentrations higher
-- Rubisco has a ‘choice’
C4 Isotope Discrimination (less)
-- Stomata more closed, internal CO2 concentrations lower
-- PEP carboxylase has very high affinity for CO2
Stable Isotopes of Carbon:
13C ~ 1.1% of total C
12C
is ~98.9% of total C
Stable Isotopes are measured by mass spectrometry
Isotope composition is expressed as a ratio (R) = 13C/12C
In practice, measure R in a sample relative to that in a standard
Rstd= fossil carbonates, Pee Dee Formation, southeastern US
-standard abbreviated PDB
- abundant fossils of Belemnita americana
Carbon isotope ratios are expressed in parts per mil (‰) = parts per thousand according to:
δ 13C = (Rsx/Rstd – 1) • 1000
The role of terrestrial plants in limiting atmospheric CO2
decline over the past 24 million years
Pagani et al. (2009) Nature 460, 85-88
Equus
Cerling, Ehleringer & Harris (1998)
What are the implications of a drop in
atmospheric CO2???
Major change in fauna spp. composition
attributed to changes in climate
East African Mammals
Dorcatherium
…reduced CO2 --> colder and drier &
changes in vegetation composition
Dryopithecus
# Observations
C4
Modern Day Plants with C3 and C4 Ps
C3
Modern day animal tooth enamel
for species consuming plants w/C3 and C4 Ps
# Observations
Fossil animal tooth enamel >8 MYA
for species consuming plants w/what Ps pathway?
C4 teeth
# Observations
C3 teeth
Web Figure 9.5.A Transition in isotope composition in fossil soil and fossil animal teeth,
indicating an expansion of C4 plants in the late Miocene. Isotope values are shown for carbonates
(black circles) extracted from fossil soil from Pakistan, for fossil mammalian tooth enamel from
Pakistan (light green squares), and for fossil horse tooth enamel from North America (green
circles). The change in isotope composition 5 to 7 million years ago suggests a shift from a flora
dominated by C3 plants to a flora dominated by C4 plants. (After Cerling et al. 1993.)
Fossil Soil carbonates,
Pakistan
Mammalian tooth enamel,
Pakistan
Horse tooth enamel,
North America