PhotosynthesisPhotosynthesis

Photosynthesis:Photosynthesis:

COCO22 + H + H22O + Energy O + Energy CHCH22O + OO + O22 + H + H22OO

Photosynthesis is a balance between competing processesPhotosynthesis is a balance between competing processes

CHCH2200

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HO-C-HHO-C-H

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COOHCOOH

CHCH2200

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HO-C-HHO-C-H

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COOHCOOH

Two molecules ofTwo molecules of3-phosphoglyceric acid3-phosphoglyceric acid(3PG)(3PG)

C3C3PhotosynthesisPhotosynthesis

RUBISCORUBISCO

C3 photosynthesis is dominant pathwayC3 photosynthesis is dominant pathway

From Chapin et al. (2003)

chloroplastchloroplast

…photosynthesis converts light energy to chemical energy

2 H2O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + 3 H2O + O2

Photosynthesis Photosynthesis

C3 PhotosynthesisC3 Photosynthesis: dominant photosynthetic pathway for land plants: dominant photosynthetic pathway for land plants

COCO22 + RuBP + RuBPRUBP Carboxylase OxygenaseRUBP Carboxylase Oxygenase

Energy (ATP, NADPH)Energy (ATP, NADPH)2 PGA2 PGA(3C sugar)(3C sugar)

Ribulose BisphosphateRibulose Bisphosphate5 Carbon sugar5 Carbon sugar

Calvin-Benson CycleCalvin-Benson Cycle:: -- PGA forms sugars and starches-- PGA forms sugars and starches

-- RuBP, NADP and ADP are regenerated-- RuBP, NADP and ADP are regenerated

Photosynthesis Photosynthesis

Rubisco binds CORubisco binds CO22 and O and O22; if binds O; if binds O22 then no C fixation then no C fixation

High COHigh CO22 favors C3 photosynthesis favors C3 photosynthesis

High Temperature / Dry environments High Temperature / Dry environments C C33 becomes inefficient becomes inefficient

Photorespiration…inefficiency in the Ps processPhotorespiration…inefficiency in the Ps process

1. Stomata relatively closed to conserve water1. Stomata relatively closed to conserve water

2. 2. CO CO22 diffusion into leaf and leaf [CO diffusion into leaf and leaf [CO22]]

3. 3. photorespiration [O photorespiration [O22 ““fixationfixation””]]

CC44 Photosynthesis has evolved in warm & dry Photosynthesis has evolved in warm & dry

3 Major differences w.r.t. C3 Ps3 Major differences w.r.t. C3 Ps

-- Spatially segregated-- Spatially segregated

-- CO-- CO22 fixed into a 4-C sugar fixed into a 4-C sugar

-- new enzymes & substrates-- new enzymes & substrates

C4 photosynthesis can dominate in grasslandsC4 photosynthesis can dominate in grasslands

C4 Photosynthesis C4 Photosynthesis

COCO22 + PEP + PEPPEP CarboxylasePEP Carboxylase

EnergyEnergyOAAOAA

Phosphoenol PyruvatePhosphoenol Pyruvate3 Carbon sugar3 Carbon sugar

Oxaloacetic AcidOxaloacetic Acid4 Carbon sugar4 Carbon sugar

Very high affinity for COVery high affinity for CO2, 2, no oxygenase functionno oxygenase function

Leaf Leaf MesophyllMesophyll

OAAOAA Pyruvate + COPyruvate + CO22 Bundle Bundle Sheath Sheath CellsCells

C3 Photosynthetic C3 Photosynthetic PathwayPathway

Now, RuBP and Now, RuBP and carboxylase are present carboxylase are present under high [COunder high [CO22]]

Returned to Returned to mesophyll cellsmesophyll cells

Photosynthesis Photosynthesis

3 Distinct Ecological Features of C4 Photosynthesis (relative to C3)3 Distinct Ecological Features of C4 Photosynthesis (relative to C3)::

1.1. C4 acid (OAA) transferred to BS where decarboxylated thereby C4 acid (OAA) transferred to BS where decarboxylated thereby ↑ ↑ [CO2] relative to [O2], ↑ rubisco efficiency[CO2] relative to [O2], ↑ rubisco efficiency

2.2. PEP carboxlase efficiently draws down internal leaf [CO2], ↑ PEP carboxlase efficiently draws down internal leaf [CO2], ↑ concentration gradient with atmosphere, absorb CO2 w/more tighly concentration gradient with atmosphere, absorb CO2 w/more tighly closed stomataclosed stomata

3.3. PEP regeneration ↑ energy requirement by 30% relative to C3PEP regeneration ↑ energy requirement by 30% relative to C3

J. Ehleringer

Distribution of C3 vs C4 photosynthetic pathways is highly sensitivityDistribution of C3 vs C4 photosynthetic pathways is highly sensitivityto atmospheric COto atmospheric CO22, temperature and precipitation, temperature and precipitation

