1. BIOL 350 Spring 10 1 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 4. C4 plants maximize CO 2 fixation using a C4 pathway that increases CO 2 concentration 5. CAM plants minimize water loss by fixing CO 2 at night.

2. BIOL 350 Spring 10 2 C4 Carbon Cycle - Concepts PHOTORESPIRATION: O 2 is consumed and CO 2 is released in the light. Why? O 2 consumption by RUBISCO C4 METABOLISM: - Mechanism to concentrate CO 2 in chloroplasts to minimize RUBISCO oxygenase activity - primary carboxylation is catalyzed by phosphoenolpyruvate carboxylase (PEPcase) → C4 acid - release of CO 2 from C4 acid for Calvin cycle - regeneration of CO 2 acceptor, PEP C4 Plants: e.g. corn, sugarcane 2 different cell types: MESOPHYLL CELL BUNDLE SHEATH CELL

3. BIOL 350 Spring 10 3 C4 Leaf Morphology Kranz anatomy Bundle sheath cells: Unstacked thylakoids Large starch granules Mesophyll cells: Stacked thylakoids Little or no starch → Cell-specific compart- mentalization of C4 cycle enzymes

4. BIOL 350 Spring 10 4 C3 vs. C4 Leaf Morphology C4 Monocot: Sugarcane C3 Monocot: Barley, Rice

5. BIOL 350 Spring 10 5 C4 Leaf Morphology C4 Dicot: Flaveria australasica Australian Yellow Weed (Asteraceae)

6. BIOL 350 Spring 10 6 Basic C4 photosynthetic carbon cycle 4 Stages: 1)Carboxylation: formation of C4 acid in mesophyll cell 2)Transport of C4 acid into bundle sheath cell 3)Decarboxylation of C4 acid → generation of high [CO 2 ]; Fixation of released CO 2 in Calvin cycle 4)Transport of C3 acid and regeneration of PEP acceptor

7. BIOL 350 Spring 10 7 C4 Carbon Cycle 4 Stages: 1. Carboxylation: in MESOPHYLL CELL CO 2 + PEP (C3) → OAA (C4) → MAL (C4) 2. Transport of MAL into BUNDLE SHEATH 3. Decarboxylation: MAL (C4) → Pyruvate + CO 2 → Calvin Cycle 4. Transport of pyruvate back to MESOPHYLL CELL, regeneration of PEP

8. BIOL 350 Spring 10 8 C4 Carbon Cycle

9. BIOL 350 Spring 10 9 C4 photosynthetic pathway CO 2 concentrates in bundle sheath cells

10. BIOL 350 Spring 10 10 Single Cell C4 photosynthesis Borszczowia Chenopodiaceae aralocaspica(goosefoot family) Bienertia cycloptera

11. BIOL 350 Spring 10 11 Single Cell C4 photosynthesis Borszczowia aralocaspicaBienertia 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. BIOL 350 Spring 10 12 CAM (Crassulacean Acid Metabolism) CAM plants fix CO 2 at night Amaranthus tricolor (pigweed) Agave Also: Pineapple Orchids Cacti

13. BIOL 350 Spring 10 13 DARK: Stomata opened

14. BIOL 350 Spring 10 14 Light: Stomata closed How do plants regulate this?

15. BIOL 350 Spring 10 15 CAM PEPcase is active in the night

16. BIOL 350 Spring 10 16 Regulation of CAM 1. 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.