Photosynthesis Photosynthesis: CO 2 + H 2 O + Energy  CH 2 O + O 2 + H 2 O Photosynthesis is a balance between competing processes

CH 2 0 CH 2 0 |HO-C-H | COOH COOH CH 2 0 CH 2 0 |HO-C-H | COOH COOH Two molecules of 3-phosphoglyceric acid (3PG) C3Photosynthesis RUBISCO

C3 photosynthesis is dominant pathway

From Chapin et al. (2003) chloroplast …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 C3 Photosynthesis: dominant photosynthetic pathway for land plants CO 2 + RuBP RUBP Carboxylase Oxygenase Energy (ATP, NADPH) 2 PGA (3C sugar) Ribulose Bisphosphate 5 Carbon sugar Calvin-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 Rubisco binds CO 2 and O 2 ; if binds O 2 then no C fixation High CO 2 favors C3 photosynthesis High Temperature / Dry environments  C 3 becomes inefficient Photorespiration…inefficiency in the Ps process 1. Stomata relatively closed to conserve water 2.  CO 2 diffusion into leaf and leaf [CO 2 ] 3.  photorespiration [O 2 “fixation”] C 4 Photosynthesis has evolved in warm & dry 3 Major differences w.r.t. C3 Ps -- Spatially segregated -- CO 2 fixed into a 4-C sugar -- new enzymes & substrates

C4 photosynthesis can dominate in grasslands

C4 Photosynthesis CO 2 + PEP PEP Carboxylase Energy OAA Phosphoenol Pyruvate 3 Carbon sugar Oxaloacetic Acid 4 Carbon sugar Very high affinity for CO 2, no oxygenase function Leaf Mesophyll

OAA Pyruvate + CO 2 Bundle Sheath Cells C3 Photosynthetic Pathway Now, RuBP and carboxylase are present under high [CO 2 ] Returned to mesophyll cells

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

J. Ehleringer Distribution of C3 vs C4 photosynthetic pathways is highly sensitivity to atmospheric CO 2, temperature and precipitation

Photosynthesis CAM Photosynthesis (Crassulacean Acid Metabolism) Based on C4 photosynthetic pathway Desert Species: Extremely hot and dry At night: Stomates open CO 2  mesophyll cells  CO 2 + PEP  OAA, stored in vaculoes During Day: Stomates closed OAA + enzymes  CO 2 (enters Calvin Cycle) + Pyruvate

Photosynthesis Favored PsEcosystem Pathway TemperaturePrecipitationType C 3 cold-warmwetforests, arctic, tundra C 4 warm – hotdrysemi-aridgrasslands, tropical savanna CAMcold - hotextremely dry desserts

Stable Isotopes of Carbon: 13 C ~ 1.1% of total C 12 C is ~98.9% of total C Two classes of C-isotope ratios in plants: δ 13 C approximately –12‰ = C 4 Ps pathway δ 13 C approximately –28‰ = C 3 Ps pathway Plants are ‘depleted’ in 13 C 2 sources of isotope discrimination: A. Physical: 13 CO 2 diffusion slower than 12 CO 2 B. Enzymatic: Biology ‘favors’ light isotopes 12 CO 2 b/c of greater enzyme conformity C 3 Isotope Discrimination (greater) -- Stomata more open, internal CO 2 concentrations higher -- Rubisco has a ‘choice’ C 4 Isotope Discrimination (less) -- Stomata more closed, internal CO 2 concentrations lower -- PEP carboxylase has very high affinity for CO2

Isotope composition is expressed as a ratio (R) = 13 C/ 12 C In practice, measure R in a sample relative to that in a standard R std = 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 13 C = (R sx /R std – 1) 1000 Stable Isotopes of Carbon: 13 C ~ 1.1% of total C 12 C is ~98.9% of total C Stable Isotopes are measured by mass spectrometry

The role of terrestrial plants in limiting atmospheric CO 2 decline over the past 24 million years Pagani et al. (2009) Nature 460, 85-88

Cerling, Ehleringer & Harris (1998) What are the implications of a drop in atmospheric CO 2 ??? Major change in fauna spp. composition attributed to changes in climate East African Mammals …reduced CO 2 --> colder and drier & changes in vegetation composition Dryopithecus Equus Dorcatherium

Modern Day Plants with C3 and C4 Ps C3 C4 Modern day animal tooth enamel for species consuming plants w/C3 and C4 Ps C3 teeth C4 teeth Fossil animal tooth enamel >8 MYA for species consuming plants w/what Ps pathway? # Observations

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 ) Fossil Soil carbonates, Pakistan Horse tooth enamel, North America Mammalian tooth enamel, Pakistan