Other Types of Photosynthesis C 4 Photosynthesis and CAM Photosynthesis

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3 Calvin Cycle Reactions: Carbon Dioxide Fixation CO 2 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) CO 2 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 CO 2 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 CO 2

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

Other Types of Photosynthesis C 4 Photosynthesis and CAM Photosynthesis

In C 3 plants, the Calvin cycle fixes CO 2 directly; the first molecule following CO 2 fixation is 3PG. In hot weather, stomata close to save water; CO 2 concentration decreases in leaves; O 2 increases. O 2 combines with RuBP instead of CO 2 This is called photorespiration since oxygen is taken up and CO 2 is produced; this produces less 3PG. Most plants are C 3 plants

In a C 3 plant, mesophyll cells contain well ‑ formed chloroplasts, arranged in parallel layers. In C 4 plants, bundle sheath cells as well as the mesophyll cells contain chloroplasts. In C 4 leaf, mesophyll cells are arranged concentrically around the bundle sheath cells. C 4 Photosynthesis

Remember C 3 plants use RuBP carboxylase to fix CO 2 to RuBP in mesophyll; the first detected molecule is 3PG. C 4 plants use the enzyme PEP carboxylase (PEPCase) to fix CO 2 to PEP (phosphoenolpyruvate, a C 3 molecule); the end product is oxaloacetate (a C 4 molecule). In C 4 plants, CO 2 is taken up in mesophyll cells and malate, a reduced form of oxaloacetate, is pumped into the bundle ‑ sheath cells; here CO 2 enters Calvin cycle. In hot, dry climates, net photosynthetic rate of C 4 plants (e.g., corn) is 2–3 times that of C 3 plants. Photorespiration does not occur in C 4 leaves because PEP does not combine with O 2 ; even when stomata are closed, CO 2 is delivered to the Calvin cycle in bundle sheath cells. C 4 plants have advantage over C 3 plants in hot and dry weather because photorespiration does not occur; e.g., bluegrass (C 3 ) dominates lawns in early summer, whereas crabgrass (C 4 ) takes over in the hot midsummer. C 4 Photosynthesis

CAM (crassulacean ‑ acid metabolism) plants form a C 4 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 CO 2 by forming C 4 molecule stored in large vacuoles in mesophyll. C 4 formed at night is broken down to CO 2 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 CO 2 to enter photosynthesizing tissues; during the day, stomata are closed to conserve water but now CO 2 cannot enter photosynthesizing tissues. Photosynthesis in a CAM plant is minimal, due to limited amount of CO 2 fixed at night; but this does allow CAM plants to live under stressful conditions. CAM Photosynthesis