Photosynthesis

1. Photosynthesis Photosynthesis: CO2 + H2O + Energy CH2O + O2 + H2O Photosynthesis is a balance between competing processes

2. CH20 | HO-C-H | COOH CH20 | HO-C-H | COOH Two molecules of 3-phosphoglyceric acid (3PG) C3 Photosynthesis RUBISCO

3. C3 photosynthesis is dominant pathway

4. …photosynthesis converts light energy to chemical energy chloroplast From Chapin et al. (2003)

5. 2 H2O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + 3 H2O + O2

7. Photosynthesis RUBP Carboxylase Oxygenase CO2 + RuBP 2 PGA (3C sugar) C3 Photosynthesis: dominant photosynthetic pathway for land plants Energy (ATP, NADPH) Ribulose Bisphosphate 5 Carbon sugar Calvin-Benson Cycle: -- PGA forms sugars and starches -- RuBP, NADP and ADP are regenerated

8. 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

9. C4 photosynthesis can dominate in grasslands

10. C4 Photosynthesis Very high affinity for CO2, no oxygenase function Leaf Mesophyll PEP Carboxylase CO2 + PEP OAA Energy Oxaloacetic Acid 4 Carbon sugar Phosphoenol Pyruvate 3 Carbon sugar

11. OAA Pyruvate + CO2 Bundle Sheath Cells C3 Photosynthetic Pathway Returned to mesophyll cells Now, RuBP and carboxylase are present under high [CO2]

12. Photosynthesis • 3 Distinct Ecological Features of C4 Photosynthesis (relative to C3): • C4 acid (OAA) transferred to BS where decarboxylated thereby ↑ [CO2] relative to [O2], ↑ rubisco efficiency • PEP carboxlase efficiently draws down internal leaf [CO2], ↑ concentration gradient with atmosphere, absorb CO2 w/more tighly closed stomata • PEP regeneration ↑ energy requirement by 30% relative to C3

13. Distribution of C3 vs C4 photosynthetic pathways is highly sensitivity to atmospheric CO2, temperature and precipitation J. Ehleringer

14. 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

15. Photosynthesis Favored Ps Ecosystem Pathway TemperaturePrecipitationType C3 cold-warm wet forests, arctic, tundra C4 warm – hot dry semi-arid grasslands, tropical savanna CAM cold - hot extremely dry desserts

16. 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

17. 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

19. The role of terrestrial plants in limiting atmospheric CO2 decline over the past 24 million years Pagani et al. (2009) Nature 460, 85-88

20. Equus Dorcatherium 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 …reduced CO2 --> colder and drier & changes in vegetation composition Dryopithecus

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

23. 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 Horse tooth enamel, North America Mammalian tooth enamel, Pakistan