PhotosynthesisPhotosynthesis

CAM Photosynthesis (Crassulacean Acid CAM Photosynthesis (Crassulacean Acid Metabolism)Metabolism)

Based on C4 photosynthetic pathwayBased on C4 photosynthetic pathway

Desert Species: Extremely hot and dryDesert Species: Extremely hot and dry

At nightAt night: Stomates open: Stomates open

COCO22 mesophyll cells mesophyll cells CO CO22 + PEP + PEP OAA, stored in vaculoesOAA, stored in vaculoes

During DayDuring Day: Stomates closed: Stomates closed

OAA + enzymes OAA + enzymes COCO22 (enters Calvin (enters Calvin Cycle) + PyruvateCycle) + Pyruvate

PhotosynthesisPhotosynthesis

Favored PsFavored Ps EcosystemEcosystem

Pathway Pathway TemperatureTemperature PrecipitationPrecipitation TypeType

CC33 cold-warmcold-warm wetwet forests, forests, arctic, tundraarctic, tundra

CC44 warm – hotwarm – hot drydry semi-aridsemi-aridgrasslands, grasslands, tropical savannatropical savanna

CAMCAM cold - hotcold - hot extremely dry extremely dry dessertsdesserts

Stable Isotopes of CarbonStable Isotopes of Carbon::1313C ~ 1.1% of total CC ~ 1.1% of total C 1212C is ~98.9% of total CC is ~98.9% of total C

Two classes of C-isotope ratios in plants:Two classes of C-isotope ratios in plants:

δ δ 1313C approximately –12‰ = CC approximately –12‰ = C44 Ps pathway Ps pathwayδ δ 1313C approximately –28‰ = CC approximately –28‰ = C33 Ps pathway Ps pathway

Plants are Plants are ‘‘depleteddepleted’’ in in 1313CC

2 sources of isotope discrimination:2 sources of isotope discrimination:A. Physical: A. Physical: 1313COCO22 diffusion slower than diffusion slower than 1212COCO22

B. Enzymatic: Biology B. Enzymatic: Biology ‘‘favorsfavors’’ light isotopes light isotopes 1212COCO22 b/c of greater enzyme conformity b/c of greater enzyme conformity

CC33 Isotope Discrimination (greater) Isotope Discrimination (greater)-- Stomata more open, internal CO-- Stomata more open, internal CO22 concentrations higher concentrations higher-- Rubisco has a -- Rubisco has a ‘‘choicechoice’’

CC44 Isotope Discrimination (less) Isotope Discrimination (less)-- Stomata more closed, internal CO-- Stomata more closed, internal CO22 concentrations lower concentrations lower-- PEP carboxylase has very high affinity for CO2-- PEP carboxylase has very high affinity for CO2

Isotope composition is expressed as a ratio (R) = Isotope composition is expressed as a ratio (R) = 1313C/C/1212CC

In practice, measure R in a sample relative to that in a standardIn practice, measure R in a sample relative to that in a standard

RRstdstd= fossil carbonates, Pee Dee Formation, southeastern US= fossil carbonates, Pee Dee Formation, southeastern US

-standard abbreviated PDB-standard abbreviated PDB

- abundant fossils of - abundant fossils of Belemnita americanaBelemnita americana

Carbon isotope ratios are expressed in parts per mil (‰) = parts per thousand according to:Carbon isotope ratios are expressed in parts per mil (‰) = parts per thousand according to:

δ δ 1313C = (RC = (Rsxsx/R/Rstdstd – 1) • 1000 – 1) • 1000

Stable Isotopes of CarbonStable Isotopes of Carbon::1313C ~ 1.1% of total CC ~ 1.1% of total C 1212C is ~98.9% of total CC is ~98.9% of total C

Stable Isotopes are measured by mass spectrometryStable Isotopes are measured by mass spectrometry

The role of terrestrial plants in limiting atmospheric CO2

decline over the past 24 million yearsPagani et al. (2009) Nature 460, 85-88

Cerling, Ehleringer & Harris Cerling, Ehleringer & Harris (1998)(1998)

What are the implications of a drop What are the implications of a drop in atmospheric COin atmospheric CO22??????

Major change in fauna spp. Major change in fauna spp. composition attributed to changes in composition attributed to changes in climateclimate

East African MammalsEast African Mammals

……reduced COreduced CO22 --> colder and drier & --> colder and drier &

changes in vegetation compositionchanges in vegetation compositionDryopithecusDryopithecus

EquusEquus

DorcatheriumDorcatherium

Modern Day Plants with C3 and C4 PsC3

C4

Modern day animal tooth enamelfor species consuming plants w/C3 and C4 Ps

C3 teethC4 teeth

Fossil animal tooth enamel >8 MYAfor species consuming plants w/what Ps pathway?

# O

bser

vatio

ns# O

bservations# O

bservations

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,Fossil Soil carbonates,PakistanPakistan

Horse tooth enamel,Horse tooth enamel,North AmericaNorth America

Mammalian tooth enamel,Mammalian tooth enamel,PakistanPakistan