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Fig. 8.2

An outline of C3 photosynthesis. The Calvin Cycle (reductive pentose phosphate cycle) 3 Stages Carboxylation Reduction Regeneration. Fig. 8.2. A 3 carbon molecule. Carboxylation The key initial step in C3 photosynthesis RUBP + CO 2 ---> 3-PGA

jessamine-menachem
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Fig. 8.2

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  1. An outline of C3 photosynthesis • The Calvin Cycle • (reductive • pentose phosphate • cycle) • 3 Stages • Carboxylation • Reduction • Regeneration Fig. 8.2 A 3 carbon molecule

  2. Carboxylation • The key initial step in C3 photosynthesis • RUBP + CO2 ---> 3-PGA • Catalyzed by “Rubisco”: ribulose 1,5-bisphosphate carboxylase-oxygenase • binds the 5C RUBP molecule and 1C CO2, making two 3C molecules. 5 C + 1 C -----> 2 x 3C molecules Fig. 8.3 (partial)

  3. Carboxylation • Reduction • Regeneration Fig. 8.2

  4. Reduction Reduction steps of the Calvin Cycle use ATP and NADPH to produce a carbohydrate, glyceraldehyde 3 phosphate. 3PGA + ATP + NADPH --> G3P G3P can be used to make sucrose or starch

  5. Fig. 8.3 (partial) - the reduction steps

  6. Carboxylation • Reduction • Regeneration Fig. 8.2

  7. Regeneration The regeneration steps of the Calvin Cycle use ATP to regenerate RUBP from some of the glyceraldehyde-3-P so the cycle can continue. Some of the carbohydrate is converted back into ribulose 1,5 bisphosphate, the initial CO2 receptor molecule.

  8. Fig. 8.3 (partial) - the regeneration steps

  9. 3-PGA RUBP 3 carbon molecules, hence “C3” photosynthesis Fig. 8.4

  10. Reviewing the Calvin cycle and counting carbon (C) atoms associated with one carboxylation. Carboxylation. 1 CO2 binds to 1 RuBP (5C) producing two molecules of 3-PGA (total of 6 C). 2. Reduction. The two 3-PGA (3 C each) are reduced to two glyceraldehyde 3 phosphate (G3P, 3 C each) using ATP and NADPH produced by the light reactions (still 6 C). 3. Regeneration. 5 of the 6 C in the 2 molecules of G3P are used to regenerate one RuBP (5C) using ATP. A total of 6 turns of the Calvin cycle are required to make one hexose (6C). This requires 18 ATP + 12 NADPH.

  11. 6 turns (6 CO2) of the Calvin cycle are required to make one hexose (6C). This requires 18 ATP + 12 NADPH. • How much light energy is required to produce hexose? • Minimum of 8 (often 9 to 10) photons required per CO2 fixed (remember quantum yield?) • Red light (680nm) = 175kJ/mol photons (from E = hn) • 6 CO2/hexose x 8photons/CO2 x 175 kJ/photon = • 8400 kJ/mole hexose

  12. What is the energy efficiency of hexose production? 8400 kJ/mole hexose (for the red light example!) One mole of hexose (e.g. glucose or fructose) yields about 2800 kJ when it’s oxidized. (The heat of combustion) Efficiency = energy output/energy input = 2800kJ/8400kJ = 33% This is the maximum overall thermodynamic efficiency of photosynthesis. Actual efficiency is much lower because: 1) quantum yield is < 1 CO2/8 photons 2) higher energy light (l < 680nm) is used

  13. Fig 9.8 Typical light response of photosynthesis for a C3 plant Quantum yield =CO2 fixed/photon absorbed

  14. In low O2 air, 2%. In standard air, 21% O2.

  15. Why does decreasing the O2 concentration around a C3 leaf increase the uptake of CO2? Why is this effect not seen in some plants such as corn, sugar cane, and many grasses common in warm environments?

