Other Types of Photosynthesis C 4 Photosynthesis and CAM Photosynthesis

1. Other Types of Photosynthesis C4 Photosynthesis and CAM Photosynthesis

3. Calvin Cycle Reactions:Carbon Dioxide Fixation • CO2 is attached to 5-carbon RuBP molecule • Result in a 6-carbon molecule • This splits into two 3-carbon molecules (3PG) • Reaction accelerated by RuBP Carboxylase (Rubisco) • CO2 now “fixed” because it is part of a carbohydrate

4. Calvin Cycle Reactions:Carbon Dioxide Reduction • 3PG reduced to BPG • BPG then reduced to G3P • Utilizes NADPH and some ATP produced in light reactions

5. Calvin Cycle Reactions:Regeneration of RuBP • RuBP used in CO2 fixation must be replaced • Every three turns of Calvin Cycle, • Five G3P (a 3-carbon molecule) used To remake three RuBP (a 5-carbon molecule)

6. The Calvin Cycle: Fixation of CO2

7. Importance of Calvin Cycle • G3P (glyceraldehyde-3-phosphate) can be converted to many other molecules • The hydrocarbon skeleton of G3P can form • Fatty acids and glycerol to make plant oils • Glucose phosphate (simple sugar) • Fructose (which with glucose = sucrose) • Starch and cellulose • Amino acids

9. Other Types of Photosynthesis C4 Photosynthesis and CAM Photosynthesis

10. Most plants are C3 plants • In C3 plants, the Calvin cycle fixes CO2 directly; the first molecule following CO2 fixation is 3PG. • In hot weather, stomata close to save water; CO2 concentration decreases in leaves; O2 increases. • O2 combines with RuBP instead of CO2 • This is called photorespiration since oxygen is taken up and CO2 is produced; this produces less 3PG.

11. C4 Photosynthesis • In a C3 plant, mesophyll cells contain well‑formed chloroplasts, arranged in parallel layers. • In C4 plants, bundle sheath cells as well as the mesophyll cells contain chloroplasts. • In C4 leaf, mesophyll cells are arranged concentrically around the bundle sheath cells.

12. C4 Photosynthesis • Remember C3 plants use RuBP carboxylase to fix CO2 to RuBP in mesophyll; the first detected molecule is 3PG. • C4 plants use the enzyme PEP carboxylase (PEPCase) to fix CO2 to PEP (phosphoenolpyruvate, a C3 molecule); the end product is oxaloacetate (a C4 molecule). • In C4 plants, CO2 is taken up in mesophyll cells and malate, a reduced form of oxaloacetate, is pumped into the bundle‑sheath cells; here CO2 enters Calvin cycle. • In hot, dry climates, net photosynthetic rate of C4 plants (e.g., corn) is 2–3 times that of C3 plants. • Photorespiration does not occur in C4 leaves because PEP does not combine with O2; even when stomata are closed, CO2 is delivered to the Calvin cycle in bundle sheath cells. • C4 plants have advantage over C3 plants in hot and dry weather because photorespiration does not occur; e.g., bluegrass (C3) dominates lawns in early summer, whereas crabgrass (C4) takes over in the hot midsummer.

13. CAM Photosynthesis • CAM(crassulacean‑acid metabolism) plants form a C4 molecule at night when stomata can open without loss of water; found in many succulent desert plants including the family Crassulaceae. • At night, CAM plants use PEPCase to fix CO2 by forming C4 molecule stored in large vacuoles in mesophyll. • C4 formed at night is broken down to CO2 during the day and enters the Calvin cycle which now has NADPH and ATP available to it from the light‑dependent reactions. • CAM plants open stomata only at night, allowing CO2 to enter photosynthesizing tissues; during the day, stomata are closed to conserve water but now CO2 cannot enter photosynthesizing tissues. • Photosynthesis in a CAM plant is minimal, due to limited amount of CO2 fixed at night; but this does allow CAM plants to live under stressful conditions.