Photosynthetic Pathways C3 Photosynthesis:  Used by most plants and algae.  CO 2 + 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 CO 2 - Water conc. gradient allows some water to escape.

C 3 Photosynthesis

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 CO 2 uptake in the C4 pathway has a higher affinity for CO 2 than the enzyme used in the C3 pathway. The effect is to reduce the internal CO 2 concentrations and create a steeper concentration gradient. Therefore:  Increases rate of CO 2 diffusion inward.  Need fewer stomata open.  Conserves water

C 4 Photosynthesis

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

Why haven’t C4 plants taken over the world? C4 photsynthesis requires more energy to fix CO 2  C4 spp. not well adapted to low light conditions  Not found in forested or understory habitats

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

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.

CAM Photosynthesis

C3 Leaf Anatomy C4 Leaf Anatomy