  16. I. Photorespiration II. CO2 concentrating mechanisms - variation on the “C3” photosynthetic metabolism.

  17. Plant of the day, Zea mays (Poaceae)

  18. How does the photosynthetic response to light compare in corn and beans?

  19. Corn Bean Corn vs. bean Corn has: Lower QY 2. Higher max. photosynthesis 3. Higher light saturation 4. O2 insensitive

  20. The first step in the Calvin cycle is the carboxylation of RUBP by Rubisco. Remember Rubisco’s full name? Ribulose 1,5 bisphosphate carboxylase-oxygenase

  21. Rubisco can catalyze the oxygenation (O2) of RuBP and the carboxylation (CO2) of RuBP. Rubisco Fig. 8.8

  22. The set of reactions that begins with Rubisco oxygenation of RUBP is called photorespiration. When Rubisco oxygenates RUBP, a CO2 is lost from the leaf, reducing the net uptake of CO2.

  23. CO2 Carbon gain + RuBP + O2 Carbon loss, photorespiration What determines the rate of carboxylation vs. oxygenation? What determines the reaction rates for any two competing substrates in an enzyme-catalyzed reaction?

  24. Determinants of carboxylation vs. oxygenation. 1. Concentration of CO2 & O2 2. Rubisco specificity for CO2 vs. O2 Rubisco CO2 O2 Concentration of O2 >> CO2, but Rubisco specificity favors CO2 binding. Chloroplast stroma

  25. Oxygenation of RuBP causes a loss of CO2 and reduces CO2 uptake. In low O2 air, 2%. In standard air, 21% O2.

  26. So why does Rubisco have this inefficient property? Consider Earth’s atmosphere 3 billion years ago. High CO2/low O2 20% CO2 no O2 Oxygenation was not a problem CO2/O2 ratio has decreased greatly over Earth’s history 0.04% CO2 (and rising) 21% O2

  27. The O2 inhibition of CO2 uptake represents a huge selective pressure for plant characteristics to prevent carboxylation. How to avoid oxygenation? 1. Develop new Rubisco that’s insensitive to O2 2. Reduce O2 concentration in chloroplast 3. Increase CO2 concentration in chloroplast

  28. Plants like corn show no effect of O2 concentration; apparently no oxygenation by Rubisco. They also have different initial products; 14C label shows up first in 4 carbon organic acids - malic acid, aspartic acid. These are called “C4” plants. C4 plants have Rubisco, so how do they avoid oxygenation? a) Initial carboxylation is not by Rubisco in C4 plants b) C4 leaf anatomy differs

  29. How does C4 biochemistry differ from C3? • Primary carbon fixation step uses different substrates and enzymes. HCO3- + PEP --------> 4 carbon organic acids PEP carboxylase Phosphenol pyruvate = PEP Phosphenol pyruvate carboxylase = PEPcase PEPcase activity is not affected by O2. PEPcase uses HCO3-, not CO2. [HCO3-] > [CO2]

  30. C4 leaf anatomy model (Fig 8.8d)

  31. C4 leaf anatomy (Fig. 8.9a)

  32. C4 leaf anatomy differs from C3 Primary carboxylation is spatially separated from the Calvin cycle.

  33. The C4 system concentrates CO2 at Rubisco. This is particularly useful in warm environments because 1) the solubility of CO2 decreases more with temperature than the solubility of O2. 2) Allows C4 plants to operate with lower stomatal aperture (conductance), thereby losing less water.

  34. Extra ATP cost of regenerating PEP means that C4 CO2 fixation requires more light energy. 1. Quantum yield of C4 < C3 Extra ATP (light) cost is not a problem in high light environments, but is in low light environments. Few C4 “shade” plants.

  35. Corn, a C4 plant Bean, a C3 plant Corn vs. bean Lower QY 2. Higher max. photosynthesis 3. Higher light saturation 4. O2 insensitive

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