Alternative mechanisms of carbon fixation

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Alternative mechanisms of carbon fixation
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Metabolic challenges water vs. CO2

• Ex. Photosynthesis and preventing excessive water
  loss
• CO2 comes in via stomata, but water goes out
  at same time (transpiration)
• so, plants close stomata during hot, dry days
  BUT, decreases photosynthetic yield

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Normal pathwayC3 plants

• Initial fixation of carbon occurs using rubisco
  enzyme in Calvin cycle that adds CO2 to
  ribulose biphosphate
• Produces first organic product called
  3-phosphoglycerate (thus, C3 plant.)
• Ex. Rice, wheat, soybeans

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Figure 10.17 The Calvin cycle (Layer 3)
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Photorespiration occurs in the light, and
consumes O2

• When plants close stomata, get declining levels
  of CO2 starves the Calvin Cycle
• Rubisco can also accept O2 in place of CO2, so if
  rubisco adds O2, the product splits, and the 2-C
  cmpd is exported out of the chloroplast
• Mitochondria and peroxisomes break this 2-C cmpd
  down
• DOES NOT PRODUCE ANY ATP!!!
• Process actually takes organic materials AWAY
  from the Calvin Cycle

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So why photorespiration?

• Evolutionary baggage originated when early
  atmosphere had less O2 and more CO2

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C4 plants spatial separation of processes
(mesophyll cells vs. bundle-sheath cells)

• Preface the Calvin Cycle with carbon fixation
  that forms a 4-C compound as its first product
  CO2 is added to PEP to form oxaloacetate
• Requires unique leaf anatomy see page 192
• PEP has a very high affinity for CO2, so when it
  is hot and dry (and stomata close), PEP can fix
  CO2 when rubisco cannot.
• Mesophyll cells of a C4 plant pump CO2 into the
  bundle sheath, keeping CO2 high enough for
  rubisco to accept carbon dioxide
• C4 photosynthesis minimizes photorespiration and
  enhances sugar production
• Ex. Sugarcane, corn, members of the grass family

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Figure 10.18 C4 leaf anatomy and the C4 pathway
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CAM plants temporal separation night vs. day

• Succulents, cacti, pineapples
• Open stomata during night and close them during
  day
• Take up CO2 during night and make organic acids
  (this is the mode of carbon fixation)
• Mesophyll cells of CAM plants store organic acids
  in their vacuoles until day, when light reactions
  can supply ATP and NADPH for Calvin Cycle
• During day, the CO2 is released from the organic
  acids

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Figure 10.19 C4 and CAM photosynthesis compared