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Photosynthesis Has Two Sets of Reactions

Photosynthesis Has Two Sets of Reactions. Light-dependent reactions takes place in the thylakoids ; cannot take place unless light is present, traps energy. Light-independent reactions take place in the stroma; can occur in either the light or the dark, fixes carbon.

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Photosynthesis Has Two Sets of Reactions

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  1. Photosynthesis Has Two Sets of Reactions • Light-dependent reactions takes place in the thylakoids; cannot take place unless light is present, traps energy. • Light-independent reactions take place in the stroma; can occur in either the light or the dark, fixes carbon.

  2. 7.3. Solar Energy Is Captured

  3. Photosynthesis Light Dependent

  4. Carbohydrate Is Synthesizedby the Calvin Cyclein the Stroma

  5. A. Light-Independent Reactions • 1. The second stage of photosynthesis; light is not directly required. • 2. Require CO2, which enters through leaf and NADPH and ATP, which have been produced by light-dependent reactions.

  6. C. The Calvin Cycle Has Three Stages

  7. C14 Labeled

  8. Photosynthesis Light Independent

  9. 1. Fixing Carbon Dioxide • a. CO2 fixation is the attachment of CO2 to an organic compound. • b. RuBP (ribulose bisphosphate) is a five-carbon molecule that combines with carbon dioxide.

  10. 2. Reducing PGA • a. Six-carbon molecule immediately breaks down, forms two PGA (3-phosphoglycerate[C3]) molecules.

  11. 2. Reducing PGA • b. Each of two PGA molecules undergoes reduction to PGAL in two steps. • c. Light-dependent reactions provide NADPH (electrons) and ATP (energy) to reduce PGA to PGAL.

  12. 3. Regenerating RuBP • a. Every three turns of Calvin cycle, five molecules of PGAL are used to re-form three molecules of RuBP (ribulose bisphosphate).

  13. 3. Regenerating RuBP • b. Every three turns of Calvin cycle, there is net gain of one PGAL molecule; five PGAL regenerate RuBP. • c.First molecule identified by Calvin was PGA [C3], a three-carbon product; Calvin cycle is also known as C3 cycle.

  14. B. The Importance of PGAL • 1. PGAL (glyceraldehyde-3-phosphate) is product of Calvin cycle; is converted to many organic molecules.

  15. Photosynthesis Takes Other Routes;C3 and C4 Plants • 1. In C3 plants, Calvin cycle fixes CO2 directly; first molecule following CO2 fixation is PGA, a C3 molecule. • 2. C4leaves fix CO2 by forming a C4 molecule prior to the involvement of the Calvin cycle.

  16. C3 and C4 Plants • C3 Plants • Wheat • Rice • Oats • C4 Plants • Corn • Sugarcane • Bermuda grass

  17. Photorespiration • a. In hot weather, stomates close to save water; CO2 concentration decreases in leaves; O2 increases. • b. In C3 plants, O2 competes with CO2, resulting in production of only one molecule of PGA. • c. Called "photorespiration" since oxygen is taken up and CO2 is produced; produces only one PGA.

  18. C4 Plants Flourish When It Is Hot and Dry • Photorespiration does not occur in C4 leaves even when stomates are closed. • C4 plants have advantage over C3 plants: in hot and dry weather, photorespiration does not occur (e.g., bluegrass dominates lawns in early summer, crabgrass takes over in hot midsummer).

  19. The End. Practice Quiz Chapter 10

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