Photosynthetic Pathways

1. Photosynthetic Pathways • C3 Photosynthesis: • Used by most plants and algae. • CO2 + ribulose bisphosphate (5 carbon sugar) = phosphoglyceric acid (3 carbon acid). This is called the C3 pathway because the product (phosphoglyceric acid) is a 3-carbon molecule. • To fix carbon, plants must open stomata to let in CO2 - Water conc. gradient allows some water to escape.

2. C3 Photosynthesis

3. Photosynthetic Pathways • C4 Photosynthesis • Initial product of photosynthesis for this pathway is a 4-carbon molecule • Acids produced during carbon fixation diffuse from mesophyll cells to bundle sheath cells. • The enzyme that catalyzes CO2 uptake in the C4 pathway has a higher affinity for CO2 than the enzyme used in the C3 pathway. The effect is to reduce the internal CO2 concentrations and create a steeper concentration gradient. Therefore: • Increases rate of CO2 diffusion inward. • Need fewer stomata open. • Conserves water

4. C4 Photosynthesis

5. C4 plants usually outperform C3 plants • C4 plants capable of higher photosynthetic rates • Environments where C4 plants are more efficient: • High light intensity • High temperature • Where water is limiting

6. Why haven’t C4 plants taken over the world? • C4 photsynthesis requires more energy to fix CO2 • C4 spp. not well adapted to low light conditions • Not found in forested or understory habitats

7. C4 plants are most abundant in hot dry areas • Percentage of C4 species in the grass flora

8. Photosynthetic Pathways • CAM Photosynthesis • (Crassulacean Acid Metabolism) • Limited to succulent plants in arid and semi-arid environments. • Carbon fixation takes place at night. • Reduced water loss. • Low rates of photosynthesis. • Extremely high rates of water use efficiency.

9. CAM Photosynthesis

10. C3 Leaf Anatomy C4 Leaf Anatomy