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Other CO 2 fixation mechanisms

Other CO 2 fixation mechanisms. 1. C3 pathway to fix CO 2 to make sugars C3 reduction: or Calvin cycle (Melvin Calvin 1950s, Nobel prize in 1961) CO 2 → C3 → C6 2. Regulation of key enzymes by light 3. Many plants have photorespiration

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Other CO 2 fixation mechanisms

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  1. Other CO2 fixation mechanisms 1. C3 pathway to fix CO2 to make sugars C3 reduction: or Calvin cycle (Melvin Calvin 1950s, Nobel prize in 1961) CO2→ C3 → C6 2. Regulation of key enzymes by light 3. Many plants have photorespiration 4. C4 plants maximize CO2 fixation using a C4 pathway that increases CO2 concentration 5. CAM plants minimize water loss by fixing CO2 at night.

  2. C4 Carbon Cycle - Concepts PHOTORESPIRATION: O2 is consumed and CO2 is released in the light. Why? O2 consumption by RUBISCO C4 METABOLISM: - Mechanism to concentrate CO2 in chloroplasts to minimize RUBISCO oxygenase activity - primary carboxylation is catalyzed by phosphoenolpyruvate carboxylase (PEPcase) → C4 acid - release of CO2 from C4 acid for Calvin cycle - regeneration of CO2 acceptor, PEP C4 Plants: e.g. corn, sugarcane 2 different cell types: MESOPHYLL CELL BUNDLE SHEATH CELL

  3. C4 Leaf Morphology Bundle sheath cells: Unstacked thylakoids Large starch granules Mesophyll cells: Stacked thylakoids Little or no starch → Cell-specific compart-mentalization of C4 cycle enzymes Kranz anatomy

  4. C3 vs. C4 Leaf Morphology C3 Monocot: Barley, Rice C4 Monocot: Sugarcane

  5. C4 Leaf Morphology C4 Dicot: Flaveria australasica Australian Yellow Weed (Asteraceae)

  6. Basic C4 photosynthetic carbon cycle • 4 Stages: • Carboxylation: formation of C4 acid in mesophyll cell • Transport of C4 acid into bundle sheath cell • Decarboxylation of C4 acid → generation of high [CO2]; Fixation of released CO2 in Calvin cycle • Transport of C3 acid and regeneration of PEP acceptor

  7. C4 Carbon Cycle 4 Stages: 1. Carboxylation: in MESOPHYLL CELL CO2 + PEP (C3) → OAA (C4) → MAL (C4) 2. Transport of MAL into BUNDLE SHEATH 3. Decarboxylation: MAL (C4) → Pyruvate + CO2→ Calvin Cycle 4. Transport of pyruvate back to MESOPHYLL CELL, regeneration of PEP

  8. C4 Carbon Cycle

  9. C4 photosynthetic pathway CO2 concentrates in bundle sheath cells

  10. Single Cell C4 photosynthesis Borszczowia Chenopodiaceae aralocaspica (goosefoot family) Bienertia cycloptera

  11. Single Cell C4 photosynthesis Borszczowia aralocaspica Bienertia cycloptera Confocal fluorescence of a chlorenchyma cell of Borszczowia aralocaspica (A) and Bienertia cycloptera (B) illustrating the chloroplasts in the two cytoplasmic compartments. The broken white lines show the outline of a single cell. Scale bars = 20 mm.

  12. CAM (Crassulacean Acid Metabolism) CAM plants fix CO2 at night Also: Pineapple Orchids Cacti Amaranthus tricolor (pigweed) Agave

  13. DARK: Stomata opened

  14. Light: Stomata closed How do plants regulate this?

  15. CAM PEPcase is active in the night

  16. Regulation of CAM • PEP Carboxylase: is shut down during the day. • 2. MAL product inhibits PEPcase in the day. • Summary: • CAM reduces water loss by separating reactions in time. • CAM plants are adapted to dry habitats.

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