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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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7.3. Solar Energy Is Captured
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Photosynthesis Light Dependent
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Carbohydrate Is Synthesized by the Calvin Cycle in the Stroma
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A. Light-Independent Reactions 1.The second stage of photosynthesis; light is not directly required. 2.Require CO 2, which enters through leaf and NADPH and ATP, which have been produced by light-dependent reactions.
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C.The Calvin Cycle Has Three Stages
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C14 Labeled
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Photosynthesis Light Independent
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1.Fixing Carbon Dioxide a. CO 2 fixation is the attachment of CO 2 to an organic compound. b.RuBP (ribulose bisphosphate) is a five- carbon molecule that combines with carbon dioxide.
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2.Reducing PGA a.Six-carbon molecule immediately breaks down, forms two PGA (3- phosphoglycerate[C 3 ]) molecules.
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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.
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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).
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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 [C 3 ], a three-carbon product; Calvin cycle is also known as C 3 cycle.
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B. The Importance of PGAL 1.PGAL (glyceraldehyde-3- phosphate) is product of Calvin cycle; is converted to many organic molecules.
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Photosynthesis Takes Other Routes; C3 and C4 Plants 1. In C 3 plants, Calvin cycle fixes CO 2 directly; first molecule following CO 2 fixation is PGA, a C 3 molecule. 2. C 4 leaves fix CO 2 by forming a C 4 molecule prior to the involvement of the Calvin cycle.
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C3 and C4 Plants C3 Plants –Wheat –Rice –Oats C4 Plants –Corn –Sugarcane –Bermuda grass
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Photorespiration a.In hot weather, stomates close to save water; CO 2 concentration decreases in leaves; O 2 increases. b.In C 3 plants, O 2 competes with CO 2, resulting in production of only one molecule of PGA. c.Called "photorespiration" since oxygen is taken up and CO 2 is produced; produces only one PGA.
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C 4 Plants Flourish When It Is Hot and Dry Photorespiration does not occur in C 4 leaves even when stomates are closed. C 4 plants have advantage over C 3 plants: in hot and dry weather, photorespiration does not occur (e.g., bluegrass dominates lawns in early summer, crabgrass takes over in hot midsummer).
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The End. Practice Quiz Chapter 10
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