photosynthesis has two sets of reactions light-dependent reactions takes place in the thylakoids ;...
TRANSCRIPT
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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 Synthesizedby 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 CO2, 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. 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.
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2. Reducing PGA
• a. Six-carbon molecule immediately breaks down, forms two PGA (3-phosphoglycerate[C3]) 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 [C3], a three-carbon product;
Calvin cycle is also known as C3 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 C3 plants, Calvin cycle fixes CO2
directly; first molecule following CO2 fixation is PGA, a C3 molecule.
• 2. C4 leaves fix CO2 by forming a C4
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; 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.
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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).
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The End.
Practice Quiz Chapter 10
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